java - 检索 map 中的对象

Question

当我更改了一个对象的实例变量时，我试图检索一个对象作为 Map 中的键。就我而言，当我在 Map 结构中搜索关键对象时，它应该在 hashCode 方法返回与最初相同的值时出现。但是，根据以下代码，我无法通过更改了变量权重值的方法 get() 检索对象:

public class Car  implements Comparable<Car> {

 int weight;
 String name;

 public Car(int w, String n) {
    weight=w;
    name=n;
 }

public boolean equals(Object o){
    if(o instanceof Car){
        Car d = (Car)o;
        return ((d.name.equals(name)) && (d.weight==weight));
    }
    return false;

}

public int hashCode(){
    return weight/2 + 17;
}

public String toString(){
    return "I am " +name+ " with weight: "+weight;
}


public int compareTo(Car d){
    if(this.weight>d.weight)
        return 1;
    else if(this.weight<d.weight)
        return -1;
    else
        return this.name.compareTo(d.name);
}

}


public static void main(String[] args) {
    Car d1 = new Car(107, "a");
    Car d2 = new Car(110, "b");
    Car d3 = new Car(110, "c");
    Car d4 = new Car(107, "a");

    Map<Car, Integer> m = new HashMap<Car, Integer>();
    m.put(d1, 1);
    m.put(d2, 2);
    m.put(d3, 3);
    m.put(d4, 16);

    System.out.println(m.get(d1).toString());
    d1.weight = 34;
    System.out.println(m.get(new Car(34, "a")));

    for(Map.Entry<Car, Integer> me : m.entrySet())
        System.out.println(me.getKey().toString() + " value: " +me.getValue());

}

输出是:

16

null

I am a with weight: 34 16

I am c with weight: 110 3

I am b with weight: 110 2

但是，如果我不更改权重值(省略行:d1.weight = 34;)，仅使用代码行:

System.out.println(m.get(new Car(107, "a")));

输出是:

16

16

I am a with weight: 107 value: 16

I am c with weight: 110 value: 3

I am b with weight: 110 value: 2

它实际上找到了对象。它是否以两种方式达到预期的输出，或者它是否也应该在第一个版本中找到并检索到对象？

Answer 1

将可变对象作为键存储在哈希表中时应该非常小心。哈希码仅在对象插入时计算一次。如果您稍后以影响其哈希码的方式更改对象，则很可能无法找到它。

每当进行此类更改时，您都需要删除该对象并重新插入它。