CharSequence.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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package java.lang;

import java.util.NoSuchElementException;
import java.util.Objects;
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;

/**
 * A {@code CharSequence} is a readable sequence of {@code char} values. This
 * interface provides uniform, read-only access to many different kinds of
 * {@code char} sequences.
 * A {@code char} 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 testing two objects
 * that implement {@code CharSequence} for equality 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 {@code CharSequence} instances as elements in a set or as keys in
 * a map. </p>
 *
 * @author Mike McCloskey
 * @spec JSR-51
 * @since 1.4
 */

// 字符序列接口，封装了对字符序列的一些操作（包括转换为流的操作）
public interface CharSequence {
    
    /**
     * Returns the length of this character sequence.  The length is the number
     * of 16-bit {@code char}s in the sequence.
     *
     * @return the number of {@code char}s in this sequence
     */
    // 返回字符序列长度
    int length();
    
    /**
     * Returns the {@code char} value at the specified index.  An index ranges from zero
     * to {@code length() - 1}.  The first {@code char} 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} value specified by the index is a
     * <a href="{@docRoot}/java.base/java/lang/Character.html#unicode">surrogate</a>, the surrogate
     * value is returned.
     *
     * @param index the index of the {@code char} value to be returned
     *
     * @return the specified {@code char} value
     *
     * @throws IndexOutOfBoundsException if the {@code index} argument is negative or not less than
     *                                   {@code length()}
     */
    // 返回索引index处的字符
    char charAt(int index);
    
    /**
     * Returns a {@code CharSequence} that is a subsequence of this sequence.
     * The subsequence starts with the {@code char} value at the specified index and
     * ends with the {@code char} value at index {@code end - 1}.  The length
     * (in {@code char}s) of the
     * returned sequence is {@code end - start}, so if {@code start == end}
     * 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 {@code start} or {@code end} are negative,
     *                                   if {@code end} is greater than {@code length()},
     *                                   or if {@code start} is greater than {@code end}
     */
    // 返回该字符序列的子序列
    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
     */
    // 返回由这个字符序列组成的字符串
    String toString();
    
    /**
     * Compares two {@code CharSequence} instances lexicographically. Returns a
     * negative value, zero, or a positive value if the first sequence is lexicographically
     * less than, equal to, or greater than the second, respectively.
     *
     * <p>
     * The lexicographical ordering of {@code CharSequence} is defined as follows.
     * Consider a {@code CharSequence} <i>cs</i> of length <i>len</i> to be a
     * sequence of char values, <i>cs[0]</i> to <i>cs[len-1]</i>. Suppose <i>k</i>
     * is the lowest index at which the corresponding char values from each sequence
     * differ. The lexicographic ordering of the sequences is determined by a numeric
     * comparison of the char values <i>cs1[k]</i> with <i>cs2[k]</i>. If there is
     * no such index <i>k</i>, the shorter sequence is considered lexicographically
     * less than the other. If the sequences have the same length, the sequences are
     * considered lexicographically equal.
     *
     * @param cs1 the first {@code CharSequence}
     * @param cs2 the second {@code CharSequence}
     *
     * @return the value {@code 0} if the two {@code CharSequence} are equal;
     * a negative integer if the first {@code CharSequence}
     * is lexicographically less than the second; or a
     * positive integer if the first {@code CharSequence} is
     * lexicographically greater than the second.
     *
     * @since 11
     */
    // 按字典顺序比较两个字符序列
    @SuppressWarnings("unchecked")
    static int compare(CharSequence cs1, CharSequence cs2) {
        if(Objects.requireNonNull(cs1) == Objects.requireNonNull(cs2)) {
            return 0;
        }
        
        if(cs1.getClass() == cs2.getClass() && cs1 instanceof Comparable) {
            return ((Comparable<Object>) cs1).compareTo(cs2);
        }
        
        for(int i = 0, len = Math.min(cs1.length(), cs2.length()); i<len; i++) {
            char a = cs1.charAt(i);
            char b = cs2.charAt(i);
            if(a != b) {
                return a - b;
            }
        }
        
        return cs1.length() - cs2.length();
    }
    
    /**
     * 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.base/java/lang/Character.html#unicode">surrogate code
     * point</a> is passed through uninterpreted.
     *
     * <p>The stream binds to this sequence when the terminal stream operation
     * commences (specifically, for mutable sequences the spliterator for the
     * stream is <a href="../util/Spliterator.html#binding"><em>late-binding</em></a>).
     * If the sequence is modified during that operation then the result is
     * undefined.
     *
     * @return an IntStream of char values from this sequence
     *
     * @since 1.8
     */
    // 将当前char序列转为流序列，序列中每个元素是char
    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>The stream binds to this sequence when the terminal stream operation
     * commences (specifically, for mutable sequences the spliterator for the
     * stream is <a href="../util/Spliterator.html#binding"><em>late-binding</em></a>).
     * If the sequence is modified during that operation then the result is
     * undefined.
     *
     * @return an IntStream of Unicode code points from this sequence
     *
     * @since 1.8
     */
    // 将当前Unicode符号序列转为流序列，序列中每个元素是Unicode符号
    default IntStream codePoints() {
        class CodePointIterator implements PrimitiveIterator.OfInt {
            int cur = 0;
            
            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++;
                        // 返回值是Unicode符号编码值
                        return Character.toCodePoint(c1, c2);
                    }
                }
                return c1;
            }
            
            @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;
                }
            }
            
        }
        
        return StreamSupport.intStream(
            () -> Spliterators.spliteratorUnknownSize(new CodePointIterator(), Spliterator.ORDERED),
            Spliterator.ORDERED,
            false
        );
    }
}