Deeply understand the operation results and operating principles of ""i=i++;" in Java

A brief talk about "i=i;" in java

First of all, here is the code:

    public class Demo{
        public static void main(String []args){
            int i=12;
            i=i++;
            System.out.println(i);
        }
    }

You can imagine how much this code is. Most students may answer 13, but This is not the case, as shown in the figure below:

In the figure above, I ran it first, and you can see the results. Our i is not what most people guessed. The output is 13, but it seems that this auto-increment operation is not performed, but why is this?

So, there is the subsequent operation of decompiling into bytecode. After we use the javap instruction to decompile, we can clearly see what is happening. Some students are not clear about the JVM instructions. It doesn’t matter. I am here. Let’s first popularize the JVM instructions involved in the code of this article;

(The key instruction code here is after the main method, so only the following instructions are explained. The ones before it are automatically built by the virtual machine. The instruction code of the default construction method)

1.bipush: Push the single-byte constant value (-128~127) to the top of the stack
2.istore: Push the int type on the top of the stack The value is stored in the specified local variable
3.iload: Push the specified int type local variable to the top of the stack
4.iinc: This instruction is used to perform auto-increment and decrement operations on local (local) variables. The first parameter of this instruction is the number of the local variable, and the second parameter is the number of auto-increments and decrements
5.getstatic gets the static domain of the specified class and pushes its value onto the top of the stack (after reaching this position, static is called The operation of object out printing will not be described in detail. Interested students can learn more about it at the link attached at the end of this article)

After knowing the basic instructions, let’s analyze the meaning of the key instructions in the above picture. ：

1. bipush 12 Here is pushing 12 to the top of the stack. The int value of 12 is stored in the stack

2. istore_1 Here is popping 12 to The second (note: the first local variable should be this) local variable is i=12

3. iload_1. Here is the value of the second local variable (i) pushed onto the stack. At this time, the top of the stack is 12

4. iinc 1,1. Naturally, i=i 1, that is, i. At this time, i=13

5. istore_1 here The top value of the stack is popped to the local variable i. At this time, i is assigned the value 12

, so i is 12 when it is finally printed.

A collection of JVM instructions compiled by a CSDN guy, which is very convenient for query and learning

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JAVA Tutorial Manual

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