public class MutableKey {private int i;private int j;public MutableKey(int i, int j) {this.i = i;this.j = j;}public final int getI() {return i;}public final void setI(int i) {this.i = i;}public final int getJ() {return j;}public final void setJ(int j) {this.j = j;}@Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result + i;result = prime * result + j;return result;}@Overridepublic boolean equals(Object obj) {if (this == obj) {return true;}if (obj == null) {return false;}if (!(obj instanceof MutableKey)) {return false;}MutableKey other = (MutableKey) obj;if (i != other.i) {return false;}if (j != other.j) {return false;}return true;}
}

public class MutableDemo {public static void main(String[] args) {// Object createdMutableKey key = new MutableKey(10, 20);System.out.println("Hash code: " + key.hashCode());// Object State is changed after object creation.key.setI(30);key.setJ(40);System.out.println("Hash code: " + key.hashCode());}
}

public V get(Object key)
{   // 如果 key 是 null，调用 getForNullKey 取出对应的 value   if (key == null)   return getForNullKey();   // 根据该 key 的 hashCode 值计算它的 hash 码  int hash = hash(key.hashCode());   // 直接取出 table 数组中指定索引处的值，  for (Entry<K,V> e = table[indexFor(hash, table.length)];   e != null;   // 搜索该 Entry 链的下一个 Entr   e = e.next)         // ①
 {   Object k;   // 如果该 Entry 的 key 与被搜索 key 相同  if (e.hash == hash && ((k = e.key) == key   || key.equals(k)))   return e.value;   }   return null;
}   

import java.util.HashMap;
import java.util.Map;public class MutableDemo1 {public static void main(String[] args) {// HashMapMap<MutableKey, String> map = new HashMap<>();// Object createdMutableKey key = new MutableKey(10, 20);// Insert entry.map.put(key, "Robin");// This line will print 'Robin'
        System.out.println(map.get(key));// Object State is changed after object creation.// i.e. Object hash code will be changed.key.setI(30);// This line will print null as Map would be unable to retrieve the// entry.
        System.out.println(map.get(key));}
}

Robin
null

import java.util.HashMap;
import java.util.Map;public class MutableSafeKeyDemo {public static void main(String[] args) {Employee emp = new Employee(2);emp.setName("Robin");// Put object in HashMap.Map<Employee, String> map = new HashMap<>();map.put(emp, "Showbasky");System.out.println(map.get(emp));// Change Employee name. Change in 'name' has no effect// on hash code.emp.setName("Lily");System.out.println(map.get(emp));}
}class Employee {// It is specified while object creation.// Cannot be changed once object is created. No setter for this field.private int id;private String name;public Employee(final int id) {this.id = id;}public final String getName() {return name;}public final void setName(final String name) {this.name = name;}public int getId() {return id;}// Hash code depends only on 'id' which cannot be// changed once object is created. So hash code will not change// on object's state change
    @Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result + id;return result;}@Overridepublic boolean equals(Object obj) {if (this == obj)return true;if (obj == null)return false;if (getClass() != obj.getClass())return false;Employee other = (Employee) obj;if (id != other.id)return false;return true;}
}

Showbasky
Showbasky