今天无意中发现一个怪事,当时没理解,后来跟网友讨论了才知道原理,是关于int值的加法算法,两段代码如下:
代码1:
@Test
public void test1() {
int stackLength = ;
int count = ;
try {
while(true){
count++;
stackLength += stackLength; //每次打印的值为0
System.out.println("stackLength="+count+"次:: "+stackLength);
}
} catch (Exception e) {
System.out.println("occur exception!!! stackLength="+stackLength);
}
}
控制台输出结果很奇怪,全都是0,:
stackLength=21719次:: 0
stackLength=21720次:: 0
stackLength=21721次:: 0
stackLength=21722次:: 0
stackLength=21723次:: 0
stackLength=21724次:: 0
stackLength=21725次:: 0
…… 由于打印太快,已经计算了2万多次。
代码2:
@Test
public void test2() {
int stackLength = 1;
int count = 0;
try {
while(true){
count++;
stackLength += 1;
System.out.println("stackLength="+count+"次:: "+stackLength);
}
} catch (Exception e) {
System.out.println("occur exception!!! stackLength="+stackLength);
}
}
控制台打印结果正常,是一个数一个数加上去的:
stackLength=21211次:: 21212
stackLength=21212次:: 21213
stackLength=21213次:: 21214
stackLength=21214次:: 21215
stackLength=21215次:: 21216
stackLength=21216次:: 21217
stackLength=21217次:: 21218
stackLength=21218次:: 21219
stackLength=21219次:: 21220
stackLength=21220次:: 21221
stackLength=21221次:: 21222
stackLength=21222次:: 21223
……
思路
这就很奇怪了,代码1和代码2中,只有一行代码不一样,
在代码1中为stackLength += stackLength;
在代码2中为stackLength += 1;
但是运行2万多次后,结果却大相径庭。后来与网友讨论才明白其中原理如下:
初始值1 就是 0000 0000 0000 0000 0000 0000 0000 0001 ,
这个值是最小负数。
当是33位时,两个最大负数相加,连33位都变成了0
因为int只能取32位
所以就是0
然后后面就一直是0+0了
@Test
public void test3() {
int stackLength = 1;
int count = 0;
try {
while(true){
count++;
stackLength += stackLength;
System.out.println("stackLength="+count+"次:: "+stackLength);
if(count > 40){
break;
}
}
} catch (Exception e) {
System.out.println("occur exception!!! stackLength="+stackLength);
}
}
控制台打印如预期之中:
stackLength=1次:: 2
stackLength=2次:: 4
stackLength=3次:: 8
stackLength=4次:: 16
stackLength=5次:: 32
stackLength=6次:: 64
stackLength=7次:: 128
stackLength=8次:: 256
stackLength=9次:: 512
stackLength=10次:: 1024
stackLength=11次:: 2048
stackLength=12次:: 4096
stackLength=13次:: 8192
stackLength=14次:: 16384
stackLength=15次:: 32768
stackLength=16次:: 65536
stackLength=17次:: 131072
stackLength=18次:: 262144
stackLength=19次:: 524288
stackLength=20次:: 1048576
stackLength=21次:: 2097152
stackLength=22次:: 4194304
stackLength=23次:: 8388608
stackLength=24次:: 16777216
stackLength=25次:: 33554432
stackLength=26次:: 67108864
stackLength=27次:: 134217728
stackLength=28次:: 268435456
stackLength=29次:: 536870912
stackLength=30次:: 1073741824
stackLength=31次:: -2147483648
stackLength=32次:: 0
stackLength=33次:: 0
stackLength=34次:: 0
stackLength=35次:: 0
stackLength=36次:: 0
stackLength=37次:: 0
stackLength=38次:: 0
stackLength=39次:: 0
stackLength=40次:: 0
stackLength=41次:: 0