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
