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 = new int[1000000]; 2         String[] strArr1 = new String[1000000]; 3  4         Long s0 = System.currentTimeMillis(); 5         for (int i = 0; i < 1000000; i++) { 6             intArr[i] = i + 1; 7         } 8         Long e0 = System.currentTimeMillis(); 9 10         Long s1 = e0;11         for (int i = 0; i < 1000000; i++) {12             strArr1[i] = String.valueOf(intArr[i]);13         }14         Long e1 = System.currentTimeMillis();15 16         Long s2 = e1;17         for (int i = 0; i < 1000000; i++) {18             strArr1[i] = Integer.toString(intArr[i]);19         }20         Long e2 = System.currentTimeMillis();21 22         Long s3 = e2;23         for (int i = 0; i < 1000000; i++) {24             strArr1[i] = intArr[i] + "";25         }26         Long e3 = System.currentTimeMillis();27 28         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 = 1539999876082e0 = 1539999876085s1 = 1539999876085e1 = 1539999876173s2 = 1539999876173e2 = 1539999876243s3 = 1539999876243e3 = 1539999876291String.valueOf(i):88Integer.toString(i):70num+"":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) {2         if (i == Integer.MIN_VALUE)3             return "-2147483648";4         int size = (i < 0) ? stringSize(-i) + 1 : stringSize(i); //判断i位数5         char[] buf = new char[size];6         getChars(i, size, buf);7         return new String(buf, true);8 }

getChars()

1 static void getChars(int i, int index, char[] buf) { 2         int q, r; 3         int charPos = index; 4         char sign = 0; 5 　　　　　// 初始化符号 6         if (i < 0) { 7             sign = '-'; 8             i = -i; 9         }10 11         // Generate two digits per iteration12         while (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         }20 21         // Fall thru to fast mode for smaller numbers22         // assert(i <= 65536, i);23         for (;;) {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;28             if (i == 0) break;29         }30         if (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,String.valueOf(),是逐位转换;而字符串拼接则是使用StringBuilder.