package hashMap;

import java.util.HashMap;
import java.util.Map;

/* 
 * 可变对象是指创建后自身状态能改变的对象。换句话说，可变对象是该对象在创建后它的哈希值可能被改变。

在下面的代码中，对象MutableKey的键在创建时变量 i=10 j=20，哈希值是1291。

然后我们改变实例的变量值，该对象的键 i 和 j 从10和20分别改变成30和40。现在Key的哈希值已经变成1931。

显然，这个对象的键在创建后发生了改变。所以类MutableKey是可变的。
 */
class MutableKey {
    private int i;
    private int j;

    public MutableKey(int i, int j) {
        this.i = i;
        this.j = j;
    }

    public final int getI() {
        return i;
    }

    public final void setI(int i) {
        this.i = i;
    }

    public final int getJ() {
        return j;
    }

    public final void setJ(int j) {
        this.j = j;
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + i;
        result = prime * result + j;
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }

        if (obj == null) {
            return false;
        }
        if (!(obj instanceof MutableKey)) {
            return false;
        }
        MutableKey other = (MutableKey) obj;
        if (i != other.i) {
            return false;
        }
        if (j != other.j) {
            return false;
        }
        return true;
    }
}


public class objectAsKey {

    private static void simulateObjectLost() {
        // HashMap
        Map<MutableKey, String> map = new HashMap<>();
        
        // Object created
        MutableKey key = new MutableKey(10, 20);
        
        // Insert entry.
        map.put(key, "Robin");
        
        // This line will print 'Robin'
        System.out.println(map.get(key));
        
        // Object State is changed after object creation.
        // i.e. Object hash code will be changed.
        key.setI(30);
        
        // This line will print null as Map would be unable to retrieve the entry.
        System.out.println(map.get(key));
    }
    
    public static void main(String[] args) {
        // Object created
        MutableKey key = new MutableKey(10, 20);
        System.out.println("Hash code: " + key.hashCode());
        // Object State is changed after object creation.
        key.setI(30);
        key.setJ(40);
        System.out.println("Hash code: " + key.hashCode());
        
        simulateObjectLost();
    }
}