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When and why use Java @Override annotation

15 Jan 2017
Java

When should we use @Override annotation

  • When you want to override a method of parent class
  • When you want to implement a method of an interface

And if we forgot to use @Override annotation

It’s just fine, no error happens. But there will be potential unexpected behaviours due to human mistakes.

So why should we use @Override annotation

  • For code readability.
  • To take advantage of compiler error checking.
  • If Parent class changed, we can immediately see error when compiling.

Check this example: suppose we have a class Vehicle:

class Vehicle {
  void hello() {
    System.out.print("Hello from Vehicle");
  }
}

This is case when we don’t use @Override annotation:

class Car extends Vehicle {
  // Misspelling: No error at compile time
  void halo() {
    System.out.print("Hello from Car");
  }
}

This code is wrong: we intend to override method hello, but we misspelling it, into halo. There is no error at compile time, but only when run time. Let’s run a test to confirm:

class Test {
  void main(String[] args) {
    Car car = new Car();
    car.hello(); // Will print "Hello from Vehicle"
  }
}

This type of error is bad, because it leaves potential bug, until late phase like test or even worse, production phase. The best practice is to avoid it, by using @Override annotation, let’s rewrite the code:

class Car extends Vehicle {

  // Compile error: override non existed method
  @Override
  void halo() {

  }

  // Compile error: override non existed method
  // Correct method name, wrong method param
  @Override
  void halo(int param) {

  }
}

The error can be detected asap, at compile time!

So it’s best practice to use @Override annotation whenever:

  • You want to override a method of parent class
  • You want to implement a method of an interface

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