charAt（）或子字符串？哪个更快？

像往常一样：没关系，但是如果您坚持花时间进行微优化，或者如果您真的想针对非常特殊的用例进行优化，请尝试以下 *** 作：

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());    }}