String

1.類聲明：

public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {}

2.變量：

private final char value[];//存儲字符串
public static final Comparator<String> CASE_INSENSITIVE_ORDER
                                         = new CaseInsensitiveComparator();//靜態內部類，比較器

3.方法：

  public String(char value[]) {
        this.value = Arrays.copyOf(value, value.length);
    }
    //通過String構造String
     public String(String original) {
        this.value = original.value;
        this.hash = original.hash;
    }
    //通過int數組構造，考慮每個int是不是一個char，是不是一個Unicode，如果都不是則拋出異常
     public String(int[] codePoints, int offset, int count) {
        if (offset < 0) {
            throw new StringIndexOutOfBoundsException(offset);
        }
        if (count <= 0) {
            if (count < 0) {
                throw new StringIndexOutOfBoundsException(count);
            }
            if (offset <= codePoints.length) {
                this.value = "".value;
                return;
            }
        }
        // Note: offset or count might be near -1>>>1.
        if (offset > codePoints.length - count) {
            throw new StringIndexOutOfBoundsException(offset + count);
        }

        final int end = offset + count;

        // Pass 1: Compute precise size of char[]
        int n = count;
        for (int i = offset; i < end; i++) {
            int c = codePoints[i];
            if (Character.isBmpCodePoint(c))
                continue;
            else if (Character.isValidCodePoint(c))
                n++;//如果不是char,但是是unicode字符，則添加一個字符存儲
            else throw new IllegalArgumentException(Integer.toString(c));
        }

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

        for (int i = offset, j = 0; i < end; i++, j++) {
            int c = codePoints[i];
            if (Character.isBmpCodePoint(c))
                v[j] = (char)c;
            else
                Character.toSurrogates(c, v, j++);//將一個int存儲爲2個字符
        }

        this.value = v;
    }
    //通過字節數組進行構造，調用SringCoding將其解碼爲字符
      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);
    }
    //通過StringBuffer構造，加入同步，多線程可以同時使用同一個buffer進行構造
    public String(StringBuffer buffer) {
        synchronized(buffer) {
            this.value = Arrays.copyOf(buffer.getValue(), buffer.length());
        }
    }
    //通過StringBuilder進行構造
    public String(StringBuilder builder) {
        this.value = Arrays.copyOf(builder.getValue(), builder.length());
    }
    //charAt方法直接返會下標位置的字符
       public char charAt(int index) {
        if ((index < 0) || (index >= value.length)) {
            throw new StringIndexOutOfBoundsException(index);
        }
        return value[index];
    }
    //複製字符串到字符數組指定位置
    void getChars(char dst[], int dstBegin) {
        System.arraycopy(value, 0, dst, dstBegin, value.length);
    }
    //將字符串編碼爲指定字符集的字節數組
       public byte[] getBytes(String charsetName)
            throws UnsupportedEncodingException {
        if (charsetName == null) throw new NullPointerException();
        return StringCoding.encode(charsetName, value, 0, value.length);
    }
    //覆寫equals方法，同一個對象，或者每個字符都相等，才返回true
        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;
    }
    //將字符串與CharSequence比較，若是StringBuffer，還需要加鎖比較，對StringBuffer對象加鎖
    public boolean contentEquals(CharSequence cs) {
        // Argument is a StringBuffer, StringBuilder
        if (cs instanceof AbstractStringBuilder) {
            if (cs instanceof StringBuffer) {
                synchronized(cs) {
                   return nonSyncContentEquals((AbstractStringBuilder)cs);
                }
            } else {
                return nonSyncContentEquals((AbstractStringBuilder)cs);
            }
        }
        // Argument is a String
        if (cs instanceof String) {
            return equals(cs);
        }
        // Argument is a generic CharSequence
        char v1[] = value;
        int n = v1.length;
        if (n != cs.length()) {
            return false;
        }
        for (int i = 0; i < n; i++) {
            if (v1[i] != cs.charAt(i)) {
                return false;
            }
        }
        return true;
    }
    //跟另一個String比較
       public boolean equalsIgnoreCase(String anotherString) {
        return (this == anotherString) ? true
                : (anotherString != null)
                && (anotherString.value.length == value.length)
                && regionMatches(true, 0, anotherString, 0, value.length);
    }
    //覆蓋Comparable接口方法cmpareTo,按字典順序比較
        public int compareTo(String anotherString) {
        int len1 = value.length;
        int len2 = anotherString.value.length;
        int lim = Math.min(len1, len2);
        char v1[] = value;
        char v2[] = anotherString.value;

        int k = 0;
        while (k < lim) {
            char c1 = v1[k];
            char c2 = v2[k];
            if (c1 != c2) {
                return c1 - c2;
            }
            k++;
        }
        return len1 - len2;
    }
    //通過比較器比較（忽視大小寫）
    public int compareToIgnoreCase(String str) {
        return CASE_INSENSITIVE_ORDER.compare(this, str);
    }
    //在某個下標是否以某個字符串開始
       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;
    }
    //轉換爲在某個位置以什麼字符串開始
     public boolean endsWith(String suffix) {
        return startsWith(suffix, value.length - suffix.value.length);
    }
    //從fromIndex往前找，找到第一個ch則返回下標
    public int lastIndexOf(int ch, int fromIndex) {
        if (ch < Character.MIN_SUPPLEMENTARY_CODE_POINT) {
            // handle most cases here (ch is a BMP code point or a
            // negative value (invalid code point))
            final char[] value = this.value;
            int i = Math.min(fromIndex, value.length - 1);
            for (; i >= 0; i--) {
                if (value[i] == ch) {
                    return i;
                }
            }
            return -1;
        } else {
            return lastIndexOfSupplementary(ch, fromIndex);
        }
    }
    //如果常量池中已經有了這個字符串，返回它的引用，否則將這個字符串加入常量池同時返回引用
       public native String intern();
       //正則匹配
       public boolean matches(String regex) {
        return Pattern.matches(regex, this);
    }