extends vs implements in Java

The extends and implements keywords are part of the inheritance feature in Java.

The extends keyword is used whenever you want to extend a class or an interface as shown below:

The code below shows how the two keywords work:

// for classes
class Dog {}

class Poodle extends Dog {}

// for interfaces 
interface Cat {}

interface Tabby extends Cat {}

A Java class can only extend one class, while an interface can extend multiple interfaces:

class Dog {}
class Animal {}
class Poodle extends Dog, Animal {} // ERROR

interface Cat {}
interface Feline {}
interface Tabby extends Cat, Feline {} // OK

The extends keyword can only be used between the same reference type:

  • A class with a class
  • An interface with one or more interfaces

When you want to inherit an interface to a class, you need to use the implements keyword:

interface Cat {}

class Tom implements Cat {}

You can implement one or more interfaces to a class, but you can’t extend more than one class.

Now you’ve learned the differences between extends and implements keywords. Nice work! 😉

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