Exception Handling in Java

Java Tutorial
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What is Java ?	History of Java
Features of Java	Simple Program of Java
Internal Details of Hello Java Program	How to set path in Java
Difference between JDK, JRE and JVM	JVM (Java Virtual Machine)
Variable and Datatype in Java	Unicode System
Operators in Java

Java OOP's Concepts
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Java OOP's Concepts	Java Naming conventions
Object and Class in Java	Method Overloading in Java
Constructor in Java	Java static keyword
this keyword in Java	Inheritance in Java
Aggregation in Java	Method Overriding in Java
Covariant Return Type	Super keyword in Java
Instance initializer block	Final Keyword In Java
Polymorphism in Java	Static Binding and Dynamic Binding
Java instanceof	Abstract class in Java
Interface in Java	Abstract V/S Interface
Java Package	Access Modifiers in Java
Encapsulation in Java	Object class in Java
Object Cloning in Java	Java Array
Call by Value and Call by Reference in Java	Creating API Document | javadoc tool

Java String
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Java String	Java String charAt
Java String concat	Java String contains
Java String equals	Java String format
Java String getBytes()	Java String indexOf
Java String intern	Java String isEmpty
Java String join	Java String length
Java String replace	Java String split
Java String substring	Java String toLowerCase()
Java String toUpperCase()	Java String trim

Exception Handling in Java
Produced by Ashish Goel
Exception Handling in Java	Java try block
Java catch multiple exceptions	Java Nested try block
Java finally block	Java throw exception
Java Exception propagation	Java throws keyword
Java String intern	Java String isEmpty
Java String join	Java String length
Java String replace	Java String split
Java String substring	Java String toLowerCase()
Java String toUpperCase()	Java String trim

Java Custom Exception

If you are creating your own Exception that is known as custom exception or user-defined exception. Java custom exceptions are used to customize the exception according to user need.

By the help of custom exception, you can have your own exception and message.

Let's see a simple example of java custom exception.

1.

class InvalidAgeException extends Exception{  

 InvalidAgeException(String s){  

  super(s);  

 }  

}  

2.

class TestCustomException1{  

  

   static void validate(int age)throws InvalidAgeException{  

     if(age<18)  

      throw new InvalidAgeException("not valid");  

     else  

      System.out.println("welcome to vote");  

   }  

   public static void main(String args[]){  

      try{  

      validate(13);  

      }catch(Exception m){System.out.println("Exception occured: "+m);}  

  

      System.out.println("rest of the code...");  

  }  

}

Output:Exception occured: InvalidAgeException:not valid

       rest of the code...

ExceptionHandling with MethodOverriding in Java

ExceptionHandling with MethodOverriding in Java

There are many rules if we talk about methodoverriding with exception handling. The Rules are as follows: If the superclass method does not declare an exception If the superclass method does not declare an exception, subclass overridden method cannot declare the checked exception but it can declare unchecked exception. If the superclass method declares an exception If the superclass method declares an exception, subclass overridden method can declare same, subclass exception or no exception but cannot declare parent exception.

If the superclass method does not declare an exception

1) Rule: If the superclass method does not declare an exception, subclass overridden method cannot declare the checked exception.

import java.io.*;  

class Parent{  

  void msg(){System.out.println("parent");}  

}  

  

class TestExceptionChild extends Parent{  

  void msg()throws IOException{  

    System.out.println("TestExceptionChild");  

  }  

  public static void main(String args[]){  

   Parent p=new TestExceptionChild();  

   p.msg();  

  }  

}  

Output:Compile Time Error

2) Rule: If the superclass method does not declare an exception, subclass overridden method cannot declare the checked exception but can declare unchecked exception.


import java.io.*;  

class Parent{  

  void msg(){System.out.println("parent");}  

}  

  

class TestExceptionChild1 extends Parent{  

  void msg()throws ArithmeticException{  

    System.out.println("child");  

  }  

  public static void main(String args[]){  

   Parent p=new TestExceptionChild1();  

   p.msg();  

  }  

}





Output:child

If the superclass method declares an exception

1) Rule: If the superclass method declares an exception, subclass overridden method can declare same, subclass exception or no exception but cannot declare parent exception.

Example in case subclass overridden method declares parent exception


import java.io.*;  

class Parent{  

  void msg()throws ArithmeticException{System.out.println("parent");}  

}  

  

class TestExceptionChild2 extends Parent{  

  void msg()throws Exception{System.out.println("child");}  

  

  public static void main(String args[]){  

   Parent p=new TestExceptionChild2();  

   try{  

   p.msg();  

   }catch(Exception e){}  

  }  

}  





Output:Compile Time Error



Example in case subclass overridden method declares same exception


import java.io.*;  

class Parent{  

  void msg()throws Exception{System.out.println("parent");}  

}  

  

class TestExceptionChild3 extends Parent{  

  void msg()throws Exception{System.out.println("child");}  

  

  public static void main(String args[]){  

   Parent p=new TestExceptionChild3();  

   try{  

   p.msg();  

   }catch(Exception e){}  

  }  

}





Output:child

Example in case subclass overridden method declares subclass exception


import java.io.*;  

class Parent{  

  void msg()throws Exception{System.out.println("parent");}  

}  

  

class TestExceptionChild4 extends Parent{  

  void msg()throws ArithmeticException{System.out.println("child");}  

  

  public static void main(String args[]){  

   Parent p=new TestExceptionChild4();  

   try{  

   p.msg();  

   }catch(Exception e){}  

  }  

}  





Output:child

Example in case subclass overridden method declares no exception


import java.io.*;  

class Parent{  

  void msg()throws Exception{System.out.println("parent");}  

}  

  

class TestExceptionChild5 extends Parent{  

  void msg(){System.out.println("child");}  

  

  public static void main(String args[]){  

   Parent p=new TestExceptionChild5();  

   try{  

   p.msg();  

   }catch(Exception e){}  

  }  

}  





Output:child

Java throws keyword

The Java throws keyword is used to declare an exception. It gives an information to the programmer that there may occur an exception so it is better for the programmer to provide the exception handling code so that normal flow can be maintained.

Exception Handling is mainly used to handle the checked exceptions. If there occurs any unchecked exception such as NullPointerException, it is programmers fault that he is not performing check up before the code being used.

Syntax of java throws

return_type method_name() throws exception_class_name{  

 ...   

} 

Which exception should be declared

Ans) checked exception only, because:

• unchecked Exception: under your control so correct your code.
• error: beyond your control e.g. you are unable to do anything if there occurs VirtualMachineError or StackOverflowError.

Advantage of Java throws keyword

Now Checked Exception can be propagated (forwarded in call stack).

It provides information to the caller of the method about the exception.

Java throws example

Let's see the example of java throws clause which describes that checked exceptions can be propagated by throws keyword.

import java.io.IOException;  

class Testthrows1{  

  void m()throws IOException{  

    throw new IOException("device error");//checked exception  

  }  

  void n()throws IOException{  

    m();  

  }  

  void p(){  

   try{  

    n();  

   }catch(Exception e){System.out.println("exception handled");}  

  }  

  public static void main(String args[]){  

   Testthrows1 obj=new Testthrows1();  

   obj.p();  

   System.out.println("normal flow...");  

  }  

}

Output:

exception handled

normal flow...

Rule: If you are calling a method that declares an exception, you must either caught or declare the exception.


There are two cases:


Case1:You caught the exception i.e. handle the exception using try/catch.
Case2:You declare the exception i.e. specifying throws with the method.




Case1: You handle the exception

In case you handle the exception, the code will be executed fine whether exception occurs during the program or not.


import java.io.*;  


class M{  

 void method()throws IOException{  





  throw new IOException("device error");  



 }  


}  

public class Testthrows2{  

   public static void main(String args[]){  

    try{  

     M m=new M();  

     m.method();  

    }catch(Exception e){System.out.println("exception handled");}     

  

    System.out.println("normal flow...");  

  }  

}  


Output:exception handled


normal flow...


Case2: You declare the exception

A)In case you declare the exception, if exception does not occur, the code will be executed fine.
B)In case you declare the exception if exception occures, an exception will be thrown at runtime because throws does not handle the exception.




A)Program if exception does not occur




import java.io.*;  

class M{  

 void method()throws IOException{  

  System.out.println("device operation performed");  

 }  

}  

class Testthrows3{  

   public static void main(String args[])throws IOException{//declare exception  

     M m=new M();  

     m.method();  

  

    System.out.println("normal flow...");  

  }  

}  




Output:

device operation performed

normal flow...

B)Program if exception occurs

import java.io.*;  

class M{  

 void method()throws IOException{  

  throw new IOException("device error");  

 }  

}  

class Testthrows4{  

   public static void main(String args[])throws IOException{//declare exception  

     M m=new M();  

     m.method();  

  

    System.out.println("normal flow...");  

  }  

}


Output:Runtime Exception

Difference between throw and throws:


throw keyword
throws keyword
1)throw is used to explicitly throw an exception.
throws is used to declare an exception.
2)checked exception can not be propagated without throws.
checked exception can be propagated with throws.
3)throw is followed by an instance.
throws is followed by class.
4)throw is used within the method.
throws is used with the method signature.
5)You cannot throw multiple exception
You can declare multiple exception e.g.
public void method()throws IOException,SQLException.