不抛出越界异常的求子串方法

?? 用String.substring方法，不小心会有越界异常。现实现一个没抛出越界异常，越界就返回null，不过直接返回的再用其它方法，可能有Null异常。现还实现可以负index的，可能逆向的。

package com.chenlb.util;public class StringUtil {/** * start与end均可负数<br/> * start < end正向取, start > end逆向取<br/> * 示例:str="I am chenlb"<br/> * StringUtil.substring(str, 0, 12) -> null<br/> * StringUtil.substring(str, 12, 12) -> null<br/> * StringUtil.substring(str, 12, 13) -> null<br/> * StringUtil.substring(str, 4, 4) -> ""<br/> * StringUtil.substring(str, 0, 4) -> "I am"<br/> * StringUtil.substring(str, -4, -1) -> "enl"<br/> * StringUtil.substring(str, -2, 4) -> "lbI am"<br/> * StringUtil.substring(str, 4, 0) -> "ma I"<br/> * StringUtil.substring(str, -1, -4) -> "lne"<br/> * StringUtil.substring(str, 1, -4) -> "Iblne"<br/> * StringUtil.substring(str, 0, -4) -> "blne"<br/> * StringUtil.substring(str, -4, 0) -> "enlb"<br/> * @return 越界返回null, start==end返回空 * @author chenlb 2008-6-18 下午12:39:51 */public static String substring(String str, int start, int end) {if(str == null) {return null;}int len = str.length();if(Math.abs(start) >= len) {return null;}if(Math.abs(end) > len) {return null;}StringBuilder sb = new StringBuilder();if(end > start) {//正向substring(sb, str, start, end);} else if(end == start) {return "";} else {//逆向 end < startsubstring(sb, str, end, start);sb.reverse();}return sb.toString();}private static void substring(StringBuilder sb, String str, int start, int end) {int len = str.length();if(start < 0) {if(end < 0) {sb.append(str.substring(len+start, len+end));} else {sb.append(str.substring(len+start, len));sb.append(str.substring(0, end));}} else {sb.append(str.substring(start, end));}}}

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测试代码：

public void testSubstring() {String str = "I am chenlb";assertEquals(null, StringUtil.substring(str, 0, 12));assertEquals(null, StringUtil.substring(str, 12, 12));assertEquals(null, StringUtil.substring(str, 12, 13));assertEquals("", StringUtil.substring(str, 4, 4));assertEquals("I am", StringUtil.substring(str, 0, 4));assertEquals("am", StringUtil.substring(str, 2, 4));assertEquals("I am chenlb", StringUtil.substring(str, 0, 11));assertEquals("enl", StringUtil.substring(str, -4, -1));assertEquals("lbI am", StringUtil.substring(str, -2, 4));assertEquals("ma I", StringUtil.substring(str, 4, 0));assertEquals("lne", StringUtil.substring(str, -1, -4));assertEquals("Iblne", StringUtil.substring(str, 1, -4));assertEquals("blne", StringUtil.substring(str, 0, -4));assertEquals("enlb", StringUtil.substring(str, -4, 0));}

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