Java Jdk8 String阅读

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写在前言:String类是由字符数组实现的,拥有诸多模式的构造函数(空、字符数组、String、Unicode编码、ascii编码(停止使用)、字节数组等),重写了equals、hashcode等方法,除了提供各基础数据转换String的方法,还提供了大小写转换、对字符插入分隔符、按字符拆分、包含、替换、组合、截取、比较等一系列方法。
属性

value[]字节数组。
hash hashcode的缓存。
serialPersistentFields 无默认序列化的数据。

构造函数

String() 创建一个空字节数组。
String(String original) 赋值数组和hash缓存。
String(char value[]) 字符数组构造函数。
String(char value[], int offset, int count) 字符数组区间构造函数。
String(int[] codePoints, int offset, int count) 字符的16位Unicode编码构造函数
String(byte ascii[], int hibyte, int offset, int count)已不再使用的构造函数
String(byte ascii[], int hibyte)已不再使用的构造函数
String(byte bytes[], int offset, int length, String charsetName) 字节构造方法,需要编码格式,之后转Charset
String(byte bytes[], int offset, int length, Charset charset)字节构造方法
String(byte bytes[], String charsetName)字节构造方法
String(byte bytes[], int offset, int length)字节构造方法
String(byte bytes[])字节构造方法
String(StringBuffer buffer)需要同步锁,拷贝字符数组的构造函数
String(StringBuilder builder)构造函数
String(char[] value, boolean share)直接给字符数组对象地址构造函数,内部使用,replace

私有方法

checkBounds(byte[] bytes, int offset, int length)检测范围
nonSyncContentEquals(AbstractStringBuilder sb):比较方法
indexOfSupplementary(int ch, int fromIndex):unicode高低位比较
lastIndexOfSupplementary(int ch, int fromIndex):unicode高低位比较

内部方法

getChars(char dst[], int dstBegin):拷贝数组
indexOf(char[] source, int sourceOffset, int sourceCount,String target, int fromIndex):字符数组下标
indexOf(char[] source, int sourceOffset, int sourceCount,char[] target, int targetOffset, int targetCount,int fromIndex):字符数组下标
lastIndexOf(char[] source, int sourceOffset, int sourceCount,char[] target, int targetOffset, int targetCount,int fromIndex):往后找字符数组下标

公有方法

length:字符数组长度
isEmpty:字符数组长度和0比较
charAt(int index):取字符数组其中一个字符
codePointAt(int index):取字符数组的Unicode代码点
codePointBefore(int index):取字符数组的Unicode编码
codePointCount(int beginIndex, int endIndex):取字符数组的Unicode编码
offsetByCodePoints(int index, int codePointOffset):取字符数组的Unicode编码
getChars(int srcBegin, int srcEnd, char dst[], int dstBegin):拷贝数组
getBytes(int srcBegin, int srcEnd, byte dst[], int dstBegin):已经禁用了,拷贝字节
getBytes(String charsetName):按格式解码
getBytes(Charset charset):按格式解码
getBytes():默认解码
equals(Object anObject):比较方法
contentEquals(StringBuffer sb):比较方法
contentEquals(CharSequence cs):比较方法
equalsIgnoreCase(String anotherString):忽略大小写比较方法
compareTo(String anotherString):比较下标
compareToIgnoreCase(String str):CaseInsensitiveComparator类的实例。 regionMatches(int toffset, String other, int ooffset,int len):忽略大小写比较方法
regionMatches(boolean ignoreCase, int toffset, String other, int ooffset, int len):忽略大小写比较方法
startsWith(String prefix, int toffset):是否从下标处以指定字符串开头
startsWith(String prefix):是否以指定字符串开头
endsWith(String suffix):是否以指定字符串结束
hashCode():没有属性hash的时候会进行计算
indexOf(int ch):代码点下标
indexOf(int ch, int fromIndex):代码点下标
lastIndexOf(int ch):代码点倒序找下标
lastIndexOf(int ch, int fromIndex):代码点倒序找下标
indexOf(String str):字符串下标
indexOf(String str, int fromIndex):字符串下标
lastIndexOf(String str):往后找字符数组下标
lastIndexOf(String str, int fromIndex):往后找字符数组下标
substring(int beginIndex):截取字符串
substring(int beginIndex, int endIndex):截取字符串
subSequence(int beginIndex, int endIndex):截取字符串
concat(String str):组合字符串
replace(char oldChar, char newChar):替换字符
matches(String regex)
contains(CharSequence s):包含字符
replaceFirst(String regex, String replacement):替换
replaceAll(String regex, String replacement):替换
replace(CharSequence target, CharSequence replacement):替换
split(String regex, int limit):分割
split(String regex):分割
join(CharSequence delimiter, CharSequence... elements):字符插入符合
join(CharSequence delimiter,Iterable<? extends CharSequence> elements):字符插入符合
toLowerCase(Locale locale):小写
toLowerCase():小写
toUpperCase(Locale locale):大写
toUpperCase():大写
trim():去除前后空格
toString():返回当前类
toCharArray():转字符数组
format(String format, Object... args):使用本地语言环境生成固定格式
format(Locale l, String format, Object... args):使用固定语言环境生成固定格式
intern()字符串常量池中对象

CaseInsensitiveComparator:忽略大小写比较类

类型转String

valueOf(Object obj)
valueOf(char data[])
valueOf(char data[], int offset, int count)
copyValueOf(char data[], int offset, int count)
copyValueOf(char data[])
valueOf(boolean b)
valueOf(char c)
valueOf(int i)
valueOf(long l)
valueOf(float f)
valueOf(double d)

// 存储字节数组,即字符串内容
private final char value[];

// 是String hashcode的一个缓存,hashcode进行了重写
private int hash; // Default to 0

// java序列化机制,序列化和反序列化验证版本的一致
private static final long serialVersionUID = -6849794470754667710L;

// 用于指定哪些字段需要被默认序列号,创建一个长度为0的数据,因为String类型的序列化在流处理中是个特例,有自己的序列号逻辑,所以不需要默认序列号
private static final ObjectStreamField[] serialPersistentFields =
    new ObjectStreamField[0];

// 构造函数,定义一个空字符数组给存储字符数组
public String() {
    this.value = new char[0];
}

// 构造函数,赋予字符数组,以及hashcode缓存
public String(String original) {
    this.value = original.value;
    this.hash = original.hash;
}

// 构造函数,传参字符数组,重新拷贝这个数组赋予存储字符数组
public String(char value[]) {
    this.value = Arrays.copyOf(value, value.length);
}

// 构造函数,传参字符数组,开始下标,拷贝数量
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);
}

// 构造函数,codepoints范围:000000 - 10FFFF,代码点:字符的16位Unicode编码,开始下标,拷贝数量
public String(int[] codePoints, 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 > codePoints.length - count) {
        throw new StringIndexOutOfBoundsException(offset + count);
    }

    final int end = offset + count;

    // Pass 1: Compute precise size of char[]
    int n = count;
    for (int i = offset; i < end; i++) {
        int c = codePoints[i];
        if (Character.isBmpCodePoint(c))
            continue;
        else if (Character.isValidCodePoint(c))
            n++;
        else throw new IllegalArgumentException(Integer.toString(c));
    }

    // Pass 2: Allocate and fill in char[]
    final char[] v = new char[n];

    for (int i = offset, j = 0; i < end; i++, j++) {
        int c = codePoints[i];
        if (Character.isBmpCodePoint(c))
            v[j] = (char)c;
        else
            Character.toSurrogates(c, v, j++);
    }

    this.value = v;
}

// ascii编码构造函数,传参ascii数组,补码,开始下标,数量
@Deprecated // 代表已经不用了
public String(byte ascii[], int hibyte, int offset, int count) {
    checkBounds(ascii, offset, count);
    char value[] = new char[count];

    if (hibyte == 0) {
        for (int i = count; i-- > 0;) {
            value[i] = (char)(ascii[i + offset] & 0xff);
        }
    } else {
        hibyte <<= 8;
        for (int i = count; i-- > 0;) {
            value[i] = (char)(hibyte | (ascii[i + offset] & 0xff));
        }
    }
    this.value = value;
}

@Deprecated // 代表已经不用了
public String(byte ascii[], int hibyte) {
    this(ascii, hibyte, 0, ascii.length);
}

// 检查字节数组的范围
private static void checkBounds(byte[] bytes, int offset, int length) {
    if (length < 0)
        throw new StringIndexOutOfBoundsException(length);
    if (offset < 0)
        throw new StringIndexOutOfBoundsException(offset);
    if (offset > bytes.length - length)
        throw new StringIndexOutOfBoundsException(offset + length);
}

// 字节数组构造函数,传参字节数组,开始下标,数量,编码格式charsetName(utf-8),调用方法会转换成Charset
public String(byte bytes[], int offset, int length, String charsetName)
        throws UnsupportedEncodingException {
    if (charsetName == null)
        throw new NullPointerException("charsetName");
    checkBounds(bytes, offset, length);
    this.value = StringCoding.decode(charsetName, bytes, offset, length);
}

// 字节数组构造函数,传参字节数组,开始下标,数量,编码格式charset(utf-8)
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) {
    this(bytes, 0, bytes.length, charset);
}

// 按默认ISO-8859-1编码格式
public String(byte bytes[], int offset, int length) {
    checkBounds(bytes, offset, length);
    this.value = StringCoding.decode(bytes, offset, length);
}

public String(byte bytes[]) {
    this(bytes, 0, bytes.length);
}

// StringBuffer构造函数,线程不安全,通过同步锁,保证同一个时刻只有一个线程执行
public String(StringBuffer buffer) {
    synchronized(buffer) {
        this.value = Arrays.copyOf(buffer.getValue(), buffer.length());
    }
}

// StringBuilder构造函数
public String(StringBuilder builder) {
    this.value = Arrays.copyOf(builder.getValue(), builder.length());
}

// 直接赋予值数组地址,加个share使用来区分构造函数,无实际意义
String(char[] value, boolean share) {
    // assert share : "unshared not supported";
    this.value = value;
}

// 字符数组长度,utf-16代码单元数量(辅助字符一个代码点是两个代码单元),不一定是实际字符个数
public int length() {
    return value.length;
}

public boolean isEmpty() {
    return value.length == 0;
}

// 取出对应下标的字符
public char charAt(int index) {
    if ((index < 0) || (index >= value.length)) {
        throw new StringIndexOutOfBoundsException(index);
    }
    return value[index];
}

// 返回指定下标的Unicode代码点
public int codePointAt(int index) {
    if ((index < 0) || (index >= value.length)) {
        throw new StringIndexOutOfBoundsException(index);
    }
    return Character.codePointAtImpl(value, index, value.length);
}

// 返回指定索引之前的字符Unicode编码
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);
}

// 返回两个下标之间字符串Unicode的数量,实际的字符个数(代码点数),可以当做length使用
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);
}

// 返回从下标开始偏移codePointOffset个代码点的索引
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);
}

// 将字符数组从下标0开始拷贝到目标数组中
void getChars(char dst[], int dstBegin) {
    System.arraycopy(value, 0, dst, dstBegin, value.length);
}

public void getChars(int srcBegin, int srcEnd, char dst[], int dstBegin) {
    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);
}

@Deprecated // 不再使用
public void getBytes(int srcBegin, int srcEnd, byte dst[], int dstBegin) {
    if (srcBegin < 0) {
        throw new StringIndexOutOfBoundsException(srcBegin);
    }
    if (srcEnd > value.length) {
        throw new StringIndexOutOfBoundsException(srcEnd);
    }
    if (srcBegin > srcEnd) {
        throw new StringIndexOutOfBoundsException(srcEnd - srcBegin);
    }
    Objects.requireNonNull(dst);

    int j = dstBegin;
    int n = srcEnd;
    int i = srcBegin;
    char[] val = value;   /* avoid getfield opcode */

    while (i < n) {
        dst[j++] = (byte)val[i++];
    }
}

// 按照编码格式进行编码
public byte[] getBytes(String charsetName)
        throws UnsupportedEncodingException {
    if (charsetName == null) throw new NullPointerException();
    return StringCoding.encode(charsetName, value, 0, value.length);
}

// 按照编码格式进行编码
public byte[] getBytes(Charset charset) {
    if (charset == null) throw new NullPointerException();
    return StringCoding.encode(charset, value, 0, value.length);
}

// 按默认ISO-8859-1编码格式
public byte[] getBytes() {
    return StringCoding.encode(value, 0, value.length);
}

// 判断地址,判断类型,判断长度,循环比较
public boolean equals(Object anObject) {
    if (this == anObject) {
        return true;
    }
    if (anObject instanceof String) {
        String anotherString = (String)anObject;
        int n = value.length;
        if (n == anotherString.value.length) {
            char v1[] = value;
            char v2[] = anotherString.value;
            int i = 0;
            while (n-- != 0) {
                if (v1[i] != v2[i])
                    return false;
                i++;
            }
            return true;
        }
    }
    return false;
}

// 转成字符序列比较
public boolean contentEquals(StringBuffer sb) {
    return contentEquals((CharSequence)sb);
}

// 字符序列是AbstractStringBuilder的之类比较,比较长度,循环比较
private boolean nonSyncContentEquals(AbstractStringBuilder sb) {
    char v1[] = value;
    char v2[] = sb.getValue();
    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;
}

// 比较字符序列方法,AbstractStringBuilder比较,String比较,判断长度,代码点比较
public boolean contentEquals(CharSequence cs) {
    // Argument is a StringBuffer, StringBuilder
    if (cs instanceof AbstractStringBuilder) {
        if (cs instanceof StringBuffer) {
            synchronized(cs) {
               return nonSyncContentEquals((AbstractStringBuilder)cs);
            }
        } else {
            return nonSyncContentEquals((AbstractStringBuilder)cs);
        }
    }
    // Argument is a String
    if (cs instanceof String) {
        return equals(cs);
    }
    // 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;
}

// 比较地址,比较长度、不考虑大小写比较
public boolean equalsIgnoreCase(String anotherString) {
    return (this == anotherString) ? true
            : (anotherString != null)
            && (anotherString.value.length == value.length)
            && regionMatches(true, 0, anotherString, 0, value.length);
}

// 比较字符内容,如果不等,返回字符ascii码差值,相等,返回长度差值
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;
}

// 根据比较类创建一个对象,引用不可变,内容可变
public static final Comparator<String> CASE_INSENSITIVE_ORDER
                                     = new CaseInsensitiveComparator();
									 
// 字符串比较类,如果字符或大写或小写只要有个相同就遍历下一个,否则返回两个ascii的差值
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);
                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;
    }

    /** Replaces the de-serialized object. */
    private Object readResolve() { return CASE_INSENSITIVE_ORDER; }
}

// 当前字符串和参数比较
public int compareToIgnoreCase(String str) {
    return CASE_INSENSITIVE_ORDER.compare(this, str);
}

// 和参数比较在各自区域内是否相等
public boolean regionMatches(int toffset, String other, int ooffset,
        int len) {
    char ta[] = value;
    int to = toffset;
    char pa[] = other.value;
    int po = ooffset;
    // 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) {
        if (ta[to++] != pa[po++]) {
            return false;
        }
    }
    return true;
}

// 是否开启忽略大小写的区域比较
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) {
            // 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;
}

// 从下标开始,是否以参数开头
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 startsWith(String prefix) {
    return startsWith(prefix, 0);
}

public boolean endsWith(String suffix) {
    return startsWith(suffix, value.length - suffix.value.length);
}

// hashcode的计算方式,已计算的放入缓存
public int hashCode() {
    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) {
    return indexOf(ch, 0);
}

// 检测范围,进行比较,Unicode字符,返回开始下标
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;
    }
	//最小代码点,直接比较值,一字节
    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);
    }
}

// Unicode字符高低位比较,2字节
private int indexOfSupplementary(int ch, int fromIndex) {
    if (Character.isValidCodePoint(ch)) {
        final char[] value = this.value;
        final char hi = Character.highSurrogate(ch);
        final char lo = Character.lowSurrogate(ch);
        final int max = value.length - 1;
        for (int i = fromIndex; i < max; i++) {
            if (value[i] == hi && value[i + 1] == lo) {
                return i;
            }
        }
    }
    return -1;
}

// 反着找下标
public int lastIndexOf(int ch) {
    return lastIndexOf(ch, value.length - 1);
}

// 检测范围,一个字节和多个字节比较(一个代码点可能有多个代码单元)
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);
    }
}

// Unicode字符高低位比较,2字节
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;
}

// 返回下标
public int indexOf(String str) {
    return indexOf(str, 0);
}

// 从开始下标处找该字符串下标
public int indexOf(String str, int fromIndex) {
    return indexOf(value, 0, value.length,
            str.value, 0, str.value.length, fromIndex);
}

// 开原数组,开始下标处到长度找目标字符串
static int indexOf(char[] source, int sourceOffset, int sourceCount,
        String target, int fromIndex) {
    return indexOf(source, sourceOffset, sourceCount,
                   target.value, 0, target.value.length,
                   fromIndex);
}

// 实现方法,如果开始下标大于等于源数组数量,如果目标数组数量等于0返回,不然返回-1,先判断第一个,再判断后面字符
static int indexOf(char[] source, int sourceOffset, int sourceCount,
        char[] target, int targetOffset, int targetCount,
        int fromIndex) {
    if (fromIndex >= sourceCount) {
        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;
}

// 倒着查最后出现的
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,
        String target, int fromIndex) {
    return lastIndexOf(source, sourceOffset, sourceCount,
                   target.value, 0, target.value.length,
                   fromIndex);
}

// 从后往前查,用了递归方法
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;
    }
}

// 从开始下标截取字符串
public String substring(int beginIndex) {
    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);
}

// 开始-结束中间字符串
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);
}

// 返回类型区分
public CharSequence subSequence(int beginIndex, int endIndex) {
    return this.substring(beginIndex, endIndex);
}

// 组合字符串
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);
}

// 替换字符,返回新字符串,过滤相等的,再进行判断
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;
}

// 根据正则表达式判断是否匹配((.*)值(.*))
public boolean matches(String regex) {
    return Pattern.matches(regex, this);
}

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

// 根据正则表达式替换首次出现的,成功则返回替换的字符串,失败则返回原始字符串。
public String replaceFirst(String regex, String replacement) {
    return Pattern.compile(regex).matcher(this).replaceFirst(replacement);
}

// 根据正则表达式替换所有出现的,成功则返回替换的字符串,失败则返回原始字符串。
public String replaceAll(String regex, String replacement) {
    return Pattern.compile(regex).matcher(this).replaceAll(replacement);
}

// 字符序列替换
public String replace(CharSequence target, CharSequence replacement) {
    return Pattern.compile(target.toString(), Pattern.LITERAL).matcher(
            this).replaceAll(Matcher.quoteReplacement(replacement.toString()));
}

// 根据正则表达式进行拆分,参数拆分正则表达式、返回数组最大长度,验证分割规则,防止未转义和编码错误,验证长度进行截取放入数组,根据限制长度未分割的放至最后一个,最后去除空格的
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[] split(String regex) {
    return split(regex, 0);
}

// 在字符串里面插入分隔符,可变参数
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();
}

// 在字符串里面插入分隔符,可迭代参数
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();
}

// 转换小写
public String toLowerCase(Locale locale) {
    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
            srcChar == '\u0130') { // LATIN CAPITAL LETTER I WITH DOT ABOVE
            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() {
    return toLowerCase(Locale.getDefault());
}

// 转换成大写
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);
}

public String toUpperCase() {
    return toUpperCase(Locale.getDefault());
}

// 去除前后空格
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 String toString() {
    return this;
}

// 拷贝数组返回
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;
}

// 使用本地语言环境
public static String format(String format, Object... args) {
    return new Formatter().format(format, args).toString();
}

// 使用指定语言环境,制定字符串格式和参数生成格式化字符串
public static String format(Locale l, String format, Object... args) {
    return new Formatter(l).format(format, args).toString();
}

// 对象
public static String valueOf(Object obj) {
    return (obj == null) ? "null" : obj.toString();
}

// 字符数组
public static String valueOf(char data[]) {
    return new String(data);
}

// 字符数组,开始下标,长度
public static String valueOf(char data[], int offset, int count) {
    return new String(data, offset, count);
}

// 字符数组,开始下标,长度
public static String copyValueOf(char data[], int offset, int count) {
    return new String(data, offset, count);
}

// 字符数组
public static String copyValueOf(char data[]) {
    return new String(data);
}

// boolean
public static String valueOf(boolean b) {
    return b ? "true" : "false";
}

// char
public static String valueOf(char c) {
    char data[] = {c};
    return new String(data, true);
}

// int
public static String valueOf(int i) {
    return Integer.toString(i);
}

// long
public static String valueOf(long l) {
    return Long.toString(l);
}

// float
public static String valueOf(float f) {
    return Float.toString(f);
}

// double
public static String valueOf(double d) {
    return Double.toString(d);
}

// 从堆的字符串常量池中取出来,堆中对象也是指向它,唯一
public native String intern();