The document discusses user defined exceptions and assertion statements in Java. It provides examples of how to create a custom exception class that extends the Exception class, and how to throw that exception from a method. It also explains how to write assertion statements to verify assumptions, including the simple and complex forms. Assertions fail if the expression is false, and can provide a message in the complex form.

1. Lecture-22
Instructor Name:
Object Oriented Programming

2. Today’s Lecture
 User Defined Exceptions
 Assertion Statement
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3. User Defined Exception
What is User Defined Exception?
 User-defined exception class is used to show custom messages to the
end users.
 It is used to hide the other exceptions like null pointer exception;
arithmetic exception etc.
 User-defined exception class is so common that it will be used in
every project.
 Only one step will be enough to create user-defined exception class
i.e. extend the Exception class.
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4. User Defined Exception
Key Points
 Keep the following points in mind when writing your own exception
classes:
 All exceptions must be a child of Throwable.
 If you want to write a checked exception that is automatically
enforced by the Handle or Declare Rule, you need to extend the
Exception class.
 If you want to write a runtime exception, you need to extend the
RuntimeException class.
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5. User Defined Exception
Syntax
 We can define our own Exception class as below:
class MyException extends Exception{
. . . . .
}
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6. User Defined Exception
User Defined Exception - Example
 The Example InsufficientFundsException class is a user-defined
exception that extends the Exception class, making it a checked
exception.
 An exception class is like any other class, containing useful fields and
methods.
Example is taken from the following link
http://www.tutorialspoint.com/java/java_exceptions.htm
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7. User Defined Exception
User Defined Exception – Example
// File Name InsufficientFundsException.
import java.io.*;
public class InsufficientFundsException extends Exception {
private double amount;
public InsufficientFundsException(double amount) {
this.amount = amount;
}
public double getAmount() {
return amount;
}
}
InsufficientFundsException is user defined exception class
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8. User Defined Exception
User Defined Exception - Example
 To demonstrate using our user-defined exception, the following
CheckingAccount class contains a withdraw() method that throws an
InsufficientFundsException.
Example is taken from the following link
http://www.tutorialspoint.com/java/java_exceptions.htm
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9. User Defined Exception
import java.io.*;
public class CheckingAccount {
private double balance;
private int number;
public CheckingAccount(int number) {
this.number = number;
}
public void deposit(double amount) {
balance += amount;
}
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10. User Defined Exception
public void withdraw(double amount) throws
InsufficientFundsException {
if(amount <= balance) {
balance -= amount;
}
else {
double needs = amount - balance;
throw new InsufficientFundsException(needs);
}
}
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11. User Defined Exception
public double getBalance() {
return balance;
}
public int getNumber() {
return number;
}
}
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12. User Defined Exception
public class BankDemo {
public static void main(String [] args) {
CheckingAccount c = new CheckingAccount(101);
System.out.println("Depositing $500...");
c.deposit(500.00);
try {
System.out.println("nWithdrawing $100...");
c.withdraw(100.00);
System.out.println("nWithdrawing $600...");
c.withdraw(600.00);
}
catch(InsufficientFundsException e) {
System.out.println("Sorry, but you are short $" +
e.getAmount()); e.printStackTrace(); } } }
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13. assert Statement
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What is an assert Statement?
 An assertion is a statement in Java that enables you to test your
assumptions about your program.
 Each assertion contains a boolean expression that you believe will be
true when the assertion executes.
 By verifying that the boolean expression is indeed true, the assertion
confirms your assumptions about the behavior of your program,
increasing your confidence that the program is free of errors.

14. assert Statement
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Why use assertions?
 Programming by contract
 Pre-conditions
– Assert precondition as requirement of client
 Post-conditions
– Assert post-condition as effect of client method

15. assert Statement
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Simple Assertion Form
 The assertion statement has two forms
 The first is:
assert Expression1 ;
 Where Expression1 is a boolean expression
 When the system runs the assertion, it evaluates Expression1 and if it
is false throws an AssertionError with no details

16. assert Statement
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Complex Assertion Form
 The second form of the assertion statement is:
assert Expression1 : Expression2 ;
 where:
– Expression1 is a boolean expression
– Expression2 is an expression that has a value
– It cannot invoke of a method that is declared void

17. assert Statement
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Complex Assertion Form
 Use the second version of the assert statement to provide a detailed
message for the AssertionError
 The system passes the value of Expression2 to the appropriate
AssertionError constructor, which uses the string error message
 The purpose of the message is to communicate the reason for the
assertion failure
 Don’t use assertions to flag user errors—why not?

18. assert Statement
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When an Assertion Fails
 Assertion failures are labeled in stack trace with the file and line
number from which they were thrown
 Second form of the assertion statement should be used in preference
to the first when the program has some additional information that
might help diagnose the failure

19. assert Statement
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Simple Example
import java.util.Scanner;
class AssertionExample{
public static void main( String args[] ){
Scanner scanner = new Scanner( System.in );
System.out.print("Enter ur age ");
int value = scanner.nextInt();
assert value>=18:" Not valid";
System.out.println("value is "+value);
}
}

20. assert Statement
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Compiling and Running
 If you use assertion, It will not run simply because assertion is disabled by
default. To enable the assertion, -ea or -enableassertions switch of java must
be used.
 Compile it by: javac AssertionExample.java
 Run it by: java -ea AssertionExample

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