Let’s talk about overcalculation! Is your calculation method overcalculated?

Let’s talk about overcalculation! Is your calculation method overcalculated?

The game is about to be officially launched, and today I found a bug that made me laugh and cry. The data calculation overflowed; the gold recharged by the player became 0; this is a big deal, after all, no one can bear this responsibility;

Let me explain the reason. The development language is Java and the tool is NetBeans IDE 8.0.2

The player object has an attribute called gold which is of int type;

The calculation method when a player recharges is as follows.

   int gold = 20000 ; //The player's original gold  
 int tempGold = 20000 ; //The current recharge amount of the player   
 
 if (Integer.MAX_VALUE >= gold + tempGold) {
 gold = gold + tempGold;
 } else {
 gold = Integer.MAX_VALUE;
 }

It seems like there is nothing wrong with it, right? Of course, the above is a simulation;

If you are experienced, or can see some clues below, then it will be over-calculated;

Maybe you can't see any problem. I didn't find any problem at the beginning, so I wrote the code like this. Well, let's simulate the copy code

 int gold = Integer .MAX_VALUE - 1800; //The player's original gold
 int tempGold = 20000 ; //The current recharge amount of the player
 if (Integer.MAX_VALUE > = gold + tempGold) {
 gold gold = gold + tempGold;
 System.out.println("1");
 } else {
 gold = Integer.MAX_VALUE ;
 System.out.println("2");
 }

Well, guess what the output will be?

Maybe you will answer output 2. Yes, I thought it would output 2.

But why is the running result 1?

First, let's analyze why we thought it would output 2? Then it is obvious that we put

gold + tempGold

The calculation of these two values ​​is assumed to be long type and greater than Integer.MAX_VALUE

However, this is not the case. Let me take a look at the output. Copy code

 nt gold = Integer.MAX_VALUE - 1800 ; //The player's original  
 int tempGold = 20000 ; //The current recharge amount of the player  
 if (Integer.MAX_VALUE >= gold + tempGold) {
 gold = gold + tempGold;
 System.out.println( "1" );
 } else {
 gold = Integer.MAX_VALUE;
 System.out.println( "2" );
 }

 --- exec-maven-plugin: 1.2 . 1 :exec ( default -cli) @ game-gamesr ---
 1  
 - 2147445449  
 --------------------------------------------------------------------------------
 BUILD SUCCESS

In Java, the addition of gold + tempGold does not become a long but a negative number.

Seeing this, maybe you will laugh at me. Well, I admit that I have not verified this problem. Fortunately, the game has not been launched yet. The problem was found in the test.

Anyway, now that I have discovered the problem and understand what the problem is, I will try to solve it.

That is, the problem of converting int to long

 int gold = Integer.MAX_VALUE - 1800 ; //The player's original  
 int tempGold = 20000 ; //The current recharge amount of the player  
 long tempLGold = tempGold;
 if (Integer.MAX_VALUE >= gold + tempLGold) {
 gold = gold + tempGold;
 System.out.println( "1" );
 } else {
 gold = Integer.MAX_VALUE;
 System.out.println( "2" );
 }
 System.out.println(gold + tempGold);

How about testing the current output?

 --- exec-maven-plugin: 1.2 . 1 :exec ( default -cli) @ game-gamesr ---
 2  
 - 2147463649  
 --------------------------------------------------------------------------------
 BUILD SUCCESS

These are correct, ok. . The stupid things are over. But I found that there is an extra variable long tempLGold; the properties and operations are not very convenient. Is there any better operation?

 nt gold = Integer.MAX_VALUE - 1800 ; //The player's original  
 int tempGold = 20000 ; //The current recharge amount of the player  
 if (Integer.MAX_VALUE >= gold + tempGold + 0L) {
 gold = gold + tempGold;
 System.out.println( "1" );
 } else {
 gold = Integer.MAX_VALUE;
 System.out.println( "2" );
 }

Note the 0L at the end

Look at the output

 --- exec-maven-plugin:1.2.1:exec (default-cli) @ game-gamesr ---
 1
 --------------------------------------------------------------------------------
 BUILD SUCCESS

The result is still output 1. Maybe you will laugh at me for being stupid. I am indeed stupid. After further investigation, I found out that it is a problem of operator priority.

Okay, then change it.

 int gold = Integer.MAX_VALUE - 1800 ; //The player's original  
 int tempGold = 20000 ; //The current recharge amount of the player  
 if (Integer.MAX_VALUE >= 0L + gold + tempGold) {
 gold = gold + tempGold;
 System.out.println( "1" );
 } else {
 gold = Integer.MAX_VALUE;
 System.out.println( "2" );

Output

 --- exec-maven-plugin: 1.2 . 1 :exec ( default -cli) @ game-gamesr ---
 2  
 --------------------------------------------------------------------------------
 BUILD SUCCESS

Well enough

 int gold = Integer.MAX_VALUE - 1800 ; //The player's original  
 int tempGold = 20000 ; //The current recharge amount of the player  
 if (Integer.MAX_VALUE >= gold + tempGold * 1L) {
 gold = gold + tempGold;
 System.out.println( "1" );
 } else {
 gold = Integer.MAX_VALUE;
 System.out.println( "2" );

 nt gold = Integer.MAX_VALUE - 1800 ; //The player's original  
 int tempGold = 20000 ; //The current recharge amount of the player  
 if (Integer.MAX_VALUE >= gold + tempGold * 1L) {
 gold = gold + tempGold;
 System.out.println( "1" );
 } else {
 gold = Integer.MAX_VALUE;
 System.out.println( "2" );
 }

 --- exec-maven-plugin: 1.2 . 1 :exec ( default -cli) @ game-gamesr ---
 2  
 --------------------------------------------------------------------------------
 BUILD SUCCESS

This is correct now.

These are the mistakes made by a programmer who has made mistakes...