Throwing Exceptions

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This section is a follow up to Exception Handling. After seeing how we handle exceptions, now maybe we wish to throw exceptions ourselves. In this section, we will explore how to manually throw exceptions.

To throw exceptions, first of all, we must notify that our method is capable of throwing such error. Let's say we have this program which calculates the sum of two integers.

public class AnotherDayAnotherClass {
    public int sum(int i1, int i2) {
        return i1 + i2;
    }
}

Really simple code. What if a user inputs a null object? Maybe we wish to throw a NullPointerException. To do this, first modify the function signature.

import java.util.NullPointerException;

public class AnotherDayAnotherClass {
    public int sum(int i1, int i2) throws NullPointerException {
        return i1 + i2;
    }
}

Then, we can use an if-clause to check if the inputs are valid.

import java.util.NullPointerException;

public class AnotherDayAnotherClass {
    public int sum(int i1, int i2) throws NullPointerException {
        if (i1 == null || i2 == null) {
            throw new NullPointerException("Don't anyhow please");
        }
        
        return i1 + i2;
    }
}

Now, if any of i1 or i2 are null objects, the method (not class!) will throw a NullPointerException with a message Don't anyhow please.