摘要：如果起始地址小于或者起始地址所比較對象長度大于自身對象長度，返回假從所比較對象的末尾開始比較起始比較和末尾比較都是比較經常用得到的方法，例如在判斷一個字符串是不是協議的，或者初步判斷一個文件是不是文件，都可以采用這個方法進行比較。

(28) public boolean startsWith(String prefix, int toffset) {

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      char ta[] = value;
      int to = toffset;
      char pa[] = prefix.value;
      int po = 0;
      int pc = prefix.value.length;
      // Note: toffset might be near -1>>>1.
      //如果起始地址小于0或者（起始地址+所比較對象長度）大于自身對象長度，返回假
      if ((toffset < 0) || (toffset > value.length - pc)) {
          return false;
      }
      //從所比較對象的末尾開始比較
      while (--pc >= 0) {
          if (ta[to++] != pa[po++]) {
              return false;
          }
      }
      return true;
  }

(29) public boolean startsWith(String prefix) {

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      return startsWith(prefix, 0);
  }

(30) public boolean endsWith(String suffix) {

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      return startsWith(suffix, value.length - suffix.value.length);
  }
 起始比較和末尾比較都是比較經常用得到的方法，例如在判斷一個字符串是不是http協議的，
 或者初步判斷一個文件是不是mp3文件，都可以采用這個方法進行比較。

(31) public native String intern();

intern方法是Native調用，它的作用是在方法區中的常量池里通過equals方法尋找等值的對象，
如果沒有找到則在常量池中開辟一片空間存放字符串并返回該對應String的引用，
否則直接返回常量池中已存在String對象的引用

(32) public String toLowerCase() {

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    return toLowerCase(Locale.getDefault());
 }

(33) public String toUpperCase(Locale locale) {

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    if (locale == null) {
        throw new NullPointerException();
    }
    int firstLower;
    final int len = value.length;
    /* Now check if there are any characters that need to be changed. */
    scan: {
        for (firstLower = 0 ; firstLower < len; ) {
            int c = (int)value[firstLower];
            int srcCount;
            if ((c >= Character.MIN_HIGH_SURROGATE)
                    && (c <= Character.MAX_HIGH_SURROGATE)) {
                c = codePointAt(firstLower);
                srcCount = Character.charCount(c);
            } else {
                srcCount = 1;
            }
            int upperCaseChar = Character.toUpperCaseEx(c);
            if ((upperCaseChar == Character.ERROR)
                    || (c != upperCaseChar)) {
                break scan;
            }
            firstLower += srcCount;
        }
        return this;
    }
    /* result may grow, so i+resultOffset is the write location in result */
    int resultOffset = 0;
    char[] result = new char[len]; /* may grow */
    /* Just copy the first few upperCase characters. */
    System.arraycopy(value, 0, result, 0, firstLower);
    String lang = locale.getLanguage();
    boolean localeDependent =
            (lang == "tr" || lang == "az" || lang == "lt");
    char[] upperCharArray;
    int upperChar;
    int srcChar;
    int srcCount;
    for (int i = firstLower; i < len; i += srcCount) {
        srcChar = (int)value[i];
        if ((char)srcChar >= Character.MIN_HIGH_SURROGATE &&
            (char)srcChar <= Character.MAX_HIGH_SURROGATE) {
            srcChar = codePointAt(i);
            srcCount = Character.charCount(srcChar);
        } else {
            srcCount = 1;
        }
        if (localeDependent) {
            upperChar = ConditionalSpecialCasing.toUpperCaseEx(this, i, locale);
        } else {
            upperChar = Character.toUpperCaseEx(srcChar);
        }
        if ((upperChar == Character.ERROR)
                || (upperChar >= Character.MIN_SUPPLEMENTARY_CODE_POINT)) {
            if (upperChar == Character.ERROR) {
                if (localeDependent) {
                    upperCharArray =
                            ConditionalSpecialCasing.toUpperCaseCharArray(this, i, locale);
                } else {
                    upperCharArray = Character.toUpperCaseCharArray(srcChar);
                }
            } else if (srcCount == 2) {
                resultOffset += Character.toChars(upperChar, result, i + resultOffset) - srcCount;
                continue;
            } else {
                upperCharArray = Character.toChars(upperChar);
            }
            /* Grow result if needed */
            int mapLen = upperCharArray.length;
            if (mapLen > srcCount) {
                char[] result2 = new char[result.length + mapLen - srcCount];
                System.arraycopy(result, 0, result2, 0, i + resultOffset);
                result = result2;
            }
            for (int x = 0; x < mapLen; ++x) {
                result[i + resultOffset + x] = upperCharArray[x];
            }
            resultOffset += (mapLen - srcCount);
        } else {
            result[i + resultOffset] = (char)upperChar;
        }
    }
    return new String(result, 0, len + resultOffset);
}
將字符串的大寫轉換為小寫，將字符串小寫轉換為大寫

（34）public char[] toCharArray() {

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      char result[] = new char[value.length];
      System.arraycopy(value, 0, result, 0, value.length);
      return result;
  }
將字符串轉化為數組，由于本身就是數組的形式，只需將其拷貝即可。

（35） public String[] split(String regex, int limit) {

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    char ch = 0;
    if (((regex.value.length == 1 &&
         ".$|()[{^?*+".indexOf(ch = regex.charAt(0)) == -1) ||
         (regex.length() == 2 &&
          regex.charAt(0) == "" &&
          (((ch = regex.charAt(1))-"0")|("9"-ch)) < 0 &&
          ((ch-"a")|("z"-ch)) < 0 &&
          ((ch-"A")|("Z"-ch)) < 0)) &&
        (ch < Character.MIN_HIGH_SURROGATE ||
         ch > Character.MAX_LOW_SURROGATE))
    {
        int off = 0;
        int next = 0;
        boolean limited = limit > 0;
        ArrayList list = new ArrayList<>();
        while ((next = indexOf(ch, off)) != -1) {
            if (!limited || list.size() < limit - 1) {
                list.add(substring(off, next));
                off = next + 1;
            } else {    // last one
                //assert (list.size() == limit - 1);
                list.add(substring(off, value.length));
                off = value.length;
                break;
            }
        }
        // If no match was found, return this
        if (off == 0)
            return new String[]{this};
        // Add remaining segment
        if (!limited || list.size() < limit)
            list.add(substring(off, value.length));
        // Construct result
        int resultSize = list.size();
        if (limit == 0) {
            while (resultSize > 0 && list.get(resultSize - 1).length() == 0) {
                resultSize--;
            }
        }
        String[] result = new String[resultSize];
        return list.subList(0, resultSize).toArray(result);
    }
    return Pattern.compile(regex).split(this, limit);
}

（36）public String[] split(String regex) {

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    return split(regex, 0);
 }
對于字符串 "boo:and:foo",regex為o,limit為5時，
splite方法首先去字符串里查找regex——o，然后把o做為分隔符，
逐個把o去掉并且把字符串分開，比如，發現b后面有一個o，于是把這個o去掉，
并且把字符串拆成"b", "o:and:foo"兩個字符串（注意：b后面的兩個o已經去掉了一個），
接下來看"o:and:foo"這個字符串，第一個字符就是o，
于是o前面相當于一個空串，把這個o去掉，"o:and:foo"被分開成"", ":and:foo"這樣兩個字符串，
以此類推循環5次就是splite("o", 5)方法的作用

(37) public static String join(CharSequence delimiter, CharSequence... elements) {

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    Objects.requireNonNull(delimiter);
    Objects.requireNonNull(elements);
    // Number of elements not likely worth Arrays.stream overhead.
    StringJoiner joiner = new StringJoiner(delimiter);
    for (CharSequence cs: elements) {
        joiner.add(cs);
    }
    return joiner.toString();
 }

(38) public static String join(CharSequence delimiter,

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        Iterable elements) {
    Objects.requireNonNull(delimiter);
    Objects.requireNonNull(elements);
    StringJoiner joiner = new StringJoiner(delimiter);
    for (CharSequence cs: elements) {
        joiner.add(cs);
    }
    return joiner.toString();
}
將已給的數組用給定的字符進行分開，返回一個特定的字符串，例如
String [] strings = {"a","b","c","d"};
 info =  String.join( "a",strings );
 System.out.println( info );
打印的結果為 aabacad

（39） public static String format(String format, Object... args) {

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    return new Formatter().format(format, args).toString();
  }

（40） public static String format(Locale l, String format, Object... args) {

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    return new Formatter(l).format(format, args).toString();
  }
  將字符串串格式化為一定格式的形式返回給

（41） 各種各樣的 valueOf(Object obj) 方法

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  作用就是將相應格式的對象轉換為你字符串的格式