零基础java自学流程-Java语言进阶123

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

public class R {
    private int count;

    public R(int count) {
        this.count = count;
    }

    @Override
    public String toString() {
          return "R{" + "count=" + count + " # hashCode=" + this.hashCode() + '}';
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        R r = (R) o;
        return count == r.count;
    }

//    @Override
//    public int hashCode() {
//        return Objects.hash(count);
//    }

    public static void main(String[] args) {
        Set set = new HashSet();
        set.add(new R(5));
        set.add(new R(-3));
        set.add(new R(9));
        set.add(new R(-2));
        System.out.println(set.contains(new R(-3)));
        System.out.println(set);

    }
}

上面注释了hashCode方法，所以你将会看到下面的结果。

false
[R{count=9 # hashCode=2003749087}, R{count=5 # hashCode=1283928880}, R{count=-3 # hashCode=295530567}, R{count=-2 # hashCode=1324119927}]

取消注释，则结果就正确了

true
[R{count=5 # hashCode=36}, R{count=9 # hashCode=40}, R{count=-3 # hashCode=28}, R{count=-2 # hashCode=29}]