throw keyword in java with example

Throw keyword in java

• Throw keyword is used to throw exception manually.
• Whenever it is required to suspend the execution of the functionality based on the user defined logical error condition we will use this throw keyword to throw exception.
• So we need to handle these exceptions also using try catch blocks.

 Java simple example Program to explain use of throw keyword in java

1. package exceptions;
2.  
3. public class ThrowKeyword {
4.  
5.     
6. public static void main(String[] args) {
8.  
9. try {
10.             
11.    throw new ArithmeticException();
12.             
13.             
14. } catch (Exception e) {
15.  
16.    System.out.println(e);
17.    e.printStackTrace();
18. }
19.  
20. }
21.  
22. }

Output:

1. java.lang.ArithmeticException
2. java.lang.ArithmeticException
3.     at exceptions.ThrowKeyword.main(ThrowKeyword.java:11)

Rules to use "throw" keyword in java 

• throw keyword must follow Throwable type of object.
• It must be used only in method logic.
• Since it is a transfer statement , we can not place statements after throw statement. It leads to compile time error Unreachable code

•  We can throw user defined exception using throw keyword.

1. //Custom exception
2. package com.instanceofjavaforus;
3. public class InvalidAgeException extends Exception {
4.  InvaidAgeException(String msg){
5.  super(msg);
6.  }
7. }
    

1. package com.exceptions;
2. public class ThrowDemo {
3. 
   
4. public boolean isValidForVote(int age){
5.  
6.  try{
7.    if(age<18){
8.   throw new InvalidAgeException ("Invalid age for voting");
9. }
10.  }catch(Exception e){
11.  System.out.println(e);
12.  }
13.   return false;
14.  }
15.  
16. public static void main(String agrs[]){
17.  
18.  ThrowDemo obj= new ThrowDemo();
19.    obj.isValidForVote(17);
20.  
21.   }
22. }

 Output:

1.  exceptions.InvalidAgeException: Invalid age for voting

• We can throw predefined exceptions using throw keyword

1. package com.instanceofjava;
2.  
3. public class ThrowKeyword{
4. public void method(){
5.  
6.  try{
7.   
8.   throw new NullPointerException("Invalid age for voting");
9. }
10.  }catch(Exception e){
11.  System.out.println(e);
12.  }
13.  }
14.  
15.   public static void main(String agrs[]){
16.  
17.  ThrowKeyword obj= new ThrowKeyword();
18.  
19.    obj.method();
20.  
21.   }
22. }

 Output:

1.  java.lang.NullPointerException: Invalid age for voting