多型就是通過繼承和動態繫結來決定程式使用哪個實現.

一個簡單的例子。

interface Animal {
    public void eat();
}

class Whale extends SeaAnimal {
    int age = 20;

    @Override
    public void eat() {
        System.out.println("Whale is eatting a small fish");
    }

    private void swimming() {
        System.out.println("Whale is swimming");
    }

    @Override
    public int getAge() {
        return age;
    }

    public static void staticSwimming() {
        System.out.println("Static whale is swimming .");
    }

}

public class SeaAnimal implements Animal {
    int age = 10;

    private void swimming() {
        System.out.println("SeaAnimal is swimming.");

    }

    public static void staticSwimming() {
        System.out.println("Static seaAnimal is swimming.");
    }

    public int getAge() {
        return age;
    }

    public static void main(String[] args) {
        SeaAnimal animal = new Whale();
        animal.swimming();
        animal.eat();
        animal.staticSwimming();
        System.out.println("The age is " + animal.age);
        System.out.println("The age is " + animal.getAge());
    }

    @Override
    public void eat() {
        System.out.println("SeaAnimal is eatting a small fish");
    }
}
 

輸出結果：

SeaAnimal is swimming.
Whale is eatting a small fish
Static seaAnimal is swimming.
The age is 10
The age is 20

從例子可以看出：

• 成員變數不具備多型
• private 方法不具備多型
• static方法不具備多型
• 構造方法不具備多型