java中int->String 3种方式效率分析

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

        int[] intArr = new int[1000000];
        String[] strArr1 = new String[1000000];

        Long s0 = System.currentTimeMillis();
        for (int i = 0; i < 1000000; i++) {
            intArr[i] = i + 1;
        }
        Long e0 = System.currentTimeMillis();

        Long s1 = e0;
        for (int i = 0; i < 1000000; i++) {
            strArr1[i] = String.valueOf(intArr[i]);
        }
        Long e1 = System.currentTimeMillis();

        Long s2 = e1;
        for (int i = 0; i < 1000000; i++) {
            strArr1[i] = Integer.toString(intArr[i]);
        }
        Long e2 = System.currentTimeMillis();

        Long s3 = e2;
        for (int i = 0; i < 1000000; i++) {
            strArr1[i] = intArr[i] + "";
        }
        Long e3 = System.currentTimeMillis();

        System.out.println("s0 = " + s0);
        System.out.println("e0 = " + e0);
        System.out.println("s1 = " + s1);
        System.out.println("e1 = " + e1);
        System.out.println("s2 = " + s2);
        System.out.println("e2 = " + e2);
        System.out.println("s3 = " + s3);
        System.out.println("e3 = " + e3);
        System.out.println("String.valueOf(i):" + (e1 - s1));
        System.out.println("Integer.toString(i):" + (e2 - s2));
        System.out.println("num + "":" + (e3 - s3));

测试结果如下

"D:Program FilesJavajdk1.8.0_181injava.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()

public static String toString(int i) {
        if (i == Integer.MIN_VALUE)
            return "-2147483648";
        int size = (i < 0) ? stringSize(-i) + 1 : stringSize(i); //判断i位数
        char[] buf = new char[size];
        getChars(i, size, buf);
        return new String(buf, true);
}

getChars()

static void getChars(int i, int index, char[] buf) {
        int q, r;
        int charPos = index;
        char sign = 0;
　　　　　// 初始化符号
        if (i < 0) {
            sign = '-';
            i = -i;
        }

        // Generate two digits per iteration
        while (i >= 65536) {
            q = i / 100;
        // really: r = i - (q * 100); 获得 十位 个位
            r = i - ((q << 6) + (q << 5) + (q << 2));
            i = q;
            buf [--charPos] = DigitOnes[r];
            buf [--charPos] = DigitTens[r];
        }

        // Fall thru to fast mode for smaller numbers
        // assert(i <= 65536, i);
        for (;;) {
            q = (i * 52429) >>> (16+3);
            r = i - ((q << 3) + (q << 1));  // r = i-(q*10) ...
            buf [--charPos] = digits [r];
            i = q;
            if (i == 0) break;
        }
        if (sign != 0) {
            buf [--charPos] = sign;
        }
    }

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.