JDK源码解析string之二

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

      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+所比较对象长度）大于自身对象长度，返回假
      if ((toffset < 0) || (toffset > value.length - pc)) {
          return false;
      }
      //从所比较对象的末尾开始比较
      while (--pc >= 0) {
          if (ta[to++] != pa[po++]) {
              return false;
          }
      }
      return true;
  }

(29) public boolean startsWith(String prefix) {

      return startsWith(prefix, 0);
  }

(30) public boolean endsWith(String suffix) {

      return startsWith(suffix, value.length - suffix.value.length);
  }
 起始比较和末尾比较都是比较经常用得到的方法，例如在判断一个字符串是不是http协议的，
 或者初步判断一个文件是不是mp3文件，都可以采用这个方法进行比较。

(31) public native String intern();

intern方法是Native调用，它的作用是在方法区中的常量池里通过equals方法寻找等值的对象，
如果没有找到则在常量池中开辟一片空间存放字符串并返回该对应String的引用，
否则直接返回常量池中已存在String对象的引用

(32) public String toLowerCase() {

    return toLowerCase(Locale.getDefault());
 }

(33) public String toUpperCase(Locale locale) {

    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);
}
将字符串的大写转换为小写，将字符串小写转换为大写

（34）public char[] toCharArray() {

      char result[] = new char[value.length];
      System.arraycopy(value, 0, result, 0, value.length);
      return result;
  }
将字符串转化为数组，由于本身就是数组的形式，只需将其拷贝即可。

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

    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<String> 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) {

    return split(regex, 0);
 }

对于字符串 "boo:and:foo",regex为o,limit为5时，
splite方法首先去字符串里查找regex——o，然后把o做为分隔符，
逐个把o去掉并且把字符串分开，比如，发现b后面有一个o，于是把这个o去掉，
并且把字符串拆成"b", "o:and:foo"两个字符串（注意：b后面的两个o已经去掉了一个），
接下来看"o:and:foo"这个字符串，第一个字符就是o，
于是o前面相当于一个空串，把这个o去掉，"o:and:foo"被分开成"", ":and:foo"这样两个字符串，
以此类推循环5次就是splite("o", 5)方法的作用

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

    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,

        Iterable<? extends CharSequence> elements) {
    Objects.requireNonNull(delimiter);
    Objects.requireNonNull(elements);
    StringJoiner joiner = new StringJoiner(delimiter);
    for (CharSequence cs: elements) {
        joiner.add(cs);
    }
    return joiner.toString();
}
将已给的数组用给定的字符进行分开，返回一个特定的字符串，例如
String [] strings = {"a","b","c","d"};
 info =  String.join( "a",strings );
 System.out.println( info );
打印的结果为 aabacad

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

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

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

    return new Formatter(l).format(format, args).toString();
  }
  将字符串串格式化为一定格式的形式返回给

（41） 各种各样的 valueOf(Object obj) 方法

  作用就是将相应格式的对象转换为你字符串的格式