charAt（）或substring？哪个更快？ [英] charAt() or substring? Which is faster?

问题描述

我想遍历一个字符串中的每个字符，并将字符串的每个字符作为一个字符串传递给另一个函数。

I want to go through each character in a String and pass each character of the String as a String to another function.

String s = "abcdefg";
for(int i = 0; i < s.length(); i++){
    newFunction(s.substring(i, i+1));}

或

String s = "abcdefg";
for(int i = 0; i < s.length(); i++){
    newFunction(Character.toString(s.charAt(i)));}

最后的结果需要是一个String。

The final result needs to be a String. So any idea which will be faster or more efficient?

推荐答案

像往常一样：没关系，但如果你坚持支出时间微优化或如果你真的想优化你的非常特殊的用例，请尝试这：

As usual: it doesn't matter but if you insist on spending time on micro-optimization or if you really like to optimize for your very special use case, try this:

import org.junit.Assert;
import org.junit.Test;

public class StringCharTest {

    // Times:
    // 1. Initialization of "s" outside the loop
    // 2. Init of "s" inside the loop
    // 3. newFunction() actually checks the string length,
    // so the function will not be optimized away by the hotstop compiler

    @Test
    // Fastest: 237ms / 562ms / 2434ms
    public void testCacheStrings() throws Exception {
        // Cache all possible Char strings
        String[] char2string = new String[Character.MAX_VALUE];
        for (char i = Character.MIN_VALUE; i < Character.MAX_VALUE; i++) {
            char2string[i] = Character.toString(i);
        }

        for (int x = 0; x < 10000000; x++) {
            char[] s = "abcdefg".toCharArray();
            for (int i = 0; i < s.length; i++) {
                newFunction(char2string[s[i]]);
            }
        }
    }

    @Test
    // Fast: 1687ms / 1725ms / 3382ms
    public void testCharToString() throws Exception {
        for (int x = 0; x < 10000000; x++) {
            String s = "abcdefg";
            for (int i = 0; i < s.length(); i++) {
                // Fast: Creates new String objects, but does not copy an array
                newFunction(Character.toString(s.charAt(i)));
            }
        }
    }

    @Test
    // Very fast: 1331 ms/ 1414ms / 3190ms
    public void testSubstring() throws Exception {
        for (int x = 0; x < 10000000; x++) {
            String s = "abcdefg";
            for (int i = 0; i < s.length(); i++) {
                // The fastest! Reuses the internal char array
                newFunction(s.substring(i, i + 1));
            }
        }
    }

    @Test
    // Slowest: 2525ms / 2961ms / 4703ms
    public void testNewString() throws Exception {
        char[] value = new char[1];
        for (int x = 0; x < 10000000; x++) {
            char[] s = "abcdefg".toCharArray();
            for (int i = 0; i < s.length; i++) {
                value[0] = s[i];
                // Slow! Copies the array
                newFunction(new String(value));
            }
        }
    }

    private void newFunction(String string) {
        // Do something with the one-character string
        Assert.assertEquals(1, string.length());
    }

}

这篇关于charAt（）或substring？哪个更快？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！