Java源码解析：hashCode与相同对象的关系

逃离我推掉我的手 2022-06-12 12:57 184阅读 0赞

## **1.普通类对象** ##

**1. hashCode相同，不一定是同一个对象**  
**2. 同一个对象的，hashCode值一定相同**

![这里写图片描述][SouthEast]

## **2. 数值型的原始数据类型对应的包装类** ##

只要值是一样的，hashCode就会是相同的。尽管不同的数值类型的包装类，计算hashCode的算法不一样，但是底层都是拿对应的原始数据类型的值去进行hashCode计算。

**以Double类为例**

![这里写图片描述][SouthEast 1]

--------------------

## **3. 测试代码如下** ##

/**
     *hashCode相同，不一定是同一个对象
     *同一个对象的，hashCode值一定相同 
     *
     *-------------------------------------------------------------------------------
     *普通对象的HashCode值源码解释：
     *If two objects are equal according to the equals(Object) method, then calling 
     *the hashCode method on each of the two objects must produce the same integer result. 
     *-------------------------------------------------------------------------------
     *It is not required that if two objects are unequal according to the 
     *java.lang.Object.equals(java.lang.Object) method, then calling the hashCode 
     *method on each of the two objects must produce distinct integer results.
     *-------------------------------------------------------------------------------
     *However, the programmer should be aware that producing distinct integer results 
     *for unequal objects may improve the performance of hash tables. 
     *-------------------------------------------------------------------------------
     *
     */
    public class Test {
        
        private String name;
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        public Test() {
            super();
        }
    
        public Test(String name) {
            super();
            this.name = name;
        }
    
        public static void main(String[] args) {
            System.out.println("--------------------普通对象-----------------------");
    
            Test test3=new Test();
            Test test4=new Test();
            System.out.println(test3.hashCode());//2018699554
            System.out.println(test4.hashCode());//1311053135
    
            Test test=new Test("我");
            Test test2=new Test("我");
            System.out.println(test.hashCode());//366712642
            System.out.println(test2.hashCode());//1829164700
    
            System.out.println("--------------------String-----------------------");
    
            String s1="abc";
            String s2="abc";
            System.out.println(s1.hashCode());//96354
            System.out.println(s2.hashCode());//96354
    
            String s5=new String("abc");
            String s6=new String("abc");
            System.out.println(s5.hashCode());//96354
            System.out.println(s6.hashCode());//96354
    
            String s3=new String();
            String s4=new String();
            System.out.println(s3.hashCode());//0
            System.out.println(s4.hashCode());//0
    
            /**
             * 数值型原始类型对应的包装类(Byte,Short,Integer,Float,Double)，hashCode算法都是基于
             * 对应的原始数据类型，所以只要包装类的数值相同，那么hashCode必然相同
             * 
             * Double类关于hashCode源码说明：
             * Double类的hashCode是根据对应的double值计算获得的。
             * 
             * Returns a hash code for a {@code double} value; compatible with
             * {@code Double.hashCode()}.
             *
             * @param value the value to hash
             * @return a hash code value for a {@code double} value.
             * @since 1.8
             */
            /*Double类关于hashCode源码：
             * public static int hashCode(double value) {
                long bits = doubleToLongBits(value);
                return (int)(bits ^ (bits >>> 32));
            }*/
            System.out.println("--------------------原始类型对应的包装类-----------------------");
            Double d1=5.0;
            Double d2=5.0;
            System.out.println(d1.hashCode());//1075052544
            System.out.println(d2.hashCode());//1075052544
    
            Double d3=new Double(5.0);
            Double d4=new Double(5.0);
            System.out.println(d3.hashCode());//1075052544
            System.out.println(d4.hashCode());//1075052544
    
        }
    }

[SouthEast]: /images/20220612/1c24f709e2334f61833c12673a0573ae.png
[SouthEast 1]: /images/20220612/2d3bc481c809437c8dd28be87cf57e71.png