JDK源码阅读之String

String是平常使用最多的结构之一，String提供来很多方便的接口来处理字符串，同时其支持正则表达式，所以字符串处理能力很强，下面我们简单介绍下String的内部实现，重点关注下常用的一些方法实现，String的一些方法涉及到字符编码，这里不介绍字符编码相关的。

//String类是final的，即不可继承public final class String implements java.io.Serializable, Comparable<String>, CharSequence {    private final char value[];//内部存储结构，即以数组来存储其内容    private int hash;//标识一个字符串//实现空字符串public String() {    this.value = new char[0];}//用字符串构造字符串的public String(String original) {     this.value = original.value;     this.hash = original.hash;}//用字符数组构造字符串public String(char value[]) {     this.value = Arrays.copyOf(value, value.length);}//如果以字节数组构造，则涉及到字符编码的问题public String(byte bytes[], Charset charset) {      this(bytes, 0, bytes.length, charset);}//获取长度信息，因为数组提供了长度信息，所以直接获取，否则可以提供一个长度信息public int length() {     return value.length;}//判断是否为空public boolean isEmpty() {     return value.length == 0;}//String也不是完全不可变的，这里可以直接修改内容public char charAt(int index) {   if ((index < 0) || (index >= value.length)) {            throw new StringIndexOutOfBoundsException(index);   }   return value[index];}//比较两字符串public boolean equals(Object anObject) {        if (this == anObject) {//判断是否指向同一个对象            return true;        }        if (anObject instanceof String) {//判断是否是String类的对象            String anotherString = (String) anObject;            int n = value.length;//判断长度信息            if (n == anotherString.value.length) {                char v1[] = value;                char v2[] = anotherString.value;                int i = 0;                while (n-- != 0) {                    if (v1[i] != v2[i])//判断内容                            return false;                    i++;                }                return true;            }        }        return false;}//比较两字符串的大小，比较过程和equals类似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];            char c2 = v2[k];            if (c1 != c2) {                return c1 - c2;            }            k++;        }        return len1 - len2;}//忽略大小写的一种判断方法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) {//判断是否相等                    c1 = Character.toUpperCase(c1);//如果不相等，则都转换成大写进行判断                    c2 = Character.toUpperCase(c2);                    if (c1 != c2) {                        c1 = Character.toLowerCase(c1);//转换成小写判断                        c2 = Character.toLowerCase(c2);                        if (c1 != c2) {                            return c1 - c2;                        }                    }                }            }            return n1 - n2;}//比较两字符串是否相等public boolean regionMatches(int toffset, String other, int ooffset,            int len) {        char ta[] = value;        int to = toffset;        char pa[] = other.value;        int po = ooffset;        if ((ooffset < 0) || (toffset < 0) //判断参数有效性                || (toffset > (long)value.length - len)                || (ooffset > (long)other.value.length - len)) {            return false;        }        while (len-- > 0) { //比较内容            if (ta[to++] != pa[po++]) {                return false;            }        }        return true;}//判断是否以字串开始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;        if ((toffset < 0) || (toffset > value.length - pc)) {//判断参数有效性            return false;        }        while (--pc >= 0) { //比较内容            if (ta[to++] != pa[po++]) {                return false;            }        }        return true;}//startWith的逆操作public boolean endsWith(String suffix) {        return startsWith(suffix, value.length - suffix.value.length);}//查找字符public int indexOf(int ch, int fromIndex) {        final int max = value.length;        if (fromIndex < 0) {//参数有效性判断            fromIndex = 0;        } else if (fromIndex >= max) {            return -1;        }        //if是执行字符有效性判断        if (ch < Character.MIN_SUPPLEMENTARY_CODE_POINT) {            final char[] value = this.value;//获取字符串内容            for (int i = fromIndex; i < max; i++) {                if (value[i] == ch) {//比较内容                    return i;                }            }            return -1;        } else {            return indexOfSupplementary(ch, fromIndex);        }}//删除空格public String trim() {        int len = value.length;        int st = 0;        char[] val = value;        while ((st < len) && (val[st] <= ' ')) {//删除头部空格            st++;        }        while ((st < len) && (val[len - 1] <= ' ')) {//删除尾部空格            len--;        }        return ((st > 0) || (len < value.length)) ? substring(st, len) : this;//重新构造字符串} //返回字符数组public char[] toCharArray() {        char result[] = new char[value.length];        System.arraycopy(value, 0, result, 0, value.length);//执行内存copy        return result;}//执行字符串分割操作public String[] split(String regex, int limit) {        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) {                    list.add(substring(off, next));//从主串里面substring分割元素                    off = next + 1;                } else {//判断模式是否启用，而且已经使用的次数大于limit                    list.add(substring(off, value.length));                    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));            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);}