1. 代码示例

package demo01;

class Grain {

    @Override
    public String toString() {
        return "Grain []";
    }

}

class Wheat extends Grain {
    @Override
    public String toString() {
        return "Wheat []";
    }
}

class Mill {
    Grain process() {
        return new Grain();
    }
}

class WheatMill extends Mill {
    @Override
    Grain process() {
        return new Wheat();
    }
}

public class Poppet {

    public static void main(String[] args) {
        Mill m = new WheatMill();
        System.out.println(m.process());
    }

}

2.解析

如代码所示,基类方法的返回类型如果也是基类的话,子类重写的方法可以返回基类的子类.表示子类重写方法的返回值可以是基类方法返回值的子类型.