Analysis of i=i++ problems in Java

http://www.ticmy.com/?p=43

Focus: The execution of the local variable table and the operand stack.

Use Javac to compile and then use JAVAP-C test to decompile this class to see its bytecode, as follows (only the main method is extracted):

public static void Main (java.lang.string[]);

Code:

0:iconst_0

1:istore_1

2:iload_1

3:iinc 1, 1

6:istore_1

7:getstatic #2; Field Java/lang/system.out:ljava/io/printstream;

10:iload_1

11:invokevirtual #3; Method java/io/printstream.println: (I) V

14:return

Here, I start analysis from line No. 0 (analysis "" means the stack, the bottom of the stack is on the left, the top is on the right):

0: Press the constant 0 into the stack, the contents of the stack: "0"

1: Pop the element at the top of the stack, that is, 0, and save it to the local variable area index to 1 (that is, the variable i). Stack content: ""

2: Press the value of the local variable area index to 1 (that is, the variable i) into the stack, the stack content: "0"

3: Add one to the value of the local variable area index of 1 (that is, constant I), at which point the local variable area index is 1 (that is, the value of i) is 1. Stack content: "0"

6: The top element of the stack pops up and is saved to the local variable area index of 1 (that is, I), at which I becomes 0. Stack content: ""

7: Gets the class variable represented by index 2 in the constant pool, which is System.out. Stack element: ""

10: Press the value of the local variable area index to 1 (that is, I) into the stack. Stack element: "0"

11: Call the constant pool index to 3 method, that is, System.out.println

14: Return to Main method

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Java uses the intermediate cache variable mechanism:

i=i++; equivalent to:

Temp=i ( i on the right side of the equals sign)

i=i+1; ( I on the right side of the equals sign )

I=temp; ( I on the left of the equals sign )

and I=++i; is equivalent to:

i=i+1;

Temp=i;

I=temp;

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15:iload_2// The variable i value 0 of the local variable area index 2 is pressed into the operand stack /c10>

16:iinc 2, 1//Add 1 to the value of the local variable area 2 index , and the operand stack does not change

19:istore_2// 0 popup at the top of the stack , assigned to the i of the local variable area index 2

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The local variable table of main and fremin I at the respective ( runtime's current stack frame <StackFrame>) <local Variable in table>, two variables are different. main call fremin after the end of the stack, but did not change the main stack frame of the local variable table I value.  

i=i++;i++ 's Assembly instruction is (iinc 1,1), he only changes the value of the local variable table , but when i= assignment, the stack top of the operand stack is still 0 , save to the local variable table, and then cover the slot1 to 0, so get 0.

iconst_0//0

Istore_1//slot 1:i=0

Iload_1// remove 0

Iinc//slot 1:i=1

istore_1//Slot 1 = 0 overwrite

Iload_1//println 's 0

Output.

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Analysis of i=i++ problems in Java