Interfaces in Java

The contract, plus behavior the contract can ship itself.

Shape[] shapes = { new Circle(1.5), new Square(2) };
for (var shape : shapes) {
    System.out.printf("%.2f / %s%n", shape.area(), shape.label());
}

interface Shape {
    double area();

    // a default method ships behavior with the contract
    default String label() { return "a " + getClass().getSimpleName(); }
}

record Circle(double radius) implements Shape {
    public double area() { return Math.PI * radius * radius; }
}

record Square(double side) implements Shape {
    public double area() { return side * side; }
}

How it works

1. interface Shape demands area; records implement it.
2. A default method gives every implementer label for free.
3. The array of shapes dispatches to each implementation.

Keywords and builtins used here

• Circle
• Shape
• Square
• area
• default
• double
• for
• implements
• interface
• label
• new
• public
• record
• return
• var

The run, in numbers

Lines

19

Characters to type

507

Tokens

129

Three-star pace

85 tpm

At the three-star pace of 85 tokens a minute, this run takes about 91 seconds.

Type this snippet

Step 4 of 4 in Classes & records, step 21 of 29 in Language basics.

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Interfaces in other languages

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