Java的String

  String的底层是char类型的数组，所以先需要看一下char类型。

1、char类型

  Java中char类型采用16位的Unicode编码，后来由于字符集太多，超过了65536个，所以16位的Unicode已经不能满足需要了。于是自JDK5.0开始，采用代码点表示某个字符对应的代码值，代码点规定十六进制书写，并加上前缀U+，例如U+0041表示A的代码点。Unicode的代码点分为17个代码级别。第一个代码级别是基本的多语言级别(bmp)，代码点从U+0000到U+FFFF，其中包括经典的Unicode代码；其余的16个附加级别，代码点从U+10000到U+10FFFF，其中包括了一些辅助字符。

  在基本的多语言级别中，每个字符采用16位表示，通常称为代码单元。如下图所示：此时一个代码点只需要一个代码单元(两个字节)即可表示，即一个char字符。而辅助字符则采用一对连续的代码单元表示进行编码(即两个代码单元(四个字节，两个char字符)表示一个代码点)。

2、Sring类型

  String底层是由char数组组成，length方法将返回一个UTF-16编码表示的给定字符串所需要的代码单元数量。例如：

   String greeting = "Hello";

   int n = greeting.length();//5

 要想得到实际的长度，即代码点的数量，可以调用：

  int cpCount = greeting.length(0,greeting.length());

 调用s.charAt(n)将返回位置n的代码单元，n介于0 - s.length()-1之间。例如：

  char first = greeting.charAt(0);//H

  char last = greeting.charAt(4);//o

 要想得到第i个代码点，应该使用下列语句：

  int index = greeting.offsetByCodePoint(0,i);

  int cp = greeting.codePointAt(index);

3、String源码

  JDK：1.7；package：java.lang。省去部分重载方法。

public final class String    //final,不能给被继承。    implements java.io.Serializable, Comparable<String>, CharSequence {//可序列化，可排序//String底层是char数组,被final修饰:value保存的数组引用不能改变，又没有get/set方法能访问到这个引用，//就更不可能去修改这个引用所指向的数组里的值(实际反射可以做到，但是不推荐)，所以一般认为String不可变。    private final char value[];     private int hash; // Default to 0    public String() {          this.value = new char[0];  //初始化为空字符串""，因为String是不可变的，所以基本不用    }    public String(String original) {        this.value = original.value;     //初始化Sring，新字符串是该字符串的副本，基本不用        this.hash = original.hash;    }    public String(char value[]) {   //根据char数组构建String        this.value = Arrays.copyOf(value, value.length);    }        public String(char value[], int offset, int count) { //截取一部分创建一个字符串        if (offset < 0) {   //参数有效性检查            throw new StringIndexOutOfBoundsException(offset);        }        if (count < 0) {            throw new StringIndexOutOfBoundsException(count);        }        // Note: offset or count might be near -1>>>1.        if (offset > value.length - count) {            throw new StringIndexOutOfBoundsException(offset + count);        }        this.value = Arrays.copyOfRange(value, offset, offset+count);    }  //分配一个新的 String，它包含代码点数组参数一个子数组的字符。offset 参数是该子数组第一个代码点的索引，count 参数指定子数组的长度。    public String(int[] codePoints, int offset, int count) {        if (offset < 0) {            throw new StringIndexOutOfBoundsException(offset);        }        if (count < 0) {            throw new StringIndexOutOfBoundsException(count);        }        if (offset > codePoints.length - count) {            throw new StringIndexOutOfBoundsException(offset + count);        }        final int end = offset + count;        int n = count;        for (int i = offset; i < end; i++) {            int c = codePoints[i];            if (Character.isBmpCodePoint(c))  //判断是否属于基本多语言级别，是则true，不会溢出直接下次循环，否则false                continue;            else if (Character.isValidCodePoint(c)) //判断codePoint是不是一个有效的Unicode码值，即是否在0000-10FFFF之间                n++;     //n初始值为count，在count基础上增加            else throw new IllegalArgumentException(Integer.toString(c));        }        final char[] v = new char[n];        for (int i = offset, j = 0; i < end; i++, j++) {            int c = codePoints[i];            if (Character.isBmpCodePoint(c))   //同上进行判断是否基本多语言级别                v[j] = (char)c;                //若是直接转换类型即可，不会溢出            else                Character.toSurrogates(c, v, j++); //否则就存储两个代码单元表示一个代码点        }        this.value = v;    }    private static void checkBounds(byte[] bytes, int offset, int length) { //参数有效性检查        if (length < 0)            throw new StringIndexOutOfBoundsException(length);        if (offset < 0)            throw new StringIndexOutOfBoundsException(offset);        if (offset > bytes.length - length)            throw new StringIndexOutOfBoundsException(offset + length);    }    public String(byte bytes[], int offset, int length, String charsetName)//使用charsetName编码方式进行解码，构造String            throws UnsupportedEncodingException {        if (charsetName == null)            throw new NullPointerException("charsetName");        checkBounds(bytes, offset, length);  //参数有效性检查        this.value = StringCoding.decode(charsetName, bytes, offset, length);    }    public String(byte bytes[], int offset, int length, Charset charset) {        if (charset == null)            throw new NullPointerException("charset");        checkBounds(bytes, offset, length);        this.value =  StringCoding.decode(charset, bytes, offset, length);    }    public String(byte bytes[], String charsetName)            throws UnsupportedEncodingException {        this(bytes, 0, bytes.length, charsetName);    }        public String(byte bytes[], Charset charset) {//通过使用指定的 charset 解码指定的 byte 数组，构造一个新的 String        this(bytes, 0, bytes.length, charset);    }        public String(byte bytes[], int offset, int length) {        checkBounds(bytes, offset, length);        this.value = StringCoding.decode(bytes, offset, length);    }      public int length() {    //字符串长度，代码单元        return value.length;    }    public boolean isEmpty() {        return value.length == 0;   //空串    }    public char charAt(int index) {        if ((index < 0) || (index >= value.length)) {            throw new StringIndexOutOfBoundsException(index);        }        return value[index];   //返回指定位置代码单元的值    }        public int codePointAt(int index) {        if ((index < 0) || (index >= value.length)) {            throw new StringIndexOutOfBoundsException(index);        }        return Character.codePointAtImpl(value, index, value.length);  //返回指定位置的代码点    }       public int codePointBefore(int index) {  //index的前一个代码点的值        int i = index - 1;        if ((i < 0) || (i >= value.length)) {            throw new StringIndexOutOfBoundsException(index);        }        return Character.codePointBeforeImpl(value, index, 0);    }    public int codePointCount(int beginIndex, int endIndex) {    //指定范围的代码点数量        if (beginIndex < 0 || endIndex > value.length || beginIndex > endIndex) {            throw new IndexOutOfBoundsException();        }        return Character.codePointCountImpl(value, beginIndex, endIndex - beginIndex);    }    public int offsetByCodePoints(int index, int codePointOffset) {//偏移指定位置的代码点索引        if (index < 0 || index > value.length) {            throw new IndexOutOfBoundsException();        }        return Character.offsetByCodePointsImpl(value, 0, value.length,                index, codePointOffset);    }       public void getChars(int srcBegin, int srcEnd, char dst[], int dstBegin) {//字符串复制到数组中        if (srcBegin < 0) {            throw new StringIndexOutOfBoundsException(srcBegin);        }        if (srcEnd > value.length) {            throw new StringIndexOutOfBoundsException(srcEnd);        }        if (srcBegin > srcEnd) {            throw new StringIndexOutOfBoundsException(srcEnd - srcBegin);        }        System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);    }    public byte[] getBytes(String charsetName)              throws UnsupportedEncodingException {  //字符串转换成给定字符集的字节数组        if (charsetName == null) throw new NullPointerException();        return StringCoding.encode(charsetName, value, 0, value.length);    }    public byte[] getBytes(Charset charset) {   //字符串转换成给定charset转换成字节数组        if (charset == null) throw new NullPointerException();        return StringCoding.encode(charset, value, 0, value.length);    }    public byte[] getBytes() {   //用系统默认的编码方式进行转换        return StringCoding.encode(value, 0, value.length);    }    public boolean equals(Object anObject) {        if (this == anObject) {  //引用相等，则必然相等            return true;        }        if (anObject instanceof String) {   //必须是String的实例            String anotherString = (String) anObject;            int n = value.length;            if (n == anotherString.value.length) {  //比较长度                char v1[] = value;                char v2[] = anotherString.value;                int i = 0;                while (n-- != 0) {                    if (v1[i] != v2[i])   //每一位相等                            return false;                    i++;                }                return true;            }        }        return false;    }    public boolean contentEquals(StringBuffer sb) {   //与StringBuffer的内容比较        synchronized (sb) {            return contentEquals((CharSequence) sb);        }    }    public boolean contentEquals(CharSequence cs) {//与CharSequence的内容比较        if (value.length != cs.length())            return false;        // Argument is a StringBuffer, StringBuilder        if (cs instanceof AbstractStringBuilder) {            char v1[] = value;            char v2[] = ((AbstractStringBuilder) cs).getValue();            int i = 0;            int n = value.length;            while (n-- != 0) {                if (v1[i] != v2[i])                    return false;                i++;            }            return true;        }        // Argument is a String        if (cs.equals(this))            return true;        // Argument is a generic CharSequence        char v1[] = value;        int i = 0;        int n = value.length;        while (n-- != 0) {            if (v1[i] != cs.charAt(i))                return false;            i++;        }        return true;    }    public boolean equalsIgnoreCase(String anotherString) {  //忽略大小写        return (this == anotherString) ? true   //是否同一个引用                : (anotherString != null)                     && (anotherString.value.length == value.length)  //长度相等                && regionMatches(true, 0, anotherString, 0, value.length);//正则表达式匹配    }    public int compareTo(String anotherString) {        int len1 = value.length;        int len2 = anotherString.value.length;        int lim = Math.min(len1, len2);         char v1[] = value;        char v2[] = anotherString.value;        int k = 0;        while (k < lim) {            char c1 = v1[k];//进行比较            char c2 = v2[k];            if (c1 != c2) {                return c1 - c2; //考虑大小写，直接相减            }            k++;        }        return len1 - len2;    }    public static final Comparator<String> CASE_INSENSITIVE_ORDER                                         = new CaseInsensitiveComparator();    private static class CaseInsensitiveComparator            implements Comparator<String>, java.io.Serializable {        // use serialVersionUID from JDK 1.2.2 for interoperability        private static final long serialVersionUID = 8575799808933029326L;        public int compare(String s1, String s2) {            int n1 = s1.length();            int n2 = s2.length();            int min = Math.min(n1, n2);            for (int i = 0; i < min; i++) {                char c1 = s1.charAt(i);                char c2 = s2.charAt(i);                if (c1 != c2) {                    c1 = Character.toUpperCase(c1);  //大写比较                    c2 = Character.toUpperCase(c2);                    if (c1 != c2) {                        c1 = Character.toLowerCase(c1);  //小写比较                        c2 = Character.toLowerCase(c2);                        if (c1 != c2) {                            return c1 - c2;                        }                    }                }            }            return n1 - n2;        }    }    public int compareToIgnoreCase(String str) {   //忽略大小写        return CASE_INSENSITIVE_ORDER.compare(this, str);    }    public boolean regionMatches(int toffset, String other, int ooffset,            int len) {    //字符串从toffset开始，与other从ooffset开始，长度均为len，进行比较        char ta[] = value;        int to = toffset;  //字符串的起始位置        char pa[] = other.value;          int po = ooffset;  //other字符串的起始位置        // Note: toffset, ooffset, or len might be near -1>>>1.        if ((ooffset < 0) || (toffset < 0)     //小于0则false                || (toffset > (long)value.length - len)   //长度超过范围则false                || (ooffset > (long)other.value.length - len)) {            return false;        }        while (len-- > 0) {            if (ta[to++] != pa[po++]) {                return false;            }        }        return true;    }    public boolean regionMatches(boolean ignoreCase, int toffset,            String other, int ooffset, int len) { //重载方法，是否忽略大小写进行比较        char ta[] = value;        int to = toffset;        char pa[] = other.value;        int po = ooffset;        // Note: toffset, ooffset, or len might be near -1>>>1.        if ((ooffset < 0) || (toffset < 0)                || (toffset > (long)value.length - len)                || (ooffset > (long)other.value.length - len)) {            return false;        }        while (len-- > 0) {            char c1 = ta[to++];            char c2 = pa[po++];            if (c1 == c2) {                continue;            }            if (ignoreCase) {                // If characters don't match but case may be ignored,                // try converting both characters to uppercase.                // If the results match, then the comparison scan should                // continue.                char u1 = Character.toUpperCase(c1);                char u2 = Character.toUpperCase(c2);                if (u1 == u2) {                    continue;                }                // Unfortunately, conversion to uppercase does not work properly                // for the Georgian alphabet, which has strange rules about case                // conversion.  So we need to make one last check before                // exiting.                if (Character.toLowerCase(u1) == Character.toLowerCase(u2)) {                    continue;                }            }            return false;        }        return true;    }    public boolean startsWith(String prefix, int toffset) {  //指定位置是否以某个前缀开始        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.        if ((toffset < 0) || (toffset > value.length - pc)) {            return false;        }        while (--pc >= 0) {            if (ta[to++] != pa[po++]) {  //与前缀的每一位进行比较                return false;            }        }        return true;    }    public boolean startsWith(String prefix) {  //同上，重载方法        return startsWith(prefix, 0);    }    public boolean endsWith(String suffix) {//结束        return startsWith(suffix, value.length - suffix.value.length);    }    public int hashCode() {    //hash值        int h = hash;        if (h == 0 && value.length > 0) {            char val[] = value;            for (int i = 0; i < value.length; i++) {                h = 31 * h + val[i];            }            hash = h;        }        return h;    }    public int indexOf(int ch) { //第一次出现的索引，ch为代码点        return indexOf(ch, 0);    }    public int indexOf(int ch, int fromIndex) {   //第一次出现的索引，从指定位置开始算，ch为代码点        final int max = value.length;        if (fromIndex < 0) {            fromIndex = 0;        } else if (fromIndex >= max) {            return -1;    //越界了返回-1        }        if (ch < Character.MIN_SUPPLEMENTARY_CODE_POINT) { //MIN_SUPPLEMENTARY_CODE_POINT = 0x010000;            final char[] value = this.value;                //此时直接比较值即可            for (int i = fromIndex; i < max; i++) {                if (value[i] == ch) {                    return i;                }            }            return -1;        } else {            return indexOfSupplementary(ch, fromIndex);    //如果为辅助字符，则要比较两个代码单元        }    }    /**     * Handles (rare) calls of indexOf with a supplementary character.     */    private int indexOfSupplementary(int ch, int fromIndex) {        if (Character.isValidCodePoint(ch)) {  //是否在0x010000到0x10FFFF之间            final char[] value = this.value;            final char hi = Character.highSurrogate(ch);  //高位            final char lo = Character.lowSurrogate(ch);   //低位            final int max = value.length - 1;            for (int i = fromIndex; i < max; i++) {                if (value[i] == hi && value[i + 1] == lo) {  //同时比较两个                    return i;                }            }        }        return -1;    }       public int lastIndexOf(int ch) {  //最后一次出现的索引        return lastIndexOf(ch, value.length - 1);    }    public int lastIndexOf(int ch, int fromIndex) {//最后一次出现的索引，与第一次出现的类似        if (ch < Character.MIN_SUPPLEMENTARY_CODE_POINT) {            // handle most cases here (ch is a BMP code point or a            // negative value (invalid code point))            final char[] value = this.value;            int i = Math.min(fromIndex, value.length - 1);            for (; i >= 0; i--) {                if (value[i] == ch) {                    return i;                }            }            return -1;        } else {            return lastIndexOfSupplementary(ch, fromIndex);        }    }    private int lastIndexOfSupplementary(int ch, int fromIndex) {        if (Character.isValidCodePoint(ch)) {            final char[] value = this.value;            char hi = Character.highSurrogate(ch);            char lo = Character.lowSurrogate(ch);            int i = Math.min(fromIndex, value.length - 2);            for (; i >= 0; i--) {                if (value[i] == hi && value[i + 1] == lo) {                    return i;                }            }        }        return -1;    }    public int indexOf(String str) {  //str第一次出现的索引        return indexOf(str, 0);    }    public int indexOf(String str, int fromIndex) {  //同前        return indexOf(value, 0, value.length,                str.value, 0, str.value.length, fromIndex);    }    static int indexOf(char[] source, int sourceOffset, int sourceCount,            char[] target, int targetOffset, int targetCount,            int fromIndex) {        if (fromIndex >= sourceCount) {  //指定位置大于等于数组长度            return (targetCount == 0 ? sourceCount : -1);        }        if (fromIndex < 0) {            fromIndex = 0;        }        if (targetCount == 0) {            return fromIndex;        }        char first = target[targetOffset];        int max = sourceOffset + (sourceCount - targetCount);        for (int i = sourceOffset + fromIndex; i <= max; i++) {            /* Look for first character. */            if (source[i] != first) {                while (++i <= max && source[i] != first);            }            /* Found first character, now look at the rest of v2 */            if (i <= max) {                int j = i + 1;                int end = j + targetCount - 1;                for (int k = targetOffset + 1; j < end && source[j]                        == target[k]; j++, k++);                if (j == end) {                    /* Found whole string. */                    return i - sourceOffset;                }            }        }        return -1;    }    public int lastIndexOf(String str) {        return lastIndexOf(str, value.length);    }    public int lastIndexOf(String str, int fromIndex) {        return lastIndexOf(value, 0, value.length,                str.value, 0, str.value.length, fromIndex);    }    static int lastIndexOf(char[] source, int sourceOffset, int sourceCount,            char[] target, int targetOffset, int targetCount,            int fromIndex) {        /*         * Check arguments; return immediately where possible. For         * consistency, don't check for null str.         */        int rightIndex = sourceCount - targetCount;        if (fromIndex < 0) {            return -1;        }        if (fromIndex > rightIndex) {            fromIndex = rightIndex;        }        /* Empty string always matches. */        if (targetCount == 0) {            return fromIndex;        }        int strLastIndex = targetOffset + targetCount - 1;        char strLastChar = target[strLastIndex];        int min = sourceOffset + targetCount - 1;        int i = min + fromIndex;        startSearchForLastChar:        while (true) {            while (i >= min && source[i] != strLastChar) {                i--;            }            if (i < min) {                return -1;            }            int j = i - 1;            int start = j - (targetCount - 1);            int k = strLastIndex - 1;            while (j > start) {                if (source[j--] != target[k--]) {                    i--;                    continue startSearchForLastChar;                }            }            return start - sourceOffset + 1;        }    }    public String substring(int beginIndex) {//截取该字符串获得子串        if (beginIndex < 0) {            throw new StringIndexOutOfBoundsException(beginIndex);        }        int subLen = value.length - beginIndex;        if (subLen < 0) {            throw new StringIndexOutOfBoundsException(subLen);        }        return (beginIndex == 0) ? this : new String(value, beginIndex, subLen);//相同则返回自身，不同则new一个    }    public String substring(int beginIndex, int endIndex) {  //重载方法，同上        if (beginIndex < 0) {            throw new StringIndexOutOfBoundsException(beginIndex);        }        if (endIndex > value.length) {            throw new StringIndexOutOfBoundsException(endIndex);        }        int subLen = endIndex - beginIndex;        if (subLen < 0) {            throw new StringIndexOutOfBoundsException(subLen);        }        return ((beginIndex == 0) && (endIndex == value.length)) ? this                : new String(value, beginIndex, subLen);    }    public CharSequence subSequence(int beginIndex, int endIndex) {        return this.substring(beginIndex, endIndex);  //继承CharSequence接口实现其中的方法    }    public String concat(String str) {  //将指定字符串连接到此字符串的结尾        int otherLen = str.length();        if (otherLen == 0) {            return this;   //str为空串则返回自身        }        int len = value.length;        char buf[] = Arrays.copyOf(value, len + otherLen);//扩充数组长度        str.getChars(buf, len);        return new String(buf, true);  //创建一个新串    }    public String replace(char oldChar, char newChar) {//用newChar替代oldChar返回新串        if (oldChar != newChar) {            int len = value.length;            int i = -1;            char[] val = value; /* avoid getfield opcode */            while (++i < len) {                if (val[i] == oldChar) {//找到字符串中第一次出现oldChar的位置i                    break;                }            }            if (i < len) {                char buf[] = new char[len];                for (int j = 0; j < i; j++) {                    buf[j] = val[j]; //保存第一次出现oldChar字符串之前的字符串                }                while (i < len) {                    char c = val[i];                    buf[i] = (c == oldChar) ? newChar : c; //替换之后的oldChar并存储                    i++;                }                return new String(buf, true);            }        }        return this; //相等则返回自身    }    public boolean matches(String regex) {//正则表达式匹配        return Pattern.matches(regex, this);    }    public boolean contains(CharSequence s) {  //包含s则true        return indexOf(s.toString()) > -1;    }    public String replaceFirst(String regex, String replacement) {//替换正则表达式匹配到的第一个字符串        return Pattern.compile(regex).matcher(this).replaceFirst(replacement);    }    public String replaceAll(String regex, String replacement) {//替换全部        return Pattern.compile(regex).matcher(this).replaceAll(replacement);    }    public String replace(CharSequence target, CharSequence replacement) {//替换序列        return Pattern.compile(target.toString(), Pattern.LITERAL).matcher(                this).replaceAll(Matcher.quoteReplacement(replacement.toString()));    }        public String[] split(String regex, int limit) {  //使用指定正则表达式分割字符串        char ch = 0;      //如果分割符是这些字符，则自行进行分割        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<>();//借助链表来存储分割的元素              while ((next = indexOf(ch, off)) != -1) {//定位元素                  if (!limited || list.size() < limit - 1) {//从主串里面substring分割元素                      list.add(substring(off, next));                    off = next + 1;                } else {    // last one                    //assert (list.size() == limit - 1);                    list.add(substring(off, value.length));//判断模式是否启用，而且已经使用的次数大于limit                      off = value.length;                    break;                }            }          //没有该字符，则返回完整的串              if (off == 0)                return new String[]{this};          //模式阀值未超过，则添加剩余的串            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);//返回字符数组          }        //否则直接调用正则表达式进行分割          return Pattern.compile(regex).split(this, limit);    }    public String[] split(String regex) {        return split(regex, 0);    }       public String toLowerCase(Locale locale) {  //使用给定 Locale 的规则进行字符串小写转化        if (locale == null) {            throw new NullPointerException();        }        int firstUpper;        final int len = value.length;        /* Now check if there are any characters that need to be changed. */        scan: {            for (firstUpper = 0 ; firstUpper < len; ) {                char c = value[firstUpper];                if ((c >= Character.MIN_HIGH_SURROGATE)                        && (c <= Character.MAX_HIGH_SURROGATE)) {                    int supplChar = codePointAt(firstUpper);                    if (supplChar != Character.toLowerCase(supplChar)) {                        break scan;                    }                    firstUpper += Character.charCount(supplChar);                } else {                    if (c != Character.toLowerCase(c)) {                        break scan;                    }                    firstUpper++;                }            }            return this;        }        char[] result = new char[len];        int resultOffset = 0;  /* result may grow, so i+resultOffset                                * is the write location in result */        /* Just copy the first few lowerCase characters. */        System.arraycopy(value, 0, result, 0, firstUpper);        String lang = locale.getLanguage();        boolean localeDependent =                (lang == "tr" || lang == "az" || lang == "lt");        char[] lowerCharArray;        int lowerChar;        int srcChar;        int srcCount;        for (int i = firstUpper; 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 || srcChar == '\u03A3') { // GREEK CAPITAL LETTER SIGMA                lowerChar = ConditionalSpecialCasing.toLowerCaseEx(this, i, locale);            } else {                lowerChar = Character.toLowerCase(srcChar);            }            if ((lowerChar == Character.ERROR)                    || (lowerChar >= Character.MIN_SUPPLEMENTARY_CODE_POINT)) {                if (lowerChar == Character.ERROR) {                    lowerCharArray =                            ConditionalSpecialCasing.toLowerCaseCharArray(this, i, locale);                } else if (srcCount == 2) {                    resultOffset += Character.toChars(lowerChar, result, i + resultOffset) - srcCount;                    continue;                } else {                    lowerCharArray = Character.toChars(lowerChar);                }                /* Grow result if needed */                int mapLen = lowerCharArray.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] = lowerCharArray[x];                }                resultOffset += (mapLen - srcCount);            } else {                result[i + resultOffset] = (char)lowerChar;            }        }        return new String(result, 0, len + resultOffset);    }    public String toLowerCase() {   //默认语言环境转化成小写        return toLowerCase(Locale.getDefault());    }    public String toUpperCase(Locale locale) {  //转化为大写        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;        }        char[] result = new char[len]; /* may grow */        int resultOffset = 0;  /* result may grow, so i+resultOffset         * is the write location in result */        /* 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);    }        public String toUpperCase() {        return toUpperCase(Locale.getDefault());    }        public String trim() {//忽略前导空白和后导空白        int len = value.length;        int st = 0;        char[] val = value;    /* avoid getfield opcode */        while ((st < len) && (val[st] <= ' ')) {//去掉前导空白' '            st++;        }        while ((st < len) && (val[len - 1] <= ' ')) {//去掉后岛空白 ' '            len--;        }        return ((st > 0) || (len < value.length)) ? substring(st, len) : this;//若存在空白就截取返回，否则返回自身    }    public String toString() {  //直接返回自身        return this;    }    public char[] toCharArray() {//转化成char数组        char result[] = new char[value.length];        System.arraycopy(value, 0, result, 0, value.length);        return result;    }       public static String format(String format, Object... args) {//使用指定的格式字符串和参数返回一个格式化字符串        return new Formatter().format(format, args).toString();    }  //使用指定的语言环境、格式字符串和参数返回一个格式化字符串。     public static String format(Locale l, String format, Object... args) {        return new Formatter(l).format(format, args).toString();    }    public static String valueOf(Object obj) {//返回Object的字符串形式        return (obj == null) ? "null" : obj.toString();    }    public static String valueOf(char data[]) {        return new String(data);//char 数组参数的字符串表示形式，新字符串，不影响原数组    }    public static String valueOf(char data[], int offset, int count) {        return new String(data, offset, count);  //数组一部分变成新串    }    public static String copyValueOf(char data[], int offset, int count) {        return new String(data, offset, count);    }    public static String copyValueOf(char data[]) {        return new String(data);    }        public static String valueOf(boolean b) {//布尔值转化为字符串值        return b ? "true" : "false";    }    //当调用 intern 方法时，如果常量池已经包含一个等于此 String 对象的字符串（用 equals(Object) 方法确定），则返回池中的字符串    //否则，将此 String 对象添加到池中，并返回此 String 对象的引用。    public native String intern();}