2.String

• String 实现了 java.io.Serializable, Comparable<String>, CharSequence
• 参数

 private final char value[];//用于字符存储。
 private int hash; // 记录字符串的哈希值  默认 0
 private static final long serialVersionUID = -6849794470754667710L;//序列化
 private static final ObjectStreamField[] serialPersistentFields =
        new ObjectStreamField[0];

1.String()

    public String() {
        this.value = "".value;//初始化是一个空字符串
    }

2.String(String original)

   public String(String original) {
        this.value = original.value;//String参数非空初始化
        this.hash = original.hash;
    }

3.String(char value[])

 public String(char value[]) {//char []参数非空初始化
        this.value = Arrays.copyOf(value, value.length);
    }

4.String(char value[], int offset, int count)

public String(char value[], int offset, int count) {
//offset开始的value的下标
//count要复制长度
        if (offset < 0) {
            throw new StringIndexOutOfBoundsException(offset);
        }
        if (count <= 0) {//复制0个
            if (count < 0) {
                throw new StringIndexOutOfBoundsException(count);
            }
            if (offset <= value.length) {//且长度在范围内
                this.value = "".value;//返回一个空
                return;
            }
        }
        // Note: offset or count might be near -1>>>1.
        if (offset > value.length - count) {//offset  +count> value.length//超出范围
            throw new StringIndexOutOfBoundsException(offset + count);
        }
        this.value = Arrays.copyOfRange(value, offset, offset+count);
    }

4.length()

length() {
        return value.length;//返回长度
    }

5.isEmpty()

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

6.charAt(int index)

   public char charAt(int index) {
        if ((index < 0) || (index >= value.length)) {//判断下标是否越界
            throw new StringIndexOutOfBoundsException(index);
        }
        return value[index];//返回下标值
    }

7.codePointAt(int index)

    public int codePointAt(int index) {
        if ((index < 0) || (index >= value.length)) {
            throw new StringIndexOutOfBoundsException(index);
        }
        return Character.codePointAtImpl(value, index, value.length);//返回对应Unicode值
    }

8.compareTo(String anotherString)

两个值对比

ublic int compareTo(String anotherString) {
        int len1 = value.length;
        int len2 = anotherString.value.length;
        int lim = Math.min(len1, len2);//获取较小值
        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;//不等时返回差值
            }
            k++;
        }
        return len1 - len2;//若最后都相等，则返回0
    }

9.getBytes()

返回这个字符串的字节码数组，"aA".getBytes() 返回97和65，默认编码为ISO-8859-1，也可以自己指定，如UTF-8

    public byte[] getBytes() {
        return StringCoding.encode(value, 0, value.length);
    }

10.equals()

见Object

11.contentEquals(StringBuffer sb)

比较String和StringBuffer值是否相等，相等返回true

  public boolean contentEquals(StringBuffer sb) {
        return contentEquals((CharSequence)sb);
    }

 public boolean contentEquals(CharSequence cs) {
        // 如果参数是 StringBuffer, StringBuilder
        if (cs instanceof AbstractStringBuilder) {
            if (cs instanceof StringBuffer) {
                //StringBuffer是线程安全的，所有对它的处理都加锁
                synchronized(cs) {
                   return nonSyncContentEquals((AbstractStringBuilder)cs);
                }
            } else {
                //StringBuilder 不是，没有加锁
                return nonSyncContentEquals((AbstractStringBuilder)cs);
            }
        }
        // 如果参数是 String
        if (cs instanceof String) {
            return equals(cs);
        }
        // 如果参数是通用 CharSequence 则对长度和元素逐一比较
        char v1[] = value;
        int n = v1.length;
        if (n != cs.length()) {
            return false;
        }
        for (int i = 0; i < n; i++) {
            if (v1[i] != cs.charAt(i)) {
                return false;
            }
        }
        return true;
    }

   // 如果参数是AbstractStringBuilder  则对长度和元素逐一比较
private boolean nonSyncContentEquals(AbstractStringBuilder sb) {
        char v1[] = value;
        char v2[] = sb.getValue();
        int n = v1.length;
        if (n != sb.length()) {
            return false;
        }
        for (int i = 0; i < n; i++) {
            if (v1[i] != v2[i]) {
                return false;
            }
        }
        return true;
    }

12.equalsIgnoreCase(String anotherString)

   public boolean equalsIgnoreCase(String anotherString) {
        return (this == anotherString) ? true//地址相等返回true
                : (anotherString != null)//anotherString = null 返回false
                && (anotherString.value.length == value.length)//长度不等返回false
                && regionMatches(true, 0, anotherString, 0, value.length);
    }

/ * @param   ignoreCase   if {@code true}, true忽略大小写比较
*/
public boolean regionMatches(boolean ignoreCase, int toffset,
            String other, int ooffset, int len) {
        char ta[] = value;
        int to = toffset;
        char pa[] = other.value;
        int po = ooffset;
        // Note: toffset, ooffset, or len might be near -1>>>1.
        if ((ooffset < 0) || (toffset < 0)
                || (toffset > (long)value.length - len)
                || (ooffset > (long)other.value.length - len)) {
            return false;
        }
        while (len-- > 0) {
            char c1 = ta[to++];
            char c2 = pa[po++];
            if (c1 == c2) {
                continue;
            }
            if (ignoreCase) {
                // If characters don't match but case may be ignored,
                // try converting both characters to uppercase.
                // If the results match, then the comparison scan should
                // continue.
                char u1 = Character.toUpperCase(c1);
                char u2 = Character.toUpperCase(c2);
                if (u1 == u2) {
                    continue;
                }
                // Unfortunately, conversion to uppercase does not work properly
                // for the Georgian alphabet, which has strange rules about case
                // conversion.  So we need to make one last check before
                // exiting.
                if (Character.toLowerCase(u1) == Character.toLowerCase(u2)) {
                    continue;
                }
            }
            return false;
        }
        return true;
    }

13.startsWith(String prefix, int toffset)

 public boolean startsWith(String prefix, int toffset) {
        char ta[] = value;
        int to = toffset;//指定位置
        char pa[] = prefix.value;//比较值
        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;
    }

14.hashCode()

s[0]31^(n-1) + s[1]31^(n-2) + ... + s[n-1] 计算公式

public int hashCode() {
        int h = hash;
        if (h == 0 && value.length > 0) {
            char val[] = value;

            for (int i = 0; i < value.length; i++) {
                h = 31 * h + val[i];
            }
            hash = h;
        }
        return h;
    }

15.substring

new String(value, beginIndex, subLen); 居然是重新new 了一个。。。。

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);
    }

16.concat

在原字符串后面追加一段

   public String concat(String str) {
        int otherLen = str.length();
        if (otherLen == 0) {
            return this;
        }
        int len = value.length;
        char buf[] = Arrays.copyOf(value, len + otherLen);//建一个新的char数组长度为两个旧的之和
        str.getChars(buf, len);//数组拷贝
        return new String(buf, true);
    }

   public static char[] copyOf(char[] original, int newLength) {
        char[] copy = new char[newLength];
        System.arraycopy(original, 0, copy, 0,
                         Math.min(original.length, newLength));
        return copy;
    }

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

17.indexOf(int ch, int fromIndex)

返回指定字符的位置

public int indexOf(int ch, int fromIndex) {
        final int max = value.length;
        if (fromIndex < 0) {
            fromIndex = 0;
        } else if (fromIndex >= max) {
            // Note: fromIndex might be near -1>>>1.
            return -1;
        }

        if (ch < Character.MIN_SUPPLEMENTARY_CODE_POINT) {
            // for循环处理，从指定位置fromIndex开始往后找
            final char[] value = this.value;
            for (int i = fromIndex; i < max; i++) {
                if (value[i] == ch) {/如果地址相等，就返回下标位置
                    return i;
                }
            }
            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;
    }

18.replace(char oldChar, char newChar)

  public String replace(char oldChar, char newChar) {
        if (oldChar != newChar) {
            int len = value.length;
            int i = -1;
            char[] val = value; /* avoid getfield opcode */

            while (++i < len) {
                if (val[i] == oldChar) {
                    break;//找到oldChar字符的位置下标i
                }
            }
            if (i < len) {
                char buf[] = new char[len];
                for (int j = 0; j < i; j++) {
                    buf[j] = val[j];//复制i位置之前的字符
                }
                while (i < len) {
                    char c = val[i];//往后遍历取值，判断值是否等于oldChar，如果等就换成newChar
                    buf[i] = (c == oldChar) ? newChar : c;
                    i++;
                }
                return new String(buf, true);
            }
        }
        return this;
    }

19.matches(String regex)

在字符串匹配给定的正则表达式时，返回 true。

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

   public static void main(String[] args) throws UnsupportedEncodingException {
        String Str = new String("www.runoob.com");
        System.out.print("返回值 :" );
        System.out.println(Str.matches("(.*)runoob(.*)"));
    }
返回值 :true

20.contains(CharSequence s)

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

21.split(String regex, int limit)

将字符串根据regex拆分成数组返回

public String[] split(String regex, int limit) {
     
        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))//判断是不是符合要求的regex
        {
            int off = 0;
            int next = 0;
            boolean limited = limit > 0;
            ArrayList<String> list = new ArrayList<>();//新建一个list
            while ((next = indexOf(ch, off)) != -1) {//判断是否含有指定字符
                if (!limited || list.size() < limit - 1) {
                    list.add(substring(off, next));//有就截取并添加到list
                    off = next + 1;//下标后移
                } else {    // 不含就是最后一个
                    //assert (list.size() == limit - 1);
                    list.add(substring(off, value.length));
                    off = value.length;
                    break;
                }
            }
            // 没有符合返回原来的字符串
            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);
    }

22.join(CharSequence delimiter, CharSequence... elements)

将数组elements的元素以delimiter拼接成一个新的字符串返回

 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 StringJoiner add(CharSequence newElement) {
        prepareBuilder().append(newElement);//StringJoiner 是jdk1.8的，内部是一个StringBuilder实现的
        return this;
    }

    public static void main(String[] args) {
        String delimiter = "-";
        String[] elements = {"JDK", "8", "String", "join"};
        List<String> list = new ArrayList<String>(Arrays.asList(elements));
        System.out.println(String.join(delimiter, elements));
        System.out.println(String.join(delimiter, "JDK", "8", "String", "join"));
        System.out.println(String.join(delimiter, list));
    }
返回值：JDK-8-String-join

23.toLowerCase(Locale locale)

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: {//scan,自定义循环结束，配合break使用
            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);
    }

24.format(String format, Object... args)

   public static String format(String format, Object... args) {
        return new Formatter().format(format, args).toString();
    }
String.format("你可以成为%s","平凡的人")  ------>   你可以成为平凡的人（字符串替换）

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。

String.format("你可以成为%2$s，他也可以成为%1$s","平凡的人","不平凡的人")  ------>   你可以成为不平凡的人，他也可以成为平凡的人。（位置控制通过[argument_index$]）

String.format("%o", 12)); ---------->14  （10进制转8进制）

String.format("%x", 12)); ----------> c  （10进制转16进制）

String.format("%1$,d", 12302562); -------------> 12,302,562  (flag 的用法，这里用都好隔开，并转换成10进制。）

String.format("%1$08d", 123456);--------------> 00123456  (width的用法，用0填充（flag），最少8位。