Java @Override 注解

1. Overview

In this quick tutorial, we’ll have a look at how to use the @Override annotation.

2. @Override Annotation

In a subclass, we can override or overload instance methods. Overriding indicates that the subclass is replacing inherited behavior. Overloading is when a subclass is adding new behavior.

Sometimes, we’ll overload by accident when we actually intended to override. It’s easy to make this mistake in Java:

public class Machine {
    public boolean equals(Machine obj) {
        return true;
    }

    @Test
    public void whenTwoDifferentMachines_thenReturnTrue() {
        Object first = new Machine();
        Object second = new Machine();
        assertTrue(first.equals(second));
    }
}

Surprisingly, the test above fails. This is because this equals method is overloading Object#equals, not overriding it.

We can use the @Override annotation on inherited methods to protect us from this mistake.

In this example, we can add the @Override annotation above the equals method:

@Override
public boolean equals(Machine obj) {
    return true;
}

At this point, the compiler will raise an error, informing us that we aren’t overriding equals like we think.

Then, we can correct our mistake:

@Override
public boolean equals(Object obj) {
    return true;
}

Because of how easy it’s to accidentally overload, it’s a common recommendation to use the @Override annotation on all inherited methods.

3. Conclusion

In this guide, we saw how the @Override annotation works in Java.

The full source code for the examples can be found over on GitHub.