自定義類型是否可以作爲Map的Key問題

 雖說經常用map，但是我用它的時候，key都是用String或者Integer類型，今天偶然想到，可不可以用自己定義的對象作爲map的key呢？

talk is cheap give me code

class Student{
    private String name;
    private String sex;
    public String getName() {
        return name;
    }
    public Student(String name,String sex){
        this.name=name;
        this.sex=sex;
    }
    public Student(){}
    public void setName(String name) {
        this.name = name;
    }
    public String getSex() {
        return sex;
    }
    public void setSex(String sex) {
        this.sex = sex;
    }

    public static void main(String[] args) {
        Map<Student,String> map = new HashMap();
        Student s1 = new Student();
        s1.setName("一號");
        s1.setSex("男");
        Student s2 = new Student();
        s2.setName("二號");
        s2.setSex("女");
        map.put(s1, "111");
        map.put(s2, "222");

        Set set = map.keySet(); //試了試遍歷key，然後獲取
        for(Object s:set){
            System.out.println(map.get(s));
        }
        map.put(new Student("二號","女"),"333");
        System.out.println(map.get(new Student("二號","女"))); //試了試單獨根據key獲取值
    }
}

 

結果是這個，爲何會存在一個null呢？？？？

因爲map存儲獲取都是根據hashcode值和equals方法有聯繫，前邊將map的key拿到再放進去獲取value，並沒有發生對象的變化，hashcode值也不變，下邊我又以新的對象去map中獲取，此時的hashcode值已經變了，所以返回爲null，因爲那是我新new的對象所以，map已經不認識了。

怎麼才能解決這個問題呢。重寫Student的hashcode方法和equals方法就可以了,如下：

class Student{
    private String name;
    private String sex;
    public String getName() {
        return name;
    }
    public Student(String name,String sex){
        this.name=name;
        this.sex=sex;
    }
    public Student(){}
    public void setName(String name) {
        this.name = name;
    }
    public String getSex() {
        return sex;
    }
    public void setSex(String sex) {
        this.sex = sex;
    }@Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        result = prime * result + ((sex == null) ? 0 : sex.hashCode());
        return result;
    }
    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Student other = (Student) obj;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        if (sex == null) {
            if (other.sex != null)
                return false;
        } else if (!sex.equals(other.sex))
            return false;
        return true;
    }

    public static void main(String[] args) {
        Map<Student,String> map = new HashMap();
        Student s1 = new Student();
        s1.setName("一號");
        s1.setSex("男");
        Student s2 = new Student();
        s2.setName("二號");
        s2.setSex("女");
        map.put(s1, "111");
        map.put(s2, "222");

        Set set = map.keySet(); //試了試遍歷key，然後獲取
        for(Object s:set){
            System.out.println(map.get(s));
        }
        map.put(new Student("二號","女"),"333");
        System.out.println(map.get(new Student("二號","女"))); //試了試單獨根據key獲取值
    }
}

 

綜上：如果要以可變對象作爲key的話，需要重寫hashcode和equal方法來達到這個目的。

本篇博客是剛剛踩坑之後寫的，之前的博主坑還是很明顯的。附上鍊接https://blog.csdn.net/wohaqiyi/article/details/57083444