Java中的char、Character和CharSequence的区别

char 与 Character

char是一种基本的数据类型，Character是char类型的包装类，即通过Character创建出来的是一种对象。 

Character是char的包装类，就像Integer和int，以及Long和long一样。

包装类和基本类型可以自动转换，这是jdk1.5(5.0)的新特性，叫做自动封箱和自动解封

自动拆箱、自动装箱

char ch='a';
 
Character ch1=ch;//自动封箱
 
Character c=new Character(a);
 
char c1=c;//自动解封

如何理解Java中的自动拆箱和自动装箱？ 

定义：基本数据类型和包装类之间可以自动地相互转换

理解：装箱就是自动将基本数据类型转换为封装类型，拆箱就是自动将封装类型转换为基本数据类型。

// 自动装箱
1. Integer a = 100;
// 自动拆箱
2. int b = a;

自动装箱，相当于Java编译器替我们执行了 Integer.valueOf(XXX);

自动拆箱，相当于Java编译器替我们执行了 Integer.intValue(XXX);

CharSequence

CharSequence是一个描述字符串结构的接口，在这个接口里面一般发现有三种常用的子类：

• 获取指定索引的字符：public char charAt​(int index) 
• 获取字符串长度：public int length​()
• 截取部分字符串：public CharSequence subSequence​(int start, int end)

截取部分字符串代码样例

public static void main(String[] args) {
    CharSequence str = "hello world";
    CharSequence sub = str.subSequence(6,11);
    System.out.println(sub);
}

CharSequence接口源码

package java.lang;
 
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;
 
/**
 * A CharSequence is a readable sequence of char values. This
 * interface provides uniform, read-only access to many different kinds of
 * char sequences.
 * A char value represents a character in the Basic
 * Multilingual Plane (BMP) or a surrogate. Refer to 
 * href="Character.html#unicode">Unicode Character Representation for details.
 *
 * 
 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 CharSequence 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 CharSequence instances as elements in a set or as keys in
 * a map. 
 *
 * @author Mike McCloskey
 * @since 1.4
 * @spec JSR-51
 */
 
public interface CharSequence {
 
    /**
     * Returns the length of this character sequence.  The length is the number
     * of 16-bit chars in the sequence.
     *
     * @return  the number of chars in this sequence
     */
    int length();
 
    /**
     * Returns the char value at the specified index.  An index ranges from zero
     * to length() - 1.  The first char value of the sequence is at
     * index zero, the next at index one, and so on, as for array
     * indexing.
     *
     * 
If the char value specified by the index is a
     * @docRoot}/java/lang/Character.html#unicode">surrogate, the surrogate
     * value is returned.
     *
     * @param   index   the index of the char value to be returned
     *
     * @return  the specified char value
     *
     * @throws  IndexOutOfBoundsException
     *          if the index argument is negative or not less than
     *          length()
     */
    char charAt(int index);
 
    /**
     * Returns a CharSequence that is a subsequence of this sequence.
     * The subsequence starts with the char value at the specified index and
     * ends with the char value at index end - 1.  The length
     * (in chars) of the
     * returned sequence is end - start, so if 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 start or end are negative,
     *          if end is greater than length(),
     *          or if start is greater than end
     */
    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 
     * href="{@docRoot}/java/lang/Character.html#unicode">surrogate code
     * point is passed through uninterpreted.
     *
     * 
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.
     *
     * 
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);
    }
}