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Hello everyone , I'm cabbage , You can call me vegetable ( Worthy of the name, it belongs to , ha-ha ~), A sophomore . This article is mainly about java Interview questions ：== Operators, and equals() Knowledge points of . If you get something after reading the article , You can support bloggers three times in a row ~, Hee hee .

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One 、 Preface

Understand the difference between them , First of all, we need to understand their definitions .

• When comparing reference data types ,== The comparison is whether the memory address of the variable is the same , That is, whether it points to the same object , Is the real pointer operation .
• equals() Whether the two objects are the same , Because all classes inherit java.lang.Object Class , So it applies to all objects , If the method is not covered , The call is still Object Methods in class , and Object Medium equals Method returns == The judgment of the .
• in other words == Is a pointer operation , Used to compare equal values , and equals Is a method , stay Object Class equals Method is actually ==, If subclasses don't override equals Method , The two are no different .

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Two 、 The source code parsing

Have a look first Object Class ：

You can see it in the source code , The two compare the address value .

Look again. String Class ：

You can see it in the source code ,String Class overridden Object Class equals Method , First determine the addresses of the two , If equal , Go straight back true; Otherwise , Then judge whether the parameter is String Type of , If it is , Just compare the contents of each character one by one , The final result .

Finally, let's look at Integer Class source code ：

You can know from the source code , First, judge whether the parameter is Integer Type of ; If it is , Will call intValue(), Get the value of this parameter , Finally, the two are compared ; In a nutshell , They end up comparing numbers .

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3、 ... and 、 Comparison of basic data types

For the basic data type of , Only == The operator , Because the basic data type is not an object , No inheritance Object class , Naturally, you can't rewrite equals Method . It should be noted that , In the use of == When the operator , Automatic type conversion occurs , Look at the following code ：

    public void test1() {
    
        int i = 10;
        int j = 10;
        double d = 10.0;
        System.out.println(i == j);//true
        System.out.println(i == d);//true
        boolean b = true;
// System.out.println(i == b); There will be errors in compilation 
        char c = 10;
        System.out.println(i == c);//true
        System.out.println(c == d);//true
        char c1 = 'A';
        char c2 = 65;
        System.out.println(c1 == c2);//true
    }

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Four 、 Reference data type comparison

In the use of reference data types , The most common is String The class , So let's take a look at the following code , Try to think about the results of the run ：

    public void test2() {
    
        String s1 = "hello";
        String s2 = "hello";
        System.out.println(s1 == s2);
        System.out.println(s1.equals(s2));
        
        String str1 = new String("hello");
        String str2 = new String("hello");
        System.out.println(str1 == str2);
        System.out.println(str1.equals(str2));
        System.out.println(s1 == str1);
        System.out.println(s1.equals(str1));
    }

The end result is ：true、true、false、true、false、true

• First look at the first half of the code ,s1 == s2 The result is true, Because the value of the string is final, Its value is immutable , There is only one copy in memory , therefore s1 and s2 The address value of is the same .
• Finally, look at the second half of the code , because str1 and str2 Keyword used new, That is, two pieces of space are opened up in memory , Their address values are different , therefore str1 == str2 The result is false.

Let's look at the last set of code ：

    public void test3() {
    
        int a = 10;
        Integer b = new Integer(10);
        Integer c = new Integer(10);
        System.out.println(b == c);
        System.out.println(a == c);
        System.out.println(b.equals(c));
        System.out.println(c.equals(a));
    }

The result of the operation is ：false、true、true、true

• about a==c Why does this return true Well ？ Are variables a and c Is the memory address stored the same ？ This is obviously not true , After all a It's the basic data type , The stored address is the address in the stack , and c Is the reference data type , The storage address should point to the address in the alignment . This is because when Integer And int Conduct == When comparing ,Integer Will be unpacked into one int type , So it's still equivalent to two int Compare types , there Integer, Whether it's direct assignment , still new Objects created , Just follow int Comparison will unpack for int type , So it's equal . In fact, all wrapper classes are related to their basic data types == The box will be opened automatically during comparison .

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5、 ... and 、 Custom implementation equals Method

So here comes the question , If we want to write our own class , Also want to rewrite equals How to do it ？ It's very simple , Use Alt+insert Then select as shown in the figure ：
Write a code test ：

    public void test4() {
    
        Person person1 = new Person("cabbage",20);
        Person person2 = new Person("cabbage",20);
        System.out.println(person1.equals(person2));
    }

If the run result returns true, So congratulations , succeed ~

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6、 ... and 、 Summary

• When used equals() When comparing methods , Antitype File、String、Date And packaging , Is to compare the type and content without considering whether the reference is the same object ; Because in these classes, you override Object Class equals() Method .
• Finally, give a reference answer ：
  

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Thank you for reading , Progress together , Hee hee ~