Concept · Core

Abstract class vs Interface

An abstract class is a half-built base you EXTEND (one parent, shared code + state); an interface is a pure contract you IMPLEMENT (many of them, no state). Knowing which to reach for is a classic interview question.

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step 1 / 5
Extend one, implement many
«abstract»
Game
# players: int
+ play()
# initialize()*
# endGame()*
«interface»
Savable
+ save()
Chess
# initialize()
# endGame()
+ save()
«interface»
Rankable
+ rank()
Extend one, implement many
 1abstract class Game {
 2    protected int players;
 3    public final void play() { initialize(); endGame(); }
 4    protected abstract void initialize();
 5    protected abstract void endGame();
 6}
 7interface Savable  { void save(); }
 8interface Rankable { int  rank(); }
 9 
10class Chess extends Game implements Savable, Rankable { ... }
State
abstractcannot be new-ed
state +concrete + abstract
✦

Game is an ABSTRACT class: it holds state (# players), a CONCRETE template method play(), and ABSTRACT methods (marked *) that subclasses must fill in. You can never `new` it directly.