为什么java中具有相同值的两个对象的hashcode（）不同？_Java_Hashcode

为什么java中具有相同值的两个对象的hashcode（）不同？

java

为什么java中具有相同值的两个对象的hashcode（）不同？,java,hashcode,Java,Hashcode,Object-1: Employee employee1 = new Employee("AB12","Dhruv", 24); Employee employee2 = new Employee("AB12", "Dhruv", 24); true employee1.hashCode(): 511833308 employee3.hashCode(): 511833308 false employee2.hashCode(): 1297685781 true employee1.has

Object-1:

Employee employee1 = new Employee("AB12","Dhruv", 24);

Employee employee2 = new Employee("AB12", "Dhruv", 24);

true
employee1.hashCode(): 511833308
employee3.hashCode(): 511833308
false
employee2.hashCode(): 1297685781

true
employee1.hashCode(): 128107556
employee3.hashCode(): 128107556
false
employee2.hashCode(): 128107556

true
employee1.hashCode(): 66066760
employee1.hashCode() again: 66069457
employee3.hashCode(): 66073797
false
employee2.hashCode(): 66074882

true
employee1.hashCode(): 511833308
employee3.hashCode(): 511833308
true
employee2.hashCode(): 1297685781

true
employee1.hashCode(): 128107556
employee3.hashCode(): 128107556
true
employee2.hashCode(): 128107556

Object-2:

Employee employee1 = new Employee("AB12","Dhruv", 24);

Employee employee2 = new Employee("AB12", "Dhruv", 24);

true
employee1.hashCode(): 511833308
employee3.hashCode(): 511833308
false
employee2.hashCode(): 1297685781

true
employee1.hashCode(): 128107556
employee3.hashCode(): 128107556
false
employee2.hashCode(): 128107556

true
employee1.hashCode(): 66066760
employee1.hashCode() again: 66069457
employee3.hashCode(): 66073797
false
employee2.hashCode(): 66074882

true
employee1.hashCode(): 511833308
employee3.hashCode(): 511833308
true
employee2.hashCode(): 1297685781

true
employee1.hashCode(): 128107556
employee3.hashCode(): 128107556
true
employee2.hashCode(): 128107556

输出

HASH CODE FOR EMPLOYEE 1 :  43704527 
HASH CODE FOR EMPLOYEE 2 : 158893348

除非

@重写hashCode（）
方法并实现自己的版本，否则类将使用从对象继承的默认方法，该方法会为每个对象实例生成唯一的hashCode
如果希望equals
和/或hashCode
只关心对象的成员，则需要自己编写实现。
因为您没有重写hashCode
来替换默认实现。重写equals
不会改变hashCode
（如果重写equals
，则几乎总是必须同时重写这两者）
hashCode
说明了对象中的默认实现：
在合理可行的情况下，hashCode方法由
类对象确实为不同的对象返回不同的整数。（本
通常通过转换
对象转换为整数，但此实现技术不适用
Java所要求的™ 编程语言。）
因此，除非替换hashCode
，否则对于不同的实例，即使它们相等，也会得到不同的整数（通常）
你可以这样做
 @Override
        public int hashCode() {
            return hashId;
        }

从以下几点来看：
在Java应用程序的执行过程中，每当在同一对象上多次调用hashCode方法时，只要没有修改对象上的equals比较中使用的信息，hashCode方法必须始终返回相同的整数。从应用程序的一次执行到同一应用程序的另一次执行，该整数不必保持一致
如果根据equals（Object）方法两个对象相等，那么对两个对象中的每一个调用hashCode方法必须产生相同的整数结果
根据equals（java.lang.Object）方法，如果两个对象不相等，则对这两个对象中的每一个调用hashCode方法都必须产生不同的整数结果，这不是必需的。但是，程序员应该知道，为不相等的对象生成不同的整数结果可能会提高哈希表的性能
只要是合理可行的，类对象定义的hashCode方法确实会为不同的对象返回不同的整数。（hashCode在某个时间点可能实现为对象内存地址的函数，也可能不实现。）
现在，查看以下代码及其输出：
class MyEmployee {
    String code;
    String name;
    int age;

    public MyEmployee(String code, String name, int age) {
        super();
        this.code = code;
        this.name = name;
        this.age = age;
    }
}

public class Main {
    public static void main(String[] args) {
        MyEmployee employee1 = new MyEmployee("AB12", "Dhruv", 24);
        MyEmployee employee2 = new MyEmployee("AB12", "Dhruv", 24);
        MyEmployee employee3 = employee1;
        System.out.println(employee1.equals(employee3));
        System.out.println("employee1.hashCode(): " + employee1.hashCode());
        System.out.println("employee3.hashCode(): " + employee3.hashCode());
        System.out.println(employee1.equals(employee2));
        System.out.println("employee2.hashCode(): " + employee2.hashCode());
    }
}

class MyEmployee {
    String code;
    String name;
    int age;

    public MyEmployee(String code, String name, int age) {
        super();
        this.code = code;
        this.name = name;
        this.age = age;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        MyEmployee other = (MyEmployee) obj;
        if (code == null) {
            if (other.code != null)
                return false;
        } else if (!code.equals(other.code))
            return false;
        return true;
    }
}

public class Main {
    public static void main(String[] args) {
        MyEmployee employee1 = new MyEmployee("AB12", "Dhruv", 24);
        MyEmployee employee2 = new MyEmployee("AB12", "Dhruv", 24);
        MyEmployee employee3 = employee1;
        System.out.println(employee1.equals(employee3));
        System.out.println("employee1.hashCode(): " + employee1.hashCode());
        System.out.println("employee3.hashCode(): " + employee3.hashCode());
        System.out.println(employee1.equals(employee2));
        System.out.println("employee2.hashCode(): " + employee2.hashCode());
    }
}

输出：
Employee employee1 = new Employee("AB12","Dhruv", 24);

Employee employee2 = new Employee("AB12", "Dhruv", 24);

true
employee1.hashCode(): 511833308
employee3.hashCode(): 511833308
false
employee2.hashCode(): 1297685781

true
employee1.hashCode(): 128107556
employee3.hashCode(): 128107556
false
employee2.hashCode(): 128107556

true
employee1.hashCode(): 66066760
employee1.hashCode() again: 66069457
employee3.hashCode(): 66073797
false
employee2.hashCode(): 66074882

true
employee1.hashCode(): 511833308
employee3.hashCode(): 511833308
true
employee2.hashCode(): 1297685781

true
employee1.hashCode(): 128107556
employee3.hashCode(): 128107556
true
employee2.hashCode(): 128107556

由于employee3
指向的对象与employee1
指向的对象相同，因此当employee2
指向另一个对象时（尽管它的内容相同，关键字new
将在内存中创建一个单独的对象），因此，您可能很少从文档状态中获得与上述第4点相同的hashcode:class对象定义的hashcode方法，只要合理可行，class对象定义的hashcode方法会为不同的对象返回不同的整数。

您必须重写hashCode
方法，该方法应为具有相同内容的两个对象返回相同的hashCode，例如
class MyEmployee {
    String code;
    String name;
    int age;

    public MyEmployee(String code, String name, int age) {
        super();
        this.code = code;
        this.name = name;
        this.age = age;
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + age;
        result = prime * result + ((code == null) ? 0 : code.hashCode());
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        return result;
    }   
}

public class Main {
    public static void main(String[] args) {
        MyEmployee employee1 = new MyEmployee("AB12", "Dhruv", 24);
        MyEmployee employee2 = new MyEmployee("AB12", "Dhruv", 24);
        MyEmployee employee3 = employee1;
        System.out.println(employee1.equals(employee3));
        System.out.println("employee1.hashCode(): " + employee1.hashCode());
        System.out.println("employee3.hashCode(): " + employee3.hashCode());
        System.out.println(employee1.equals(employee2));
        System.out.println("employee2.hashCode(): " + employee2.hashCode());
    }
}

输出：
Employee employee1 = new Employee("AB12","Dhruv", 24);

Employee employee2 = new Employee("AB12", "Dhruv", 24);

true
employee1.hashCode(): 511833308
employee3.hashCode(): 511833308
false
employee2.hashCode(): 1297685781

true
employee1.hashCode(): 128107556
employee3.hashCode(): 128107556
false
employee2.hashCode(): 128107556

true
employee1.hashCode(): 66066760
employee1.hashCode() again: 66069457
employee3.hashCode(): 66073797
false
employee2.hashCode(): 66074882

true
employee1.hashCode(): 511833308
employee3.hashCode(): 511833308
true
employee2.hashCode(): 1297685781

true
employee1.hashCode(): 128107556
employee3.hashCode(): 128107556
true
employee2.hashCode(): 128107556

上面给出的hashCode
的实现为employee1
和employee2
生成相同的hashCode，即使等于返回false
（从文档中选择上面提到的点3）
重写hashCode
的错误方法可能会导致相同的对象返回不同的hashCode，例如
class MyEmployee {
    String code;
    String name;
    int age;

    public MyEmployee(String code, String name, int age) {
        super();
        this.code = code;
        this.name = name;
        this.age = age;
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + age;
        result = prime * result + ((code == null) ? 0 : (int) (code.length() * (Math.random() * 100)));
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        return result;
    }
}

public class Main {
    public static void main(String[] args) {
        MyEmployee employee1 = new MyEmployee("AB12", "Dhruv", 24);
        MyEmployee employee2 = new MyEmployee("AB12", "Dhruv", 24);
        MyEmployee employee3 = employee1;
        System.out.println(employee1.equals(employee3));
        System.out.println("employee1.hashCode(): " + employee1.hashCode());
        System.out.println("employee1.hashCode() again: " + employee1.hashCode());
        System.out.println("employee3.hashCode(): " + employee3.hashCode());
        System.out.println(employee1.equals(employee2));
        System.out.println("employee2.hashCode(): " + employee2.hashCode());
    }
}

输出：
Employee employee1 = new Employee("AB12","Dhruv", 24);

Employee employee2 = new Employee("AB12", "Dhruv", 24);

true
employee1.hashCode(): 511833308
employee3.hashCode(): 511833308
false
employee2.hashCode(): 1297685781

true
employee1.hashCode(): 128107556
employee3.hashCode(): 128107556
false
employee2.hashCode(): 128107556

true
employee1.hashCode(): 66066760
employee1.hashCode() again: 66069457
employee3.hashCode(): 66073797
false
employee2.hashCode(): 66074882

true
employee1.hashCode(): 511833308
employee3.hashCode(): 511833308
true
employee2.hashCode(): 1297685781

true
employee1.hashCode(): 128107556
employee3.hashCode(): 128107556
true
employee2.hashCode(): 128107556

这是覆盖hashCode
的错误方法，因为在Java应用程序执行期间多次调用同一对象上的hashCode
必须一致地返回相同的整数（检查上述文档中提到的点1）
现在，查看以下代码及其输出：
class MyEmployee {
    String code;
    String name;
    int age;

    public MyEmployee(String code, String name, int age) {
        super();
        this.code = code;
        this.name = name;
        this.age = age;
    }
}

public class Main {
    public static void main(String[] args) {
        MyEmployee employee1 = new MyEmployee("AB12", "Dhruv", 24);
        MyEmployee employee2 = new MyEmployee("AB12", "Dhruv", 24);
        MyEmployee employee3 = employee1;
        System.out.println(employee1.equals(employee3));
        System.out.println("employee1.hashCode(): " + employee1.hashCode());
        System.out.println("employee3.hashCode(): " + employee3.hashCode());
        System.out.println(employee1.equals(employee2));
        System.out.println("employee2.hashCode(): " + employee2.hashCode());
    }
}

class MyEmployee {
    String code;
    String name;
    int age;

    public MyEmployee(String code, String name, int age) {
        super();
        this.code = code;
        this.name = name;
        this.age = age;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        MyEmployee other = (MyEmployee) obj;
        if (code == null) {
            if (other.code != null)
                return false;
        } else if (!code.equals(other.code))
            return false;
        return true;
    }
}

public class Main {
    public static void main(String[] args) {
        MyEmployee employee1 = new MyEmployee("AB12", "Dhruv", 24);
        MyEmployee employee2 = new MyEmployee("AB12", "Dhruv", 24);
        MyEmployee employee3 = employee1;
        System.out.println(employee1.equals(employee3));
        System.out.println("employee1.hashCode(): " + employee1.hashCode());
        System.out.println("employee3.hashCode(): " + employee3.hashCode());
        System.out.println(employee1.equals(employee2));
        System.out.println("employee2.hashCode(): " + employee2.hashCode());
    }
}

输出：
Employee employee1 = new Employee("AB12","Dhruv", 24);

Employee employee2 = new Employee("AB12", "Dhruv", 24);

true
employee1.hashCode(): 511833308
employee3.hashCode(): 511833308
false
employee2.hashCode(): 1297685781

true
employee1.hashCode(): 128107556
employee3.hashCode(): 128107556
false
employee2.hashCode(): 128107556

true
employee1.hashCode(): 66066760
employee1.hashCode() again: 66069457
employee3.hashCode(): 66073797
false
employee2.hashCode(): 66074882

true
employee1.hashCode(): 511833308
employee3.hashCode(): 511833308
true
employee2.hashCode(): 1297685781

true
employee1.hashCode(): 128107556
employee3.hashCode(): 128107556
true
employee2.hashCode(): 128107556

由于employee1.equals（employee2）
返回true
，因此hashcode也应返回相同的值（检查上述文档中的点2）。但是，employee1
和employee2
的hashcode值不同，这是不正确的。这种差异是因为我们没有重写hashCode
方法。因此，无论何时重写equals
，都应该以正确的方式重写hashCode

最后，下面给出了实现hashCode
和equals
的正确方法：
class MyEmployee {
    String code;
    String name;
    int age;

    public MyEmployee(String code, String name, int age) {
        super();
        this.code = code;
        this.name = name;
        this.age = age;
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + age;
        result = prime * result + ((code == null) ? 0 : code.hashCode());
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        MyEmployee other = (MyEmployee) obj;
        if (age != other.age)
            return false;
        if (code == null) {
            if (other.code != null)
                return false;
        } else if (!code.equals(other.code))
            return false;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        return true;
    }
}

public class Main {
    public static void main(String[] args) {
        MyEmployee employee1 = new MyEmployee("AB12", "Dhruv", 24);
        MyEmployee employee2 = new MyEmployee("AB12", "Dhruv", 24);
        MyEmployee employee3 = employee1;
        System.out.println(employee1.equals(employee3));
        System.out.println("employee1.hashCode(): " + employee1.hashCode());
        System.out.println("employee3.hashCode(): " + employee3.hashCode());
        System.out.println(employee1.equals(employee2));
        System.out.println("employee2.hashCode(): " + employee2.hashCode());
    }
}

输出：
Employee employee1 = new Employee("AB12","Dhruv", 24);

Employee employee2 = new Employee("AB12", "Dhruv", 24);

true
employee1.hashCode(): 511833308
employee3.hashCode(): 511833308
false
employee2.hashCode(): 1297685781

true
employee1.hashCode(): 128107556
employee3.hashCode(): 128107556
false
employee2.hashCode(): 128107556

true
employee1.hashCode(): 66066760
employee1.hashCode() again: 66069457
employee3.hashCode(): 66073797
false
employee2.hashCode(): 66074882

true
employee1.hashCode(): 511833308
employee3.hashCode(): 511833308
true
employee2.hashCode(): 1297685781

true
employee1.hashCode(): 128107556
employee3.hashCode(): 128107556
true
employee2.hashCode(): 128107556

可能是因为您的Employee类没有重写hashCode
方法，所以它从java.lang.Object继承了该方法，该方法通常基于内部内存地址。这不是一个很好的问题，但尽管如此，这是一个非常好和彻底的答案。非常感谢。1+感谢您的精彩解释