java.lang.CharSequence

import java.util.NoSuchElementException;
import java.util.PrimitiveIterator;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.function.IntConsumer;
import java.util.stream.IntStream;
import java.util.stream.StreamSupport;

/**
 * <tt>CharSequence</tt>是一个可读的<code>char</code>序列.这个接口提供许多不同字符序列的统一,只读的方法.
 * 一个<code>char</code>值是<i>Basic Multilingual Plane (BMP)</i>中一个.具体参考
 * <a href="Character.html#unicode">Unicode Character 表示</a>
 * 
 * 
 * <p> 这个接口并不违反 {@link java.lang.Object#equals(java.lang.Object) equals} 和 
 * {@link java.lang.Object#hashCode() hashCode} 的约定. 
 * 因此比较俩个<tt>CharSequence</tt>序列的结果通常是没有定义.
 * 不同的对象由不同的类实现,因此不保证测试它们实例能够被测试是否相等.
 * 所以,不推荐在set或者map的key中使用<tt>CharSequence</tt>
 * 
 */
/**
 * A <tt>CharSequence</tt> is a readable sequence of <code>char</code> values. This
 * interface provides uniform, read-only access to many different kinds of
 * <code>char</code> sequences.
 * A <code>char</code> value represents a character in the <i>Basic
 * Multilingual Plane (BMP)</i> or a surrogate. Refer to <a
 * href="Character.html#unicode">Unicode Character Representation</a> for details.
 *
 * <p> This interface does not refine the general contracts of the {@link
 * java.lang.Object#equals(java.lang.Object) equals} and {@link
 * java.lang.Object#hashCode() hashCode} methods.  The result of comparing two
 * objects that implement <tt>CharSequence</tt> is therefore, in general,
 * undefined.  Each object may be implemented by a different class, and there
 * is no guarantee that each class will be capable of testing its instances
 * for equality with those of the other.  It is therefore inappropriate to use
 * arbitrary <tt>CharSequence</tt> instances as elements in a set or as keys in
 * a map. </p>
 *
 * @author Mike McCloskey
 * @since 1.4
 * @spec JSR-51
 */

public interface CharSequence {
    
    /**
     * 返回字符序列的长度,16-bit位char的个数.
     */
    /**
     * Returns the length of this character sequence.  The length is the number
     * of 16-bit <code>char</code>s in the sequence.
     *
     * @return  the number of <code>char</code>s in this sequence
     */
    int length();

    /**
     * Returns the <code>char</code> value at the specified index.  An index ranges from zero
     * to <tt>length() - 1</tt>.  The first <code>char</code> value of the sequence is at
     * index zero, the next at index one, and so on, as for array
     * indexing.
     *
     * <p>If the <code>char</code> value specified by the index is a
     * <a href="{@docRoot}/java/lang/Character.html#unicode">surrogate</a>, the surrogate
     * value is returned.
     *
     * @param   index   the index of the <code>char</code> value to be returned
     *
     * @return  the specified <code>char</code> value
     *
     * @throws  IndexOutOfBoundsException
     *          if the <tt>index</tt> argument is negative or not less than
     *          <tt>length()</tt>
     */
    char charAt(int index);

    /**
     * Returns a <code>CharSequence</code> that is a subsequence of this sequence.
     * The subsequence starts with the <code>char</code> value at the specified index and
     * ends with the <code>char</code> value at index <tt>end - 1</tt>.  The length
     * (in <code>char</code>s) of the
     * returned sequence is <tt>end - start</tt>, so if <tt>start == end</tt>
     * then an empty sequence is returned.
     *
     * @param   start   the start index, inclusive
     * @param   end     the end index, exclusive
     *
     * @return  the specified subsequence
     *
     * @throws  IndexOutOfBoundsException
     *          if <tt>start</tt> or <tt>end</tt> are negative,
     *          if <tt>end</tt> is greater than <tt>length()</tt>,
     *          or if <tt>start</tt> is greater than <tt>end</tt>
     */
    CharSequence subSequence(int start, int end);

    /**
     * Returns a string containing the characters in this sequence in the same
     * order as this sequence.  The length of the string will be the length of
     * this sequence.
     *
     * @return  a string consisting of exactly this sequence of characters
     */
    public String toString();

    /**
     * Returns a stream of {@code int} zero-extending the {@code char} values
     * from this sequence.  Any char which maps to a <a
     * href="{@docRoot}/java/lang/Character.html#unicode">surrogate code
     * point</a> is passed through uninterpreted.
     *
     * <p>If the sequence is mutated while the stream is being read, the
     * result is undefined.
     *
     * @return an IntStream of char values from this sequence
     * @since 1.8
     */
    public default IntStream chars() {
        class CharIterator implements PrimitiveIterator.OfInt {
            int cur = 0;

            public boolean hasNext() {
                return cur < length();
            }

            public int nextInt() {
                if (hasNext()) {
                    return charAt(cur++);
                } else {
                    throw new NoSuchElementException();
                }
            }

            @Override
            public void forEachRemaining(IntConsumer block) {
                for (; cur < length(); cur++) {
                    block.accept(charAt(cur));
                }
            }
        }

        return StreamSupport.intStream(() ->
                Spliterators.spliterator(
                        new CharIterator(),
                        length(),
                        Spliterator.ORDERED),
                Spliterator.SUBSIZED | Spliterator.SIZED | Spliterator.ORDERED,
                false);
    }

    /**
     * Returns a stream of code point values from this sequence.  Any surrogate
     * pairs encountered in the sequence are combined as if by {@linkplain
     * Character#toCodePoint Character.toCodePoint} and the result is passed
     * to the stream. Any other code units, including ordinary BMP characters,
     * unpaired surrogates, and undefined code units, are zero-extended to
     * {@code int} values which are then passed to the stream.
     *
     * <p>If the sequence is mutated while the stream is being read, the result
     * is undefined.
     *
     * @return an IntStream of Unicode code points from this sequence
     * @since 1.8
     */
    public default IntStream codePoints() {
        class CodePointIterator implements PrimitiveIterator.OfInt {
            int cur = 0;

            @Override
            public void forEachRemaining(IntConsumer block) {
                final int length = length();
                int i = cur;
                try {
                    while (i < length) {
                        char c1 = charAt(i++);
                        if (!Character.isHighSurrogate(c1) || i >= length) {
                            block.accept(c1);
                        } else {
                            char c2 = charAt(i);
                            if (Character.isLowSurrogate(c2)) {
                                i++;
                                block.accept(Character.toCodePoint(c1, c2));
                            } else {
                                block.accept(c1);
                            }
                        }
                    }
                } finally {
                    cur = i;
                }
            }

            public boolean hasNext() {
                return cur < length();
            }

            public int nextInt() {
                final int length = length();

                if (cur >= length) {
                    throw new NoSuchElementException();
                }
                char c1 = charAt(cur++);
                if (Character.isHighSurrogate(c1) && cur < length) {
                    char c2 = charAt(cur);
                    if (Character.isLowSurrogate(c2)) {
                        cur++;
                        return Character.toCodePoint(c1, c2);
                    }
                }
                return c1;
            }
        }

        return StreamSupport.intStream(() ->
                Spliterators.spliteratorUnknownSize(
                        new CodePointIterator(),
                        Spliterator.ORDERED),
                Spliterator.ORDERED,
                false);
    }
}

请勿在Set或者Map这种集合中使用CharSequence作为Key,因为equals和hashcode方法由实现者决定.