java協變類型返回

協變類型返回也是覆蓋方法的一種，jdk5開始支持的一種:子類覆蓋方法返回能夠是子類返回的子類,這個比較拗口

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*/

package com.fengli;

/**
* 糧食類
* @author Administrator
*/
public class Grain {

public String toString(){
return "Grain";
}

}

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package com.fengli;

/**
* 小麥類
* @author Administrator
*/
public class Wheat extends Grain{
@Override
public String toString(){
return "Wheat";
}

}

 

/*
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* and open the template in the editor.
*/
package com.fengli;

/**
*協變返回
* 子類覆蓋方法返回能夠是子類返回的子類,這個比較拗口
* @author Administrator
*/
public class CovariantReturnType {

class Mill {

public Grain process() {
return new Grain();
}
}

class WheatMill extends Mill {

@Override
public Wheat process() {
return new Wheat();
}
}

public static void main(String[] args) {
CovariantReturnType c = new CovariantReturnType();
CovariantReturnType.Mill m = c.new Mill();
Grain g = m.process();
System.out.println(g.toString());

CovariantReturnType.Mill wm = c.new WheatMill();
Grain g1 = wm.process();
System.out.println(g1.toString());

}}