如果 hashcode 返回一个常量值并且 equals 返回 false，get 方法如何在 hashmap 中工作？

Question

I have Dept class as follows, i have overridden hashcode and equals method. Hashcode returns a constant value and equals return false always.

public class Dept {
    
        private int depid;
        private String deptname;
    
        public Dept(int depid, String deptname) {
            super();
            this.depid = depid;
            this.deptname = deptname;
        }
    
        public int getDepid() {
            return depid;
        }
    
        public void setDepid(int depid) {
            this.depid = depid;
        }
    
        public String getDeptname() {
            return deptname;
        }
    
        public void setDeptname(String deptname) {
            this.deptname = deptname;
        }
    
        @Override
        public int hashCode() {
    
            return 100;
        }
    
        @Override
        public boolean equals(Object obj) {
    
            return false;
        }
    
        @Override
        public String toString() {
            return "Dept [depid=" + depid + ", deptname=" + deptname + "]";
        }
    }

I have a main method

public static void main(String[] args) {
        Dept dept = new Dept(1, "it");
        Dept dept1 = new Dept(1, "it");
        Dept dept2 = new Dept(1, "it");

        HashMap<Dept, String> map = new HashMap<>();
        map.put(dept, "a");
        map.put(dept1, "b");
        map.put(dept2, "c");

        System.out.println(map.get(dept2));// returns c
        System.out.println(map.get(dept1));// returns b
}

According to the theory i have read, hashcode returning a constant value will give us the same index of bucket in hashmap hence values are stored in a single bucket For equals method, it is returning false and hence logically same dept object are saved multiple times. How is get method returning the exact correct value from hashmap?

Answer 1

由于一个对象总是应该equals它自己， HashMap首先使用== (object identity) 检查，因为这要快得多，并且会在许多常见用例中匹配。