String

package java.lang;

import java.io.ObjectStreamField;
import java.io.UnsupportedEncodingException;
import java.lang.annotation.Native;
import java.lang.invoke.MethodHandles;
import java.lang.constant.Constable;
import java.lang.constant.ConstantDesc;
import java.nio.charset.Charset;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.Formatter;
import java.util.List;
import java.util.Locale;
import java.util.Objects;
import java.util.Optional;
import java.util.Spliterator;
import java.util.StringJoiner;
import java.util.function.Function;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import java.util.regex.PatternSyntaxException;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;
import jdk.internal.HotSpotIntrinsicCandidate;
import jdk.internal.vm.annotation.Stable;
import static java.util.function.Predicate.not;

/**
 * final不可被继承
 * Serializable：序列化接口，标志性接口，没有方法
 * Comparable：比较接口，方法compareTo
 * CharSequence：描述字符串结构，方法-length(),charAt(int index),截取字符串subSequence(int start, int end)
 * Constable：表示该类型的值是常量。可以在常量池中定义，方法-describeConstable
 * ConstantDesc:返回当前对象，方法-resolveConstantDesc。该接口用法目前比较迷
 */
public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence,
               Constable, ConstantDesc {

    /**
     * 注解@Stable，对于被这个注解修饰的变量或者数组，值或其中所有只能被修改一次
     * 引用类型初始为null，原生类型初始为0，他们能被修改为非null或者非0只能修改一次
     */
    @Stable
    private final byte[] value;

    /**
     * 用于编码字节的编码的标识符，分为 LATIN1 与 UTF16
     * coder一般只有两个值:LATIN1-0，UTF16-1
     * Latin1是ISO-8859-1的别名
     */
    private final byte coder;

    private int hash; // Default to 0

    private boolean hashIsZero; // Default to false;

    /**
     * Serializable接口的字段
     * 序列化机制是通过判断类的serialVersionUID来验证版本一致性的
     * 在进行反序列化时，JVM会把传来的字节流中的serialVersionUID与本地相应实体类的serialVersionUID进行比较
     * 如果相同就认为是一致的，可以进行反序列化，否则就会出现序列化版本不一致的异常，即是InvalidCastException
     */
    private static final long serialVersionUID = -6849794470754667710L;

    static final boolean COMPACT_STRINGS; 

    static {
        COMPACT_STRINGS = true; // 启用byte数组压缩，两个字节存储。在方法isLatin1进行控制。默认是UTF16
    }

    /**
     * ObjectStreamField描述串行化类中串行化字段的类
     * 
     */
    private static final ObjectStreamField[] serialPersistentFields =
        new ObjectStreamField[0];

    public String() {
        this.value = "".value;
        this.coder = "".coder;
    }

    @HotSpotIntrinsicCandidate
    public String(String original) {
        this.value = original.value;
        this.coder = original.coder;
        this.hash = original.hash;
    }

    public String(char value[]) {
        this(value, 0, value.length, null);
    }


    public String(char value[], int offset, int count) {
        this(value, offset, count, rangeCheck(value, offset, count));
    }

    /**
     * 返回的是类Void,不是void。Void表示空。
     */
    private static Void rangeCheck(char[] value, int offset, int count) {
        checkBoundsOffCount(offset, count, value.length);
        return null;
    }

    
    /**
     * 这构造器在编码上优化了内存空间
     */
    public String(int[] codePoints, int offset, int count) {
        checkBoundsOffCount(offset, count, codePoints.length);
        if (count == 0) {
            this.value = "".value;
            this.coder = "".coder;
            return;
        }
        if (COMPACT_STRINGS) {
            // 根据latin1编码
            byte[] val = StringLatin1.toBytes(codePoints, offset, count);
            if (val != null) { // 如果都是单字节字符，就以LATIN1编码
                this.coder = LATIN1;
                this.value = val;
                return;
            }
        }
        // 没有压缩的话，默认是UTF16，根据UTF16编码
        this.coder = UTF16;
        this.value = StringUTF16.toBytes(codePoints, offset, count);
    }

    /**
     * 不推荐使用
     */
    @Deprecated(since="1.1")
    public String(byte ascii[], int hibyte, int offset, int count) {
        checkBoundsOffCount(offset, count, ascii.length);
        if (count == 0) {
            this.value = "".value;
            this.coder = "".coder;
            return;
        }
        if (COMPACT_STRINGS && (byte)hibyte == 0) {
            this.value = Arrays.copyOfRange(ascii, offset, offset + count);
            this.coder = LATIN1;
        } else {
            hibyte <<= 8;
            byte[] val = StringUTF16.newBytesFor(count);
            for (int i = 0; i < count; i++) {
                StringUTF16.putChar(val, i, hibyte | (ascii[offset++] & 0xff));
            }
            this.value = val;
            this.coder = UTF16;
        }
    }

    /**
     * 不推荐使用
     * 
     */
    @Deprecated(since="1.1")
    public String(byte ascii[], int hibyte) {
        this(ascii, hibyte, 0, ascii.length);
    }

    /**
     * 根据编码生成String
     */
    public String(byte bytes[], int offset, int length, String charsetName)
            throws UnsupportedEncodingException {
        if (charsetName == null)
            throw new NullPointerException("charsetName");
        checkBoundsOffCount(offset, length, bytes.length);
        StringCoding.Result ret =
            StringCoding.decode(charsetName, bytes, offset, length);
        this.value = ret.value;
        this.coder = ret.coder;
    }

    
    /**
     * 根据编码生成String
     */
    public String(byte bytes[], int offset, int length, Charset charset) {
        if (charset == null)
            throw new NullPointerException("charset");
        checkBoundsOffCount(offset, length, bytes.length);
        StringCoding.Result ret =
            StringCoding.decode(charset, bytes, offset, length);
        this.value = ret.value;
        this.coder = ret.coder;
    }

    
    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);
    }

    
    public String(byte bytes[], int offset, int length) {
        checkBoundsOffCount(offset, length, bytes.length);
        StringCoding.Result ret = StringCoding.decode(bytes, offset, length);
        this.value = ret.value;
        this.coder = ret.coder;
    }

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

    
    public String(StringBuffer buffer) {
        this(buffer.toString());
    }

    
    public String(StringBuilder builder) {
        this(builder, null);
    }

    
    public int length() {
        return value.length >> coder();
    }

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

    
    
    public char charAt(int index) {
        if (isLatin1()) {
            return StringLatin1.charAt(value, index);
        } else {
            return StringUTF16.charAt(value, index);
        }
    }

    /**
     * 返回指定索引处的字符（Unicode 代码点）
     */
    public int codePointAt(int index) {
        if (isLatin1()) {
            checkIndex(index, value.length);
            return value[index] & 0xff;
        }
        int length = value.length >> 1;
        checkIndex(index, length);
        return StringUTF16.codePointAt(value, index, length);
    }

    
    /**
     * 提取前一个索引字符代码点
     */
    public int codePointBefore(int index) {
        int i = index - 1;
        if (i < 0 || i >= length()) {
            throw new StringIndexOutOfBoundsException(index);
        }
        if (isLatin1()) {
            return (value[i] & 0xff);
        }
        return StringUTF16.codePointBefore(value, index);
    }

    /**
     * 统计unicode字符的数量
     */
    public int codePointCount(int beginIndex, int endIndex) {
        if (beginIndex < 0 || beginIndex > endIndex ||
            endIndex > length()) {
            throw new IndexOutOfBoundsException();
        }
        if (isLatin1()) {
            return endIndex - beginIndex;
        }
        return StringUTF16.codePointCount(value, beginIndex, endIndex);
    }

    /**
     * 返回此 String 中从给定的 index 处偏移 codePointOffset 个代码点的索引
     */
    public int offsetByCodePoints(int index, int codePointOffset) {
        if (index < 0 || index > length()) {
            throw new IndexOutOfBoundsException();
        }
        return Character.offsetByCodePoints(this, index, codePointOffset);
    }

    /**
     * 将字符从字符串复制到目标字符数组
     */
    public void getChars(int srcBegin, int srcEnd, char dst[], int dstBegin) {
        checkBoundsBeginEnd(srcBegin, srcEnd, length());
        checkBoundsOffCount(dstBegin, srcEnd - srcBegin, dst.length);
        if (isLatin1()) {
            StringLatin1.getChars(value, srcBegin, srcEnd, dst, dstBegin);
        } else {
            StringUTF16.getChars(value, srcBegin, srcEnd, dst, dstBegin);
        }
    }

    
    /**
     * 不推荐使用
     */
    @Deprecated(since="1.1")
    public void getBytes(int srcBegin, int srcEnd, byte dst[], int dstBegin) {
        checkBoundsBeginEnd(srcBegin, srcEnd, length());
        Objects.requireNonNull(dst);
        checkBoundsOffCount(dstBegin, srcEnd - srcBegin, dst.length);
        if (isLatin1()) {
            StringLatin1.getBytes(value, srcBegin, srcEnd, dst, dstBegin);
        } else {
            StringUTF16.getBytes(value, srcBegin, srcEnd, dst, dstBegin);
        }
    }

    
    public byte[] getBytes(String charsetName)
            throws UnsupportedEncodingException {
        if (charsetName == null) throw new NullPointerException();
        return StringCoding.encode(charsetName, coder(), value);
    }

    
    public byte[] getBytes(Charset charset) {
        if (charset == null) throw new NullPointerException();
        return StringCoding.encode(charset, coder(), value);
     }

    
    public byte[] getBytes() {
        return StringCoding.encode(coder(), value);
    }

    
    /**
     * 重新equals，比较地址或字符串内容
     */
    public boolean equals(Object anObject) {
        if (this == anObject) {
            return true;
        }
        if (anObject instanceof String) {
            String aString = (String)anObject;
            if (!COMPACT_STRINGS || this.coder == aString.coder) {
                return StringLatin1.equals(value, aString.value);
            }
        }
        return false;
    }

    /**
     * 与equals类似，比较内容
     * equals方法只有在另一个对象是String的情况下才可能返回true
     * contentEquals只要求另一个对象是CharSequence或其子类的对象
     */
    public boolean contentEquals(StringBuffer sb) {
        return contentEquals((CharSequence)sb);
    }

    /**
     * 私有方法
     * 非同步方式（线程不安全）比较与 AbstractStringBuilder 是否相等，
     */
    private boolean nonSyncContentEquals(AbstractStringBuilder sb) {
        int len = length();
        if (len != sb.length()) {
            return false;
        }
        byte v1[] = value;
        byte v2[] = sb.getValue();
        byte coder = coder();
        if (coder == sb.getCoder()) {
            int n = v1.length;
            for (int i = 0; i < n; i++) {
                if (v1[i] != v2[i]) {
                    return false;
                }
            }
        } else {
            if (coder != LATIN1) {  // utf16 str and latin1 abs can never be "equal"
                return false;
            }
            return StringUTF16.contentEquals(v1, v2, len);
        }
        return true;
    }

    
    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
        int n = cs.length();
        if (n != length()) {
            return false;
        }
        byte[] val = this.value;
        if (isLatin1()) {
            for (int i = 0; i < n; i++) {
                if ((val[i] & 0xff) != cs.charAt(i)) {
                    return false;
                }
            }
        } else {
            if (!StringUTF16.contentEquals(val, cs, n)) {
                return false;
            }
        }
        return true;
    }

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

    /**
     * Comparable接口的比较方法
     */
    public int compareTo(String anotherString) {
        byte v1[] = value;
        byte v2[] = anotherString.value;
        byte coder = coder();
        if (coder == anotherString.coder()) {
            return coder == LATIN1 ? StringLatin1.compareTo(v1, v2)
                                   : StringUTF16.compareTo(v1, v2);
        }
        return coder == LATIN1 ? StringLatin1.compareToUTF16(v1, v2)
                               : StringUTF16.compareToLatin1(v1, v2);
     }

    
    public static final Comparator<String> CASE_INSENSITIVE_ORDER
                                         = new CaseInsensitiveComparator();
    
    /**
     * 内部类 
     */
    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) {
            byte v1[] = s1.value;
            byte v2[] = s2.value;
            byte coder = s1.coder();
            if (coder == s2.coder()) {
                return coder == LATIN1 ? StringLatin1.compareToCI(v1, v2)
                                       : StringUTF16.compareToCI(v1, v2);
            }
            return coder == LATIN1 ? StringLatin1.compareToCI_UTF16(v1, v2)
                                   : StringUTF16.compareToCI_Latin1(v1, v2);
        }

        /** Replaces the de-serialized object. */
        /**
         * 防止反射破坏单例
         */
        private Object readResolve() { return CASE_INSENSITIVE_ORDER; }
    }


    /**
     * 忽略大小写比较
     */
    public int compareToIgnoreCase(String str) {
        return CASE_INSENSITIVE_ORDER.compare(this, str);
    }

    /**
     * 检测两个字符串在一个区域内是否相等
     */
    public boolean regionMatches(int toffset, String other, int ooffset, int len) {
        byte tv[] = value;
        byte ov[] = other.value;
        // Note: toffset, ooffset, or len might be near -1>>>1.
        if ((ooffset < 0) || (toffset < 0) ||
             (toffset > (long)length() - len) ||
             (ooffset > (long)other.length() - len)) {
            return false;
        }
        byte coder = coder();
        if (coder == other.coder()) {
            if (!isLatin1() && (len > 0)) {
                toffset = toffset << 1;
                ooffset = ooffset << 1;
                len = len << 1;
            }
            while (len-- > 0) {
                if (tv[toffset++] != ov[ooffset++]) {
                    return false;
                }
            }
        } else {
            if (coder == LATIN1) {
                while (len-- > 0) {
                    if (StringLatin1.getChar(tv, toffset++) !=
                        StringUTF16.getChar(ov, ooffset++)) {
                        return false;
                    }
                }
            } else {
                while (len-- > 0) {
                    if (StringUTF16.getChar(tv, toffset++) !=
                        StringLatin1.getChar(ov, ooffset++)) {
                        return false;
                    }
                }
            }
        }
        return true;
    }

    
    /**
     * 检测两个字符串在一个区域内是否相等（忽略大小写）
     */
    public boolean regionMatches(boolean ignoreCase, int toffset,
            String other, int ooffset, int len) {
        if (!ignoreCase) {
            return regionMatches(toffset, other, ooffset, len);
        }
        // Note: toffset, ooffset, or len might be near -1>>>1.
        if ((ooffset < 0) || (toffset < 0)
                || (toffset > (long)length() - len)
                || (ooffset > (long)other.length() - len)) {
            return false;
        }
        byte tv[] = value;
        byte ov[] = other.value;
        byte coder = coder();
        if (coder == other.coder()) {
            return coder == LATIN1
              ? StringLatin1.regionMatchesCI(tv, toffset, ov, ooffset, len)
              : StringUTF16.regionMatchesCI(tv, toffset, ov, ooffset, len);
        }
        return coder == LATIN1
              ? StringLatin1.regionMatchesCI_UTF16(tv, toffset, ov, ooffset, len)
              : StringUTF16.regionMatchesCI_Latin1(tv, toffset, ov, ooffset, len);
    }

    
    /**
     * 是否以某字符串开头，可进行偏移操作
     */
    public boolean startsWith(String prefix, int toffset) {
        // Note: toffset might be near -1>>>1.
        if (toffset < 0 || toffset > length() - prefix.length()) {
            return false;
        }
        byte ta[] = value;
        byte pa[] = prefix.value;
        int po = 0;
        int pc = pa.length;
        byte coder = coder();
        if (coder == prefix.coder()) {
            int to = (coder == LATIN1) ? toffset : toffset << 1;
            while (po < pc) {
                if (ta[to++] != pa[po++]) {
                    return false;
                }
            }
        } else {
            if (coder == LATIN1) {  // && pcoder == UTF16
                return false;
            }
            // coder == UTF16 && pcoder == LATIN1)
            while (po < pc) {
                if (StringUTF16.getChar(ta, toffset++) != (pa[po++] & 0xff)) {
                    return false;
               }
            }
        }
        return true;
    }

    
    public boolean startsWith(String prefix) {
        return startsWith(prefix, 0);
    }

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

   
    public int hashCode() {
        // The hash or hashIsZero fields are subject to a benign data race,
        // making it crucial to ensure that any observable result of the
        // calculation in this method stays correct under any possible read of
        // these fields. Necessary restrictions to allow this to be correct
        // without explicit memory fences or similar concurrency primitives is
        // that we can ever only write to one of these two fields for a given
        // String instance, and that the computation is idempotent and derived
        // from immutable state
        int h = hash;
        if (h == 0 && !hashIsZero) {
            h = isLatin1() ? StringLatin1.hashCode(value)
                           : StringUTF16.hashCode(value);
            if (h == 0) {
                hashIsZero = true;
            } else {
                hash = h;
            }
        }
        return h;
    }

    
    /**
     * ch是ASCII码
     */
    public int indexOf(int ch) {
        return indexOf(ch, 0);
    }

    
    public int indexOf(int ch, int fromIndex) {
        return isLatin1() ? StringLatin1.indexOf(value, ch, fromIndex)
                          : StringUTF16.indexOf(value, ch, fromIndex);
    }

    
    public int lastIndexOf(int ch) {
        return lastIndexOf(ch, length() - 1);
    }

    
    public int lastIndexOf(int ch, int fromIndex) {
        return isLatin1() ? StringLatin1.lastIndexOf(value, ch, fromIndex)
                          : StringUTF16.lastIndexOf(value, ch, fromIndex);
    }

    
    public int indexOf(String str) {
        byte coder = coder();
        if (coder == str.coder()) {
            return isLatin1() ? StringLatin1.indexOf(value, str.value)
                              : StringUTF16.indexOf(value, str.value);
        }
        if (coder == LATIN1) {  // str.coder == UTF16
            return -1;
        }
        return StringUTF16.indexOfLatin1(value, str.value);
    }

    
    public int indexOf(String str, int fromIndex) {
        return indexOf(value, coder(), length(), str, fromIndex);
    }

    
    /**
     * [indexOf description]
     * @param  {[type]} byte[] src           代表源字符串的字节数组
     * @param  {[type]} byte   srcCoder      代表源字符串的编码方式
     * @param  {[type]} int    srcCount      代表源字符串的长度
     * @param  {[type]} String tgtStr        代表子串
     * @param  {[type]} int    fromIndex     代表源字符串开始查找的下标位置
     * @return {[type]}        [description]
     */
    static int indexOf(byte[] src, byte srcCoder, int srcCount,
                       String tgtStr, int fromIndex) {
        byte[] tgt    = tgtStr.value;
        byte tgtCoder = tgtStr.coder();
        int tgtCount  = tgtStr.length();

        if (fromIndex >= srcCount) {
            return (tgtCount == 0 ? srcCount : -1);
        }
        if (fromIndex < 0) {
            fromIndex = 0;
        }
        if (tgtCount == 0) {
            return fromIndex;
        }
        if (tgtCount > srcCount) {
            return -1;
        }
        if (srcCoder == tgtCoder) {
            return srcCoder == LATIN1
                ? StringLatin1.indexOf(src, srcCount, tgt, tgtCount, fromIndex)
                : StringUTF16.indexOf(src, srcCount, tgt, tgtCount, fromIndex);
        }
        if (srcCoder == LATIN1) {    //  && tgtCoder == UTF16
            return -1;
        }
        // srcCoder == UTF16 && tgtCoder == LATIN1) {
        return StringUTF16.indexOfLatin1(src, srcCount, tgt, tgtCount, fromIndex);
    }

    
    public int lastIndexOf(String str) {
        return lastIndexOf(str, length());
    }

    
    public int lastIndexOf(String str, int fromIndex) {
        return lastIndexOf(value, coder(), length(), str, fromIndex);
    }

    
    static int lastIndexOf(byte[] src, byte srcCoder, int srcCount,
                           String tgtStr, int fromIndex) {
        byte[] tgt = tgtStr.value;
        byte tgtCoder = tgtStr.coder();
        int tgtCount = tgtStr.length();
        /*
         * Check arguments; return immediately where possible. For
         * consistency, don't check for null str.
         */
        int rightIndex = srcCount - tgtCount;
        if (fromIndex > rightIndex) {
            fromIndex = rightIndex;
        }
        if (fromIndex < 0) {
            return -1;
        }
        /* Empty string always matches. */
        if (tgtCount == 0) {
            return fromIndex;
        }
        if (srcCoder == tgtCoder) {
            return srcCoder == LATIN1
                ? StringLatin1.lastIndexOf(src, srcCount, tgt, tgtCount, fromIndex)
                : StringUTF16.lastIndexOf(src, srcCount, tgt, tgtCount, fromIndex);
        }
        if (srcCoder == LATIN1) {    // && tgtCoder == UTF16
            return -1;
        }
        // srcCoder == UTF16 && tgtCoder == LATIN1
        return StringUTF16.lastIndexOfLatin1(src, srcCount, tgt, tgtCount, fromIndex);
    }

    
    public String substring(int beginIndex) {
        if (beginIndex < 0) {
            throw new StringIndexOutOfBoundsException(beginIndex);
        }
        int subLen = length() - beginIndex;
        if (subLen < 0) {
            throw new StringIndexOutOfBoundsException(subLen);
        }
        if (beginIndex == 0) {
            return this;
        }
        return isLatin1() ? StringLatin1.newString(value, beginIndex, subLen)
                          : StringUTF16.newString(value, beginIndex, subLen);
    }

    
    public String substring(int beginIndex, int endIndex) {
        int length = length();
        checkBoundsBeginEnd(beginIndex, endIndex, length);
        int subLen = endIndex - beginIndex;
        if (beginIndex == 0 && endIndex == length) {
            return this;
        }
        return isLatin1() ? StringLatin1.newString(value, beginIndex, subLen)
                          : StringUTF16.newString(value, beginIndex, subLen);
    }

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

    
    public String concat(String str) {
        if (str.isEmpty()) {
            return this;
        }
        return StringConcatHelper.simpleConcat(this, str);
    }

    
    public String replace(char oldChar, char newChar) {
        if (oldChar != newChar) {
            String ret = isLatin1() ? StringLatin1.replace(value, oldChar, newChar)
                                    : StringUTF16.replace(value, oldChar, newChar);
            if (ret != null) {
                return ret;
            }
        }
        return this;
    }

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

   
    public boolean contains(CharSequence s) {
        return indexOf(s.toString()) >= 0;
    }

    
    public String replaceFirst(String regex, String replacement) {
        return Pattern.compile(regex).matcher(this).replaceFirst(replacement);
    }

    
    public String replaceAll(String regex, String replacement) {
        return Pattern.compile(regex).matcher(this).replaceAll(replacement);
    }

    
    public String replace(CharSequence target, CharSequence replacement) {
        String trgtStr = target.toString();
        String replStr = replacement.toString();
        int thisLen = length();
        int trgtLen = trgtStr.length();
        int replLen = replStr.length();

        if (trgtLen > 0) {
            if (trgtLen == 1 && replLen == 1) {
                return replace(trgtStr.charAt(0), replStr.charAt(0));
            }

            boolean thisIsLatin1 = this.isLatin1();
            boolean trgtIsLatin1 = trgtStr.isLatin1();
            boolean replIsLatin1 = replStr.isLatin1();
            String ret = (thisIsLatin1 && trgtIsLatin1 && replIsLatin1)
                    ? StringLatin1.replace(value, thisLen,
                                           trgtStr.value, trgtLen,
                                           replStr.value, replLen)
                    : StringUTF16.replace(value, thisLen, thisIsLatin1,
                                          trgtStr.value, trgtLen, trgtIsLatin1,
                                          replStr.value, replLen, replIsLatin1);
            if (ret != null) {
                return ret;
            }
            return this;

        } else { // trgtLen == 0
            int resultLen;
            try {
                resultLen = Math.addExact(thisLen, Math.multiplyExact(
                        Math.addExact(thisLen, 1), replLen));
            } catch (ArithmeticException ignored) {
                throw new OutOfMemoryError();
            }

            StringBuilder sb = new StringBuilder(resultLen);
            sb.append(replStr);
            for (int i = 0; i < thisLen; ++i) {
                sb.append(charAt(i)).append(replStr);
            }
            return sb.toString();
        }
    }

    
    /**
     * limit为非正整数表示 模式被应用尽可能多的次数 比如-1
     * limit =0 表示模式应用尽可能多的次数，数组可以是任意长度，并且结尾空字符串将被丢弃
     * limit>0时 那么模式将会应用limit-1次 数组长度不会超过limit
     */
    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 ".$|()[{^?*+\\", or
         (2)two-char String and the first char is the backslash and
            the second is not the ascii digit or ascii letter.
         */
        char ch = 0;
        if (((regex.length() == 1 &&
             ".$|()[{^?*+\\".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);
                    int last = length();
                    list.add(substring(off, last));
                    off = last;
                    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, length()));

            // Construct result
            int resultSize = list.size();
            if (limit == 0) {
                while (resultSize > 0 && list.get(resultSize - 1).isEmpty()) {
                    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);
    }

    /**
     * [join description]
     * @param  {[type]} CharSequence    delimiter     分隔符
     * @param  {[type]} CharSequence... elements      需要连接的字符数组
     * @return {[type]}                 [description]
     */
    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();
    }

    
    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();
    }

    /**
     * 将字符串转换为小写
     * @param  {[type]} Locale locale        转换规则
     * @return {[type]}        [description]
     */
    public String toLowerCase(Locale locale) {
        return isLatin1() ? StringLatin1.toLowerCase(this, value, locale)
                          : StringUTF16.toLowerCase(this, value, locale);
    }

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

    
    public String toUpperCase(Locale locale) {
        return isLatin1() ? StringLatin1.toUpperCase(this, value, locale)
                          : StringUTF16.toUpperCase(this, value, locale);
    }

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

    
    /**
     * 移除字符串两侧的空白字符(空格、tab键、换行符)
     * @return {[type]} [description]
     */
    public String trim() {
        String ret = isLatin1() ? StringLatin1.trim(value)
                                : StringUTF16.trim(value);
        return ret == null ? this : ret;
    }

    
    /**
     * 去掉字符串的开始和结尾的空白字符
     * 适用于字符首尾空白是Unicode空白字符的情况
     * Character.isWhitespace(c)可以判断是否为Unicode空白字符'\u2000'。
     * @return {[type]} [description]
     */
    public String strip() {
        String ret = isLatin1() ? StringLatin1.strip(value)
                                : StringUTF16.strip(value);
        return ret == null ? this : ret;
    }

    
    /**
     * 去掉头部空格
     * @return {[type]} [description]
     */
    public String stripLeading() {
        String ret = isLatin1() ? StringLatin1.stripLeading(value)
                                : StringUTF16.stripLeading(value);
        return ret == null ? this : ret;
    }

    
    /**
     * 去掉尾部空格
     * @return {[type]} [description]
     */
    public String stripTrailing() {
        String ret = isLatin1() ? StringLatin1.stripTrailing(value)
                                : StringUTF16.stripTrailing(value);
        return ret == null ? this : ret;
    }

    
    /**
     * 字符串为空或只包含空格
     * @return {Boolean} [description]
     */
    public boolean isBlank() {
        return indexOfNonWhitespace() == length();
    }

    
    /**
     * 返回一个字符串 Stream, 可以识别 \n 和 \r 换行符换行
     * XX.lines().count() 可以计算行数
     * @return {[type]} [description]
     */
    public Stream<String> lines() {
        return isLatin1() ? StringLatin1.lines(value) : StringUTF16.lines(value);
    }

    /**
     * 控制每行开头增加或减少几个空格，n为正数时增加，n为负数时减少
     * @param  {[type]} int n             [description]
     * @return {[type]}     [description]
     */
    public String indent(int n) {
        if (isEmpty()) {
            return "";
        }
        Stream<String> stream = lines();
        if (n > 0) {
            final String spaces = " ".repeat(n);
            stream = stream.map(s -> spaces + s);
        } else if (n == Integer.MIN_VALUE) {
            stream = stream.map(s -> s.stripLeading());
        } else if (n < 0) {
            stream = stream.map(s -> s.substring(Math.min(-n, s.indexOfNonWhitespace())));
        }
        return stream.collect(Collectors.joining("\n", "", "\n"));
    }

    private int indexOfNonWhitespace() {
        return isLatin1() ? StringLatin1.indexOfNonWhitespace(value)
                          : StringUTF16.indexOfNonWhitespace(value);
    }

    private int lastIndexOfNonWhitespace() {
        return isLatin1() ? StringLatin1.lastIndexOfNonWhitespace(value)
                          : StringUTF16.lastIndexOfNonWhitespace(value);
    }


    /**
     * 不推荐使用
     */    
    @Deprecated(forRemoval=true, since="13")
    public String stripIndent() {
        int length = length();
        if (length == 0) {
            return "";
        }
        char lastChar = charAt(length - 1);
        boolean optOut = lastChar == '\n' || lastChar == '\r';
        List<String> lines = lines().collect(Collectors.toList());
        final int outdent = optOut ? 0 : outdent(lines);
        return lines.stream()
            .map(line -> {
                int firstNonWhitespace = line.indexOfNonWhitespace();
                int lastNonWhitespace = line.lastIndexOfNonWhitespace();
                int incidentalWhitespace = Math.min(outdent, firstNonWhitespace);
                return firstNonWhitespace > lastNonWhitespace
                    ? "" : line.substring(incidentalWhitespace, lastNonWhitespace);
            })
            .collect(Collectors.joining("\n", "", optOut ? "\n" : ""));
    }

    private static int outdent(List<String> lines) {
        // Note: outdent is guaranteed to be zero or positive number.
        // If there isn't a non-blank line then the last must be blank
        int outdent = Integer.MAX_VALUE;
        for (String line : lines) {
            int leadingWhitespace = line.indexOfNonWhitespace();
            if (leadingWhitespace != line.length()) {
                outdent = Integer.min(outdent, leadingWhitespace);
            }
        }
        String lastLine = lines.get(lines.size() - 1);
        if (lastLine.isBlank()) {
            outdent = Integer.min(outdent, lastLine.length());
        }
        return outdent;
    }

    /**
     * 不推荐使用
     */
    @Deprecated(forRemoval=true, since="13")
    public String translateEscapes() {
        if (isEmpty()) {
            return "";
        }
        char[] chars = toCharArray();
        int length = chars.length;
        int from = 0;
        int to = 0;
        while (from < length) {
            char ch = chars[from++];
            if (ch == '\\') {
                ch = from < length ? chars[from++] : '\0';
                switch (ch) {
                case 'b':
                    ch = '\b';
                    break;
                case 'f':
                    ch = '\f';
                    break;
                case 'n':
                    ch = '\n';
                    break;
                case 'r':
                    ch = '\r';
                    break;
                case 't':
                    ch = '\t';
                    break;
                case '\'':
                case '\"':
                case '\\':
                    // as is
                    break;
                case '0': case '1': case '2': case '3':
                case '4': case '5': case '6': case '7':
                    int limit = Integer.min(from + (ch <= '3' ? 2 : 1), length);
                    int code = ch - '0';
                    while (from < limit) {
                        ch = chars[from];
                        if (ch < '0' || '7' < ch) {
                            break;
                        }
                        from++;
                        code = (code << 3) | (ch - '0');
                    }
                    ch = (char)code;
                    break;
                default: {
                    String msg = String.format(
                        "Invalid escape sequence: \\%c \\\\u%04X",
                        ch, (int)ch);
                    throw new IllegalArgumentException(msg);
                }
                }
            }

            chars[to++] = ch;
        }

        return new String(chars, 0, to);
    }

    
    /**
     * 字符串转换
     * @param  {[type]} Function<? super         String, ? extends R> f  函数式接口 Function
     * @return {[type]}            [description]
     */
    public <R> R transform(Function<? super String, ? extends R> f) {
        return f.apply(this);
    }

    /**
     * This object (which is already a string!) is itself returned.
     *
     * @return  the string itself.
     */
    public String toString() {
        return this;
    }

    
    /**
     * 返回代表字符代码的int(IntStream)流
     */
    @Override
    public IntStream chars() {
        return StreamSupport.intStream(
            isLatin1() ? new StringLatin1.CharsSpliterator(value, Spliterator.IMMUTABLE)
                       : new StringUTF16.CharsSpliterator(value, Spliterator.IMMUTABLE),
            false);
    }


    /**
     *  返回所有代码点值的Stream
     */
    @Override
    public IntStream codePoints() {
        return StreamSupport.intStream(
            isLatin1() ? new StringLatin1.CharsSpliterator(value, Spliterator.IMMUTABLE)
                       : new StringUTF16.CodePointsSpliterator(value, Spliterator.IMMUTABLE),
            false);
    }

    
    public char[] toCharArray() {
        return isLatin1() ? StringLatin1.toChars(value)
                          : StringUTF16.toChars(value);
    }

    
    /**
     * 示例：String.format("你可以成为%s，他也可以成为%s","平凡的人","不平凡的人")  ------> 
     *      你可以成为平凡的人，他也可以成为不平凡的人  
     * @param  {[type]} String    format        [description]
     * @param  {[type]} Object... args          [description]
     * @return {[type]}           [description]
     */
    public static String format(String format, Object... args) {
        return new Formatter().format(format, args).toString();
    }

    
    public static String format(Locale l, String format, Object... args) {
        return new Formatter(l).format(format, args).toString();
    }

    /**
     * 不推荐使用
     */
    @Deprecated(forRemoval=true, since="13")
    public String formatted(Object... args) {
        return new Formatter().format(this, args).toString();
    }

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

    
    public static String valueOf(char data[]) {
        return new String(data);
    }

    
    public static String valueOf(char data[], int offset, int count) {
        return new String(data, offset, count);
    }

    
    public static String copyValueOf(char data[], int offset, int count) {
        return new String(data, offset, count);
    }

    
    public static String copyValueOf(char data[]) {
        return new String(data);
    }

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

    
    public static String valueOf(char c) {
        if (COMPACT_STRINGS && StringLatin1.canEncode(c)) {
            return new String(StringLatin1.toBytes(c), LATIN1);
        }
        return new String(StringUTF16.toBytes(c), UTF16);
    }

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

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

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

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

    
    public native String intern();

    
    /**
     * 示例 String str = "abc";
     *      String repeated = str.repeat(3);
     *      repeated.equals("abcabcabc") = true;
     * 重复字符串
     * @param  {[type]} int count         [description]
     * @return {[type]}     [description]
     */
    public String repeat(int count) {
        if (count < 0) {
            throw new IllegalArgumentException("count is negative: " + count);
        }
        if (count == 1) {
            return this;
        }
        final int len = value.length;
        if (len == 0 || count == 0) {
            return "";
        }
        if (len == 1) {
            final byte[] single = new byte[count];
            Arrays.fill(single, value[0]);
            return new String(single, coder);
        }
        if (Integer.MAX_VALUE / count < len) {
            throw new OutOfMemoryError("Repeating " + len + " bytes String " + count +
                    " times will produce a String exceeding maximum size.");
        }
        final int limit = len * count;
        final byte[] multiple = new byte[limit];
        System.arraycopy(value, 0, multiple, 0, len);
        int copied = len;
        for (; copied < limit - copied; copied <<= 1) {
            System.arraycopy(multiple, 0, multiple, copied, copied);
        }
        System.arraycopy(multiple, 0, multiple, copied, limit - copied);
        return new String(multiple, coder);
    }

    
    void getBytes(byte dst[], int dstBegin, byte coder) {
        if (coder() == coder) {
            System.arraycopy(value, 0, dst, dstBegin << coder, value.length);
        } else {    // this.coder == LATIN && coder == UTF16
            StringLatin1.inflate(value, 0, dst, dstBegin, value.length);
        }
    }

    
    String(char[] value, int off, int len, Void sig) {
        if (len == 0) {
            this.value = "".value;
            this.coder = "".coder;
            return;
        }
        if (COMPACT_STRINGS) {
            byte[] val = StringUTF16.compress(value, off, len);
            if (val != null) {
                this.value = val;
                this.coder = LATIN1;
                return;
            }
        }
        this.coder = UTF16;
        this.value = StringUTF16.toBytes(value, off, len);
    }

    
    String(AbstractStringBuilder asb, Void sig) {
        byte[] val = asb.getValue();
        int length = asb.length();
        if (asb.isLatin1()) {
            this.coder = LATIN1;
            this.value = Arrays.copyOfRange(val, 0, length);
        } else {
            if (COMPACT_STRINGS) {
                byte[] buf = StringUTF16.compress(val, 0, length);
                if (buf != null) {
                    this.coder = LATIN1;
                    this.value = buf;
                    return;
                }
            }
            this.coder = UTF16;
            this.value = Arrays.copyOfRange(val, 0, length << 1);
        }
    }

   
    String(byte[] value, byte coder) {
        this.value = value;
        this.coder = coder;
    }

    byte coder() {
        return COMPACT_STRINGS ? coder : UTF16;
    }

    byte[] value() {
        return value;
    }

    boolean isLatin1() {
        return COMPACT_STRINGS && coder == LATIN1;
    }

    @Native static final byte LATIN1 = 0;
    @Native static final byte UTF16  = 1;

    
    /**
     * 检查越界
     * @param  {[type]} int index         [description]
     * @param  {[type]} int length        [description]
     * @return {[type]}     [description]
     */
    static void checkIndex(int index, int length) {
        if (index < 0 || index >= length) {
            throw new StringIndexOutOfBoundsException("index " + index +
                                                      ",length " + length);
        }
    }

    
    static void checkOffset(int offset, int length) {
        if (offset < 0 || offset > length) {
            throw new StringIndexOutOfBoundsException("offset " + offset +
                                                      ",length " + length);
        }
    }

    
    /**
     * 检查是否越界
     */
    static void checkBoundsOffCount(int offset, int count, int length) {
        if (offset < 0 || count < 0 || offset > length - count) {
            throw new StringIndexOutOfBoundsException(
                "offset " + offset + ", count " + count + ", length " + length);
        }
    }

    
    static void checkBoundsBeginEnd(int begin, int end, int length) {
        if (begin < 0 || begin > end || end > length) {
            throw new StringIndexOutOfBoundsException(
                "begin " + begin + ", end " + end + ", length " + length);
        }
    }

    
    static String valueOfCodePoint(int codePoint) {
        if (COMPACT_STRINGS && StringLatin1.canEncode(codePoint)) {
            return new String(StringLatin1.toBytes((char)codePoint), LATIN1);
        } else if (Character.isBmpCodePoint(codePoint)) {
            return new String(StringUTF16.toBytes((char)codePoint), UTF16);
        } else if (Character.isSupplementaryCodePoint(codePoint)) {
            return new String(StringUTF16.toBytesSupplementary(codePoint), UTF16);
        }

        throw new IllegalArgumentException(
            format("Not a valid Unicode code point: 0x%X", codePoint));
    }

    /**
     * 来自接口Constable，这类型的值是常量，可以在JVMS 4.4常量池中定义
     */
    @Override
    public Optional<String> describeConstable() {
        return Optional.of(this);
    }

    
    /**
     * 来自接口ConstantDesc
     */
    @Override
    public String resolveConstantDesc(MethodHandles.Lookup lookup) {
        return this;
    }

}