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preface

• No matter when learning C++ or Java, we were all irrigated by the teacheri++and++iThe different
• The former is assigned before the +1 operation and the latter is assigned before the +1 operation. We are also sure that the improper use of both causes our program to miscalculate

Problem description

• Here’s the thing! In line with the principle of adding before adding began to recite memory blindly. Always thought thati++ 和++iThey execute differently. But then I looked at the bytecode
• We do not need to elaborate on the specific scene here.

Problem analysis

• We’re straight oni++,++iAnalyze. We need not dwell on the distinction. Today we will look at the difference between the two implementations at the bytecode level
• Right! We need to look beyond the shell to the essence

• This code should be pretty straightforward. Let’s take a look at his bytecode by compiling it. There are many ways to compile bytecode that you can go through directlyjavac. Here I use the idea plug-in to create. There are subtle differences but they don’t affect our reading. I’ve taken the bytecode from the main function for easy reading

• Let’s understand the following instructions for the sake of understanding

• With these five instructions in mind let’s read the bytecode of i++.
• First push the value 3 onto the operand stack
• Write the top element in the operand stack to the second position in the local variable table
• ③ Add 1 to the second position of the local variable table
• (4), end
• ⑤ why is it added to the second position of the local variable scale? Because the first default of the local variable table is ARGS. That’s the main program

• When I look at the++iAfter the bytecode is found withi++The execution is exactly the same. I ends up being 2 on the local variable scale. So understanding has to look at the bottom

Accustomed distinction

• Above we show it at the bytecode leveli++ 和++iThere is no difference between them. So what do we think of as differences
• The ~. We have to look at it in conjunction with assignment to see what’s going on

• Above are two pieces of code and their corresponding bytecodes. Internal is not with I ++ is first ILOAD then IINC.++ I is first IINC in ILOAD. As a result, the result of I in the local variable scale is different in the two programs

conclusion

• Sometimes a summary of experience learned from others is just experience, and does not necessarily reveal something deep inside.
• I++ is assigned plus 1. Many people would assume that two bytecodes are executed differently. Actually. You still have to look at the bottom

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