读String源码

//先看构造函数
public String() {
        this.value = new char[0];
    }


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

public String(char value[]) {
        this.value = Arrays.copyOf(value, value.length);
    }

//从offset位置开始复制count个char元素
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.
	//如果数组不够长
        if (offset > value.length - count) {
            throw new StringIndexOutOfBoundsException(offset + count);
        }
        this.value = Arrays.copyOfRange(value, offset, offset+count);
    }


 public String(int[] codePoints, int offset, int count) {
        if (offset < 0) {
            throw new StringIndexOutOfBoundsException(offset);
        }
        if (count < 0) {
            throw new StringIndexOutOfBoundsException(count);
        }
        //如果超界了
        if (offset > codePoints.length - count) {
            throw new StringIndexOutOfBoundsException(offset + count);
        }

        final int end = offset + count;

        int n = count;
        for (int i = offset; i < end; i++) {
            int c = codePoints[i];
	    //确定是否在BMP代码点
            if (Character.isBmpCodePoint(c))
                continue;
	    //确定是否是有效的Unicode代码点
            else if (Character.isValidCodePoint(c))
                n++;
            else throw new IllegalArgumentException(Integer.toString(c));
        }

        
        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;
    }


 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);
    }

//检查是否超出界限
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);
    }

static char[] decode(String charsetName, byte[] ba, int off, int len)
        throws UnsupportedEncodingException
    {
        StringDecoder sd = deref(decoder);
        String csn = (charsetName == null) ? "ISO-8859-1" : charsetName;
        if ((sd == null) || !(csn.equals(sd.requestedCharsetName())
                              || csn.equals(sd.charsetName()))) {
            sd = null;
            try {
                Charset cs = lookupCharset(csn);
                if (cs != null)
                    sd = new StringDecoder(cs, csn);
            } catch (IllegalCharsetNameException x) {}
            if (sd == null)
                throw new UnsupportedEncodingException(csn);
            set(decoder, sd);
        }
        return sd.decode(ba, off, len);
    }

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);
    }
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);
    }

 public String(StringBuffer buffer) {
      //为了保证StringBuffer的线程安全性语义
        synchronized(buffer) {
            this.value = Arrays.copyOf(buffer.getValue(), buffer.length());
        }
    }

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

//返回String的长度
 public int length() {
        return value.length;
    }

//是否为空
public boolean isEmpty() {
        return value.length == 0;
    }

//获取指定位置上的char
public char charAt(int index) {
        if ((index < 0) || (index >= value.length)) {
            throw new StringIndexOutOfBoundsException(index);
        }
        return value[index];
    }


//返回指定位置的代码点
public int codePointAt(int index) {
        if ((index < 0) || (index >= value.length)) {
            throw new StringIndexOutOfBoundsException(index);
        }
        return Character.codePointAtImpl(value, index, value.length);
    }

//返回指定位置之前的代码点
 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);
    }

//返回指定范围的代码点点数
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);
    }

//返回从指定位置瞬移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);
    }


//复制到指定字符数组
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);
    }

void getChars(char dst[], int dstBegin) {
        System.arraycopy(value, 0, dst, dstBegin, value.length);
    }

//转成byte序列并封装到数组中
 public byte[] getBytes(String charsetName)
            throws UnsupportedEncodingException {
        if (charsetName == null) throw new NullPointerException();
        return StringCoding.encode(charsetName, value, 0, value.length);
    }

//转成byte序列并封装到数组中
public byte[] getBytes(Charset charset) {
        if (charset == null) throw new NullPointerException();
        return StringCoding.encode(charset, value, 0, value.length);
    }

//使用默认字符集编码
public byte[] getBytes() {
        return StringCoding.encode(value, 0, value.length);
    }

//判断元素是否相等
 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;
    }


//是否和sb相等
public boolean contentEquals(StringBuffer sb) {
        synchronized (sb) {
            return contentEquals((CharSequence) sb);
        }
    }

//和传入的CharSequence比较
public boolean contentEquals(CharSequence cs) {
        if (value.length != cs.length())
            return false;
        // Argument is a StringBuffer, StringBuilder
        if (cs instanceof AbstractStringBuilder) {
            char v1[] = value;
            char v2[] = ((AbstractStringBuilder) cs).getValue();
            int i = 0;
            int n = value.length;
            while (n-- != 0) {
                if (v1[i] != v2[i])
                    return false;
                i++;
            }
            return true;
        }
        // Argument is a String
        if (cs.equals(this))
            return true;
        // Argument is a generic CharSequence
        char v1[] = value;
        int i = 0;
        int n = value.length;
        while (n-- != 0) {
            if (v1[i] != cs.charAt(i))
                return false;
            i++;
        }
        return true;
    }

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

//比较部分区域是否相等
 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;
                }
                
                if (Character.toLowerCase(u1) == Character.toLowerCase(u2)) {
                    continue;
                }
            }
            return false;
        }
        return true;
    }

//比较
public int compareTo(String anotherString) {
        int len1 = value.length;
        int len2 = anotherString.value.length;
	//获取比较小的长度作为下界
        int lim = Math.min(len1, len2);
        char v1[] = value;
        char v2[] = anotherString.value;

        int k = 0;
        while (k < lim) {
            char c1 = v1[k];
            char c2 = v2[k];
            if (c1 != c2) {
                return c1 - c2;
            }
            k++;
        }
        return len1 - len2;
    }

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

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;
        }

//测试两个字符串区域是否相等
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;
    }

//是否以某个前缀开始
public boolean startsWith(String prefix) {
        return startsWith(prefix, 0);
    }

//从指定位置是否已制定前缀开始
public boolean startsWith(String prefix, int toffset) {
        char ta[] = value;
        int to = toffset;
        char pa[] = prefix.value;
        int po = 0;
        int pc = prefix.value.length;
        // 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;
    }

//是否是从指定的后缀结束
public boolean endsWith(String suffix) {
        return startsWith(suffix, value.length - suffix.value.length);
    }


//返回指定的字符第一次出现的索引
public int indexOf(int ch) {
        return indexOf(ch, 0);
    }


//从指定的索引开始搜索字符第一次出现的索引
public int indexOf(int ch, int fromIndex) {
        final int max = value.length;
        if (fromIndex < 0) {
            fromIndex = 0;
        } else if (fromIndex >= max) {
            
            return -1;
        }

        if (ch < Character.MIN_SUPPLEMENTARY_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);
        }
    }


//搜索字符最后一次出现的索引
public int lastIndexOf(int ch) {
        return lastIndexOf(ch, value.length - 1);
    }


//从指定的字符位置反向搜索字符出现的位置
 public int lastIndexOf(int ch, int fromIndex) {
        if (ch < Character.MIN_SUPPLEMENTARY_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);
        }
    }


//搜索str第一次出现的位置
public int indexOf(String str) {
        return indexOf(str, 0);
    }

//从fromIndex位置搜索str第一次出现的位置
public int indexOf(String str, int fromIndex) {
        return indexOf(value, 0, value.length,
                str.value, 0, str.value.length, fromIndex);
    }

 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];
	//最大循环次数
        int max = sourceOffset + (sourceCount - targetCount);

        for (int i = sourceOffset + fromIndex; i <= max; i++) {
            //找到第一个相等的位置
            if (source[i] != first) {
                while (++i <= max && source[i] != first);
            }

            //找到了第一个
            if (i <= max) {
                int j = i + 1;
		//计算最大循环到的角标
                int end = j + targetCount - 1;
                for (int k = targetOffset + 1; j < end && source[j]
                        == target[k]; j++, k++);
               //所有的都相等
                if (j == end) {
                    //找到的i-偏移量为最后的角标
                    return i - sourceOffset;
                }
            }
        }
        return -1;
    }


//搜索字符串最后一次出现的索引
public int lastIndexOf(String str) {
        return lastIndexOf(str, value.length);
    }

//从指定位置反向搜索字符串最后一次出现的位置
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,
            char[] target, int targetOffset, int targetCount,
            int fromIndex) {
        //最右边的位置
        int rightIndex = sourceCount - targetCount;
        if (fromIndex < 0) {
            return -1;
        }
        if (fromIndex > rightIndex) {
            fromIndex = rightIndex;
        }
        //空字符串一定包含
        if (targetCount == 0) {
            return fromIndex;
        }
        //找到target最后面的index
        int strLastIndex = targetOffset + targetCount - 1;
	//找到target的最后一位char
        char strLastChar = target[strLastIndex];
	//能够循环到的最小的角标
        int min = sourceOffset + targetCount - 1;
	//从哪里开始循环
        int i = min + fromIndex;

        startSearchForLastChar:
        while (true) {
	    //从后面开始找找到和最后面一位相等的char
            while (i >= min && source[i] != strLastChar) {
                i--;
            }
	    //超界了
            if (i < min) {
                return -1;
            }
	    //source开始索引
            int j = i - 1;
	    //source最左边循环到的边界
            int start = j - (targetCount - 1);
            int k = strLastIndex - 1;

            while (j > start) {
                if (source[j--] != target[k--]) {
                    i--;
                    continue startSearchForLastChar;
                }
            }
            return start - sourceOffset + 1;
        }
    }

//返回从指定位置开始的新字符串
 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);
    }


//返回从指定位置开始到结束的字符串
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);
    }


 //返回从开始位置到结束位置的CharSequence
public CharSequence subSequence(int beginIndex, int endIndex) {
        return this.substring(beginIndex, endIndex);
    }


//将指定字符串连接到此字符串的末尾
public String concat(String str) {
        int otherLen = str.length();
        if (otherLen == 0) {
            return this;
        }
        int len = value.length;
	//复制到buf数组
        char buf[] = Arrays.copyOf(value, len + otherLen);
	//将str复制到buf中
        str.getChars(buf, len);
        return new String(buf, true);
    }

//复制数组
void getChars(char dst[], int dstBegin) {
        System.arraycopy(value, 0, dst, dstBegin, value.length);
    }

//用新值替换旧值
public String replace(char oldChar, char newChar) {
        if (oldChar != newChar) {
            int len = value.length;
            int i = -1;
            char[] val = value; /* avoid getfield opcode */
            //找到oldChar的位置
            while (++i < len) {
                if (val[i] == oldChar) {
                    break;
                }
            }
            if (i < len) {
                char buf[] = new char[len];
		//复制前半部分
                for (int j = 0; j < i; j++) {
                    buf[j] = val[j];
                }
		//复制后半部分
                while (i < len) {
                    char c = val[i];
                    buf[i] = (c == oldChar) ? newChar : c;
                    i++;
                }
                return new String(buf, true);
            }
        }
        return this;
    }

//字符串是否匹配给定的正则表达式
public boolean matches(String regex) {
        return Pattern.matches(regex, this);
    }

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

//使用给定的正则表达式替换第一个字符串
public String replaceFirst(String regex, String replacement) {
        return Pattern.compile(regex).matcher(this).replaceFirst(replacement);
    }

//使用给定的正则表达式替换所有字符串
 public String replaceAll(String regex, String replacement) {
        return Pattern.compile(regex).matcher(this).replaceAll(replacement);
    }

//使用指定的字符串替换
public String replace(CharSequence target, CharSequence replacement) {
        return Pattern.compile(target.toString(), Pattern.LITERAL).matcher(
                this).replaceAll(Matcher.quoteReplacement(replacement.toString()));
    }

//使用给定的正则表达式拆分字符串
public String[] split(String regex) {
        return split(regex, 0);
    }

//忽略空白
  public String trim() {
        int len = value.length;
        int st = 0;
        char[] val = value;    /* avoid getfield opcode */
        //找到前面有空格的最大index
        while ((st < len) && (val[st] <= ' ')) {
            st++;
        }
	 //找到后面有空格的最大index
        while ((st < len) && (val[len - 1] <= ' ')) {
            len--;
        }
        return ((st > 0) || (len < value.length)) ? substring(st, len) : this;
    }

//复制为char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;
    }


//使用指定的字符串格式化字符串
public static String format(String format, Object... args) {
        return new Formatter().format(format, args).toString();
    }

//返回Object的字符串表示形式
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);
    }

//复制数据返回一个String
 public static String copyValueOf(char data[], int offset, int count) {
        // All public String constructors now copy the data.
        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) {
        char data[] = {c};
        return new String(data, true);
    }

 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);
    }

//返回字符串在常量池中的引用
 public native String intern();