JDK源码 -- String
2019-08-18 本文已影响0人
TomyZhang
一、概念
类定义：
public final class String
implements java.io.Serializable, Comparable<String>, CharSequence
- 实现了Serializable接口，可进行序列化。
- 实现了Comparable接口，可进行比较。
- 实现了CharSequence接口，说明是个字符序列。
特点：
- 不可继承。
- 不可变。
- 线程安全。
二、使用
//TestString
public class TestString {
private static final String TAG = "TestString";
private String str = "tomorrow";
public void testEquals() {
Log.d(TAG, "zwm, tomorrow equals tommy: " + str.equals("tommy"));
Log.d(TAG, "zwm, tomorrow equals tomorrow: " + str.equals("tomorrow"));
}
public void testEqualsIgnoreCase() {
Log.d(TAG, "zwm, tomorrow equalsIgnoreCase TOMORROW: " + str.equalsIgnoreCase("TOMORROW"));
}
public void testSubSequence() {
CharSequence charSequence = str.subSequence(2, 6);
Log.d(TAG, "zwm, subSequence index 2 - 6: " + charSequence.toString());
}
public void testHashCode() {
Log.d(TAG, "zwm, hashCode: " + str.hashCode());
}
public void testStartsWith() {
Log.d(TAG, "zwm, startsWith tom: " + str.startsWith("tom"));
}
public void testStartsWith2() {
Log.d(TAG, "zwm, startsWith morr index 2: " + str.startsWith("morr", 2));
}
public void testEndsWith() {
Log.d(TAG, "zwm, endsWith rrow: " + str.endsWith("rrow"));
}
public void testContains() {
Log.d(TAG, "zwm, contains morr: " + str.contains("morr"));
}
public void testCharAt() {
Log.d(TAG, "zwm, charAt index 2: " + str.charAt(2));
}
public void testCodePointAt() {
Log.d(TAG, "zwm, codePointAt index 2: " + str.codePointAt(2));
}
public void testCodePointBefore() {
Log.d(TAG, "zwm, codePointBefore index 2: " + str.codePointBefore(2));
}
public void testCodePointCount() {
Log.d(TAG, "zwm, codePointCount index 0 - 2: " + str.codePointCount(0, 2));
}
public void testOffsetByCodePoints() {
Log.d(TAG, "zwm, offsetByCodePoints index 2, offset 1: " + str.offsetByCodePoints(2, 1));
}
public void testCompareTo() {
Log.d(TAG, "zwm, tomorrow compareTo TOMORROW: " + str.compareTo("TOMORROW"));
}
public void testCompareToIgnoreCase() {
Log.d(TAG, "zwm, tomorrow compareToIgnoreCase TOMORROW: " + str.compareToIgnoreCase("TOMORROW"));
}
public void testConcat() {
Log.d(TAG, "zwm, tomorrow concat XYZ, result: " + str.concat("XYZ"));
Log.d(TAG, "zwm, tomorrow concat XYZ, str: " + str);
}
public void testContentEquals() {
CharSequence charSequence = "tomorrow";
Log.d(TAG, "zwm, contentEquals CharSequence: " + str.contentEquals(charSequence));
}
public void testContentEquals2() {
StringBuffer stringBuffer = new StringBuffer("tomorrow");
Log.d(TAG, "zwm, contentEquals StringBuffer: " + str.contentEquals(stringBuffer));
}
public void testGetBytes() {
Log.d(TAG, "zwm, getBytes size: " + str.getBytes().length);
}
public void testGetChars() {
char[] dest = new char[10];
str.getChars(2, 6, dest, 0);
for(int i=0; i<4; i++) {
Log.d(TAG, "zwm, getChars, ch: " + dest[i]);
}
}
public void testLength() {
Log.d(TAG, "zwm, length: " + str.length());
}
public void testIsEmpty() {
Log.d(TAG, "zwm, isEmpty: " + str.isEmpty());
}
public void testIndexOf() {
Log.d(TAG, "zwm, indexOf o: " + str.indexOf('o'));
}
public void testLastIndexOf() {
Log.d(TAG, "zwm, lastIndexOf o: " + str.lastIndexOf('o'));
}
public void testIntern() {
Log.d(TAG, "zwm, intern: " + str.intern());
}
public void testMatches() {
boolean result = str.matches(".+omo.+");
Log.d(TAG, "zwm, match result: " + result);
}
public void testRegionMatches() {
Log.d(TAG, "zwm, tomorrow regionMatches xxtomoxx: " + str.regionMatches(2, "xxtomoxx", 4, 2));
}
public void testReplace() {
String result = str.replace('o', 'p');
Log.d(TAG, "zwm, tomorrow p replace o, result: " + result);
Log.d(TAG, "zwm, tomorrow p replace o, str: " + str);
}
public void testSplit() {
String[] result = str.split("o");
for(String str : result) {
Log.d(TAG, "zwm, split item: " + str);
}
Log.d(TAG, "zwm, split str: " + str);
}
public void testSubstring() {
String result = str.substring(2, 6);
Log.d(TAG, "zwm, tomorrow substring index 2 - 6, result: " + result);
Log.d(TAG, "zwm, tomorrow substring index 2 - 6, str: " + str);
}
public void testToLowerCase() {
String result = str.toLowerCase();
Log.d(TAG, "zwm, toLowerCase, result: " + result);
Log.d(TAG, "zwm, toLowerCase, str: " + str);
}
public void testToUpperCase() {
String result = str.toUpperCase();
Log.d(TAG, "zwm, toUpperCase, result: " + result);
Log.d(TAG, "zwm, toUpperCase, str: " + str);
}
public void testToCharArray() {
char[] result = str.toCharArray();
for(char ch : result) {
Log.d(TAG, "zwm, toCharArray ch: " + ch);
}
}
public void testTrim() {
String test = " , " + str + " , ";
Log.d(TAG, "zwm, test: " + test + ", length: " + test.length());
String result = test.trim();
Log.d(TAG, "zwm, result: " + result + ", length: " + result.length());
Log.d(TAG, "zwm, test: " + test + ", length: " + test.length());
}
}
//测试代码
private void testMethod() {
Log.d(TAG, "zwm, testMethod");
TestString testString = new TestString();
testString.testEquals();
testString.testEqualsIgnoreCase();
testString.testSubSequence();
testString.testHashCode();
testString.testStartsWith();
testString.testStartsWith2();
testString.testEndsWith();
testString.testContains();
testString.testCharAt();
testString.testCodePointAt();
testString.testCodePointBefore();
testString.testCodePointCount();
testString.testOffsetByCodePoints();
testString.testCompareTo();
testString.testCompareToIgnoreCase();
testString.testConcat();
testString.testContentEquals();
testString.testContentEquals2();
testString.testGetBytes();
testString.testGetChars();
testString.testLength();
testString.testIsEmpty();
testString.testIndexOf();
testString.testLastIndexOf();
testString.testIntern();
testString.testMatches();
testString.testRegionMatches();
testString.testReplace();
testString.testSplit();
testString.testSubstring();
testString.testToLowerCase();
testString.testToUpperCase();
testString.testToCharArray();
testString.testTrim();
}
//输出log
2019-08-18 17:23:01.983 zwm, testMethod
2019-08-18 17:23:01.985 zwm, tomorrow equals tommy: false
2019-08-18 17:23:01.985 zwm, tomorrow equals tomorrow: true
2019-08-18 17:23:01.986 zwm, tomorrow equalsIgnoreCase TOMORROW: true
2019-08-18 17:23:01.986 zwm, subSequence index 2 - 6: morr
2019-08-18 17:23:01.986 zwm, hashCode: -1037172987
2019-08-18 17:23:01.987 zwm, startsWith tom: true
2019-08-18 17:23:01.987 zwm, startsWith morr index 2: true
2019-08-18 17:23:01.987 zwm, endsWith rrow: true
2019-08-18 17:23:01.987 zwm, contains morr: true
2019-08-18 17:23:01.987 zwm, charAt index 2: m
2019-08-18 17:23:01.987 zwm, codePointAt index 2: 109
2019-08-18 17:23:01.987 zwm, codePointBefore index 2: 111
2019-08-18 17:23:01.987 zwm, codePointCount index 0 - 2: 2
2019-08-18 17:23:01.988 zwm, offsetByCodePoints index 2, offset 1: 3
2019-08-18 17:23:01.988 zwm, tomorrow compareTo TOMORROW: 32
2019-08-18 17:23:01.988 zwm, tomorrow compareToIgnoreCase TOMORROW: 0
2019-08-18 17:23:01.988 zwm, tomorrow concat XYZ, result: tomorrowXYZ
2019-08-18 17:23:01.988 zwm, tomorrow concat XYZ, str: tomorrow
2019-08-18 17:23:01.988 zwm, contentEquals CharSequence: true
2019-08-18 17:23:01.988 zwm, contentEquals StringBuffer: true
2019-08-18 17:23:01.989 zwm, getBytes size: 8
2019-08-18 17:23:01.989 zwm, getChars, ch: m
2019-08-18 17:23:01.989 zwm, getChars, ch: o
2019-08-18 17:23:01.989 zwm, getChars, ch: r
2019-08-18 17:23:01.989 zwm, getChars, ch: r
2019-08-18 17:23:01.989 zwm, length: 8
2019-08-18 17:23:01.989 zwm, isEmpty: false
2019-08-18 17:23:01.989 zwm, indexOf o: 1
2019-08-18 17:23:01.989 zwm, lastIndexOf o: 6
2019-08-18 17:23:01.990 zwm, intern: tomorrow
2019-08-18 17:23:01.990 zwm, match result: true
2019-08-18 17:23:01.990 zwm, tomorrow regionMatches xxtomoxx: true
2019-08-18 17:23:01.990 zwm, tomorrow p replace o, result: tpmprrpw
2019-08-18 17:23:01.990 zwm, tomorrow p replace o, str: tomorrow
2019-08-18 17:23:01.991 zwm, split item: t
2019-08-18 17:23:01.991 zwm, split item: m
2019-08-18 17:23:01.991 zwm, split item: rr
2019-08-18 17:23:01.991 zwm, split item: w
2019-08-18 17:23:01.991 zwm, split str: tomorrow
2019-08-18 17:23:01.991 zwm, tomorrow substring index 2 - 6, result: morr
2019-08-18 17:23:01.991 zwm, tomorrow substring index 2 - 6, str: tomorrow
2019-08-18 17:23:01.991 zwm, toLowerCase, result: tomorrow
2019-08-18 17:23:01.991 zwm, toLowerCase, str: tomorrow
2019-08-18 17:23:01.992 zwm, toUpperCase, result: TOMORROW
2019-08-18 17:23:01.992 zwm, toUpperCase, str: tomorrow
2019-08-18 17:23:01.992 zwm, toCharArray ch: t
2019-08-18 17:23:01.992 zwm, toCharArray ch: o
2019-08-18 17:23:01.992 zwm, toCharArray ch: m
2019-08-18 17:23:01.992 zwm, toCharArray ch: o
2019-08-18 17:23:01.992 zwm, toCharArray ch: r
2019-08-18 17:23:01.992 zwm, toCharArray ch: r
2019-08-18 17:23:01.992 zwm, toCharArray ch: o
2019-08-18 17:23:01.992 zwm, toCharArray ch: w
2019-08-18 17:23:01.993 zwm, test: , tomorrow , , length: 16
2019-08-18 17:23:01.993 zwm, result: , tomorrow ,, length: 12
2019-08-18 17:23:01.993 zwm, test: , tomorrow , , length: 16
三、原理
重要参数
//存放字符串内容的数组
private final char value[];
//hashCode值的缓存
private int hash; // Default to 0
//自定义比较器，可忽略大小写进行字符串的比较
public static final Comparator<String> CASE_INSENSITIVE_ORDER
= new CaseInsensitiveComparator();
构造函数
//无参构造函数
public String() {
this.value = new char[0]; //创建一个长度为0的新字符数组
}
//指定一个字符串参数的构造函数
public String(String original) {
this.value = original.value; //取其中的字符数组
this.hash = original.hash; //取其中的哈希值
}
//指定一个字符数组的构造函数
public String(char value[]) {
this.value = Arrays.copyOf(value, value.length); //拷贝参数中的字符数组生成一个新的字符数组
}
//指定一个字符数组、偏移量、长度的构造函数
public String(char value[], int offset, int count) {
if (offset < 0) {
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); //拷贝参数中的字符数组生成一个新的字符数组
}
//其它构造函数
...
public int hashCode()
//计算哈希值
public int hashCode() {
int h = hash; //hash为缓存的哈希值，如果为0则表示未进行缓存
if (h == 0 && value.length > 0) {
char val[] = value;
for (int i = 0; i < value.length; i++) {
h = 31 * h + val[i]; //缓存计算方式
}
hash = h; //缓存起来
}
return h;
}
public boolean equals(Object anObject)
//判定当前对象与指定的对象是否相等
public boolean equals(Object anObject) {
if (this == anObject) { //如果对象地址相同
return true;
}
if (anObject instanceof String) {
String anotherString = (String)anObject;
int n = value.length;
if (n == anotherString.value.length) { //如果字符串长度相同
char v1[] = value;
char v2[] = anotherString.value;
int i = 0;
while (n-- != 0) {
if (v1[i] != v2[i]) //如果字符不相同
return false;
i++;
}
return true;
}
}
return false;
}
public boolean equalsIgnoreCase(String anotherString)
//忽略大小写，判定当前对象与指定的对象是否相等
public boolean equalsIgnoreCase(String anotherString) {
return (this == anotherString) ? true //如果对象地址相同
: (anotherString != null)
&& (anotherString.value.length == value.length) //如果字符串长度相同
&& regionMatches(true, 0, anotherString, 0, value.length); //调用regionMatches方法进行比较
}
public boolean regionMatches(boolean ignoreCase, int toffset,
String other, int ooffset, int len) {
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) { //判断是否忽略大小写
// 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 int compareTo(String anotherString)
//当前对象与指定对象进行比较
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;
}
public int compareToIgnoreCase(String str)
//忽略大小写，比较当前字符串与指定的字符串
public int compareToIgnoreCase(String str) {
return CASE_INSENSITIVE_ORDER.compare(this, str);
}
//自定义比较器
public static final Comparator<String> CASE_INSENSITIVE_ORDER
= new CaseInsensitiveComparator();
//CaseInsensitiveComparator
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);
c2 = Character.toUpperCase(c2);
if (c1 != c2) {
c1 = Character.toLowerCase(c1);
c2 = Character.toLowerCase(c2);
if (c1 != c2) {
// No overflow because of numeric promotion
return c1 - c2;
}
}
}
}
return n1 - n2;
}
//CaseInsensitiveComparator
private Object readResolve() { return CASE_INSENSITIVE_ORDER; }
public CharSequence subSequence(int beginIndex, int endIndex)
//从beginIndex索引到endIndex索引构造新字符串
public CharSequence subSequence(int beginIndex, int endIndex) {
return this.substring(beginIndex, endIndex);
}
public boolean startsWith(String prefix)
//判断当前字符串是否以指定的字符串开头
public boolean startsWith(String prefix) {
return startsWith(prefix, 0);
}
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.
if ((toffset < 0) || (toffset > value.length - pc)) {
return false;
}
while (--pc >= 0) {
if (ta[to++] != pa[po++]) {
return false;
}
}
return true;
}
public boolean endsWith(String suffix)
//判断当前字符串是否以指定的字符串结尾
public boolean endsWith(String suffix) {
return startsWith(suffix, value.length - suffix.value.length);
}
public boolean contains(CharSequence s)
//判断当前字符串是否包含指定的字符序列
public boolean contains(CharSequence s) {
return indexOf(s.toString()) > -1;
}
public char charAt(int index)
//查找索引位置的字符
public char charAt(int index) {
if ((index < 0) || (index >= value.length)) {
throw new StringIndexOutOfBoundsException(index);
}
return value[index];
}
public int codePointAt(int index)
//获取索引位置的字符的Unicode码
public int codePointAt(int index) {
if ((index < 0) || (index >= value.length)) {
throw new StringIndexOutOfBoundsException(index);
}
return Character.codePointAtImpl(value, index, value.length);
}
//Character
static int codePointAtImpl(char[] a, int index, int limit) {
char c1 = a[index];
if (isHighSurrogate(c1) && ++index < limit) {
char c2 = a[index];
if (isLowSurrogate(c2)) {
return toCodePoint(c1, c2);
}
}
return c1;
}
//Character
public static int toCodePoint(char high, char low) {
// Optimized form of:
// return ((high - MIN_HIGH_SURROGATE) << 10)
// + (low - MIN_LOW_SURROGATE)
// + MIN_SUPPLEMENTARY_CODE_POINT;
return ((high << 10) + low) + (MIN_SUPPLEMENTARY_CODE_POINT
- (MIN_HIGH_SURROGATE << 10)
- MIN_LOW_SURROGATE);
}
public int codePointBefore(int index)
//获取指定索引的前一个位置的字符的Unicode码
public int codePointBefore(int index) {
int i = index - 1;
if ((i < 0) || (i >= value.length)) {
throw new StringIndexOutOfBoundsException(index);
}
return Character.codePointBeforeImpl(value, index, 0);
}
static int codePointBeforeImpl(char[] a, int index, int start) {
char c2 = a[--index];
if (isLowSurrogate(c2) && index > start) {
char c1 = a[--index];
if (isHighSurrogate(c1)) {
return toCodePoint(c1, c2);
}
}
return c2;
}
public int codePointCount(int beginIndex, int endIndex)
//获取beginIndex索引到endIndex索引的Unicode字节码个数
public int codePointCount(int beginIndex, int endIndex) {
if (beginIndex < 0 || endIndex > value.length || beginIndex > endIndex) {
throw new IndexOutOfBoundsException();
}
return Character.codePointCountImpl(value, beginIndex, endIndex - beginIndex);
}
static int codePointCountImpl(char[] a, int offset, int count) {
int endIndex = offset + count;
int n = count;
for (int i = offset; i < endIndex; ) {
if (isHighSurrogate(a[i++]) && i < endIndex &&
isLowSurrogate(a[i])) {
n--;
i++;
}
}
return n;
}
public int offsetByCodePoints(int index, int codePointOffset)
//获取index索引偏移codePointOffset长度后的索引位置
public int offsetByCodePoints(int index, int codePointOffset) {
if (index < 0 || index > value.length) {
throw new IndexOutOfBoundsException();
}
return Character.offsetByCodePointsImpl(value, 0, value.length,
index, codePointOffset);
}
//Character
static int offsetByCodePointsImpl(char[]a, int start, int count,
int index, int codePointOffset) {
int x = index;
if (codePointOffset >= 0) {
int limit = start + count;
int i;
for (i = 0; x < limit && i < codePointOffset; i++) {
if (isHighSurrogate(a[x++]) && x < limit &&
isLowSurrogate(a[x])) {
x++;
}
}
if (i < codePointOffset) {
throw new IndexOutOfBoundsException();
}
} else {
int i;
for (i = codePointOffset; x > start && i < 0; i++) {
if (isLowSurrogate(a[--x]) && x > start &&
isHighSurrogate(a[x-1])) {
x--;
}
}
if (i < 0) {
throw new IndexOutOfBoundsException();
}
}
return x;
}
public String concat(String str)
//在当前字符串后面拼接指定的字符串，生成新的字符串
public String concat(String str) {
int otherLen = str.length();
if (otherLen == 0) {
return this;
}
int len = value.length;
char buf[] = Arrays.copyOf(value, len + otherLen);
str.getChars(buf, len);
return new String(buf, true); //构造新的字符串
}
//构造函数
String(char[] value, boolean share) {
// assert share : "unshared not supported";
this.value = value; //将指定的字符数组的地址传递给成员变量value
}
public boolean contentEquals(CharSequence cs)
//当前字符串对象与指定的CharSequence对象进行内容比较
public boolean contentEquals(CharSequence cs) {
// Argument is a StringBuffer, StringBuilder
if (cs instanceof AbstractStringBuilder) {
if (cs instanceof StringBuffer) { //若指定对象是StringBuffer实例，则需要进行同步
synchronized(cs) {
return nonSyncContentEquals((AbstractStringBuilder)cs);
}
} else {
return nonSyncContentEquals((AbstractStringBuilder)cs);
}
}
// Argument is a String
if (cs.equals(this))
return true;
// Argument is a generic CharSequence
char v1[] = value;
int n = v1.length;
if (n != cs.length()) {
return false;
}
for (int i = 0; i < n; i++) {
if (v1[i] != cs.charAt(i)) {
return false;
}
}
return true;
}
private boolean nonSyncContentEquals(AbstractStringBuilder sb) {
char v1[] = value;
char v2[] = sb.getValue(); //获取字符数组内容
int n = v1.length;
if (n != sb.length()) {
return false;
}
for (int i = 0; i < n; i++) {
if (v1[i] != v2[i]) {
return false;
}
}
return true;
}
public boolean contentEquals(StringBuffer sb)
//当前字符串对象与指定的StringBuffer对象进行内容比较
public boolean contentEquals(StringBuffer sb) {
return contentEquals((CharSequence)sb);
}
public byte[] getBytes()
//对当前字符串使用默认的字符集获取字节数组
public byte[] getBytes() {
return StringCoding.encode(value, 0, value.length);
}
//StringCoding
static byte[] encode(char[] ca, int off, int len) {
String csn = Charset.defaultCharset().name(); //获取默认的字符集
try {
// use charset name encode() variant which provides caching.
return encode(csn, ca, off, len);
} catch (UnsupportedEncodingException x) {
warnUnsupportedCharset(csn);
}
try {
return encode("ISO-8859-1", ca, off, len);
} catch (UnsupportedEncodingException x) {
// If this code is hit during VM initialization, MessageUtils is
// the only way we will be able to get any kind of error message.
MessageUtils.err("ISO-8859-1 charset not available: "
+ x.toString());
// If we can not find ISO-8859-1 (a required encoding) then things
// are seriously wrong with the installation.
System.exit(1);
return null;
}
}
void getChars(char dst[], int dstBegin)
//获取从索引dstBegin开始组成的新字符串
void getChars(char dst[], int dstBegin) {
System.arraycopy(value, 0, dst, dstBegin, value.length);
}
public int length()
//获取字符串长度
public int length() {
return value.length;
}
public boolean isEmpty()
//判断字符串是否为空
public boolean isEmpty() {
return value.length == 0;
}
public int indexOf(int ch)
//从左到右获取第一个与指定字符相同的位置索引
public int indexOf(int ch) {
return indexOf(ch, 0);
}
public int indexOf(int ch, int fromIndex) {
final int max = value.length;
if (fromIndex < 0) {
fromIndex = 0;
} else if (fromIndex >= max) {
// Note: fromIndex might be near -1>>>1.
return -1;
}
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;
for (int i = fromIndex; i < max; i++) {
if (value[i] == ch) {
return i;
}
}
return -1;
} else {
return indexOfSupplementary(ch, fromIndex);
}
}
public int lastIndexOf(int ch)
//从右到左获取第一个与指定字符相同的位置索引
public int lastIndexOf(int ch) {
return lastIndexOf(ch, value.length - 1);
}
public int lastIndexOf(int ch, int fromIndex) {
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);
}
}
public boolean matches(String regex)
//判断当前字符串是否匹配指定的正则表达式
public boolean matches(String regex) {
return Pattern.matches(regex, this);
}
public String replace(char oldChar, char newChar)
//将当前字符串的旧字符oldChar替换为新字符newChar
public String replace(char oldChar, char newChar) {
if (oldChar != newChar) {
int len = value.length;
int i = -1;
char[] val = value; /* avoid getfield opcode */
while (++i < len) {
if (val[i] == oldChar) {
break;
}
}
if (i < len) {
char buf[] = new char[len];
for (int j = 0; j < i; j++) {
buf[j] = val[j];
}
while (i < len) {
char c = val[i];
buf[i] = (c == oldChar) ? newChar : c;
i++;
}
return new String(buf, true);
}
}
return this;
}
public String[] split(String regex)
//使用指定的正则表达式分割字符串，并返回分割后的字符串数组
public String[] split(String regex) {
return split(regex, 0);
}
public String[] split(String regex, int limit) {
/* fastpath if the regex is a
(1)one-char String and this character is not one of the
RegEx's meta characters ".$|()[{^?*+\\", or
(2)two-char String and the first char is the backslash and
the second is not the ascii digit or ascii letter.
*/
char ch = 0;
if (((regex.value.length == 1 &&
".$|()[{^?*+\\".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);
}
public String substring(int beginIndex, int endIndex)
//从索引beginIndex开始到索引endIndex为止，生成新字符串
public String substring(int beginIndex, int endIndex) {
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 String toLowerCase()
//将当前字符串转化成小写
public String toLowerCase() {
return toLowerCase(Locale.getDefault());
}
public String toUpperCase()
//将当前字符串转化成大写
public String toUpperCase() {
return toUpperCase(Locale.getDefault());
}
public char[] toCharArray()
//将当前字符串转换成字符数组
public char[] toCharArray() {
// Cannot use Arrays.copyOf because of class initialization order issues
char result[] = new char[value.length];
System.arraycopy(value, 0, result, 0, value.length);
return result;
}
public String trim()
//将当前字符串左右两边的空格去掉，生成新的字符串
public String trim() {
int len = value.length;
int st = 0;
char[] val = value; /* avoid getfield opcode */
while ((st < len) && (val[st] <= ' ')) {
st++;
}
while ((st < len) && (val[len - 1] <= ' ')) {
len--;
}
return ((st > 0) || (len < value.length)) ? substring(st, len) : this;
}
public static String format(String format, Object... args)
//格式化输出字符串
public static String format(String format, Object... args) {
return new Formatter().format(format, args).toString();
}
public static String valueOf(int i)
//将基本类型数据转为字符串数据
public static String valueOf(int i) {
return Integer.toString(i);
}
public native String intern()
//本地方法
//对于JDK 1.6及之前，当使用intern()方法时，查询字符串常量池是否存在当前字符串，若不存在则将当前字符串复制到字符串常量池中，并返回字符串常量池中的引用。
//对于JDK 1.7及之后，当使用intern()方法时，先查询字符串常量池是否存在当前字符串，若字符串常量池中不存在则再从堆中查询，然后存储并返回相关引用；若都不存在则将当前字符串复制到字符串常量池中，并返回字符串常量池中的引用。
public native String intern();
//例子1
String s = new String("180") + new String("280");
Log.d(TAG, "zwm, s.intern() == s: " + (s.intern() == s));
//输出log
//s指向堆内存字符串地址引用
//s.intern()也指向堆内存字符串地址引用，因为常量池中不存在"180280"字符串地址引用
08-18 15:48:12.401 zwm, s.intern() == s: true
//例子2
String s = new String("181") + new String("281");
Log.d(TAG, "zwm, s == 181281: " + (s == "181281"));
Log.d(TAG, "zwm, s.intern() == s: " + (s.intern() == s));
//输出log
//s指向堆内存字符串地址引用
//由于代码中使用了"181281"字符串，因此常量池中存在一份字符串地址引用
//s.intern()指向常量池字符串地址引用，即非堆内存字符串地址引用
08-18 15:51:41.535 zwm, s == 181281: false
08-18 15:51:41.535 zwm, s.intern() == s: false
