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Java - throw keyword

An exception (or exceptional event) is a problem that arises during the execution of a program. When an Exception occurs the normal flow of the program is disrupted and the program/Application terminates abnormally, which is not recommended, therefore, these exceptions are to be handled.

An exception can occur for many different reasons. Following are some scenarios where an exception occurs.

• A user has entered an invalid data.

• A file that needs to be opened cannot be found.

• A network connection has been lost in the middle of communications or the JVM has run out of memory.

Some of these exceptions are caused by user error, others by programmer error, and others by physical resources that have failed in some manner.

If a method does not handle a checked exception, the method must declare it using the throws keyword. The throws keyword appears at the end of a method's signature.

You can throw an exception, either a newly instantiated one or an exception that you just caught, by using the throw keyword.

Try to understand the difference between throws and throw keywords, throws is used to postpone the handling of a checked exception and throw is used to invoke an exception explicitly.

The following method declares that it throws a RemoteException −

public static void accesElement() throws CustomException {
   try {
      int a[] = new int[2];
      System.out.println("Access element three :" + a[3]);
   } catch(Exception e) {
      throw new CustomException(e);
   }
}

In this example, we're calling a method accessElement() method which has declared to throw CustomException. So we need to write a try-catch block over accessElement() method. When program runs, it throws exception and it is printed.

// File Name : ExcepTest.java
package com.tutorialspoint;

public class ExcepTest {

   public static void main(String args[])  {
      try {
         accesElement();
      } catch (CustomException e) {		
         e.printStackTrace();
      }
      System.out.println("Out of the block");
   }

   public static void accesElement() throws CustomException {
      try {
         int a[] = new int[2];
         System.out.println("Access element three :" + a[3]);
      } catch(Exception e) {
         throw new CustomException(e);
      }
   }
}

class CustomException extends Exception{
   private static final long serialVersionUID = 1L;
   CustomException(Exception e){
      super(e);
   }
}

com.tutorialspoint.CustomException: java.lang.ArrayIndexOutOfBoundsException: 3
	at com.tutorialspoint.ExcepTest.accesElement(ExcepTest.java:20)
	at com.tutorialspoint.ExcepTest.main(ExcepTest.java:8)
Caused by: java.lang.ArrayIndexOutOfBoundsException: 3
	at com.tutorialspoint.ExcepTest.accesElement(ExcepTest.java:18)
	... 1 more
Out of the block

Following is another example showcasing use of throws keyword.

In this example, we're calling a method accessElement() method which has declared to throw CustomException. So we are not having try-catch block over accessElement() method, the main method declares to throw the exception. When program runs, it throws exception and it is printed.

// File Name : ExcepTest.java
package com.tutorialspoint;

public class ExcepTest {

   public static void main(String args[]) throws CustomException  {
      accesElement();
      System.out.println("Out of the block");
   }

   public static void accesElement() throws CustomException {
      try {
         int a[] = new int[2];
         System.out.println("Access element three :" + a[3]);
      } catch(Exception e) {
         throw new CustomException(e);
      }
   }
}

class CustomException extends Exception{
   private static final long serialVersionUID = 1L;
   CustomException(Exception e){
      super(e);
   }
}

Exception in thread "main" com.tutorialspoint.CustomException: java.lang.ArrayIndexOutOfBoundsException: 3
	at com.tutorialspoint.ExcepTest.accesElement(ExcepTest.java:16)
	at com.tutorialspoint.ExcepTest.main(ExcepTest.java:7)
Caused by: java.lang.ArrayIndexOutOfBoundsException: 3
	at com.tutorialspoint.ExcepTest.accesElement(ExcepTest.java:14)
	... 1 more

As exception is not handled, last statement is not executed in this example.

A method can declare that it throws more than one exception, in which case the exceptions are declared in a list separated by commas. For example, the following method declares that it throws a RemoteException and an InsufficientFundsException −

import java.io.*;
public class className {

   public void withdraw(double amount) throws RemoteException, 
      InsufficientFundsException {
      // Method implementation
   }
   // Remainder of class definition
}

java_basic_syntax.htm

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