java中int->String 3种方式效率分析
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1.0 int转String方式
java中,int转String共有如下3种方式
(1) 字符串拼接(即num+"")
(2) String.valueof(num)
(3) Integer.toString(num)
其中,方法(2)内部直接调用了方法(3),效率相差无几
2.0 效率测试
1
int[] intArr = newint[1000000];
2 String[] strArr1 = new String[1000000];
3 4 Long s0 = System.currentTimeMillis();
5for (int i = 0; i < 1000000; i++) {
6 intArr[i] = i + 1;
7 }
8 Long e0 = System.currentTimeMillis();
910 Long s1 = e0;
11for (int i = 0; i < 1000000; i++) {
12 strArr1[i] = String.valueOf(intArr[i]);
13 }
14 Long e1 = System.currentTimeMillis();
1516 Long s2 = e1;
17for (int i = 0; i < 1000000; i++) {
18 strArr1[i] = Integer.toString(intArr[i]);
19 }
20 Long e2 = System.currentTimeMillis();
2122 Long s3 = e2;
23for (int i = 0; i < 1000000; i++) {
24 strArr1[i] = intArr[i] + "";
25 }
26 Long e3 = System.currentTimeMillis();
2728 System.out.println("s0 = " + s0);
29 System.out.println("e0 = " + e0);
30 System.out.println("s1 = " + s1);
31 System.out.println("e1 = " + e1);
32 System.out.println("s2 = " + s2);
33 System.out.println("e2 = " + e2);
34 System.out.println("s3 = " + s3);
35 System.out.println("e3 = " + e3);
36 System.out.println("String.valueOf(i):" + (e1 - s1));
37 System.out.println("Integer.toString(i):" + (e2 - s2));
38 System.out.println("num + \"\":" + (e3 - s3));
测试结果如下
"D:\Program Files\Java\jdk1.8.0_181\bin\java.exe"
s0 = 1539999876082
e0 = 1539999876085
s1 = 1539999876085
e1 = 1539999876173
s2 = 1539999876173
e2 = 1539999876243
s3 = 1539999876243
e3 = 1539999876291
String.valueOf(i):88
Integer.toString(i):70
num+"":48
那么为何在JDK1.8中,String.valueOf()效率为何比字符串拼接低呢?
3.0 源码分析
3.1 字符串拼接
字符串拼接解释:https://docs.oracle.com/javase/8/docs/api/
The Java language provides special support for the string concatenation operator ( + ), and for conversion of other objects to strings.
String concatenation is implemented through the StringBuilder(or StringBuffer) class and its append method.
String conversions are implemented through the method toString, defined by Object and inherited by all classes in Java.
For additional information on string concatenation and conversion, see Gosling, Joy, and Steele, The Java Language Specification.
3.2 Integer.toString()
1
public
static String toString(int i) {
2if (i == Integer.MIN_VALUE)
3return "-2147483648";
4int size = (i < 0) ? stringSize(-i) + 1 : stringSize(i); //判断i位数
5char[] buf = newchar[size];
6 getChars(i, size, buf);
7returnnew String(buf, true);
8 }
getChars()
1
static
void getChars(int i, int index, char[] buf) {
2int q, r;
3int charPos = index;
4char sign = 0;
5 // 初始化符号
6if (i < 0) {
7 sign = ‘-‘;
8 i = -i;
9 }
1011// Generate two digits per iteration12while (i >= 65536) {
13 q = i / 100;
14// really: r = i - (q * 100); 获得 十位 个位15 r = i - ((q << 6) + (q << 5) + (q << 2));
16 i = q;
17 buf [--charPos] = DigitOnes[r];
18 buf [--charPos] = DigitTens[r];
19 }
2021// Fall thru to fast mode for smaller numbers
22// assert(i <= 65536, i);23for (;;) {
24 q = (i * 52429) >>> (16+3);
25 r = i - ((q << 3) + (q << 1)); // r = i-(q*10) ...26 buf [--charPos] = digits [r];
27 i = q;
28if (i == 0) break;
29 }
30if (sign != 0) {
31 buf [--charPos] = sign;
32 }
33 }
final static char [] DigitTens = {
‘0‘, ‘0‘, ‘0‘, ‘0‘, ‘0‘, ‘0‘, ‘0‘, ‘0‘, ‘0‘, ‘0‘,
‘1‘, ‘1‘, ‘1‘, ‘1‘, ‘1‘, ‘1‘, ‘1‘, ‘1‘, ‘1‘, ‘1‘,
‘2‘, ‘2‘, ‘2‘, ‘2‘, ‘2‘, ‘2‘, ‘2‘, ‘2‘, ‘2‘, ‘2‘,
‘3‘, ‘3‘, ‘3‘, ‘3‘, ‘3‘, ‘3‘, ‘3‘, ‘3‘, ‘3‘, ‘3‘,
‘4‘, ‘4‘, ‘4‘, ‘4‘, ‘4‘, ‘4‘, ‘4‘, ‘4‘, ‘4‘, ‘4‘,
‘5‘, ‘5‘, ‘5‘, ‘5‘, ‘5‘, ‘5‘, ‘5‘, ‘5‘, ‘5‘, ‘5‘,
‘6‘, ‘6‘, ‘6‘, ‘6‘, ‘6‘, ‘6‘, ‘6‘, ‘6‘, ‘6‘, ‘6‘,
‘7‘, ‘7‘, ‘7‘, ‘7‘, ‘7‘, ‘7‘, ‘7‘, ‘7‘, ‘7‘, ‘7‘,
‘8‘, ‘8‘, ‘8‘, ‘8‘, ‘8‘, ‘8‘, ‘8‘, ‘8‘, ‘8‘, ‘8‘,
‘9‘, ‘9‘, ‘9‘, ‘9‘, ‘9‘, ‘9‘, ‘9‘, ‘9‘, ‘9‘, ‘9‘,
} ;
final static char [] DigitOnes = {
‘0‘, ‘1‘, ‘2‘, ‘3‘, ‘4‘, ‘5‘, ‘6‘, ‘7‘, ‘8‘, ‘9‘,
‘0‘, ‘1‘, ‘2‘, ‘3‘, ‘4‘, ‘5‘, ‘6‘, ‘7‘, ‘8‘, ‘9‘,
‘0‘, ‘1‘, ‘2‘, ‘3‘, ‘4‘, ‘5‘, ‘6‘, ‘7‘, ‘8‘, ‘9‘,
‘0‘, ‘1‘, ‘2‘, ‘3‘, ‘4‘, ‘5‘, ‘6‘, ‘7‘, ‘8‘, ‘9‘,
‘0‘, ‘1‘, ‘2‘, ‘3‘, ‘4‘, ‘5‘, ‘6‘, ‘7‘, ‘8‘, ‘9‘,
‘0‘, ‘1‘, ‘2‘, ‘3‘, ‘4‘, ‘5‘, ‘6‘, ‘7‘, ‘8‘, ‘9‘,
‘0‘, ‘1‘, ‘2‘, ‘3‘, ‘4‘, ‘5‘, ‘6‘, ‘7‘, ‘8‘, ‘9‘,
‘0‘, ‘1‘, ‘2‘, ‘3‘, ‘4‘, ‘5‘, ‘6‘, ‘7‘, ‘8‘, ‘9‘,
‘0‘, ‘1‘, ‘2‘, ‘3‘, ‘4‘, ‘5‘, ‘6‘, ‘7‘, ‘8‘, ‘9‘,
‘0‘, ‘1‘, ‘2‘, ‘3‘, ‘4‘, ‘5‘, ‘6‘, ‘7‘, ‘8‘, ‘9‘,
} ;
final static char[] digits = {
‘0‘ , ‘1‘ , ‘2‘ , ‘3‘ , ‘4‘ , ‘5‘ ,
‘6‘ , ‘7‘ , ‘8‘ , ‘9‘ , ‘a‘ , ‘b‘ ,
‘c‘ , ‘d‘ , ‘e‘ , ‘f‘ , ‘g‘ , ‘h‘ ,
‘i‘ , ‘j‘ , ‘k‘ , ‘l‘ , ‘m‘ , ‘n‘ ,
‘o‘ , ‘p‘ , ‘q‘ , ‘r‘ , ‘s‘ , ‘t‘ ,
‘u‘ , ‘v‘ , ‘w‘ , ‘x‘ , ‘y‘ , ‘z‘
};
由上可知,Integer.valueOf(),是逐位转换;而字符串拼接则是使用StringBuilder.
3种方式效率分析' ref='nofollow'>java中int->String 3种方式效率分析
原文：https://www.cnblogs.com/zad27/p/9820873.html
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