String源码简析（下）：

常用的方法：

1.substring(int beginIndex, int endIndex):得到一个子串.[beginIndex,endIndex)

public String substring(int beginIndex, int endIndex) {    if (beginIndex < 0) {        throw new StringIndexOutOfBoundsException(beginIndex);    }    if (endIndex > count) {        throw new StringIndexOutOfBoundsException(endIndex);    }    if (beginIndex > endIndex) {        throw new StringIndexOutOfBoundsException(endIndex - beginIndex);    }    return ((beginIndex == 0) && (endIndex == count)) ? this :        new String(offset + beginIndex, endIndex - beginIndex, value);    }

2.concat(String str) :将参数字符串添加到该字符串尾部。

 public String concat(String str) {    int otherLen = str.length();    if (otherLen == 0) {        return this;    }    char buf[] = new char[count + otherLen];    getChars(0, count, buf, 0);    str.getChars(0, otherLen, buf, count);    return new String(0, count + otherLen, buf);    }

3.replace(char oldChar, char newChar):用newchar替换字符串中所有的oldchar字符

public String replace(char oldChar, char newChar) {    if (oldChar != newChar) {        int len = count;        int i = -1;        char[] val = value; /* avoid getfield opcode */        int off = offset;   /* avoid getfield opcode */        while (++i < len) {        if (val[off + i] == oldChar) {            break;        }        }        if (i < len) {        char buf[] = new char[len];        for (int j = 0 ; j < i ; j++) {            buf[j] = val[off+j];        }        while (i < len) {            char c = val[off + i];            buf[i] = (c == oldChar) ? newChar : c;            i++;        }        return new String(0, len, buf);        }    }    return this;    }

4.对该字符串用相应的正则

public boolean matches(String regex) {        return Pattern.matches(regex, this);    }

5.contains(CharSequence s):判断字符序列s是否是该字符串的子集（实际调用的是静态的indexOf方法）

    public boolean contains(CharSequence s) {        return indexOf(s.toString()) > -1;    }

6.split(String regex, int limit)/split(String regex):用正则对该字符串进行分割,limit表示最多分割的次数

public String[] split(String regex, int limit) {    return Pattern.compile(regex).split(this, limit);    }public String[] split(String regex) {        return split(regex, 0);    }

7.toLowerCase(Locale locale):将字符串转成小写的核心代码，但是更常用的是他的重载方法toLowerCase().

public String toLowerCase(Locale locale) {    if (locale == null) {        throw new NullPointerException();        }        int     firstUpper;    /* Now check if there are any characters that need to be changed. */    scan: {        for (firstUpper = 0 ; firstUpper < count; ) {        char c = value[offset+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[count];    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, offset, 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 < count; i += srcCount) {        srcChar = (int)value[offset+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(0, count+resultOffset, result);    }<hr />public String toLowerCase() {        return toLowerCase(Locale.getDefault());    }

8.trim():去掉首尾的空格

    public String trim() {    int len = count;    int st = 0;    int off = offset;      /* avoid getfield opcode */    char[] val = value;    /* avoid getfield opcode */    while ((st < len) && (val[off + st] <= ' ')) {        st++;    }    while ((st < len) && (val[off + len - 1] <= ' ')) {        len--;    }    return ((st > 0) || (len < count)) ? substring(st, len) : this;    }

9.toString():就是他自己

  public String toString() {    return this;    }

其他方法：

1.substring(int beginIndex)：得到一个子串

public String substring(int beginIndex) {    return substring(beginIndex, count);    }

2.subSequence(int beginIndex, int endIndex)：得到一个子字符序列

public CharSequence subSequence(int beginIndex, int endIndex) {        return this.substring(beginIndex, endIndex);    }

3.toCharArray():将String转成数组，一个特殊版本的getChars

public char[] toCharArray() {    char result[] = new char[count];    getChars(0, count, result, 0);    return result;    }

注意点：

• String类中的value是不可变的，所以对一个字符串的增删改操作都会创建一个新的字符串对象，比如：

String a = new String("a");    for(int i=0;i<1000;i++)        a += "a";    //会创建1002个对象，在gc运行之间这些对象都不会回收，这是对内存极大的浪费！！！。

所以记住一个原则：
若一个字符序列会经常被增删改，那么请根据是否并发选择StringBuffer或StringBuild
若一个字符序列很少被增删改，那么请用String

• 从内存角度可以更加深刻的分析String：请看这篇博文