lecture 22
TRANSCRIPT
Lecture-22Instructor Name:
Object Oriented Programming
Today’s Lecture
User Defined Exceptions
Assertion Statement
2
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.
3
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.
4
User Defined Exception
Syntax We can define our own Exception class as below:
class MyException extends Exception{ . . . . .
}
5
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 linkhttp://www.tutorialspoint.com/java/java_exceptions.htm
6
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
7
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 linkhttp://www.tutorialspoint.com/java/java_exceptions.htm
8
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;
}
9
User Defined Exception
public void withdraw(double amount) throws
InsufficientFundsException {
if(amount <= balance) {
balance -= amount;
}
else {
double needs = amount - balance;
throw new InsufficientFundsException(needs);
}
}
10
User Defined Exception
public double getBalance() {
return balance;
}
public int getNumber() {
return number;
}
}
11
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(); } } }12
assert Statement
13
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.
assert Statement
14
Why use assertions? Programming by contract
Pre-conditions
– Assert precondition as requirement of client
Post-conditions
– Assert post-condition as effect of client method
assert Statement
15
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
assert Statement
16
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
assert Statement
17
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?
assert Statement
18
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
assert Statement
19
Simple Exampleimport 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); } }
http://www.javatpoint.com/assertion-in-java
assert Statement
20
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
21