throw and throws

In the previous tutorial, we learned how to handle an exception using try and catch block. Now we will learn how exception handling is done using throw and throws keyword.

throw keyword

The keyword throw is employed to throw an exception explicitly. It is mainly used to throw custom exceptions or user-defined exceptions. It is placed inside the method.

Syntax

throw new ExceptionClassName("Message");

JVM or method does not make the exception object like it used to do before. Here the user has to explicitly make an object of the exception using the throw keyword.

Example

class test {
  public static void DivByZero() {
    throw new ArithmeticException("Cannot divide by zero");
  }
  public static void main(String[] args) {
    DivByZero();
  }
}

Output:

Java.lang.ArithmeticException: Cannot divide by zero

We can also have a user-defined class of exceptions. Check the example given below

Example

import java.util.Scanner;
class AgeLimit extends RuntimeException {
  AgeLimit(String s) {
    super(s);
  }
}
class voting {
  public static void main(String[] args) {
    int age = 16;
    if (age < 18) {
      throw new AgeLimit("you are not eligible
      for voting");
    }
    else {
      System.out.println("you can vote");
    }
  }
  System.out.println("hello");
}

Output:

Exception in thread "main" AgeLimit:you are not eligible for voting.

We note here that exception is still not handled because our program has terminated abnormally (because it did not print hello). To handle it, you have to use try and catch block.

Example

import java.util.Scanner;
class AgeLimit extends RuntimeException {
  AgeLimit(String s) {
    super(s);
  }
}
class voting {
  public static void main(String[] args) {
    int age = 16;
    try {
      if (age < 18) {
        throw new AgeLimit("you are not eligible
        for voting");
      }
      else {
        System.out.println("you can vote");
      }
    }
    catch(Exception e) {
      e.printStackTrace();
    }
  }
  System.out.println("hello");
}

Output:

Exception in thread "main" AgeLimit:you are not eligible for voting.
hello 

Now the above code will not terminate abnormally because we have handled it using try and catch block. Remember that you cannot write any code below the throw keyword statement. If you do so then you will get a compiler error.

throws keyword

throws keyword is used at the time of method declaration which indicates to the caller method that a particular type of exception may arrive if you are using this method and accordingly the caller method has to handle that exception. throws keyword is used only for checked exceptions.

Syntax

Method () throws Exception1 ,Exception2……Exception n For better understanding, consider the example given below

Example

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.FileNotFoundException; {
class files {
    void ReadFiles() throws FileNotFoundException {
        FileInputStream fi = new FileInputStream("d:/xyz.txt");
        }
    }
    class test {
        public static void main(string[] args) {
            try {
                ReadFiles();
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            }
            System.out.println("hello");
        }
    }

Output:

Java.io.FileNotFoundException: d:/xyz.txt
hello 

Now when the main function will call the ReadFiles method then the calling method i.e. main function will get informed that the function which you are calling will throw a FileNotFoundException and therefore the main function should handle it. If we would have not used the try-catch block then the compiler would have given us warning at compile time. Since we used the try-catch block, our code will get compiled easily and will not terminate abnormally (as we can confirm from hello message above).

Difference between throw and throws