Java Library Classes

1. Blue Ridge Public School
Class X – Computer Applications
Ch 2- Library Classes
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2. Topics
• Class as a Composite Type
Since class can consist of variables of different datatypes it is called a
composite datatype
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3. Class as a Composite Type
• There are 8 primitive data types defined in Java, they
are byte, short, int, long, float, double, char, boolean
• Java allows us to create new data type using combination of
the primitive data types. Such data types are
called composite or user-defined data types
• Just as variables can be declared of any primitive data type,
similarly we can declare variables of the composite data
types
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4. Class as a Composite Type – cont.
• Class is an example of composite data type and
we can declare objects as variables of the class
• A class is known as a composite data type
because it binds both member variable and
functions as a single identity
• Composite datatypes are the datatypes which
can be constructed within a program by using
primitive datatypes and other composite types
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5. Class as a Composite Type – cont.
• Consider the following class definition:
class TypeDemo
{
byte a;
int b;
float c;
char d;
public void getData(){
.
.
}
public void display(){
.
.
}
}
You can see that primitive data types have been used for defining this class:
byte a;
int b;
float c;
char d;
Once a class is declared, variables of this class type can be declared and created e.g.,
TypeDemo obj1 = new TypeDemo();
Variables of a class type are known as objects
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6. Defining a Class
• A class is nothing but a blueprint or a template for
creating different objects which defines its
properties and behaviours
• Java class objects exhibit the properties and
behaviours defined by its class
• A class can contain fields and methods to describe
the behaviour of an object
• Methods are nothing but members of a class that
provide a service for an object or perform some
business logic
• Methods define the operations that can be
performed in java programming
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7. Defining a Class – cont.
• Below is an example showing the Objects and Classes of the
Cube class that defines 3 fields namely length, breadth and
height. Also the class contains a member function
getVolume()
public class Cube
{
int length;
int breadth;
int height;
public int getVolume()
{
return (length * breadth * height);
}
}
• Since class can consist of variables of different datatypes it is
called composite datatype
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8. Object as Instance of Class
• In object-oriented programming (OOP), an instance is a
concrete occurrence of any object, existing usually during the
runtime of a computer program
• "instance" is synonymous with "object" as they are each a
particular value (realization), and these may be called
an instance object; "instance" emphasizes the distinct identity
of the object
• An object is an instance of a class, and may be called a class
instance or class object
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9. Role of New Keyword
• As mentioned previously, a class provides the blueprints for
objects. So basically, an object is created from a class
• In Java, the new keyword is used to create new objects
• The new operator instantiates a class by allocating memory for
a new object and returning a reference to that memory
• The new operator also invokes the object constructor
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10. Role of New Keyword – cont.
• The phrase "instantiating a class" means the same thing as
"creating an object.“
• When you create an object, you are creating an "instance" of
a class, therefore "instantiating" a class
• The new operator requires a single, postfix argument: a call
to a constructor
• The name of the constructor provides the name of the class
to instantiate
• The new operator returns a reference to the object it
created. This reference is usually assigned to a variable of the
appropriate type, like:
• Point P = new Point(23, 94);
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11. Role of New Keyword – cont.
• Here's the code for the Point class:
public class Point {
public int x = 0;
public int y = 0;
//constructor
public Point(int a, int b) {
x = a;
y = b;
}
}
• This class contains a single constructor. You can recognize a
constructor because its declaration uses the same name as the
class and it has no return type
• The constructor in the Point class takes two integer arguments, as
declared by the code (int a, int b)
• The following statement provides 23 and 94 as values for those
arguments:
• Point P = new Point(23, 94);
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12. Wrapper Classes
• In Java everything is in the form of a class, except primitive
datatypes
• This is for performance reasons
• Normally, when we work with Numbers, we use primitive data
types such as byte, int, long, double, etc. For example:
• int i = 500;
• float gpa = 3.65f;
• byte mask = 0xff;
• There are, however, reasons to use objects in place of
primitives, and the Java platform provides wrapper classes for
each of the primitive data types
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13. Wrapper Classes – cont.
• A Wrapper class is a class whose object wraps
or contains a primitive data type
• When we create an object to a wrapper class, it
contains a field and in this field, we can store a
primitive data types
• In other words, we can wrap a primitive value
into a wrapper class object
• The need for wrapper class is that they convert
primitive data types into objects
• The wrapper classes encapsulate, or wrap the
primitive datatypes within a class
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14. Wrapper Classes – cont.
• The wrapper classes in java serve two primary purposes:
• Objects are needed if we wish to modify the arguments passed
into a method (because primitive types are passed by value)
• To provide an assortment of utility functions for primitives like
converting primitive types to and from string objects, converting
to various bases like binary, octal or hexadecimal, or comparing
various objects
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15. Wrapper Classes – cont.
• The following two statements illustrate the difference
between a primitive data type and an object of a wrapper
class:
int x = 25;
Integer y = new Integer(33);
• The first statement declares an int variable named x and
initializes it with the value 25
• The second statement instantiates an Integer object. The
object is initialized with the value 33 and a reference to the
object is assigned to the object variable y.
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16. Wrapper Classes – cont.
• Wrapper classes form a part of standard library of java.lang
package
• This package provides many methods that help us manipulate
the primitive datatypes
• Role of wrapper class
• Determines the nature of data of primitive datatype
• Converts them to object type
• Converts object data into data of primitive type
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17. Wrapper Classes – cont.
• Primitive Data types and their Corresponding Wrapper class
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Primitive Datatype Wrapper Class
char Character
byte Byte
short Short
int Integer
long Long
float Float
double Double
boolean Boolean

18. Wrapper Classes – cont.
• Integer Wrapper Class
• Integer class is a wrapper class for the primitive type int which
contains several methods to effectively deal with an int value like
converting it to a string representation, and vice-versa
• The Integer class wraps the int primitive data type into an object.
• This class includes helpful methods in converting value from
String to Integer
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19. Wrapper Classes – cont.
• Integer Wrapper Class
• There are two ways to instantiate the Integer object
• One is to use the new keyword.
• Integer secondInteger = new Integer(100);
• And the second method to create an Integer object is to use the
autoboxing feature of java which directly converts a primitive
data type to its corresponding wrapper class
• Below is an example on how to make use of autoboxing in order
to create a new Integer object
• Integer sampleInteger = 100;
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20. Some of the Methods of Integer
Wrapper class
Method Return
Type
Argument Description
parseInt() int String Returns int value when the function has
numbers in String format as argument
valueOf() int String Same as parseInt() but it returns Integer
object
toBinaryString() String int Returns String which is the binary
equivalent of the integer
toOctalString() String int Returns String which is the octal equivalent
of the integer in number system
toHexString() String int Returns String which is the hexadecimal
equivalent of the integer in number system
toString() String int Returns String after converting the data in
integer type passed as argument
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21. Wrapper Classes – cont.
• Integer Wrapper Class
• Convert Integer to Java String object
/*
Convert Integer object to String object
This example shows how a Integer object can be converted into String
object.
*/
public class IntegerToStringExample {
public static void main(String[] args) {
Integer intObj = new Integer(10);
//use toString method of Integer class to convert Integer into String.
String str = intObj.toString();
System.out.println("Integer converted to String as " + str);
}
}
/*
Output of the program would be
Integer converted to String as 10
*/
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22. Wrapper Classes – cont.
• Integer Wrapper Class
• Convert String to int Example
/*
Convert String to int Example
This Convert String to int Java example shows how a String object can be converted
into int primitive type
using parseInt method of Integer class.
*/
public class StringToIntExample {
public static void main(String[] args) {
//declare String object
String str = "10";
/*
use parseInt method of Integer class to convert String into int primitive data type. This
is a static method.
Please note that this method can throw a NumberFormatException if the string is not
parsable to int.
*/
int i = Integer.parseInt(str);
System.out.println(i);
}
}
/*
Output the program would be
10
*/
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23. Some of the Methods of Double
Wrapper class
Method Return
Type
Argument Description
parseDouble() double String Returns double value when the function has
numbers in String format as argument
valueOf() double String Same as parseDouble() but it returns
Double object
toString() String double Returns String after converting the data in
integer type passed as argument
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24. Some of the Methods of Float
Wrapper class
Method Return
Type
Argument Description
parseFloat() float String Returns float value when the function has
numbers in String format as argument
valueOf() float String Same as parseFloat() but it returns Float
object
toString() String float Returns String after converting the data in
integer type passed as argument
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25. Wrapper Classes – cont.
• Character Wrapper Class
• Most of the time, if you are using a single character value, you
will use the primitive char type. For example:
• char ch = 'a';
• There are times, however, when you need to use a char as an
object—for example, as a method argument where an object is
expected
• The Java programming language provides a wrapper class that
"wraps" the char in a Character object for this purpose
• You can create a Character object with
the Character constructor:
• Character ch = new Character('a');
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26. Some of the Methods of Character
Wrapper class
Method Return
Type
Argument Description
isLowerCase() boolean char Returns true if character passed as argument is of
lower case. Else returns false
isUpperCase() boolean char Returns true if character passed as argument is of
uppercase. Else returns false
isDigit() boolean char Returns true if character passed as argument is
digit. Else returns false
isLetter() boolean char Returns true if character passed as argument is
letter. Else returns false
isWhitespace() boolean char Returns true if character passed as argument is
whitespace. Else returns false
isLetterOrDigit() boolean char Returns true if character passed as argument is
letter or digit. Else returns false
toUpperCase() char char Returns character after converting it to uppercase
toLowerCase() char char Returns character after converting it to lowercase
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27. Autoboxing - Unboxing
• The automatic conversion of primitive data type into an object
of its equivalent wrapper class is known as autoboxing.
Integer wobj = new Integer(77);
• The system of converting an object of wrapper class into
primitive data type is known as unboxing.
Integer wobj = new Integer(77);
int x = wobj;
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28. Thank You!!
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