What does == mean in Java?

What does the relational operator "==" compare?

The following sentence is an excerpt from the book "Java Programming Thoughts": "Relational operators generate a boolean result, and they calculate the relationship between the values ​​of the operands. ".

This sentence seems simple, but it still needs to be understood carefully. To put it simply, == is used to compare whether values ​​are equal.

Let’s look at a few examples:

public class Main {
    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub 
        int n=3;
        int m=3;        
        System.out.println(n==m);        
        String str = new String("hello");
        String str1 = new String("hello");
        String str2 = new String("hello");        
        System.out.println(str1==str2);        
        str1 = str;
        str2 = str;
        System.out.println(str1==str2);
    }
}

The output result is:

true 
false
true

n==mThe result is true , this is easy to understand. The values ​​stored in variable n and variable m are both 3, which must be equal. And why are the results of the two comparisons of str1 and str2 different? To understand this, you only need to understand the difference between basic data type variables and non-basic data type variables.

For the variables of these 8 basic data types, the variables directly store the "value", so when the relational operator == is used for comparison, the "value" itself is compared. It should be noted that floating point types and integer types are both signed types, while char is an unsigned type (the value range of the char type is 0~2^16-1).

That is to say For example:

int n=3;
int m=3;　

Variable n and variable m both directly store the value "3", so when compared with ==, the result is true.

For variables of non-basic data types, they are called reference type variables in some books. For example, str1 above is a reference type variable. A reference type variable stores not the "value" itself, but the address of its associated object in memory.

For example, the following line of code:

String str1;

This sentence declares a reference type variable, which is not associated with any object at this time.

Use new String("hello") to generate an object (also called an instance of the String class) and bind this object to str1:

str1= new String("hello");

Then str1 points to an object (str1 is also called a reference to an object in many places). At this time, the variable str1 stores the storage address of the object it points to in memory, not the "value" itself, that is It says that the string "hello" is not stored directly. The references here are very similar to pointers in C/C.

So when you use == to compare str1 and str2 for the first time, the result is false. Therefore, they point to different objects respectively, which means that the memory addresses where they are actually stored are different.

In the second comparison, both str1 and str2 point to the object pointed to by str, so the result is undoubtedly true.

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