Downloaded 10 times
![1736210008
Main method
A public static method with the following signature can be used to start a Java
application. Such a method is typically called main method. public static void
main(String[] args){
}
Constructor
A class contains constructors that are invoked to create objects based on the class
definition. Constructor declarations look like method declarations except that they use the
name of the class and have no return type. A class can have several constructors with
different parameters. Each class must define at least one constructor.
In the following example the constructor of the class expects a parameter.
package com.vogella.javaintro.base;
public class MyConstructorExample2 {
String s;
public MyConstructorExample2(String s) {
this.s = s;
}
}
If no explicit constructor is defined the compiler adds implicitly a constructor. If the class
is sub-classed then the constructor of the super class is always implicitly called in this
case.
In the following example the definition of the constructor without parameters (also
known as the empty constructor) is unnecessary. If not specified the compiler
would create one.
package com.vogella.javaintro.base;
public class MyConstructorExample {
// Unnecessary, would be created by the compiler if left out
public MyConstructorExample() {
}
}
The naming conversion for creating a constructor is the following: classname
(Parameter p1, ..) {} .](https://image.slidesharecdn.com/industrialtrainingfinal-210321183531/85/Industrial-training-report-on-core-java-18-320.jpg)
![1736210008
}
}
Class methods and class variables
Class methods and class variables are associated with the class and not an instance of the
class, i.e. objects. To refer to these element you can use the classname and a dot (".")
followed by the class method or class variable name. Class methods and class variables
are declared with the static keyword. Class methods are also called static methods and
class variables are also called static variables or static fields.
An example for the usage of a static field is println of the following statement:
System.out.println("Hello World"). out is a static field, an object of type
PrintStream, and you call the println() method on this object. If you define a static
variable the Java runtime environment associates one class variable for a class no matter
how many instances (objects) exists. The static variable can therefore be seen as a global
variable. The following code demonstrates the usage of static fields.
package com.vogella.javaintro.base;
public class MyStaticExample {
static String PLACEHOLDER = "TEST";
static void test() {
System.out.println("Hello");
}
} package com.vogella.javaintro.base;
public class Tester {
public static void main(String[] args) {
System.out.println(MyStaticExample.PLACEHOLDER);
MyStaticExample.test();
}
}
If a variable should be defined as constant, you declare it with the static and the final
keyword. The static method runs without any instance of the class, it cannot directly
access non-static variables or methods.
Abstract class and methods
A class and method can be declared as abstract. An abstract class can not be directly
instantiated. If a class has at least one method which only contain the declaration of the](https://image.slidesharecdn.com/industrialtrainingfinal-210321183531/85/Industrial-training-report-on-core-java-21-320.jpg)
![1736210008
{ this.myvalue =
myvalue; }
public Dog getDog() {
return dog;
} public
void
setDog(Dog dog)
{ this.dog =
dog; }
}
What to do How to do it
it.
Working With Local Variable
Table 3:
What to do How to do it
Declare a (local) variable of type string. String variable1;
Declare a (local) variable of type string and assign
"Test" to it. String variable2 = "Test";
Declare a (local) variable of type Person Person person;
Declare a (local) variable of type Person, create a
new Object and assign the variable to this object. Person person = new Person();
Declare a array of type String String array[];
Declare a array of type Person and create an array
for this variable which can hold 5 Persons. Person array[]= new Person[5];
Assign 5 to the int variable var1 (which was
already declared); var1 = 5;
Assign the existing variable pers2 to the exiting
variable pers1; pers1 = pers2;
Declare a ArrayList variable which can hold
objects of type Person ArrayList<Person> persons;
Create a new ArrayList with objects of type
Person and assign it to the existing variable
persons
persons = new
ArrayList<Person>();](https://image.slidesharecdn.com/industrialtrainingfinal-210321183531/85/Industrial-training-report-on-core-java-25-320.jpg)
![1736210008
Declare a ArrayList variable which can hold
objects of type Person and create a new Object for
ArrayList<Person> persons =
new ArrayList<Person>();
Integrated Development Environment
The previous chapter explained how to create and compile a Java program on the
command line. A Java Integrated Development Environment (IDE) provides lots of ease
of use functionality for creating java programs. There are other very powerful IDE's
available, for example the Eclipse IDE. . For an introduction on how to use the Eclipse
IDE please see Eclipse IDE Tutorial. The remaining description uses the phrase: Create a
Java project called... ". This refers to creating a Java project in Eclipse. If you are using a
different IDE please follow the required steps in this IDE. Exercises - Creating Java
objects and methods
Create a Person class and instantiate it Create a new Java project called
com.vogella.javastarter.exercises1 and a package with the same name. Create a class
called Person. Add three instance variables to it, one for storing the first
name of the person, on for storing the last name and one for storing the age of the Person.
Use the constructor of the Person object to set the values to some default value. Write a
public method called writeName() which uses the System.out.println( method to print the
first name of the person to the console. Create a new class called Main with a public static
void main(String[] args). In this method create an instance of the Person class.
Use constructor
Add a constructor to your Person class which takes the first name, last name and the age
as parameter. Assign the values to your instance variables. Create in your main method
two objects of type Person and call the writeName method on it.
Define getter and setter methods
Define methods which allow you to read the values of the instance variables and to set them.
These methods are called setter and getter. Getters should start with get followed by the
variable name whereby the first letter of the variable is capitized. Setter should start with set
followed by the variable name whereby the first letter of the variable is capitized. ` For
example the variable called firstName would have the getFirstName() getter method and the
setFirstName(String s) setter method. Change your main method so that you create one
person object and use the setter method to change the last name.](https://image.slidesharecdn.com/industrialtrainingfinal-210321183531/85/Industrial-training-report-on-core-java-26-320.jpg)
![1736210008
Create an Address object
Create a new object called Address. The Address should allow you to store the address
of a person. Add a new instance variable of this type in the Person object.Also create a getter
and setter for the Address object in the Person object
Solution - Creating Java objects and methods Create a Person class
and instantiate it
The following is a potential solution for Section 11.1, “Create a Person class and instantiate
it”.
package exercises.exercise04;
class Person {
String firstname = "Jim";
String lastname = "Knopf";
int age = 12;
voi d writeName() {
// Writes the firstname
System.out.println(firstname);
// Alternatively you can combine strings with +
System.out.println(firstname + " " + lastname + "" + age);
}
}
package exercises.exercise04;
public class Main { public static void main(String[]
args) {
Person person = new Person();
}
person.writeName();](https://image.slidesharecdn.com/industrialtrainingfinal-210321183531/85/Industrial-training-report-on-core-java-27-320.jpg)
![1736210008
}
Use constructor
package com.vogella.javastarter.exercises1;
class Person {
String firstName; String
lastName;
int age;
public Person(String a, String b, int value) {
firstName = a;
lastName = b;
age=value;
}
void writeName() {
// Writes the firstname System.out.println(firstName);
// Alternatively you can combine strings with +
System.out.println(firstName + " " + lastName + "" + age);
}
} package
com.vogella.javastarter.exercises1;
public class Main { void
main(String[] args) {
Person person = new Person("Jim", "Knopf" , 12);](https://image.slidesharecdn.com/industrialtrainingfinal-210321183531/85/Industrial-training-report-on-core-java-28-320.jpg)
![1736210008
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
void writeName() {
// Writes the firstname
System.out.println(firstName);
// Alternatively you can combine strings with +
System.out.println(firstName + " " + lastName + "" + age);
}
} package
com.vogella.javastarter.exercises1; public
class Main { public static void
main(String[] args) {
Person person = new Person("Jim", "Knopf", 21);
Person person2 = new Person("Jill", "Sanders", 20);
// Jill get married to Jim
person2.setLastName("Knopf"); person2.writeName();](https://image.slidesharecdn.com/industrialtrainingfinal-210321183531/85/Industrial-training-report-on-core-java-30-320.jpg)
![1736210008
public Address getAddress() { return
address;
}
public void setAddress(Address address) {
this.address = address;
}
void writeName() {
// Writes the firstname
System.out.println(firstName);
// Alternatively you can combine strings with +
System.out.println(firstName + " " + lastName + "" + age);
}
} package
com.vogella.javastarter.exercises1;
public class Main { public static void main(String[]
args) {
// I create a person
Person pers = new Person("Jim", "Knopf", 31);
// I set the age of the person to 32](https://image.slidesharecdn.com/industrialtrainingfinal-210321183531/85/Industrial-training-report-on-core-java-34-320.jpg)
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Industrial training report on core java
- 1. Lucknow Institute ofTechnology, Lucknow INDUSTRIAL TRAINING REPORT ON Core Java SUBMITTEDIN PARTIAL FULFILLMENTFOR THE AWARD OF THE OF DEGREE OF BACHELOR OF TECHNOLOGY IN COMPUTER SCIENCE & ENGINEERING SUBMITTED BY ABHINAV KUMAR MAURYA B.Tech VII Semester 1736210001 Trainingtaken under 2020 -21
- 2. 1736210008 ACKNOWLEDGEMENT The completion ofthis training work could have been possible with continued & dedicated efforts & guidance of large number of faculty & staff members of the institute. I acknowledge our gratitude to all of them. The acknowledgement however will be incomplete without specific mention as follows I wish to acknowledge my deep gratitude to Mr. Puneet Vishwakarma, teacher at Protec for his cooperation and guidance. I am also thankful to his Lab assistant that provided staunch support throughout this training and helped me to complete the training successfully Furthermore, I would also like to acknowledge with much appreciation the crucial role of our HOD Prof. Mrs. Arifa Khan for this encouragement & providing all the facilities in the department Finally, I would like to say that I am indebted to my parents for everything that they have done for me. All of this would have been impossible without their constant support. And I also thank to God for being kind to me and driving me through this journey. Abhinav Kumar
- 3. 1736210008 LUCKNOW INSTITUTE OFTECHNOLOGY, LUCKNOW Department of Computer Science & Engineering This is to certify that the Mr. Abhinav Kumar student of Lucknow Institute of Technology, Lucknow Utter Pradesh B.tech 7th semester (computer science and engineering), has completed his Vocational training at Protec Computer Consultancy PVT.. LTD. Uttar Pradesh from Lucknow. His overall performance & behaviour during the training is found to be Satisfactory. We wish him all success for this future endeavors Signature Mrs. Arifa Khan, HOD CERTIFICATE
- 4. 1736210008 LUCKNOW INSTITUTE OFTECHNOLOGY, LUCKNOW Department of Computer Science & Engineering APPROVAL CERTIFICATE This dissertation work entitled “Industrial Training Report on Java” submitted by Mr. Abhinav Kumar is approved for the award of degree of Bachelor of Technology in Computer Science And Engineering. Internal Examiner External Examiner Date: Date:
- 5. 1736210008 LUCKNOW INSTITUTE OFTECHNOLOGY LUCKNOW Department of Computer Science & Engineering CANDIDATE DECLARATION I Abhinav Kumar, a student of Bachelor of Technology in Computer Science & Engineering, Lucknow Institute of Technology, Lucknow (U.P.), hereby declare that the work presented in this dissertation “Industrial Training on Java” is the outcome of my own work, is bonafide and correct to the best of my knowledge and this work has been carried out taking care of Engineering Ethics. The work presented does not infringe any patented work and has not been submitted to any other university or anywhere else for the award of any degree or any professional degree. Date: Abhinav Kumar. Enroll:-173621001
- 6. 1736210008 Index S. No TOPICNAME PAGE NO. SIGN. 1 Company Profile 7 2 Introduction To Java 8 3 The Java Platform 9 4 Installation Of Java 10 5 Configuring Variable 11 6 Write, Compile And Run a Java Program 13 7 Package 15 8 Class And Object 16 9 Inheritance 17 10 Variable And Method 18 11 Modifiers And Import Statement 20 12 Interface 22 13 Working With Classes 24 14 IDE 27 15 History of Java 40 16 Features of Java 43
- 7. 1736210008 PCOMPANY PROFILES Since2011, Protec Computer Consultancy in Alambagh, Lucknow has been offering professional training to students. It specialises and is well-known for training students as well as working professionals in accounting, web designing, programming languages, hardware and networking. It is run and managed by a seasoned professionals who leads a team of educators and trainers having relevant domain expertise. At this institution, one can get trained in the subject of their choice by opting from a wide range of courses. These easy-to-follow courses are primarily aimed at students, working professionals as well as IT professionals who want to enhance their knowledge and further their career prospects. Located Lucknow- Kanpur Road nh25, you can find this institution with relative ease at building no. 48c in Alambagh. Undoubtedly it is one of the best computer training institutes in Alambagh, Lucknow. Features Services at Protec Computer Consultancy ComputerTraining Institute for Java. ComputerTraining Institute for Network. ComputerTraining Institute for .net. ComputerTraining Institute for linux and so on. Protec Computer Consultancy in Alambagh offers short-term courses and certificate courses. Inclusive of comprehensive learning, the long-term programmes feature subjects such as web development, financial accountancy, computer application and programming, information technology, multimedia and web-designing. Some of the short-term courses cover topics like Windows XP, 7, 8, 10, Vista, MS Office, DTP (Desk Top Publishing), Web Designing, Multimedia, Tally ERP 9, C, C++ , and Visual Basic. Walk into this centre all through the week between 08:00 - 20:00.Pay in Cash, Cheques.
- 8. 1736210008 Introduction to Java Withthe invention of microprocessors, the world is scientifically developed with sophisticated equipments, systems, and devices. Microprocessors are used in computers, televisions, and fax machines. Even the hand-held devices such as pagers, PDAs (Personal Digital Assistant), and cell phones make use of microprocessors. All these electronic devices are helpful because of their communication capabilities. With the increasing capabilities and decreasing cost of information processing and networking technologies, the network is growing rapidly for transmitting information through electronic systems. Internet is the network of networks between different types of computers located at different places to transmit information. Information can reach to any place in the world quickly at a cheaper rate through the Internet. Thus, the Internet has made the world a global village for information exchange. The emerging infrastructure of electronic devices and interconnected computer networks create an environment that presents new challenges to software industries. for this emerging computing environment, Java process to be a well – suited programming language. it is found suitable for networked environments involving a great variety of computer and devices. Java has many characteristics that have contributed to its popularity: Platform independence - Many languages are compatible with only one platform. Java was specifically designed so that it would run on any computer, regardless if it was running Windows, Linux, Mac, Unix or any of the other operating systems. Simple and easy to use - Java's creators tried to design it so code could be written efficiently and easily. Multi-functional - Java can produce many applications from command-line programs to applets to Swing windows (basically, sophisticated graphical user interfaces). Java does have some drawbacks. Since it has automated garbage collection, it can tend to use more memory than other similar languages. There are often implementation differences on different platforms, which have led to Java being described as a "write once, test everywhere" system. Lastly, since it uses an abstract "virtual machine", a generic Java program doesn't have access to the Native API's on a system directly. None of these issues are fatal, but it can mean that Java isn't an appropriate choice for a particular piece of software. The Java Platform One thing that distinguished Java from some other languages is its ability to run the same compiled code across multiple operating systems.In other languages, the source code (code that is written by the programmer), is compiled by a compiler into an executable file. This file is in machine language, and is intended for a single operating system/processor combination, so the programmer would have to re-compile the program seperately for each new operating system/processor combination.Java is different in that it does not compile the code directly into machine language code. Compilation creates bytecode out of the source code. Bytecode generally looks something like this:
- 9. 1736210008 a7 f4 735a 1b 92 7d When the code is run by the user, it is processed by something called the Java Virtual Machine (JVM). The JVM is essentially an interpreter for the bytecode. It goes through the bytecode and runs it. There are different versions of the JVM that are compatible with each OS and can run the same code. There is virtually no difference for the end-user, but this makes it a lot easier for programmers doing software development. Java and Open Source:- In 2006 Sun started to make Java available under the GNU General Public License (GPL). Oracle continues this project called OpenJDK. Java Virtual machine :- The Java virtual machine (JVM) is a software implementation of a computer that executes programs like a real machine. The Java virtual machine is written specifically for a specific operating system, e.g. for Linux a special implementation is required as well as for Windows. Java programs are compiled by the Java compiler into bytecode. The Java virtual machine interprets this bytecode and executes the Java program. Java Runtime Environment vs. Java Development Kit A Java distribution comes typically in two flavors, the Java Runtime Environment (JRE) and the Java Development Kit (JDK). The Java runtime environment (JRE) consists of the JVM and the Java class libraries and contains the necessary functionality to start Java programs. The JDK contains in addition the development tools necessary to create Java programs. The JDK consists therefore of a Java compiler, the Java virtual machine, and the Java class libraries. Installation of Java Check installation Java might already be installed on your machine. You can test this by opening a console (if you are using Windows: Win+R, enter cmd and press Enter) and by typing in the following command: java -version If Java is correctly installed, you should see some information about your Java installation. If the command line returns the information that the program could not be
- 10. 1736210008 found, you haveto install Java. Installing the Java Development Kit Before installing the Java Development Kit (JDK), you should probably know what it is. It is distributed by Oracle. It contains the core libraries and compiler required to develop Java. The JDK should not be confused with the JRE (Java Runtime Environment). The JRE is a JVM for running, as opposed to compiling, Java programs. Downloading and Installing To download the JDK, go to http://www.oracle.com/technetwork/java/javase/downloads/index.html. Click on "JDK with NetBeans Bundle". Follow the instructions for downloading the JDK installation file. Windows: If you are running Windows, simply run the executable file and follow the installation instructions. Unix, Solaris, or Linux: For Linux and Unix, download the "jdk1 6.0" for Linux systems. Save the downloaded file in any drive. Once you have saved the file, extract it to a place that you can remember, by using Terminal or by double clicking on the file. When you have finished extracting the file, copy the JDK 1.6.0 folder and paste it in the usr/local(To paste to the usr/local directory, you have to be in root) so that every user can use the java files. You can delete the downloaded zip file so that it doesn't take up space on your drive. Macintosh: The latest available JDK is automatically installed by the operating system. Because Java for Macintosh is developed and maintained by Apple, in coordination with Sun, the current version on the Macintosh may not be the current version that is available Note on Editions The JDK comes in three editions. Java Standard Edition (JSE) – This version is the basic platform for Java. The course will focus on this edition. Java Enterprise Edition (JEE) – This edition is mainly for developing and running distributed multitier architecture Java applications, based largely on modular software components running on an application server. We will not be covering this version in the course. Java Micro Edition (JME) – This edition is primarily for developing programs to run on consumer applicances, such as PDAs and cell phones. Configuring Variables Before writing code, it is recommended that you set the Path variable on your system
- 11. 1736210008 so you cancompile your code more easily. For Windows Users From the Control Panel, double click "System" (System and Maintenance in Vista) For Windows 7 or Vista, click on "System," "Advanced System Settings" on the left, and then on "Environment Variables." For XP and 2000, click on the "Advanced" tab and click on "Environment Variables" For NT, click on the "Environment" tab. Select the Path variable and click "Edit" Add the path to the bin directory of where Java is installed on your hard drive. It should probably be: C:Program FilesJavajdk1.6.0_20bin unless you changed it during installation. Click OK For Linux and UNIX One way to set your path in Linux/Unix is to add a path export to your bash profile. In order to do this, first open your bash profile in a text editor. For example, pico ~/.bash_profile Then add this line: export PATH=$PATH:/usr/local/jdk/bin Note that the path to the java directory "/usr/local/jdk/bin" may be different on your machine. Restart your shell. For Macintosh Apple sets everything up for you. Sit back and relax. The only drawback is that because Apple handles development and maintenance of Java on the Mac, there is usually a delay from the time that a new version is released by Sun and the time that the new version is released on the Mac. Also, getting the latest version sometimes requires an operating system upgrade. Oh well, you can't have everything. Validate installation Switch again to the command line and run the following command. java -version The output should be similar to the following output. java version "1.7.0_25" OpenJDK Runtime Environment (IcedTea 2.3.10) (7u25-2.3.10-1ubuntu0.13.04.2) OpenJDK 64-Bit Server VM (build 23.7-b01, mixed mode)
- 12. 1736210008 Development Process withJava Java source files are written as plain text documents. The programmer typically writes Java source code in an Integrated Development Environment (IDE) for programming. An IDE supports the programmer in the task of writing code, e.g. it provides auto-formating of the source code, highlighting of the important keywords, etc. At some point the programmer (or the IDE) calls the Java compiler (javac). The Java compiler creates the bytecode instructions. These instructions are stored in .class files and can be executed by the Java Virtual Machine. Garbage collector The JVM automatically re-collects the memory which is not referred to by other objects. The java garbage collector checks all object references and find the objects which can be automatically released. While the garbage collector releases the programmer from the need to explicitly manage memory the programmer still need to ensure that he does not keep unneeded object references otherwise the garbage collector cannot release the associated memory. Keeping unneeded object references are typically called memory leaks. Classpath The classpath defines where the Java compiler and Java runtime look for .class files to load. This instructions can be used in the Java program. For example if you want to use an external Java library you have to add this library to your classpath to use it in your program. Write, compile and run a Java program Write source code The following Java program is developed under Linux using a text editor and the command line. The process on other operating system should be similar and but is not covered in this description. Select or create a new directory which will be used for your Java development. In this description the path homevogellajavastarter is used. On Microsoft Windows your might want to use c:tempjavastarter. This path is called javadir in the following description. Open a text editor which supports plain text, e.g. gedit under Linux or Notepad under Windows and write the following source code.
- 13. 1736210008 Save the sourcecode in your javadir directory with the HelloWorld.java filename. The name of a Java source file must always equals the class name (within the source code) and end with the .java extension. In this example the filename must be HelloWorld.java because the class is called HelloWorld. Compile and run your Java program Open a shell for command line access. Switch to the javadir directory with the command cd javadir, for example in the above example via the cd homevogellajavastarter command. Use the ls command (dir under Microsoft Windows) to verify that the source file is in the directory. Compile your Java source file into a class file with the following command. javac HelloWorld.java Afterwards list again the content of the directory with the ls or dir command. The directory contains now a file "HelloWorld.class". If you see this file you have successfully compiled your first Java source code into bytecode. You can now start your compiled Java program. Ensure that you are still in the jardir directory and enter the following command to start your Java program. java HelloWorld The system should write "Hello World" on the command line.
- 14. 1736210008 Using the classpath You can use the classpath to run the program from another place in your directory. Switch to the command line, e.g. under Windows Start-> Run -> cmd. Switch to any directory you want. Type: java HelloWorld If you are not in the directory in which the compiled class is stored then the system should result an error message Exception in thread "main" java.lang.NoClassDefFoundError: test/TestClass To use the class type the following command. Replace "mydirectory" with the directory which contains the test directory. You should again see the "HelloWorld" output. java -classpath "mydirectory" HelloWorld Java basic terms Basics: Package, Class and Object It is important to understand the base terminology of Java in terms of packages, classes and objects. This section gives an overview of these terms. Package Java groups classes into functional packages. Packages are typically used to group classes into logical units. For example all graphical views of an application might be placed in the same package called com.vogella.webapplication.views. It is common practice to use the reverse domain name of the company as top level package. For example the company might own the domain, vogella.com and in this example the Java packages of this company starts with com.vogella.
- 15. 1736210008 Other main reasonfor the usage of packages is to avoid name collisions of classes. A name collision occurs if two programmers give the same fully qualified name to a class. The fully qualified name of a class in Java consists out of the package name followed by a dot (.) and the class name. Without packages, a programmer may create a Java class called Test. Another programmer may create a class with the same name. With the usage of packages you can tell the system which class to call. For example if the first programmer puts the Test class into package report and the second programmer puts his class into package xmlreader you can distinguish between these classes by using the fully qualified name, e.g. xmlreader.Test or report.Test. Class Def.: Template that describes the data and behavior associated with an instance of that class. In Java source code a class is defined by the class keyword and must start with a capital letter. The body of a class is surrounded by {}. package test; class MyClass { } The data associated with a class is stored in variables ; the behavior associated to a class or object is implemented with methods. A class is contained in a Java source file with the same name as the class plus the .java extension. Object Def.: An object is an instance of a class. The object is the real element which has data and can perform actions. Each object is created based on the class definition. Inheritance A class can be derived from another class. In this case this class is called a subclass. Another common phrase is that a class extends another class. The class from which the subclass is derived is called a superclass. Inheritance allows a class to inherit the behavior and data definitions of another class.
- 16. 1736210008 The following codesdemonstrates how a class can extend another class. In Java a class can extend a maximum of one class. package com.vogella.javaintro.base; class MyBaseClass { void hello(){ System.out.println("Hello from MyBaseClass"); } } package com.vogella.javaintro.base; class MyExtensionClass extends MyBaseClass { } Override methods and the @override annotation If a class extends another class it inherits the methods from its superclass. If it wants to change these methods it can override these methods. To override a method you use the same method signature in the source code of the subclass. To indicate to the reader of the source code and the Java compiler that you have the intention to override a method you can use the @override annotation. The following code demonstrates how you can override a method from a superclass. package com.vogella.javaintro.base; class MyBaseClass { void hello(){ System.out.println("Hello from MyBaseClass"); } } package com.vogella.javaintro.base; class MyExtensionClass2 extends MyBaseClass { } Object has superclass Every object in Java implicitly extends the Object class. The class defines the following methods for every Java object: equals(o1) allows to check if the current object is equal to o1 getClass() returns the class of the object hashCode() returns an identifier of the current object toString() Give a string representation of the current object
- 17. 1736210008 Variables and methods Variable Variables allow the Java program to store values during the runtime of the program. A variable can either be a primitive variable or a reference variable. A primitive variable contains value while the reference variable contains a reference (pointer) to the object. Hence if you compare two reference variables, you compare if both point to the same object. To compare objects use the object1.equals(object2) method call. Instance variable Instance variable is associated with an instance of the class (also called object). Access works over these objects. Instance variables can have any access control and can be marked final or transient. Instance variables marked as final can not be changed after assigned to a value. Local variable Local (stack) variable declarations cannot have access modifiers. final is the only modifier available to local variables. This modifier defines that the variable can not be changed after first assignment. Local variables do not get default values, so they must be initialized before use. Methods A method is a block of code with parameters and a return value. It can be called on the object. package com.vogella.javaintro.base; public class MyMethodExample { void tester(String s) { System.out.println("Hello World"); } } Method can be declared with var-args. In this case the method declares a parameter which accepts from zero to many arguments (syntax: type .. name;) A method can only have one var-args parameter and this must be the last parameter in the method. Overwrite of a superclass method: A method must be of the exact same return parameter and the same arguments. Also the return parameter must be the same. Overload methods: An overloaded method is a method with the same name, but different arguments. The return type can not be used to overload a method.
- 18. 1736210008 Main method Apublic static method with the following signature can be used to start a Java application. Such a method is typically called main method. public static void main(String[] args){ } Constructor A class contains constructors that are invoked to create objects based on the class definition. Constructor declarations look like method declarations except that they use the name of the class and have no return type. A class can have several constructors with different parameters. Each class must define at least one constructor. In the following example the constructor of the class expects a parameter. package com.vogella.javaintro.base; public class MyConstructorExample2 { String s; public MyConstructorExample2(String s) { this.s = s; } } If no explicit constructor is defined the compiler adds implicitly a constructor. If the class is sub-classed then the constructor of the super class is always implicitly called in this case. In the following example the definition of the constructor without parameters (also known as the empty constructor) is unnecessary. If not specified the compiler would create one. package com.vogella.javaintro.base; public class MyConstructorExample { // Unnecessary, would be created by the compiler if left out public MyConstructorExample() { } } The naming conversion for creating a constructor is the following: classname (Parameter p1, ..) {} .
- 19. 1736210008 Every object iscreated based on a constructor. This constructor method is the first statement called before anything else can be done with the object. Modifiers Access modifiers There are three access modifiers keywords available in Java. public, protected and private. There are four access levels: public, protected, default and private. They define how the corresponding element is visible to other components. If something is declared public, e.g. classes or methods can be freely created or called by other Java objects. If something is declared private, e.g. a method, it can only be accessed within the class in which it is declared. protected and default are similar. A protected class can be accessed from the package and sub- classes outside the package while a default class can get only accessed via the same package. The following table describes the visibility: Table 1. Access Level Modifier Class Package Subclass World Public Y Y Y Y protected Y Y Y N Modifier Class Package Subclass World no modifier Y Y N N Private Y N N N Other modifiers final methods: cannot be overwritten in a subclass abstract method: no method body synchronized method: threat safe, can be final and have any access control native methods: platform dependent code, apply only to methods strictfp: class or method Import statements Usage of import statements In Java you have to access a class always via its full-qualified name, e.g. the package name and the class name. You can add import statements for classes or packages into your class file, which allow you to use the related classes in your code without the
- 20. 1736210008 package qualifier. Staticimports Static import is a feature that allows members (fields and methods) which are defined in a class with the public static access modifier to be used in Java code without specifying the class in which the member is defined. The feature provides a typesafe mechanism to include constants into code without having to reference the class that originally defined the field. More Java language constructs Interface Interfaces are contracts for what a class can do but they say nothing about the way in which the class must do it. An interface is a type similar to a class. Like a class an interface defines methods. An interface can have only abstract methods, no concrete methods are allowed. Methods defined in interfaces are by default public and abstract – explicit declaration of these modifiers is optional. Interfaces can have constants which are always implicitly public, static and final. A class can implement an interface. In this case it must provide concrete implementations of the interface methods. If you override a method defined by an interface you can also use the @override annotation. The following code shows an example implementation of an interface and its usage within a class. package com.vogella.javaintro.base; public interface MyDefinition { // constant definition String URL="http://www.vogella.com"; // define several method stubs void test(); void write(String s); } package com.vogella.javaintro.base; public class MyClassImplementation implements MyDefinition { @Override public void test() { // TODO Auto-generated method stub } @Override public void write(String s) { // TODO Auto-generated method stub
- 21. 1736210008 } } Class methods andclass variables Class methods and class variables are associated with the class and not an instance of the class, i.e. objects. To refer to these element you can use the classname and a dot (".") followed by the class method or class variable name. Class methods and class variables are declared with the static keyword. Class methods are also called static methods and class variables are also called static variables or static fields. An example for the usage of a static field is println of the following statement: System.out.println("Hello World"). out is a static field, an object of type PrintStream, and you call the println() method on this object. If you define a static variable the Java runtime environment associates one class variable for a class no matter how many instances (objects) exists. The static variable can therefore be seen as a global variable. The following code demonstrates the usage of static fields. package com.vogella.javaintro.base; public class MyStaticExample { static String PLACEHOLDER = "TEST"; static void test() { System.out.println("Hello"); } } package com.vogella.javaintro.base; public class Tester { public static void main(String[] args) { System.out.println(MyStaticExample.PLACEHOLDER); MyStaticExample.test(); } } If a variable should be defined as constant, you declare it with the static and the final keyword. The static method runs without any instance of the class, it cannot directly access non-static variables or methods. Abstract class and methods A class and method can be declared as abstract. An abstract class can not be directly instantiated. If a class has at least one method which only contain the declaration of the
- 22. 1736210008 method but notthe implementation then this class is abstract and can not be instantiated. Sub-classes need then to define the methods except if they are also declared as abstract. If a class contains an abstract method it also needs to get defined with the keyword abstract. The following example shows an abstract class. package com.vogella.javaintro.base; public abstract class MyAbstractClass { abstract double returnDouble(); } Working With Classes Table 2 What to do How to do it Create a new class called MyNewClass. package test; public class MyNewClass { } Create a new attribute (instance variable) called var1 of type String in the MyNewClass class package test; public class MyNewClass { private String var1; } Create a Constructor for your MyNewClass class which has a String parameter and assigns the value of it to the var1 instance variable. package test; public class MyNewClass { private String var1; public MyNewClass(String para1) { var1 = para1; // or this.var1= para1; } }
- 23. 1736210008 Create a newmethod called doSomeThing in your class which does not return a value and has no parameters package test; public class MyNewClass { private String var1; public MyNewClass(String para1) { var1 = para1; // or this.var1= para1; } public void doSomeThing() { } } Create a new method called doSomeThing2 in your class which does not return a value and has two parameters, a int and a Person package test; public class MyNewClass { private String var1; public MyNewClass(String para1) { var1 = para1; // or this.var1= para1; } public void doSomeThing() What to do How to do it { } public void doSomeThing2(int a, Person person) { } }
- 24. 1736210008 package test; public classMyNewClass { private String var1; public MyNewClass(String para1) { var1 = para1; // or this.var1= para1; } public void doSomeThing() { Create a new method called doSomeThing2 in your class which returns an int value and has three parameters, two Strings and a Person } public void doSomeThing2(int a, Person person) { } public int doSomeThing3(String a, String b, Person person) { return 5; // Any value will do for this example } } What to do How to do it package test; Create a class called MyOtherClass with two instance variables. One will store a String, the other will store a Dog. Create getter and setter for these variables. public class MyOtherClass { String myvalue; Dog dog; public String getMyvalue() { return myvalue; } public void setMyvalue(String myvalue)
- 25. 1736210008 { this.myvalue = myvalue;} public Dog getDog() { return dog; } public void setDog(Dog dog) { this.dog = dog; } } What to do How to do it it. Working With Local Variable Table 3: What to do How to do it Declare a (local) variable of type string. String variable1; Declare a (local) variable of type string and assign "Test" to it. String variable2 = "Test"; Declare a (local) variable of type Person Person person; Declare a (local) variable of type Person, create a new Object and assign the variable to this object. Person person = new Person(); Declare a array of type String String array[]; Declare a array of type Person and create an array for this variable which can hold 5 Persons. Person array[]= new Person[5]; Assign 5 to the int variable var1 (which was already declared); var1 = 5; Assign the existing variable pers2 to the exiting variable pers1; pers1 = pers2; Declare a ArrayList variable which can hold objects of type Person ArrayList<Person> persons; Create a new ArrayList with objects of type Person and assign it to the existing variable persons persons = new ArrayList<Person>();
- 26. 1736210008 Declare a ArrayListvariable which can hold objects of type Person and create a new Object for ArrayList<Person> persons = new ArrayList<Person>(); Integrated Development Environment The previous chapter explained how to create and compile a Java program on the command line. A Java Integrated Development Environment (IDE) provides lots of ease of use functionality for creating java programs. There are other very powerful IDE's available, for example the Eclipse IDE. . For an introduction on how to use the Eclipse IDE please see Eclipse IDE Tutorial. The remaining description uses the phrase: Create a Java project called... ". This refers to creating a Java project in Eclipse. If you are using a different IDE please follow the required steps in this IDE. Exercises - Creating Java objects and methods Create a Person class and instantiate it Create a new Java project called com.vogella.javastarter.exercises1 and a package with the same name. Create a class called Person. Add three instance variables to it, one for storing the first name of the person, on for storing the last name and one for storing the age of the Person. Use the constructor of the Person object to set the values to some default value. Write a public method called writeName() which uses the System.out.println( method to print the first name of the person to the console. Create a new class called Main with a public static void main(String[] args). In this method create an instance of the Person class. Use constructor Add a constructor to your Person class which takes the first name, last name and the age as parameter. Assign the values to your instance variables. Create in your main method two objects of type Person and call the writeName method on it. Define getter and setter methods Define methods which allow you to read the values of the instance variables and to set them. These methods are called setter and getter. Getters should start with get followed by the variable name whereby the first letter of the variable is capitized. Setter should start with set followed by the variable name whereby the first letter of the variable is capitized. ` For example the variable called firstName would have the getFirstName() getter method and the setFirstName(String s) setter method. Change your main method so that you create one person object and use the setter method to change the last name.
- 27. 1736210008 Create anAddress object Create a new object called Address. The Address should allow you to store the address of a person. Add a new instance variable of this type in the Person object.Also create a getter and setter for the Address object in the Person object Solution - Creating Java objects and methods Create a Person class and instantiate it The following is a potential solution for Section 11.1, “Create a Person class and instantiate it”. package exercises.exercise04; class Person { String firstname = "Jim"; String lastname = "Knopf"; int age = 12; voi d writeName() { // Writes the firstname System.out.println(firstname); // Alternatively you can combine strings with + System.out.println(firstname + " " + lastname + "" + age); } } package exercises.exercise04; public class Main { public static void main(String[] args) { Person person = new Person(); } person.writeName();
- 28. 1736210008 } Use constructor package com.vogella.javastarter.exercises1; classPerson { String firstName; String lastName; int age; public Person(String a, String b, int value) { firstName = a; lastName = b; age=value; } void writeName() { // Writes the firstname System.out.println(firstName); // Alternatively you can combine strings with + System.out.println(firstName + " " + lastName + "" + age); } } package com.vogella.javastarter.exercises1; public class Main { void main(String[] args) { Person person = new Person("Jim", "Knopf" , 12);
- 29. 1736210008 person.writeName(); // Reuse thesame variable and assign new object to it person = new Person("Henry", "Ford", 104); person.writeName(); } } Define getter and setter methods package com.vogella.javastarter.exercises1; class Person { String firstName; String lastName; int age; public Person(String a, String b, int value) { firstName = a; lastName = b; age = value; } public String getFirstName() { return firstName; } public void setFirstName(String firstName) { this.firstName = firstName; } public String getLastName() { return lastName;
- 30. 1736210008 } public void setLastName(StringlastName) { this.lastName = lastName; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } void writeName() { // Writes the firstname System.out.println(firstName); // Alternatively you can combine strings with + System.out.println(firstName + " " + lastName + "" + age); } } package com.vogella.javastarter.exercises1; public class Main { public static void main(String[] args) { Person person = new Person("Jim", "Knopf", 21); Person person2 = new Person("Jill", "Sanders", 20); // Jill get married to Jim person2.setLastName("Knopf"); person2.writeName();
- 31. 1736210008 } } Create an Addressobject package com.vogella.javastarter.exercises1; public class Address { private String street; private String number; private String postalCode; private String city; private string country; public String getStreet() { return street; } public void setStreet(String street) { this.street = street; } public String getNumber() { return number; } public void setNumber(String number) { this.number = number; } public String getPostalCode() { return postalCode; } PostalCode(String postalCode) { this.postalCode = postalCode; }
- 32. 1736210008 public String getCity(){ return city; } public void setCity(String city) { this.city = city; } public String getCountry() { return country; } public void setCountry(String country) { this.country = country; } public String toString() { return street + " " + number + " " + postalCode + " " + city + " " + country; } } package com.vogella.javastarter.exercises1; class Person { String firstName; String lastName; int age; private Address address; public Person(String a, String b, int value) {
- 33. 1736210008 firstName = a; lastName= b; age=value; } public String getFirstName() { return firstName; } public void setFirstName(String firstName) { this.firstName = firstName; } public String getLastName() { return lastName; } public void setLastName(String lastName) { this.lastName = lastName; } public int getAge() { return age; } public void setAge(int age) { this.age = age; }
- 34. 1736210008 public Address getAddress(){ return address; } public void setAddress(Address address) { this.address = address; } void writeName() { // Writes the firstname System.out.println(firstName); // Alternatively you can combine strings with + System.out.println(firstName + " " + lastName + "" + age); } } package com.vogella.javastarter.exercises1; public class Main { public static void main(String[] args) { // I create a person Person pers = new Person("Jim", "Knopf", 31); // I set the age of the person to 32
- 35. 1736210008 // Just fortesting I write this to the console System.out.println(pers.toString()); /* * Actually System.out.println calls always toString, if you do not * specify it so you could also have written System.out.println(pers); */ // I create an address Address address = new Address(); // I set the values for the address address.setCity("Heidelberg"); address.setCountry("Germany"); address.setNumber("104"); address.setPostalCode("69214"); address.setStreet("Musterstr."); address = null; // person is moving to the next house in the same street pers.getAddress().setNumber("105"); } }
- 36. 1736210008 HISTORY OF JAVA Itis quite interesting to know the development of Java technology, which is widely accepted in the area of networked/distributed computing. Java evolved from a project developing a language for programming consumer electronic devices at Sun Microsystems, USA. Sun’s Stealth Project in 1990 was aimed to do research in the area of application of computers in the consumer electronics market. Bill Joy, James Gosling, Mike Sheridan, Patrick Naughton (formerly the project leader of Sun’s Open Windows user environment), and several other individuals discussed the ideas for the Stealth Project in January 1991.The vision of this project was to develop smart consumer electronic devices that could all be centrally controlled and programmed from a handheld-remote-control-like device. The Stealth group began to work to achieve this goal. It was realized that a platform-independent development-environment was needed. Green Project was initiated by Patrick Naughton, Mike Sheridan, and James Gosling of Sun Microsystems in 1991 (Members of the Stealth Project, which later became known as the Green Project) The tasks of the project were divided amongst the members of Green Project. Mike Sheridan concentrated on business development, Patrick Naughton worked on the graphics system, and James Gosling was to identify the appropriate programming language for the Green Project. Even though creating a new language was not the aim of Green Project, a new language was created as a programming tool in this project since C++ was found unsuitable for the project. An intelligent remote control called *7 (Star Seven) was delivered as the first product of Green Project in 1992. It is a PDA- like device comprising of a new language oak, an operating system called GreenOS, user interface, and hardware. This device was quite impressive to Sun Executives. But no customer was interested in this project. The market was not conducive to this type of device in the consumer electronics industry. The new language Oak was created by James Gosling, a Green Team member, specifically for *7. Gosling named the new language Oak because that name was struck while looking at an oak three outside of his office window. The name Oak was later renamed to Java in order to avoid legal issues since Oak was the name of an existing language. In November 1992, the Green Project was incorporated under the name FirstPerson. In 1993. Time-Warner was demanding for proposals for set-top box operating systems and videoon- demand technology with an objective of sending the data to consumer all over the country for display on the television sets. At the same time (1993(, NCSA released the first graphical web browser Mosaic 1.0, an easy-to-use front end to the World Wide Web. When FirstPerson was bidding on the Time-Warner TV trial based on video-on-demand technology, Time-Warner chose Silicon Graphics, Inc. (SGI) over Sun. Hence, half of the members of FirstPerson left for SGI and the remaining members continued to work at Sun. Mosaic web browser revolutionized people’s perceptions. The remaining member of FirstPerson returned to work on Java (Oak was renamed Java) to develop Java-based web browser. FirstPerson was dissolved gradually. Naughton and Jonathan Payne developed WebRunner
- 37. 1736210008 (named after themovie Blade Runner). Later WevRunner was officially announced as the HotJavaTM browser in 1994. This was the turning point for Java. At that time the World Wide Web changed the face of Internet and it was winning the race on Internet. Arthur Ban Hoff implemented the Java compiler in Java itself whereas Gosling implemented it in C. The Java compiler, written in Java made the people to accept Java as a fullfeatured language. Sun Microsystems officially announced Java environment at Sun World’95 on May 23, 1995. Java entered into the mainstream of the Internet after a struggle for about four years. Netscape Communications incorporated Java into its web browser Netscape Navigator. Within a decade, Java was destined to be the most overreaching technology in the Internet. Java was not restricted to the Internet alone. The initial goal of Sun was also achieved by using Java technology in interactive set-top boxes, hand-held devices and other consumer electronics products. Sun released the first version 1.0 of Java in 1996. Java is an object-oriented programming language which evolved from C++. It is also a high-level programming language. The different forms of Java versions are discussed in the next section. History of Java Year Java - Versions and Features 1990 Sun Microsystems started Stealth project supporting application of computers in the consumer electronics market. 1991 The Green project started with the members of Stealth project such as James Gosling, Patrick Naughton, and Mike Sheridan. A new programming language, called Oak was created by Gosling. 1992 An intelligent remote control called StarSeven was delivered. The Green Project was incorporated was incorporated under the name FirstPerson. 1993 Mosaic Web browser was introduced in the world of Internet. 1994 HotJava Web browser was announced by Sun Microsystems. 1995 Oak was renamed as Java. Sun officially announced Java technology. 1996 Sun released the first version 1.0 of Java. Core Language features supporting: I/O facility, Utilities, Network Programming, User Interface – AWT, Applets, and Multithreading. 1997 Sun released JDK1.1 by including new features such as addition of inner classes to the language and capabilities such as JavaBeans, JDBC(Java Data Base Connectivity), and RMI (Remote Method Invocation). 1998 Java 2 Platform, Standard Edition (J2SE)1.2, code named as Playground, was released. It replaced JDK and distinguished the base platform from j2ee (Java 2 Platform, Enterprose Edition) and J2ME (Java 2Platform, Micro Edition). The key features include Swing graphical API, Java IDL (Interface Definition Language) to support CORBA interoperability. They also added Collections framework to support various data structures. For the first time, JVM was equipped with a JIT (Just-inTime) compiler.
- 38. 1736210008 2000 J2SE 1.3,code named as Kestrel, was released with key features such as JavaSound API to support audio operations (e.g., audio playback and capture/recording, mixing, sequencing, and synthesis), Java Naming and Directory Interface (JNDI), and Java Platform Debugger Architecture (JPDA). 2002 H2SE 1.4, code named as Merlin, was released with key features such as (a) image I/O API for manipulating images in formats like JPEG and PNG, (b) integrated XML parser and XSLT processor (JAXP), integrated security and cryptography extensions, (c) Java Web Start supporting deployment of Java software applications over the network. 2004 J2SE 5.0, code named as Tiger, was released with enhancement of Java platform and inclusion of a number of significant new language features such as Generics, Enhanced for Loop, Autoboxing/Unboxing, Typesafe Enums, Varags, Static Import, and Metadata supporting declarative style programming annotation of source code. 2006 Java SE 6.0, code named as Mustang, was released with enhancement of performance in Swing, JDBC 4.0 support, Java Compiler API, Upgrade of JAXB to version 2.0, including integration of a StAX parser, support for pluggable annotations, and many GUI improvements. 2008 Java SE 7.0, code named as Dolphin, with native support for XML recognizing its foundation for Web services. FEATURES OF JAVA According to Sun, Java is defined as a simple, object-oriented, distributed, interpreted, robust, secure, architecture –neutral, portable, high performance, multithreaded, and dynamic programming language and platform. Java is simple because the syntax of well-known languages such as C and C++ are used with modifications meant for simplification and improvement. It is easy to read and write Java code if the reader is familiar with C/C++. Almost everything in Java is centered on creating objects, manipulating the objects and making objects work together. Only the primitive operations and data types are at the sub-object level. Hence Java is an object-oriented programming language. An application can be distributed in many systems which are connected together. Since networking capability is incorporated in Java, the data/file can be sent across many systems to run the application. Java is robust because it is more reliable. It ensures the reliability by providing early checking for possible errors. It eliminates error-prone constructs such as pointer. It eliminates error-prone constructs such as pointer. It also supports runtime error handling.
- 39. 1736210008 Java programs arecompiled to a byte code format that can be read and run by interpreters on many platforms including Windows, Linux, etc. Hence, it is architecture neutral. Because of this feature it is portable to new hardware and operating systems. Although the speed of execution of Java programs is slower compared to C/C++ programs, Java is meant for distributed applications. Whenever the file/data is transmitted from one system to another, the performance is not affected. Hence, it is having high performance. Java supports multithreaded programming to perform several tasks simultaneously. Any code can be loaded without recompilation at runtime. Hence it is dynamic.