Java · Theory

Interface in java

Theory

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Interface in java

An interface is a contract: if you implement me, you must provide these behaviours. class Circle implements Drawable. A class can implement many interfaces — multiple capabilities.

Good names sound like abilities: Runnable, Comparable, Drawable. Modern Java can add default and static methods, but the core idea is still the contract: what you can do, not a family of shared fields.

If you mainly need shared fields and shared code, consider an abstract class. Interview line: interface = can-do contract; a class may implement many.

Let's take this on the board with one tiny Main class — no extra files. Interface in java — output: Draw circle. Drawable d = new Circle() is allowed — interface type, real Circle object. Start from main, go line by line, and stop at each print. That output is the proof for Interface.

Diagram

interface Drawable
    draw()
         │ implements
         ▼
      Circle.draw()

Exam tip

Multiple interfaces vs single superclass.

Example

interface Drawable {
  void draw();
}
class Circle implements Drawable {
  public void draw() { System.out.println("Draw circle"); }
}
public class Main {
  public static void main(String[] args) {
    Drawable d = new Circle();
    d.draw();
  }
}

Interface in java — output: Draw circle. Drawable d = new Circle() is allowed — interface type, real Circle object.

Short notes

DefInterface = contract. Implement me → provide these behaviours.
Ruleimplements keyword. A class can implement many interfaces.
RememberNames are capabilities — Runnable, Comparable, Drawable.
DiffNeed shared fields/code? Think abstract class. Need can-do? Interface.

Java · Theory

Interface in java

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Theory

45/270

Interface in java

An interface is a contract: if you implement me, you must provide these behaviours. class Circle implements Drawable. A class can implement many interfaces — multiple capabilities.

If you mainly need shared fields and shared code, consider an abstract class. Interview line: interface = can-do contract; a class may implement many.

Diagram

interface Drawable
    draw()
         │ implements
         ▼
      Circle.draw()

Exam tip

Multiple interfaces vs single superclass.

Example

interface Drawable {
  void draw();
}
class Circle implements Drawable {
  public void draw() { System.out.println("Draw circle"); }
}
public class Main {
  public static void main(String[] args) {
    Drawable d = new Circle();
    d.draw();
  }
}

Interface in java — output: Draw circle. Drawable d = new Circle() is allowed — interface type, real Circle object.

Short notes

DefInterface = contract. Implement me → provide these behaviours.
Ruleimplements keyword. A class can implement many interfaces.
RememberNames are capabilities — Runnable, Comparable, Drawable.
DiffNeed shared fields/code? Think abstract class. Need can-do? Interface.

Interface in java

Example

Short notes

Questions

Interface in java

Example

Short notes

Questions