从源代码的角度聊聊java中StringBuffer、StringBuilder、String中的字符串拼接

//CharSequence定义
public interface CharSequence


//StringBuffer定义
 public final class StringBuffer
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence

//StringBuilder定义
public final class StringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence

//String定义
public final class String
    implements

public String() {
        this.offset = 0;
        this.count = 0;
        this.value = new char[0];
    }

    /**
     * Initializes a newly created {@code String} object so that it represents
     * the same sequence of characters as the argument; in other words, the
     * newly created string is a copy of the argument string. Unless an
     * explicit copy of {@code original} is needed, use of this constructor is
     * unnecessary since Strings are immutable.
     *
     * @param  original
     *         A {@code String}
     */
    public String(String original) {
        int size = original.count;
        char[] originalValue = original.value;
        char[] v;

        if (originalValue.length > size) {
            // The array representing the String is bigger than the new
            // String itself.  Perhaps this constructor is being called
            // in order to trim the baggage, so make a copy of the array.
            int off = original.offset;
            v = Arrays.copyOfRange(originalValue, off, off + size);
        } else {
            // The array representing the String is the same
            // size as the String, so no point in making a copy.
            v = originalValue;
        }

        this.offset = 0;
        this.count = size;
        this.value = v;
    }

    /**
     * Allocates a new {@code String} so that it represents the sequence of
     * characters currently contained in the character array argument. The
     * contents of the character array are copied; subsequent modification of
     * the character array does not affect the newly created string.
     *
     * @param  value
     *         The initial value of the string
     */
    public String(char[] value) {
        this.offset = 0;
        this.count = value.length;
        this.value = StringValue.from(value);
    }

public String concat(String str) {
        int otherLen = str.length();

        if (otherLen == 0) {
            return this;
        }

        char[] buf = new char[count + otherLen];
        getChars(0, count, buf, 0);
        str.getChars(0, otherLen, buf, count);

        return new String(0, count + otherLen, buf);
    }

public StringBuffer() {
    super(16);
    }


    public StringBuffer(int capacity) {
    super(capacity);
    }

abstract class AbstractStringBuilder implements Appendable, CharSequence {
    final static int[] sizeTable = {
            9, 99, 999, 9999, 99999, 999999, 9999999, 99999999, 999999999,
            Integer.MAX_VALUE
        };

    /**
     * The value is used for character storage.
     */
    char[] value;

    /**
     * The count is the number of characters used.
     */
    int count;

    /**
     * This no-arg constructor is necessary for serialization of subclasses.
     */
    AbstractStringBuilder() {
    }

    /**
     * Creates an AbstractStringBuilder of the specified capacity.
     */
    AbstractStringBuilder(int capacity) {
        value = new char[capacity];
    }
//其他的一些逻辑
}

public StringBuilder append(String str) {
        super.append(str);
        return this;
    }

/**
     *  value 用来存储字符串.
     */
    char value[];

    /** 
     * 有效字符串的数目.
     */
    int count;


    public AbstractStringBuilder append(String str) {
        if (str == null) {
            str = "null";
        }

        int len = str.length();
        if (len == 0) {
            return this;
        }

        int newCount = count + len;
        if (newCount > value.length) {
            expandCapacity(newCount);
        }
       //getChars将字符串复制到指定的位置
        str.getChars(0, len, value, count);
        count = newCount;
        return this;
    }

/**
     * This implements the expansion semantics of ensureCapacity with no
     * size check or synchronization.
     */
    void expandCapacity(int minimumCapacity) {
        int newCapacity = (value.length + 1) * 2;

        if (newCapacity < 0) {
            newCapacity = Integer.MAX_VALUE;
        } else if (minimumCapacity > newCapacity) {
            newCapacity = minimumCapacity;
        }

        value = Arrays.copyOf(value, newCapacity);
    }

/**
     * Constructs a string buffer with no characters in it and an 
     * initial capacity of 16 characters. 
     */
    public StringBuffer() {
    super(16);
    }

    /**
     * Constructs a string buffer with no characters in it and 
     * the specified initial capacity. 
     *
     * @param      capacity  the initial capacity.
     * @exception  NegativeArraySizeException  if the <code>capacity</code>
     *               argument is less than <code>0</code>.
     */
    public StringBuffer(int capacity) {
    super(capacity);
    }

public synchronized StringBuffer append(String str) {
    super.append(str);
        return this;
    }

//下面只是节选一些StringBuffer中的函数
synchronized StringBuffer     append(char ch)
synchronized StringBuffer     append(char[] chars)
synchronized StringBuffer     append(char[] chars, int start, int length)
synchronized StringBuffer     append(Object obj)
synchronized StringBuffer     append(String string)
synchronized StringBuffer     append(StringBuffer sb)
synchronized StringBuffer     append(CharSequence s)
synchronized StringBuffer     append(CharSequence s, int start, int end)
synchronized StringBuffer     insert(int index, char ch)
synchronized StringBuffer     insert(int index, char[] chars)
synchronized StringBuffer     insert(int index, char[] chars, int start, int length)
synchronized StringBuffer     insert(int index, String string)
StringBuffer     insert(int index, Object obj)