Extends vs Implements in Java: Key Differences Explained
In Java, extends creates an inheritance tree: one subclass gets everything a superclass has and may override it. implements instead promises that your class will fulfill the contract defined by an interface—no code reuse, just signatures.
People swap them because both sit between the class name and something else. When you’re coding fast, “extend this behavior” and “implement this ability” feel similar, so fingers type whichever sounds right.
Key Differences
extends links one parent, supports method overrides, and carries state. implements can list many interfaces, adds no fields, and forces you to supply every method body. Java allows extends first, then implements in the same declaration.
Which One Should You Choose?
Pick extends when you’re refining an existing, concrete class. Choose implements when you need plug-and-play capabilities across unrelated hierarchies—like adding Serializable to any entity without touching its ancestors.
Examples and Daily Life
Think of extends as inheriting Grandma’s cake recipe and tweaking it. implements is promising “I can bake,” then inventing your own cake from scratch—any cake, as long as it rises.
Can a class do both extends and implements?
Yes. Single parent via extends, multiple interfaces via implements—order matters: class Dog extends Animal implements Pet, Groomable {}.
When should an interface extend another interface?
Use extends between interfaces to merge contracts. Classes never extend interfaces; they implement them.