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StringBuilder

StringBuilder extends AbstractStringBuilder

/**     * Constructs a string builder with no characters in it and an     * initial capacity of 16 characters.     * 初始化数组容量为16     */ public StringBuilder() {        super(16);    }    /**     * Constructs a string builder with no characters in it and an     * initial capacity specified by the <code>capacity</code> argument.     *     * @param      capacity  the initial capacity.     * @throws     NegativeArraySizeException  if the <code>capacity</code>     *               argument is less than <code>0</code>.     */    public StringBuilder(int capacity) {        super(capacity);    }public AbstractStringBuilder append(String str) {        if (str == null) str = "null";        int len = str.length();        ensureCapacityInternal(count + len);/确保容量最小为原来字符数量+新添加的数量        str.getChars(0, len, value, count);//复制新 String 到value数组（value数组经过容量保证判断，放得下新字符串）中，连接到count下标后面。        count += len;        return this;    } public String toString() {        // Create a copy, don't share the array        //复制value数组中的字符串，长度为count，此时value数组可能未满即count<Capacity,不共享该value数组，依旧可以继续append        return new String(value, 0, count);    }

AbstractStringBuilder

char[] vaule;//采用char数组
int count;//数组char数量

//默认容量为16AbstractStringBuilder(int capacity) {        value = new char[capacity];    }    /**     * Returns the length (character count).     *     * @return  the length of the sequence of characters currently     *          represented by this object     * 数组字符长度     */    public int length() {        return count;    }    /**     * Returns the current capacity. The capacity is the amount of storage     * available for newly inserted characters, beyond which an allocation     * will occur.     *     * @return  the current capacity     * 数组容量     */    public int capacity() {        return value.length;    } public void ensureCapacity(int minimumCapacity) {        if (minimumCapacity > 0)            ensureCapacityInternal(minimumCapacity);    }    /**     * This method has the same contract as ensureCapacity, but is     * never synchronized.     * 保证数组容量至少大于该minimumCapacity，     * 若小于该minimumCapacity，则进行扩容     */    private void ensureCapacityInternal(int minimumCapacity) {        // overflow-conscious code        if (minimumCapacity - value.length > 0)            expandCapacity(minimumCapacity);    }    /**     * This implements the expansion semantics of ensureCapacity with no     * size check or synchronization.     * 扩容至原来数组字符长度的2倍+2，若依然小于minimumCapacity，则扩容至该值minimumCapacity     */    void expandCapacity(int minimumCapacity) {        int newCapacity = value.length * 2 + 2;        if (newCapacity - minimumCapacity < 0)            newCapacity = minimumCapacity;        if (newCapacity < 0) {            if (minimumCapacity < 0) // overflow                throw new OutOfMemoryError();            newCapacity = Integer.MAX_VALUE;        }        value = Arrays.copyOf(value, newCapacity);    }