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    Dacj 2-1 b Dacj 2-1 b Presentation Transcript

    • Exception HandlingPre-assessment Questions 1. Which type of inheritance supports repeating inheritance? a. Single level inheritance b. Multiple inheritance c. Multilevel inheritance d. Multipath inheritance 2. Which keyword is used to define the constants in an interface? a. super b. public c. private d. final ©NIIT Programming in Java Lesson 1B / Slide 1 of 25
    • Exception HandlingPre-assessment Questions (Contd.) 3. Which concept of object-oriented programming is used in method overriding? a. Polymorphism b. Abstraction c. Encapsulation d. Inheritance 4. What is the return type of the methods declared in an interface? a. Default b. private c. protected d. public ©NIIT Programming in Java Lesson 1B / Slide 2 of 25
    • Exception HandlingPre-assessment Questions (Contd.) 5. Which type of inheritance enables you to inherit class data members and methods from more than one superclass? • Single level inheritance • Multiple inheritance • Multilevel inheritance • Multipath inheritance ©NIIT Programming in Java Lesson 1B / Slide 3 of 25
    • Exception HandlingSolutions to Pre-assessment Questions 1. b. Multiple inheritance 2. d. final 3. a. Polymorphism 4. d. public 5. b. Multiple inheritance ©NIIT Programming in Java Lesson 1B / Slide 4 of 25
    • Exception HandlingObjectives • In this lesson, you will learn to: • Identify the various types of exceptions in Java • Handle exceptions by using the try, catch, and finally clauses • Use the throw statement • Implement user-defined exceptions • Use assertions in Java ©NIIT Programming in Java Lesson 1B / Slide 5 of 25
    • Exception HandlingExceptions in Java • An exception can be defined as an abnormal event that occurs during program execution and disrupts the normal flow of instructions. • Errors in a Java program are categorized into two groups: • Compile-time errors • Run Time errors • Concept of Exceptions: • The unexpected situations that occur during program execution are: • Running out of memory • Resource allocation errors • Inability to find files • Problems in network connectivity ©NIIT Programming in Java Lesson 1B / Slide 6 of 25
    • Exception HandlingExceptions in Java (Contd.) • Exception Classes • The following figure shows the Exception class heirarchy: ©NIIT Programming in Java Lesson 1B / Slide 7 of 25
    • Exception HandlingExceptions in Java (Contd.) ©NIIT Programming in Java Lesson 1B / Slide 8 of 25
    • Exception HandlingExceptions in Java (Contd.) • Built-in Exceptions • Java consists of the following categories of built-in exceptions: • Checked Exceptions • Unchecked Exceptions ©NIIT Programming in Java Lesson 1B / Slide 9 of 25
    • Exception HandlingExceptions in Java (Contd.) • The following table lists the various checked exceptions in Java: Exception Cause of Creation ClassNotFoundException Occurs when the Java run time system is unable to find the class referred. IllegalAccessException Occurs when you want to refer a class that is not accessible. InstantiationException Occurs when you try to create an object of an abstract class or interface. NoSuchMethodException Occurs when you call a method that does not exist. ©NIIT Programming in Java Lesson 1B / Slide 10 of 25
    • Exception HandlingExceptions in Java (Contd.) • The following table lists the various unchecked exceptions: Exception Cause of CreationArithmeticException Occurs when you make an arithmetic error, such as dividing a number by zero.ArrayIndexOutOfBoundsException Occurs when an attempt is made to access an array element beyond the index of the array.ArrayStoreException Occurs when you assign an element to an array that is not compatible with the data type of that array. ©NIIT Programming in Java Lesson 1B / Slide 11 of 25
    • Exception HandlingExceptions in Java (Contd.) Exception Cause of Creation ClassCastException Occurs when you assign a reference variable of a class to an incompatible reference variable of another class. IllegalArgumentException Occurs when you pass an argument of incompatible data type to a method. NegativeArraySizeException Occurs when you create an array with negative size. ©NIIT Programming in Java Lesson 1B / Slide 12 of 25
    • Exception HandlingExceptions in Java (Contd.) Exception Cause of Creation NullPointerException Occurs when an application tries to use an object without allocating memory to it or calls a method of a null object. NumberFormatException Occurs when you want to convert a string in an incorrect format to a numeric format. ©NIIT Programming in Java Lesson 1B / Slide 13 of 25
    • Exception HandlingImplementing Exception Handling • You can implement exception-handling in a program by using the following keywords: • try • catch • throw • throws • finally • Using try and catch statements • The try block encloses the statements that might raise an exception within it and defines the scope of the exception handlers associated with it. • The catch block is used as an exception-handler. You enclose the code that you want to monitor inside a try block to handle a run time error. ©NIIT Programming in Java Lesson 1B / Slide 14 of 25
    • Exception HandlingImplementing Exception Handling(Contd.) • The following syntax shows how to declare the try-catch block: try { // Statements that cause an exception. } catch(ExceptionName obj) { // Error handling code. } • Using multiple catch statements • A single try block can have many catch blocks. This is necessary when the try block has statements that raise different types of exceptions. ©NIIT Programming in Java Lesson 1B / Slide 15 of 25
    • Exception HandlingImplementing Exception Handling(Contd.) • The multiple catch blocks generate unreachable code error. • If the first catch block contains the Exception class object then the subsequent catch blocks are never executed. • The Exception class being the superclass of all the exception classes catches various types of exceptions. The Java compiler gives an error stating that the subsequent catch blocks have not been reached. • This is known as the unreachable code problem. • To avoid unreachable code error, the last catch block in multiple catch blocks must contain the Exception class object.• Using the finally clause • The finally block is used to process certain statements, no matter whether an exception is raised or not. ©NIIT Programming in Java Lesson 1B / Slide 16 of 25
    • Exception HandlingImplementing Exception Handling(Contd.) • The following syntax shows how to declare the try-finally block: try { // Block of code } finally { // Block of code that is always executed irrespective of an exception being raised or not. } ©NIIT Programming in Java Lesson 1B / Slide 17 of 25
    • Exception HandlingThrowing an Exception• Using the throw statement • The throw statement causes termination of the normal flow of control of the Java code and stops the execution of the subsequent statements if an exception is thrown when the throw statement is executed. • The throw clause transfers the control to the nearest catch block handling the type of exception object throws. • The following syntax shows how to declare the throw statement: throw ThrowableObj• Using the throws statement • The throws statement is used by a method to specify the types of exceptions the method throws. • If a method is capable of raising an exception that it does not handle, the method must specify that the exception has to be handled by the calling method. • This is done using the throws statement. ©NIIT Programming in Java Lesson 1B / Slide 18 of 25
    • Exception HandlingImplementing User-Defined Exception • The user-defined Exception class also inherits the methods defined in the Throwable class. • The following table lists the various methods defined by the Throwable class: Methods Description String getMessage() Returns a description of the exception String toString() Returns a string object containing a description of the exception. Throwable fillInStackTrace() Returns a Throwable object that contains a stack trace void printStackTrace() Prints the stack trace ©NIIT Programming in Java Lesson 1B / Slide 19 of 25
    • Exception HandlingDemonstration-Exception Handling in Java • Problem Statement • System Software Solutions Inc. needs to store the details, such as name, age, and ID of three employees of the company. While storing the details of the employees, the record of an employee was entered twice and hence the record of the third employee could not be displayed. Identify the exception that is raised if the details of all the employees were stored. Use the try- catch block to handle the exception. ©NIIT Programming in Java Lesson 1B / Slide 20 of 25
    • Exception HandlingDemonstration-Exception Handling inJava (Contd.) • Solution • To solve the above problem, perform the following tasks: • Identify the type of error and the code segment in which the error occurs. • Identify the mechanism of trapping the exception. • Code the application • Compile and execute the application ©NIIT Programming in Java Lesson 1B / Slide 21 of 25
    • Exception HandlingAssertions in Java • Assertions are the checks provided by the Java language to ensure that any assumption made at the start of a program is true throughout the program. • Assertions: • Assertions are statements in Java that enable you to test the assumptions made in a program during program execution. • An assertion statement contains a boolean expression that the programmer believes to be true at the time the statement is executed. • Assertions are used during testing of a program to ensure that the specified condition is true. • The following syntax shows how to declare an assertion using assert statement in Java: assert expressionA; • The following syntax shows the compilation of the Java program named AssertDemo.java that is using assert statement: javac -source 1.4 AssertDemo.java ©NIIT Programming in Java Lesson 1B / Slide 22 of 25
    • Exception HandlingAssertions in Java (Contd.) • The syntax to enable assertion in a specified class with –ea option is: . java –ea: AssertDemo • The syntax to disable assertion in a specified class with –da option is: java –da: AssertDemo ©NIIT Programming in Java Lesson 1B / Slide 23 of 25
    • Exception HandlingSummary In this lesson, you learned: • Errors can be broadly categorized into two groups on the basis of whether the compiler is able to handle the error or not, such as compile time errors and run time errors. • An exception is a run time error that can be defined as an abnormal event that occurs during the execution of a program and disrupts the normal flow of instructions. • In Java, the Throwable class is the superclass of all the exception classes. It is the class at the top position in the exception class hierarchy. The Exception class and the Error class are two direct subclasses of the Throwable class. • The built-in exceptions in Java are divided into two types on the basis of the conditions where the exception is raised: • Checked Exceptions or Compiler-enforced Exceptions • Unchecked exceptions or Runtime Exceptions ©NIIT Programming in Java Lesson 1B / Slide 24 of 25
    • Exception HandlingSummary (Contd.) • You can implement exception handling in your program by using the following keywords: • try • catch • throw • throws • finally • You use multiple catch blocks to throw more than one type of exception. • The finally clause is used to execute the statements that need to be executed whether or not an exception has been thrown. • The throw statement causes termination of the normal flow of control of the Java code and stops the execution of subsequent statements after the throw statement. • The throws clause is used by a method to specify the types of exceptions the method throws. • You can create your own exception classes to handle the situations specific to an application ©NIIT Programming in Java Lesson 1B / Slide 25 of 25