Java Set中定制类的唯一值 [英] Unique values of custom class in Java Set

问题描述

我希望我的Set中只有2个元素，但是在打印时我会收到3个元素！如何定义唯一性?

I expect to have only 2 elements in my Set but I receive 3 elements while printing! How can I define uniqueness?

public class test {

    public static void main(String[] args) {

        class bin {
            int a;
            int b;
            bin (int a, int b){
                this.a=a;
                this.b=b;
            }
            public boolean Equals(bin me) {
                if(this.a==me.a && this.b==me.b)
                    return true;
                else 
                    return false;
            }   
            @Override
            public String toString() {
                return a+" "+b;
            }
        }

        Set<bin> q= new HashSet<bin>();
        q.add(new bin(11,23));
        q.add(new bin(11,23));
        q.add(new bin(44,25));

        System.out.println(q);
    }
}

推荐答案

这里有两个问题

• equals应该为小写并接受Object
• 您还必须覆盖hashCode

• equals should be lowercase and accept an Object
• You have to override hashCode as well

修改后的代码如下所示.请注意，实现远非完美，如equals一样，您应检查是否为null以及是否可以进行类型转换等.hashCode仅仅是示例，但如何实现此类事情是另一个主题.

The modified code could look like below. Note that the implementation is far from being perfect as in equals you should check for null and whether a type cast is possible etc. Also hashCode is just an example but how to implement such things is another topic.

import java.util.Set;
import java.util.HashSet;

public class test {

    public static void main(String[] args) {

        class bin{
            int a;
            int b;
            bin (int a, int b){
                this.a=a;
                this.b=b;
            }

            @Override
            public boolean equals(Object me) {
                bin binMe = (bin)me;
                if(this.a==binMe.a && this.b==binMe.b)
                    return true;
                else 
                    return false;
            }   

            @Override
            public int hashCode() {
                return this.a + this.b;
            }

            @Override
            public String toString() {
                return a+" "+b;
            }
        }

        Set<bin> q= new HashSet<bin>();
        q.add(new bin(11,24));
        q.add(new bin(11,24));
        q.add(new bin(10,25));
        q.add(new bin(44,25));

        System.out.println(q);
    }
}

---

结果:

---

Result:

[11 24，10 25，44 25]

[11 24, 10 25, 44 25]

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