The effect of an object with or without euqals and hashCode as a hashmap key

gistfile1.txt

package interview;

import org.junit.Test;

import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;

import static org.junit.Assert.*;

/**
 * 面试题：
 * 考察作为HashMap的key的对象有无复写equals和hashCode时的表现   
 * Created by zhuguowei on 5/16/17.
 */
public class HashMapTest {
    @Test
    public void test_StudentWithOutEqualAndHashCode() throws Exception {
        StudentWithOutEqualAndHashCode student1 = new StudentWithOutEqualAndHashCode("zhangsan");
        StudentWithOutEqualAndHashCode student2 = new StudentWithOutEqualAndHashCode("zhangsan");

        assertFalse(student1 == student2); // false

        assertFalse(student1.equals(student2)); // false

        Map<StudentWithOutEqualAndHashCode, Integer> map = new HashMap<>();
        map.put(student1, 1);
        map.put(student2, 2);

        assertEquals(2,map.size()); // 2

        assertEquals(1,map.get(student1).intValue());
        assertEquals(2,map.get(student2).intValue());

        printTable(map);

    }
    @Test
    public void test_StudentWithHashCode() throws Exception {
        StudentWithHashCode student1 = new StudentWithHashCode("zhangsan");
        StudentWithHashCode student2 = new StudentWithHashCode("zhangsan");

        assertFalse(student1 == student2); // false

        assertFalse(student1.equals(student2)); // false

        HashMap<StudentWithHashCode, Integer> map = new HashMap<>();
        map.put(student1, 1);
        map.put(student2, 2);

        assertEquals(2,map.size()); // 2

        assertEquals(1,map.get(student1).intValue());
        assertEquals(2,map.get(student2).intValue());

        printTable(map);

        Object[] table = getTable(map);
        assertEquals(student1, getStudent(student1.hashCode(), table,0));
        assertEquals(student2, getStudent(student2.hashCode(), table,1));

    }
    @Test
    public void test_StudentWithHashCodeAndEquals() throws Exception {
        StudentWithHashCodeAndEquals student1 = new StudentWithHashCodeAndEquals("zhangsan");
        StudentWithHashCodeAndEquals student2 = new StudentWithHashCodeAndEquals("zhangsan");

        assertFalse(student1 == student2); // false

        assertTrue(student1.equals(student2)); // true

        HashMap<StudentWithHashCodeAndEquals, Integer> map = new HashMap<>();
        map.put(student1, 1);
        map.put(student2, 2);

        assertEquals(1,map.size());

        assertEquals(2,map.get(student1).intValue());
        assertEquals(2,map.get(student2).intValue());

        printTable(map);

        Object[] table = getTable(map);
        assertEquals(student1, getStudent(student1.hashCode(), table,0));
        assertNull(getStudent(student1.hashCode(), table,1));

    }

    private Student getStudent(int hashCode, Object[] table, int nextIndex) throws NoSuchFieldException, IllegalAccessException {
        int i = (hashCode ^ (hashCode >>> 16)) & (16 - 1);
        Object node = table[i];
        for (int j = 0; j < nextIndex; j++) {
            Field nextField = node.getClass().getDeclaredField("next");
            nextField.setAccessible(true);

            node = nextField.get(node);
        }
        if(node == null){
            return null;
        }
        Field keyField = node.getClass().getDeclaredField("key");
        keyField.setAccessible(true);
        return (Student) keyField.get(node);
    }

    private void printTable(Map map) throws NoSuchFieldException, IllegalAccessException, NoSuchMethodException, InvocationTargetException {
        Object[] table = getTable(map);
        System.out.println(Arrays.toString(table));
    }

    private Object[] getTable(Map map) throws NoSuchFieldException, IllegalAccessException {
        Field tableField = HashMap.class.getDeclaredField("table");
        tableField.setAccessible(true);
        return (Object[]) tableField.get(map);
    }

    interface Student{}
    static class StudentWithOutEqualAndHashCode implements Student{
        private String name;

        public StudentWithOutEqualAndHashCode(String name) {
            this.name = name;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

    }
    static class StudentWithHashCode implements Student{
        private String name;

        public StudentWithHashCode(String name) {
            this.name = name;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        @Override
        public int hashCode() {
            return 1;
        }
    }
    static class StudentWithHashCodeAndEquals implements Student{
        private String name;

        public StudentWithHashCodeAndEquals(String name) {
            this.name = name;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        @Override
        public int hashCode() {
            return 1;
        }

        @Override
        public boolean equals(Object obj) {
            return true;
        }
    }
}