Exceptions and overriding

Method overriding has certain rules for exceptions. Let’s see what those are.

For quick look, you may refer the below table

Note -> Child class’ overridden method can narrow down the exception from parent class’ method. But it cannot widen the same.

Scenario	Parent Class	Child Class
1.	Parent’s method doesn’t throw any exception	-> Overridden method can throw no exception -> May throw only unchecked exceptions -> Cannot throw checked exceptions
2.	Parent’s method throws unchecked exception	-> Overridden method can throw no exception -> May throw the same unchecked exception -> May throw any other unchecked exception -> Cannot throw checked exceptions
3.	Parent’s method throws checked exception	-> Overridden method can throw no exception -> May throw the same checked exception ->May throw any subclass of the checked exception thrown by the parent class -> May throw any unchecked exception
4.	Parent’s method throws Exception class	-> Overridden method can throw no exception -> May throw any exception class (checked or unchecked) including Exception class

Case 1 – When parent class method doesn’t throw an exception (Compiler error)

public class Parent {
	
	public void display() {
		System.out.println("Parent's display");
	}
}

import java.io.IOException;

public class Child extends Parent {

	@Override
	public void display() throws IOException {   //x compilation error
		System.out.println("Child's display");
	}
}

Case 2 – when parent class throws an exception and child also throws same exception (success scenario)

import java.io.IOException;

public class Parent {
	
	public void display() throws IOException {
		System.out.println("Parent's display");
	}
}

import java.io.IOException;

public class Child extends Parent {

	@Override
	public void display() throws IOException {
		System.out.println("Child's display");
	}
}

Case 3 – when parent class throws an exception and child throws subclass exception (success scenario)

public class Parent {
	
	public void display() throws Exception {
		System.out.println("Parent's display");
	}
}

import java.io.IOException;

public class Child extends Parent {

	@Override
	public void display() throws IOException {
		System.out.println("Child's display");
	}
}

Case 4 – when parent class throws an exception and child throws superclass exception (failed scenario)

import java.io.IOException;

public class Parent {
	
	public void display() throws IOException {
		System.out.println("Parent's display");
	}
}

public class Child extends Parent {

	@Override
	public void display() throws Exception {  //X compilation error
		System.out.println("Child's display");
	}
}