java.lang.*中String类源代码详解

红太狼 2023-02-10 01:21 9阅读 0赞

### java.lang.\*中String类 源代码详解 ###

*   *  核心方法
     *  String()构造函数
     *  checkBounds()私有方法
     *  indexOf方法
     *  equals方法
     *  hashCode方法
     *  charAt方法
     *  replace方法
     *  replaceAll方法
     *  replaceFirst方法
     *  trim方法
     *  split方法
     *  getBytes方法
     *  length方法
     *  toLowerCase方法
     *  toUpperCase方法
     *  substring方法
     *  compareTo()方法
     *  lastIndexOf()方法
     *  indexOf()方法
     *  regionMatches()方法
     *  contentEquals()方法
     *  startsWith()方法
     *  isEmpty()方法

## 核心方法 ##

> String()构造函数\[OK\]  
> void checkBounds()私有方法\[OK\]  
> int indexOf() 获取指定字符串的索引位置\[OK\]  
> boolean equals() 字符串比较\[OK\]  
> int hashCode() 获取字符串的hash值\[OK\]  
> char charAt() 获取指定字符的索引位置\[OK\]  
> String replace() 字符串的替代方法\[OK\]  
> String replaceAll() 字符串的替换方法\[OK\]  
> String replaceFirst() 字符串替换方法\[OK\]  
> String trim() 字符串的两端去空格\[OK\]  
> String\[\] split() 字符串的分割方法\[OK\]  
> byte\[\] getBytes() 字符串的字节数组\[OK\]  
> int length() 字符串的长度\[OK\]  
> String toLowerCase() 字符串字母转小写\[OK\]  
> String toUpperCase() 字符串字母转大写\[OK\]  
> String substring() 字符串截取\[OK\]  
> int compareTo() 比较两个字符串是否相等\[OK\]  
> int lastIndexOf() 反向检索字符的索引位置\[OK\]  
> int indexOf() 正向检索字符的索引位置\[OK\]  
> boolean regionMatches() 比较两个字符串的子串\[OK\]  
> boolean contentEquals() 比较两个字符串序列是否相等\[OK\]  
> boolean startsWith() 判断字符串是否以某个字符开头\[OK\]  
> boolean isEmpty() 判断字符串是否为空串\[OK\]

## String()构造函数 ##

/** * 空参构造函数,默认值为""空字符串 */
    public String() { 
        this.value = "".value;
    }
    
    /** * 带参构造函数 * 参数类型：字符串 * @param original * A {@code String} */
    public String(String original) { 
        this.value = original.value;
        this.hash = original.hash;
    }
    
    /** * 带参构造函数 * 参数类型：字符数组 */
    public String(char value[]) { 
        this.value = Arrays.copyOf(value, value.length);
    }
    
    /** * @param value 字符数组 * * @param offset 字符数组的起始位置 * * @param count 偏移量 * */
    public String(char value[], int offset, int count) { 
        if (offset < 0) { 
            throw new StringIndexOutOfBoundsException(offset);
        }
        if (count <= 0) { 
            if (count < 0) { 
                throw new StringIndexOutOfBoundsException(count);
            }
            if (offset <= value.length) { 
                this.value = "".value;
                return;
            }
        }
        // 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);
    }
    
    /** * * @param codePoints int数组 * @param offset 索引开始位置 * @param count 偏移量 * int[] n1 = {1,2,3,4,5,6}; * System.out.println(Arrays.toString(new String(n1, 2, 3).getBytes())); * * @since 1.5 */
    public String(int[] codePoints, int offset, int count) { 
    	//判断角标越界
        if (offset < 0) { 
            throw new StringIndexOutOfBoundsException(offset);
        }
        //如果偏移量为0,且offset小于等于int数组长度,返回""
        if (count <= 0) { 
            if (count < 0) { 
                throw new StringIndexOutOfBoundsException(count);
            }
            if (offset <= codePoints.length) { 
                this.value = "".value;
                return;
            }
        }
        //判断offset索引开始位置是否角标越界(个人认为这个要放在前面)
        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];
            //判断是否为16位常用字符集
            if (Character.isBmpCodePoint(c))
            	//将int数字强转为char字符
                v[j] = (char)c;
            else
                Character.toSurrogates(c, v, j++);
        }
    
        this.value = v;
    }
    
    /** * * @param bytes 字节数组 * @param offset 索引开始位置 * @param length 截图的字节数组长度 * @param charsetName 按照指定字符集进行解码 * * System.out.println(new String(b1,0,2,"UTF-8")); */
    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);
    }
    
    /** * @param bytes 字节数组 * @param offset 索引开始位置 * @param length 截图的字节数组长度 * @param charset 按照指定字符集进行解码 */
    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);
    }
    
    /** * @param bytes 字节数组 * @param charsetName 指定解码字符集 */
    public String(byte bytes[], String charsetName)
            throws UnsupportedEncodingException { 
        //调用同名构造函数
        this(bytes, 0, bytes.length, charsetName);
    }
    
    /** * * @param bytes 字节数组 * @param charset 字符集对象 * @since 1.6 */
    public String(byte bytes[], Charset charset) { 
        this(bytes, 0, bytes.length, charset);
    }
    
    /** * * @param bytes 字节数组 * @param offset 起始位置索引 * @param length 偏移量 * 采用默认字符集进行解码,如果未获取到文件字符集,则用"UTF-8" * @since JDK1.1 */
    public String(byte bytes[], int offset, int length) { 
        checkBounds(bytes, offset, length);
        this.value = StringCoding.decode(bytes, offset, length);
    }
    
    /** * @param bytes 字节数组 * 默认调用的是String(byte bytes[], int offset, int length)构造方法 * @since JDK1.1 */
    public String(byte bytes[]) { 
        this(bytes, 0, bytes.length);
    }
    
    /** * Allocates a new string that contains the sequence of characters * currently contained in the string buffer argument. The contents of the * string buffer are copied; subsequent modification of the string buffer * does not affect the newly created string. * * @param buffer * A {@code StringBuffer} */
    public String(StringBuffer buffer) { 
        synchronized(buffer) { 
            this.value = Arrays.copyOf(buffer.getValue(), buffer.length());
        }
    }
    
    /** * Allocates a new string that contains the sequence of characters * currently contained in the string builder argument. The contents of the * string builder are copied; subsequent modification of the string builder * does not affect the newly created string. * * <p> This constructor is provided to ease migration to {@code * StringBuilder}. Obtaining a string from a string builder via the {@code * toString} method is likely to run faster and is generally preferred. * * @param builder * A {@code StringBuilder} * * @since 1.5 */
    public String(StringBuilder builder) { 
        this.value = Arrays.copyOf(builder.getValue(), builder.length());
    }
    
    /* * Package private constructor which shares value array for speed. * this constructor is always expected to be called with share==true. * a separate constructor is needed because we already have a public * String(char[]) constructor that makes a copy of the given char[]. */
    String(char[] value, boolean share) { 
        // assert share : "unshared not supported";
        this.value = value;
    }

## checkBounds()私有方法 ##

/* * 私有内部方法，检测角标是否越界 */
    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);
    }

## indexOf方法 ##

/** * String s1 = "ab1cdefgb"; * s1.indexOf(99) * s1.indexOf('c') * 这两个方法都是执行的这个方法,其中99是ASCII的值 * 调用的是indexOf(ch, 0)方法 */
    public int indexOf(int ch) { 
        return indexOf(ch, 0);
    }
    
    /** * * @param ch 查找的字符 * @param fromIndex 索引开始位置 * String s1 = "ab1cdefgb"; * System.out.println(s1.indexOf(99,2)); * System.out.println(s1.indexOf('c',2)); * 如何不存在返回 -1 */
    public int indexOf(int ch, int fromIndex) { 
    	//获取当前检索对象本身的长度s1.length
    	//用final修饰,在编译时即可获取值,同时防止max被篡改
        final int max = value.length;
        //判断索引是否越界,如果索引位置超过max大小,直接返回-1
        //如果索引位置小于0,默认从0号位置进行检索
        if (fromIndex < 0) { 
            fromIndex = 0;
        } else if (fromIndex >= max) { 
            // Note: fromIndex might be near -1>>>1.
            return -1;
        }
    //Character.MIN_SUPPLEMENTARY_CODE_POINT
    //这个数代表十进制中62355,刚好是2个字节
        if (ch < Character.MIN_SUPPLEMENTARY_CODE_POINT) {   
            final char[] value = this.value;
            //从[fromIndex,max),在value中匹配对应的ch值
            //如果匹配上,返回i号位置的索引值
            for (int i = fromIndex; i < max; i++) { 
                if (value[i] == ch) { 
                    return i;
                }
            }
            //如果上面for循环遍历,没有匹配上,返回-1
            return -1;
        } else { 
            return indexOfSupplementary(ch, fromIndex);
        }
    }
    
    /** * Handles (rare) calls of indexOf with a supplementary character. */
    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;
    }

## equals方法 ##

/** * 比较两个对象的值是否相等 * @see #compareTo(String) * @see #equalsIgnoreCase(String) */
    public boolean equals(Object anObject) { 
    	//1.首先判断如果两个对象相等，直接返回true
        if (this == anObject) { 
            return true;
        }
        //判断anObject是否是Strnig对象
        if (anObject instanceof String) { 
            String anotherString = (String)anObject;
            //获取当前字符串对象的长度
            int n = value.length;
            //判断当前字符串的长度是否等于比较字符串对象
            //如果不等于直接返回false
            if (n == anotherString.value.length) { 
                char v1[] = value;
                char v2[] = anotherString.value;
                int i = 0;
                //循环n次，递减
                while (n-- != 0) { 
                	//比较两个字符数组i号位置的值是否相等，
                    if (v1[i] != v2[i])
                        return false;
                    i++;
                }
                return true;
            }
        }
        return false;
    }

## hashCode方法 ##

/** * 返回当前字符串对象的hash * 计算公式如下： * 1. s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1] * s[0]代表ASCII对应的int值,如a对应97,A对应65 * 2. 默认空字符串的hashCode值为0 * 3. 对于相同的String对象,第二次取hash值时,取得缓存的值,不会再次计算 * 4. 对于乘以31,这个考虑是,它是质数,只有1和自己是因子,减少hashCode重复而设计的,后续有考虑定更大的值 * 5. 样例计算：计算"123"的hashCode值 * 公式：s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1] * 49*31^2+50*31+51 = 48690 * @return 返回对象的hash值 */
    public int hashCode() { 
        int h = hash;
        //判断是否直接取缓存中的值
        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;
    }

## charAt方法 ##

/** * 返回指定索引位置的字符 * @param index 索引位置 * @return 返回字符 */
    public char charAt(int index) { 
    	//判断角标是否越界,一旦越界抛出角标越界异常
        if ((index < 0) || (index >= value.length)) { 
            throw new StringIndexOutOfBoundsException(index);
        }
        return value[index];
    }

## replace方法 ##

/** * 将所有匹配上的旧的字符,替换成新的字符 * 方法参数为char * 例子： * String t1 = "12345abcabcABC2"; * t1.replace('a','A') * @param oldChar 老的字符 * @param newChar 新的字符 * @return a string derived from this string by replacing every * occurrence of {@code oldChar} with {@code newChar}. */
    public String replace(char oldChar, char newChar) { 
    	//1.首先判断替换的oldChar是否等于newChar
        if (oldChar != newChar) { 
            int len = value.length;
            int i = -1;
            char[] val = value; /* avoid getfield opcode */
    
            while (++i < len) { //2.本次while循环是为了找到第一个匹配到的oldChar的位置,避免重复比较,一次到位
                if (val[i] == oldChar) { 
                    break;
                }
            }
            if (i < len) { //3.如果i<len不满足,说明没有找到需要替换的字符串,直接返回就好
                char buf[] = new char[len];//4.构造一个新的char数组
                for (int j = 0; j < i; j++) { //5.将第一命中的索引i,前面索引位置的数据,装进新的buf数组中
                    buf[j] = val[j];
                }
                while (i < len) { 
                    char c = val[i];
                    //6.比较当前位置的字符是否和oldChar匹配,如果匹配上,就用newChar替换
                    buf[i] = (c == oldChar) ? newChar : c;
                    i++;
                }
                return new String(buf, true);
            }
        }
        return this;//7.返回字符串本身
    }
    
    /** * 将字符串中匹配的字符替换(区分重载方法) * 例子： * String t1 = "12345abcabcABC2"; * t1.replace("a","A") * @param target 目标CharSequence字符 * @param replacement 新的CharSequence字符 * @return 替换后的结果 * @since 1.5 */
    public String replace(CharSequence target, CharSequence replacement) { 
        return Pattern.compile(target.toString(), Pattern.LITERAL).matcher(
                this).replaceAll(Matcher.quoteReplacement(replacement.toString()));
    }

## replaceAll方法 ##

/** * 将匹配的所有字符(可以使正则匹配)，替换成新的字符或者字符串 * 例子： * String t1 = "12345abcabcABC2"; * t1.replaceAll("a","A") * t1.replaceAll("\\d","*") * t1.replaceAll("[a-zA-Z]","*") * @param regex 正则表达式,或者字符串 * @param replacement 被替换成的新的字符串 * * @return The resulting {@code String} * * @throws PatternSyntaxException * if the regular expression's syntax is invalid * @since 1.4 */
    public String replaceAll(String regex, String replacement) { 
    	//1.Pattern.compile(regex) 编译正则表达式,获得Pattern对象
    	//2.Pattern.matcher(CharSequence input) 获取匹配器
    	//3.Matcher.replaceAll(String replacement) 替换字符串
        return Pattern.compile(regex).matcher(this).replaceAll(replacement);
    }

## replaceFirst方法 ##

/** * 替换第一个匹配上的字符 * String t1 = "12345abcabcABC2"; * t1.replaceFirst("a","*") * t1.replaceFirst("\\d","*") * t1.replaceFirst("[a-zA-Z]","*") * @param regex 正则表达式或者匹配字符 * @param replacement * * @return The resulting {@code String} * * @throws PatternSyntaxException * if the regular expression's syntax is invalid * * @see java.util.regex.Pattern * * @since 1.4 * @spec JSR-51 */
    public String replaceFirst(String regex, String replacement) { 
        return Pattern.compile(regex).matcher(this).replaceFirst(replacement);
    }

## trim方法 ##

/** * 去除字符串两边的空格 */
    public String trim() { 
    	//1.定义起始位置、长度
        int len = value.length;
        int st = 0;
        char[] val = value;    /* avoid getfield opcode */
    	//2.while循环找头位置
        while ((st < len) && (val[st] <= ' ')) { 
            st++;
        }
        //3.while循环找尾位置
        while ((st < len) && (val[len - 1] <= ' ')) { 
            len--;
        }
        //4.检测两个位置是否移动,移动则调用subString截取函数,否则返回本身
        return ((st > 0) || (len < value.length)) ? substring(st, len) : this;
    }

## split方法 ##

/** * 目标字符串 "boo:and:foo" * : 2 -> ["boo", "and:foo"] * : 5 -> ["boo", "and", "foo"] * : -2 -> ["boo", "and", "foo"] * o 5 -> ["b", "", ":and:f", "", ""] * o -2 -> ["b", "", ":and:f", "", ""] * o 0 -> ["b", "", ":and:f"] * @param regex * 正则表达式、切割器 * * @param limit * 切割后长度 * (当limit大于本身能被切割的长度时,以本身为准) * 比如 “1,2,3,,”最多被切割成长度为5 * * * @return 返回切割后的字符数组 * @since 1.4 */
    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);
    }
    
    /** * @since 1.4 * @spec JSR-51 */
    public String[] split(String regex) { 
        return split(regex, 0);
    }

## getBytes方法 ##

/** * 返回字符串的byte数组(ASCII值) * @param charsetName 指定字符集类型 * @since JDK1.1 */
    public byte[] getBytes(String charsetName)
            throws UnsupportedEncodingException { 
        if (charsetName == null) throw new NullPointerException();
        return StringCoding.encode(charsetName, value, 0, value.length);
    }
    
    /** * Encodes this {@code String} into a sequence of bytes using the given * {@linkplain java.nio.charset.Charset charset}, storing the result into a * new byte array. * * <p> This method always replaces malformed-input and unmappable-character * sequences with this charset's default replacement byte array. The * {@link java.nio.charset.CharsetEncoder} class should be used when more * control over the encoding process is required. * * @param charset * The {@linkplain java.nio.charset.Charset} to be used to encode * the {@code String} * * @return The resultant byte array * * @since 1.6 */
    public byte[] getBytes(Charset charset) { 
        if (charset == null) throw new NullPointerException();
        return StringCoding.encode(charset, value, 0, value.length);
    }
    
    /** * Encodes this {@code String} into a sequence of bytes using the * platform's default charset, storing the result into a new byte array. * * <p> The behavior of this method when this string cannot be encoded in * the default charset is unspecified. The {@link * java.nio.charset.CharsetEncoder} class should be used when more control * over the encoding process is required. * * @return The resultant byte array * * @since JDK1.1 */
    public byte[] getBytes() { 
        return StringCoding.encode(value, 0, value.length);
    }

## length方法 ##

/** * 返回字符串对象的长度 */
    public int length() { 
        return value.length;
    }

## toLowerCase方法 ##

/** * Converts all of the characters in this {@code String} to lower * case using the rules of the given {@code Locale}. Case mapping is based * on the Unicode Standard version specified by the {@link java.lang.Character Character} * class. Since case mappings are not always 1:1 char mappings, the resulting * {@code String} may be a different length than the original {@code String}. * <p> * Examples of lowercase mappings are in the following table: * <table border="1" summary="Lowercase mapping examples showing language code of locale, upper case, lower case, and description"> * <tr> * <th>Language Code of Locale</th> * <th>Upper Case</th> * <th>Lower Case</th> * <th>Description</th> * </tr> * <tr> * <td>tr (Turkish)</td> * <td>\u0130</td> * <td>\u0069</td> * <td>capital letter I with dot above -> small letter i</td> * </tr> * <tr> * <td>tr (Turkish)</td> * <td>\u0049</td> * <td>\u0131</td> * <td>capital letter I -> small letter dotless i </td> * </tr> * <tr> * <td>(all)</td> * <td>French Fries</td> * <td>french fries</td> * <td>lowercased all chars in String</td> * </tr> * <tr> * <td>(all)</td> * <td><img src="doc-files/capiota.gif" alt="capiota"><img src="doc-files/capchi.gif" alt="capchi"> * <img src="doc-files/captheta.gif" alt="captheta"><img src="doc-files/capupsil.gif" alt="capupsil"> * <img src="doc-files/capsigma.gif" alt="capsigma"></td> * <td><img src="doc-files/iota.gif" alt="iota"><img src="doc-files/chi.gif" alt="chi"> * <img src="doc-files/theta.gif" alt="theta"><img src="doc-files/upsilon.gif" alt="upsilon"> * <img src="doc-files/sigma1.gif" alt="sigma"></td> * <td>lowercased all chars in String</td> * </tr> * </table> * * @param locale use the case transformation rules for this locale * @return the {@code String}, converted to lowercase. * @see java.lang.String#toLowerCase() * @see java.lang.String#toUpperCase() * @see java.lang.String#toUpperCase(Locale) * @since 1.1 */
    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);
    }
    
    /** * 将目标字符串中的字母转成小写 * 对a-z A-Z之外的字符不产生任何作用 * @return the {@code String}, converted to lowercase. * @see java.lang.String#toLowerCase(Locale) */
    public String toLowerCase() { 
        return toLowerCase(Locale.getDefault());
    }

## toUpperCase方法 ##

/** * Converts all of the characters in this {@code String} to upper * case using the rules of the given {@code Locale}. Case mapping is based * on the Unicode Standard version specified by the {@link java.lang.Character Character} * class. Since case mappings are not always 1:1 char mappings, the resulting * {@code String} may be a different length than the original {@code String}. * <p> * Examples of locale-sensitive and 1:M case mappings are in the following table. * * <table border="1" summary="Examples of locale-sensitive and 1:M case mappings. Shows Language code of locale, lower case, upper case, and description."> * <tr> * <th>Language Code of Locale</th> * <th>Lower Case</th> * <th>Upper Case</th> * <th>Description</th> * </tr> * <tr> * <td>tr (Turkish)</td> * <td>\u0069</td> * <td>\u0130</td> * <td>small letter i -> capital letter I with dot above</td> * </tr> * <tr> * <td>tr (Turkish)</td> * <td>\u0131</td> * <td>\u0049</td> * <td>small letter dotless i -> capital letter I</td> * </tr> * <tr> * <td>(all)</td> * <td>\u00df</td> * <td>\u0053 \u0053</td> * <td>small letter sharp s -> two letters: SS</td> * </tr> * <tr> * <td>(all)</td> * <td>Fahrvergnügen</td> * <td>FAHRVERGNÜGEN</td> * <td></td> * </tr> * </table> * @param locale use the case transformation rules for this locale * @return the {@code String}, converted to uppercase. * @see java.lang.String#toUpperCase() * @see java.lang.String#toLowerCase() * @see java.lang.String#toLowerCase(Locale) * @since 1.1 */
    public String toUpperCase(Locale locale) { 
        if (locale == null) { 
            throw new NullPointerException();
        }
    
        int firstLower;
        final int len = value.length;
    
        /* Now check if there are any characters that need to be changed. */
        scan: { 
            for (firstLower = 0 ; firstLower < len; ) { 
                int c = (int)value[firstLower];
                int srcCount;
                if ((c >= Character.MIN_HIGH_SURROGATE)
                        && (c <= Character.MAX_HIGH_SURROGATE)) { 
                    c = codePointAt(firstLower);
                    srcCount = Character.charCount(c);
                } else { 
                    srcCount = 1;
                }
                int upperCaseChar = Character.toUpperCaseEx(c);
                if ((upperCaseChar == Character.ERROR)
                        || (c != upperCaseChar)) { 
                    break scan;
                }
                firstLower += srcCount;
            }
            return this;
        }
    
        /* result may grow, so i+resultOffset is the write location in result */
        int resultOffset = 0;
        char[] result = new char[len]; /* may grow */
    
        /* Just copy the first few upperCase characters. */
        System.arraycopy(value, 0, result, 0, firstLower);
    
        String lang = locale.getLanguage();
        boolean localeDependent =
                (lang == "tr" || lang == "az" || lang == "lt");
        char[] upperCharArray;
        int upperChar;
        int srcChar;
        int srcCount;
        for (int i = firstLower; i < len; i += srcCount) { 
            srcChar = (int)value[i];
            if ((char)srcChar >= Character.MIN_HIGH_SURROGATE &&
                (char)srcChar <= Character.MAX_HIGH_SURROGATE) { 
                srcChar = codePointAt(i);
                srcCount = Character.charCount(srcChar);
            } else { 
                srcCount = 1;
            }
            if (localeDependent) { 
                upperChar = ConditionalSpecialCasing.toUpperCaseEx(this, i, locale);
            } else { 
                upperChar = Character.toUpperCaseEx(srcChar);
            }
            if ((upperChar == Character.ERROR)
                    || (upperChar >= Character.MIN_SUPPLEMENTARY_CODE_POINT)) { 
                if (upperChar == Character.ERROR) { 
                    if (localeDependent) { 
                        upperCharArray =
                                ConditionalSpecialCasing.toUpperCaseCharArray(this, i, locale);
                    } else { 
                        upperCharArray = Character.toUpperCaseCharArray(srcChar);
                    }
                } else if (srcCount == 2) { 
                    resultOffset += Character.toChars(upperChar, result, i + resultOffset) - srcCount;
                    continue;
                } else { 
                    upperCharArray = Character.toChars(upperChar);
                }
    
                /* Grow result if needed */
                int mapLen = upperCharArray.length;
                if (mapLen > srcCount) { 
                    char[] result2 = new char[result.length + mapLen - srcCount];
                    System.arraycopy(result, 0, result2, 0, i + resultOffset);
                    result = result2;
                }
                for (int x = 0; x < mapLen; ++x) { 
                    result[i + resultOffset + x] = upperCharArray[x];
                }
                resultOffset += (mapLen - srcCount);
            } else { 
                result[i + resultOffset] = (char)upperChar;
            }
        }
        return new String(result, 0, len + resultOffset);
    }
    
    /** * * 将目标字符串中的字母转成大写 * 对a-z A-Z之外的字符不产生任何作用 * @return the {@code String}, converted to uppercase. * @see java.lang.String#toUpperCase(Locale) */
    public String toUpperCase() { 
        return toUpperCase(Locale.getDefault());
    }

## substring方法 ##

/** * 字符串截取 * @param beginIndex 起始位置 * @return the specified substring. * @exception IndexOutOfBoundsException if * {@code beginIndex} is negative or larger than the * length of this {@code String} object. */
    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);
    }
    
    /** * 字符串 * @param beginIndex * @param endIndex the ending index, exclusive. * @return the specified substring. * @exception IndexOutOfBoundsException if the * {@code beginIndex} is negative, or * {@code endIndex} is larger than the length of * this {@code String} object, or * {@code beginIndex} is larger than * {@code endIndex}. */
    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);
    }

## compareTo()方法 ##

/** * 比较两个字符串是否相等 * 1.如果相等返回0 * 2.如果返回为正数,说明当前字符串的ASCII码值总和大于比较字符串 * 3.如果返回为负数,说明当前字符串的ASCII码值总和小于比较字符串 * @param anotherString the {@code String} to be compared. * @return the value */
    public int compareTo(String anotherString) { 
        int len1 = value.length;//取当前字符串的长度
        int len2 = anotherString.value.length;//取比较字符串的长度
        int lim = Math.min(len1, len2);//取两个字符串长度最小值
        char v1[] = value;
        char v2[] = anotherString.value;
    
        int k = 0;
        while (k < lim) { //从0号位置开始比较,到lim结束
            char c1 = v1[k];
            char c2 = v2[k];
            if (c1 != c2) { //如果两个字符不相等,则返回两则的ASCII差值
                return c1 - c2;
            }
            k++;
        }
        return len1 - len2;//如果在0-lim索引位置内,对应字符都相等,返回两者长度的差值
    }

## lastIndexOf()方法 ##

/** * 参考lastIndexOf(int ch, int fromIndex)方法 * @param ch */
    public int lastIndexOf(int ch) { 
        return lastIndexOf(ch, value.length - 1);
    }
    
    /** * 从fromIndex位置反向检索ch的索引位置 * 如果fromIndex大于当前对象的length-1索引,则从length-1开始检索 * @param ch * @param fromIndex */
    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;
            //i取fromIndex和value.length - 1中的较小值
            int i = Math.min(fromIndex, value.length - 1);
            //由于i已经提前赋值,所以可以下面写法
            for (; i >= 0; i--) { 
                if (value[i] == ch) { 
                    return i;
                }
            }
            return -1;
        } else { 
            return lastIndexOfSupplementary(ch, fromIndex);
        }
    }
    
    /** * Handles (rare) calls of lastIndexOf with a supplementary character. */
    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;
    }
    
    /** * 默认是从目标字符串最后一个位置,倒序检索 * @param str */
    public int lastIndexOf(String str) { 
        return lastIndexOf(str, value.length);
    }
    
    /** * 关注indexOf(value, 0, value.length,str.value, 0, str.value.length, fromIndex)方法 * @param str * @param fromIndex */
    public int lastIndexOf(String str, int fromIndex) { 
        return lastIndexOf(value, 0, value.length,
                str.value, 0, str.value.length, fromIndex);
    }
    
    /** * 用在AbstractStringBuilder类中 * @param source * @param sourceOffset * @param sourceCount * @param target * @param 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);
    }
    
    /** * * @param source 原字符数组 * @param sourceOffset 原字符数组索引开始位置 * @param sourceCount 原字符数组长度 * @param target 目标字符数组 * @param targetOffset 目标字符数组索引开始位置 * @param targetCount 目标字符数组长度 * @param fromIndex 检索索引开始位置 */
    static int lastIndexOf(char[] source, int sourceOffset, int sourceCount,
            char[] target, int targetOffset, int targetCount,
            int fromIndex) { 
        /* * 计算最右检索边界索引的位置 */
        int rightIndex = sourceCount - targetCount;
        //如果检索的位置小于0,认为非法,直接返回-1
        if (fromIndex < 0) { 
            return -1;
        }
        //如果检索位置大于最右边索引的位置,默认就从最右边位置开始检索
        if (fromIndex > rightIndex) { 
            fromIndex = rightIndex;
        }
        /* 如果检索的字符串为空串,直接返回fromIndex检索的值*/
        if (targetCount == 0) { 
            return fromIndex;
        }
    //计算目标字符数组最后一个字符的索引位置
        int strLastIndex = targetOffset + targetCount - 1;
        //计算目标字符数组最后一个字符的ASCII值
        char strLastChar = target[strLastIndex];
        int min = sourceOffset + targetCount - 1;
        //计算检索索引开始的位置
        int i = min + fromIndex;
    
    startSearchForLastChar:
        while (true) { 
            //查找到最后一个字母的索引位置
            while (i >= min && source[i] != strLastChar) { 
                i--;
            }
            if (i < min) { 
                return -1;
            }
            //检索目标字段索引位置的前一个位置
            int j = i - 1;
            int start = j - (targetCount - 1);
            int k = strLastIndex - 1;
    
            while (j > start) { 
                if (source[j--] != target[k--]) { 
                    i--;
                    continue startSearchForLastChar;
                }
            }
            //如果全部匹配中,返回检索的位置
            return start - sourceOffset + 1;
        }
    }

## indexOf()方法 ##

/** * 参考indexOf(str, 0) * @param str 检索的字符串 */
    public int indexOf(String str) { 
        return indexOf(str, 0);
    }
    
    /** * 参考indexOf(char[] source, int sourceOffset, int sourceCount,char[] target, int targetOffset, int targetCount,int fromIndex)方法 * @param str 检索的字符串 * @param fromIndex 索引开始位置 */
    public int indexOf(String str, int fromIndex) { 
        return indexOf(value, 0, value.length,
                str.value, 0, str.value.length, fromIndex);
    }
    
    /** * @param source 原字符数组 * @param sourceOffset 原字符数组长度 * @param sourceCount 目标字符数组 * @param target 目标字符数组长度 * @param fromIndex 检索索引开始位置 */
    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);
    }
    
    /** * @param source 原字符数组 * @param sourceOffset 原字符数组索引开始位置 * @param sourceCount 原字符数组长度 * @param target 目标字符数组 * @param targetOffset 目标字符数组索引开始位置 * @param targetCount 目标字符数组长度 * @param fromIndex 检索索引开始位置 */
    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);
    //检索从fromIndex位置,到max位置 
        for (int i = sourceOffset + fromIndex; i <= max; i++) { 
            /*查找目标字符数组第一个字符的位置 */
            if (source[i] != first) { 
            	//如果没找到,就继续找,一旦找到,结束循环
            	//否则一直到i>max时,退出while循环
                while (++i <= max && source[i] != first);
            }
    
            /* 找到第一个字符的位置,现在检索目标字符数组剩下的内容是否匹配 */
            if (i <= max) { 
            	//剩余匹配内容的开始位置(第一个字符的匹配的位置+1)
                int j = i + 1;
                //获取目标字符数组最后一个字符在原字符数组的对应位置
                int end = j + targetCount - 1;
                /** * 下面代码的艺术解读： * 1.for();的用法类似于while();写法 * 2.通过k标记目标字符数组的字符角标 * 3.通过j标记标记目标字符数组的字符角标 * 4.比较两个字符是否相等 * 5.如果不相等j++,k++,相当于比较下一个字符 * 6.if(j == end)表明所有字符都比对上了 * 7.容易迷惑:sourceOffset传入时默认为0 * 所以i-sourceOffset = i; * 8.如果j != end 说明中间for()循环比对的时候 * 退出了,最终返回-1即可 */
                for (int k = targetOffset + 1; j < end && source[j]
                        == target[k]; j++, k++);
    
                if (j == end) { 
                    /* Found whole string. */
                    return i - sourceOffset;
                }
            }
        }
        return -1;
    }

## regionMatches()方法 ##

/** * 比较两个子串是否相等 * 应用：字符串中某个子串“da”出现了多少次？ * int number = 0; * String str = "fdafdadfadf"; * for (int i = 0; i < str.length(); i++) { * if (str.regionMatches(i, "da", 0, 2)) { * number++; * } * } * @param toffset 当前字符串的起始位置 * @param other 比较字符串 * @param ooffset 比较字符串的起始位置 * @param len 比较的元素个数 */
    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.
        //判断toffset、toffset是否越界
        if ((ooffset < 0) || (toffset < 0)
                || (toffset > (long)value.length - len)
                || (ooffset > (long)other.value.length - len)) { 
            return false;
        }
        while (len-- > 0) { //遍历len次
            if (ta[to++] != pa[po++]) { //比较字符位置是否相等
                return false;
            }
        }
        return true;
    }
    
    /** * 比较两个子串是否相等 * 应用：字符串中某个子串“da”出现了多少次？ * int number = 0; * String str = "fdafdadfadf"; * for (int i = 0; i < str.length(); i++) { * if (str.regionMatches(i, "da", 0, 2)) { * number++; * } * } * @param ignoreCase 是否忽略大小写比较 * @param toffset 当前字符串的起始位置 * @param other 比较字符串 * @param ooffset 比较字符串的起始位置 * @param len 比较的元素个数 */
    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.
        //判断toffset、toffset是否越界
        if ((ooffset < 0) || (toffset < 0)
                || (toffset > (long)value.length - len)
                || (ooffset > (long)other.value.length - len)) { 
            return false;
        }
        while (len-- > 0) { //遍历len次
            char c1 = ta[to++];
            char c2 = pa[po++];
            if (c1 == c2) { 
                continue;//如果两个字符相等,则结束本次循环
            }
            if (ignoreCase) { //是否需要忽略大小写比较
           		//将两个字符转成大写进行比较
                char u1 = Character.toUpperCase(c1);
                char u2 = Character.toUpperCase(c2);
                if (u1 == u2) { 
                    continue;
                }
                将两个字符转成小写进行比较
                if (Character.toLowerCase(u1) == Character.toLowerCase(u2)) { 
                    continue;
                }
            }
            return false;
        }
        return true;
    }

## contentEquals()方法 ##

/** * 比较两个字符串序列是否相等 * @since 1.4 */
    public boolean contentEquals(StringBuffer sb) { 
        return contentEquals((CharSequence)sb);
    }
    
    /** * 非同步方法,比较两个字符串序列是否相等 */
    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++) { //循环比较n次,n代表当前字符的长度
            if (v1[i] != v2[i]) { //判断当前位置字符是否相等
                return false;
            }
        }
        return true;
    }
    
    /** * 比较两个字符串序列是否相等 * * @param cs 字符序列对象 * @since 1.5 */
    public boolean contentEquals(CharSequence cs) { 
        if (cs instanceof AbstractStringBuilder) { 
        	//如果参数是StringBuffer, StringBuilder类型
            if (cs instanceof StringBuffer) { 
                synchronized(cs) { //如果是StringBuffer,加线程锁
                   return nonSyncContentEquals((AbstractStringBuilder)cs);
                }
            } else { 
                return nonSyncContentEquals((AbstractStringBuilder)cs);
            }
        }
        //如果是字符串对象,直接调用equals方法
        if (cs instanceof String) { 
            return equals(cs);
        }
        //如果是一个普通的字符序列对象
        char v1[] = value;
        int n = v1.length;
        if (n != cs.length()) { 
            return false;
        }
        for (int i = 0; i < n; i++) { //循环n次,n代表当前字符长度
            if (v1[i] != cs.charAt(i)) { //比较当前位置的两个字符
                return false;
            }
        }
        return true;
    }

## startsWith()方法 ##

/** * 判断当前字符串是否以prefix开头 * @param prefix 匹配的字符 * @param toffset 开始检索的位置 */
    public boolean startsWith(String prefix, int toffset) { 
        char ta[] = value;
        int to = toffset;
        char pa[] = prefix.value;
        int po = 0;//prefix匹配字符串的起始检索位置
        int pc = prefix.value.length;
        // 判断toffset是否合法
        if ((toffset < 0) || (toffset > value.length - pc)) { 
            return false;
        }
        while (--pc >= 0) { //循环pc次,跟while(pc-- > 0)效果一样
            if (ta[to++] != pa[po++]) { //比较当前两边索引位置的字符是否一致
                return false;
            }
        }
        return true;
    }
    
    /** * 判断字符是否以prefix开头 * @param prefix the prefix. * @since 1. 0 */
    public boolean startsWith(String prefix) { 
        return startsWith(prefix, 0);
    }

## isEmpty()方法 ##

/** * 判断字符串是否为空串,区别开null * @since 1.6 */
    public boolean isEmpty() { 
        return value.length == 0;
    }

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