操作String的几个Method

    public static String join(String[] array, String sep) {
        if (array == null) {
            return null;
        }
        if (array.length == 0) {
            return "";
        }
        if (sep == null) {
            sep = "";
        }
        // 预处理，得到长度。
        int capacity = 0;
        for (int i = 0; i < array.length; i++) {
            array[i] = String.valueOf(array[i]);
            capacity = capacity + array[i].length() + sep.length();
        }
        char[] result = new char[capacity - sep.length()];
        int begin = 0;
        for (int i = 0; i < array.length; i++) {
            array[i].getChars(0, array[i].length(), result, begin);
            begin = begin + array[i].length();
            if (begin == result.length) {
                break;
            }
            sep.getChars(0, sep.length(), result, begin);
            begin = begin + sep.length();
        }
        return new String(result);
    }

使用结果：
StringExt.join(null, *)                = null
StringExt.join([], *)                  = ""
StringExt.join([null], *)              = "null"
StringExt.join(["a", "b", "c"], "--")  = "a--b--c"
StringExt.join(["a", "b", "c"], null)  = "abc"
StringExt.join(["a", "b", "c"], "")    = "abc"
StringExt.join([null, "", "a"], ",")   = "null,,a"

第二个

    public static String join2(String[] array, String sep) {
        if (array == null) {
            return null;
        }
        if (array.length == 0) {
            return "";
        }
        if (sep == null) {
            sep = "";
        }
        StringBuilder sb = new StringBuilder();
        for (String tkey : array) {
            sb.append(tkey);
            sb.append(sep);
        }
        int len = sb.length();
        sb.delete(len - sep.length(), len);
        return sb.toString();
    }

两个join使用结果是一样了，第一个可能快一点点，第二个看起来更简捷。
org.apache.commons.lang.StringUtils中有相似的方法。

    /**
     * 
     * @param source
     * @param separator
     * @return
     */
    public static String[] split(String source, String... separator) {
        if (source == null) {
            return null;
        }
        if (source.length() == 0) {
            return new String[] { "" };
        }
        if (separator == null) {
            return splitfields(source, null, -1);
        }
        ArrayList<String> result = new ArrayList<String>();
        char[] arrsource = source.toCharArray();
        char[][] arrseps = new char[separator.length][];
        for (int i = 0; i < arrseps.length; i++) {
            arrseps[i] = separator[i].toCharArray();
            if (arrseps[i].length == 0) {
                return splitfields(source, null, -1);
            }
        }

        int preend = 0;
        int begin = 0;
        boolean cansep = false;
        int end = 0;
        for (int i = 0; i < arrsource.length; i++) {
            begin = i;
            NextSep: for (int j = 0; j < arrseps.length; j++) {
                end = i + arrseps[j].length;
                if (end > arrsource.length) {
                    cansep = false;
                    continue;
                }
                int m = i, n = 0;
                for (; m < arrsource.length && n < arrseps[j].length; m++, n++) {
                    if (arrsource[m] != arrseps[j][n]) {
                        cansep = false;
                        continue NextSep;
                    }
                }
                cansep = true;
                break;
            }
            if (cansep) {
                int len = begin - preend;
                char[] preChar = new char[len];
                System.arraycopy(arrsource, preend, preChar, 0, len);
                result.add(new String(preChar));

                preend = end;
                cansep = false;
                i = end - 1;
                begin = i;
            }
        }
        int len = begin - preend + 1;
        char[] preChar = new char[len];
        System.arraycopy(arrsource, preend, preChar, 0, len);
        result.add(new String(preChar));

        return result.toArray(new String[0]);
    }

使用结果：
StringExt.split(null, *)                      = null
StringExt.split("", *)                        = [""]
StringExt.split("ab de fg", null)             = ["ab", "cd", "ef"]
StringExt.split("ab    de fg", null)          = ["ab", "cd", "ef"]
StringExt.split("ab de fg", null)       = ["ab", "cd", "ef"]
StringExt.split("ab de fg", "")               = ["ab", "cd", "ef"]
StringExt.split("ab    de fg", "")            = ["ab", "cd", "ef"]
StringExt.split("ab de fg", "")         = ["ab", "cd", "ef"]
StringExt.split("ab de fg", *, "")             = ["ab", "cd", "ef"]
StringExt.split("ab    de fg", *, "")          = ["ab", "cd", "ef"]
StringExt.split("ab de fg", *, "")       = ["ab", "cd", "ef"]
StringExt.split("ab de fg", "", *)             = ["ab", "cd", "ef"]
StringExt.split("ab    de fg", "", *)          = ["ab", "cd", "ef"]
StringExt.split("ab de fg", "", *)       = ["ab", "cd", "ef"]
StringExt.split("ab:cd:ef", ":")              = ["ab", "cd", "ef"]
StringExt.split("ab,,,cd,,,ef", ",,,")        = ["ab", "cd", "ef"]
StringExt.split("ab,,,cd;;;ef", ",,,", ";;;") = ["ab", "cd", "ef"]
StringExt.split("ab,,,,,,ef", ",,,")          = ["ab", "", "ef"]

split中的分割符可以是多个，为了分割文件方便。
org.apache.commons.lang.StringUtils中有相似的split方法，但是用来分割csv格式的文件，不是太好用，如：
1;2;3;4;5
a;b;;d;e
9;8;7;6;5
如果要用“;”分割，第二列就对不齐，把每二列中分割为空的扔掉了，不明白apache为啥要这样实现，难道是为了跟String.split保持一致？

    public static String[] split(String source) {
        return split(source, null, -1);
    }

    public static String[] split(String source, String sep) {
        return split(source, sep, -1);
    }

    public static String[] split(String source, String sep, int maxsplit) {
        if (source == null) {
            return null;
        }
        if (source.length() == 0) {
            return new String[] { "" };
        }
        if (sep != null && sep.length() != 0)
            return splitfields(source, sep, maxsplit);

        ArrayList<String> list = new ArrayList<String>();
        char[] chars = source.toCharArray();
        int n = chars.length;
        if (maxsplit < 0)
            maxsplit = n;

        int splits = 0;
        int index = 0;
        while (index < n && splits < maxsplit) {
            while (index < n && Character.isWhitespace(chars[index]))
                index++;
            if (index == n)
                break;
            int start = index;

            while (index < n && !Character.isWhitespace(chars[index]))
                index++;
            list.add(source.substring(start, index));
            splits++;
        }
        while (index < n && Character.isWhitespace(chars[index]))
            index++;
        if (index < n) {
            list.add(source.substring(index, n));
        }
        return list.toArray(new String[0]);
    }

    private static String[] splitfields(String source, String sep, int maxsplit) {
        ArrayList<String> list = new ArrayList<String>();
        int length = source.length();
        if (maxsplit < 0)
            maxsplit = length;

        int lastbreak = 0;
        int splits = 0;
        int sepLength = sep.length();
        while (splits < maxsplit) {
            int index = source.indexOf(sep, lastbreak);
            if (index == -1)
                break;
            splits += 1;
            list.add(source.substring(lastbreak, index));
            lastbreak = index + sepLength;
        }
        if (lastbreak <= length) {
            list.add(source.substring(lastbreak, length));
        }
        return list.toArray(new String[0]);
    }

    public static String[] splitlines(String source) {
        return splitlines(source, false);
    }
    
    public static String[] splitlines(String source, boolean keepends) {
        ArrayList<String> list = new ArrayList<String>();

        char[] chars = source.toCharArray();
        int n = chars.length;

        int j = 0;
        for (int i = 0; i < n;) {
            /* Find a line and append it */
            while (i < n && chars[i] != '\n' && chars[i] != '\r'
                    && Character.getType(chars[i]) != Character.LINE_SEPARATOR)
                i++;

            /* Skip the line break reading CRLF as one line break */
            int eol = i;
            if (i < n) {
                if (chars[i] == '\r' && i + 1 < n && chars[i + 1] == '\n')
                    i += 2;
                else
                    i++;
                if (keepends)
                    eol = i;
            }
            list.add(source.substring(j, eol));
            j = i;
        }
        if (j < n) {
            list.add(source.substring(j, n));
        }
        return list.toArray(new String[0]);
    }

splitlines按行分割，换行符可以是“\n”或“\r\n”。
org.apache.commons.lang.StringUtils中也有相似的方法。
以上方法有些是自己实现的，有些是从源码中Copy的。

对齐的实现，左对齐，右对齐等

    public static String ljust(String src, int width) {
        return expand(src, width, ' ', true);
    }

    public static String rjust(String src, int width) {
        return expand(src, width, ' ', false);
    }

    public static String ljust(String src, int width, char fillchar) {
        return expand(src, width, fillchar, true);
    }

    public static String rjust(String src, int width, char fillchar) {
        return expand(src, width, fillchar, false);
    }

    public static String expand(String src, int width, char fillchar, boolean postfix) {
        String result = src;
        if (result.length() < width) {
            char[] temp = new char[width - result.length()];
            for (int i = 0; i < temp.length; i++) {
                temp[i] = fillchar;
            }
            if (postfix) {
                result = result + new String(temp);
            } else {
                result = new String(temp) + result;
            }
        }
        return result;
    }

替换的实现，嘿嘿，很搞笑吧：

    public static String repalce(String source, String oldStr, String newStr) {
        String[] split = split(source, oldStr);
        return join(split, newStr);
    }

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