将<Object,Object>放入TreeMap时出错

Question

我有以下两个类来定义我想要放入TreeMap的对象:

class GeneKey {

    String PN;
    int PW;

    // Generator makes unique TreeMap key.
    GeneKey(String a, int b){
        this.PN = a;
        this.PW = b;
    }
}   

然后是第二个对象:

class GeneValue {

    String info;
    String date;

    // Generator makes TreeMap value
    GeneValue(String a, String b){
        this.info = a;
        this.date = b;
    }
}   

我想制作一个TreeMap:

import java.util.TreeMap;

// In main ...
TreeMap<GeneKey, GeneValue> samples = new TreeMap<GeneKey, GeneValue>();  

String a = "test";
int b = 100;

String c = "test again";
String d = "test yet again";

// Try to put these objects into the tree map.
samples.put(new GeneKey(a, b) ,new GeneValue(c,d))

但是我收到以下错误:

Exception in thread "main" java.lang.ClassCastException: GeneKey cannot be cast to java.lang.Comparable

我想知道为什么我无法使用key:GeneKey:GeneValue的值建立TreeMap,即使我在初始化TreeMap时指定了这些对象.如何初始化地图以便.put()这两个对象.

谢谢

Answer 1

TreeMap 是一个有序的容器:当您请求其密钥或其条目时,您可以按特定顺序获取它们.

订单取决于您提供的密钥.为了使容器能够对密钥进行排序,每个密钥都需要实现Comparable接口:

class GeneKey implements Comparable<GeneKey> {

    String PN;
    int PW;

    // Generator makes unique TreeMap key.
    GeneKey(String a, int b){
        this.PN = a;
        this.PW = b;
    }
    public int compareTo(GenKey other) {
        int res = PN.compareTo(other.PN);
        return (res != 0) ? res : Integer.compare(PW, other.PW);
    }
}

这不是基于散列的容器的要求,因为所有东西都来自Object,提供hashCode和equals.