JDK源码解析之String二

摘要：如果起始地址小于或者起始地址所比较对象长度大于自身对象长度，返回假从所比较对象的末尾开始比较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协议的，或者初步判断一个文件是不是文件，都可以采用这个方法进行比较。

(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 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 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