package com.mufeng.theeighthchapter;

class Grain {
	@Override
	public String toString() {
		// TODO Auto-generated method stub
		return "Grain";
	}
}

class Wheat extends Grain {
	@Override
	public String toString() {
		// TODO Auto-generated method stub
		return "Wheat";
	}
}

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

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

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

}

输出结果

Grain
Wheat