The difference between the equals() method and the == sign when comparing two objects in Java

In Java, from time to time we compare two objects, but the result we get is not what we want. What's going on?

1. Two forms of comparison: When comparing, we need to figure out which kind of comparison it is.

1. Value type comparison

That is, if the contents are the same, we consider them to be equal. For example: int i=5; int j =5; at this time, we say that i and j are equal, which actually means that the contents of i and j are the same.



2. Reference type comparison

But in Java, in addition to the value type, there is another reference type, and the reference values ​​of different objects are not actually the same. Equality, that is, in different address locations in memory. For example, we have defined the student class and have two student object instances respectively:

Student stu= new Student(); Student stu1= new Student();

At this time, whether we use the stu==stu1 symbol or the stu.equals(stu1) method, When two objects are compared, the results obtained are not equal, because for reference types, the default is to compare the addresses referenced by the two objects. It shows that the reference of each object has its own unique address, so it is not equal. .

2. Sometimes, when we compare two objects, if their contents are the same, then we consider the two objects to be equal, such as the two student objects above. At this time, what should we do? In fact, it is very simple. Just rewrite the equals() method in the class and access the contents of the object. As above, we only need to override the equals() method in the Student class.

Now, let’s take a look at an example! Comparison without overriding the equals() method:

Student class: Student class

package com.bluesky;  
  
public class Student {  
      
    String name;  
      
    public Student(){  
          
    }  
      
    public Student(String name){  
          
        this.name=name;  
          
    }</strong>  
      
  
}</span>

Test class Test:

package com.bluesky;  
  
public class Test {  
  
    public static void main(String[] args) {  
  
      int i=5;  
      int j=5;  
        
      if(i==j) System.out.println("i和j相等！");  
      else System.out.println("不相等！");  
        
      Student s = new Student("BlueSky");  
      Student s1=new Student("BlueSky");  
        
      if(s==s1)  System.out.println("s和是s1相等！");  
      else System.out.println("s和是s1不相等！");  
        
      if(s.equals(s1)) System.out.println("s和是s1相等！");  
      else System.out.println("s和是s1不相等！");  
    }  
}

Run result:

Rewrite the equals() method and compare again:

Student class:

package com.bluesky;  
  
public class Student {  
      
    String name;  
      
    public Student(){  
          
    }  
      
    public Student(String name){  
          
        this.name=name;  
          
    }  
  
      
    public boolean equals(Object obj) {  
        if (this == obj)      //传入的对象就是它自己，如s.equals(s)；肯定是相等的；  
            return true;   
        if (obj == null)     //如果传入的对象是空，肯定不相等  
            return false;  
        if (getClass() != obj.getClass())  //如果不是同一个类型的，如Studnet类和Animal类，  
                                           //也不用比较了，肯定是不相等的  
            return false;  
        Student other = (Student) obj;       
        if (name == null) {  
            if (other.name != null)  
                return false;  
        } else if (!name.equals(other.name))   //如果name属性相等，则相等  
            return false;  
        return true;  
    }

Test class Test:

package com.bluesky;  
  
public class Test {  
  
    public static void main(String[] args) {  
  
      int i=5;  
      int j=5;  
        
      if(i==j) System.out.println("i和j相等！");  
      else System.out.println("不相等！");  
        
      Student s = new Student("BlueSky");  
      Student s1=new Student("BlueSky");  
        
      if(s==s1)  System.out.println("s和是s1相等！");  
      else System.out.println("s和是s1不相等！");  
        
      if(s.equals(s1)) System.out.println("s和是s1相等！");  
      else System.out.println("s和是s1不相等！");  
    }  
}

Running result:

##After rewriting the equals() method, we get that s and s1 are equal. == Only addresses can be compared for reference types, so they are still not equal.

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