[OCPJP]介面的預設方法

Given:

public interface Moveable<Integer> {

    public default void walk(Integer distance) {
        System.out.println("Walking");
    }

    public void run(Integer distance);
}

Which statement is true?

A. Moveable can be used as below:

Moveable<Integer> animal = n -> System.out.println("Running" + n);
animal.run(100);
animal.walk(20);

B. Moveable can be used as below:

Moveable<Integer> animal = n -> n + 10;
animal.run(100);
animal.walk(20);

C. Moveable can be used as below:

Moveable animal = (Integer n) -> System.out.println(n);
animal.run(100);
Moveable.walk(20);

D. Movable cannot be used in a lambda expression.

題解

Moveable介面需要實作的方法為僅接受傳入一個整數參數，且沒有回傳值的的run方法。walk方法是Moveable介面的預設方法，只能在實體化出Moveable介面的物件後才能使用，無法直接使用Moveable介面來呼叫。

選項A，正確。

選項B，run方法不能有回傳值。

選項C，walk方法不能用這樣的方式呼叫。

選項D，Moveable介面可以使用Lambda表示式來實作。