What do the relationships "is-a" and "has" mean in object-oriented programming?

The "is-a" relationship — inheritance. A subclass "is" a superclass and inherits its properties and behavior.

// Superclass
class Animal {
    void eat() {
        System.out.println("Animal eats");
    }
}

// Subclass
class Dog extends Animal {
    void bark() {
        System.out.println("Dog barks");
    }
}

In this example, Dog "is a" Animal.

The "has-a" relationship — composition or aggregation. One object "has" a reference to another object as part of its state.

class Engine {
    void start() {
        System.out.println("Engine started");
    }
}

class Car {
    private Engine engine; // Car "has" an engine

    public Car() {
        this.engine = new Engine();
    }

    void drive() {
        engine.start();
        System.out.println("Car is driving");
    }
}

Here, Car "has" an Engine. Composition implies a strong relationship (the lifecycle of the contained object depends on the container), while aggregation is a weaker relationship (objects can exist independently). The example shows composition.

Key difference:

• "Is-a" (is-a) — inheritance, defines a type hierarchy.
• "Has-a" (has-a) — composition/aggregation, defines the structure of an object through inclusion of other objects.