AbstractStringBuilder

看了下StringBuilder得源码，发现是继承了AbstractStringBuilder，StringBuilder里面得功能比如append，length（）等函数得实现是通过调用了AbstractStringBuilder类里面得函数来实现了，所以就看下AbstractStringBuilder是如何实现这些功能得。

AbstractStringBuilder 里面是通过char [] 来保存数据，在构造函数中指定缓冲区得大小

  

  /**
     * Creates an AbstractStringBuilder of the specified capacity.
     */
    AbstractStringBuilder(int capacity) {
        value = new char[capacity];
    }

既然是数组，那么大capacity 大小不够得时候，需要扩容，调用ensureCapacity来实现得。

  

  /**
     * Ensures that the capacity is at least equal to the specified minimum.
     * If the current capacity is less than the argument, then a new internal
     * array is allocated with greater capacity. The new capacity is the
     * larger of:
     * <ul>
     * <li>The {@code minimumCapacity} argument.
     * <li>Twice the old capacity, plus {@code 2}.
     * </ul>
     * If the {@code minimumCapacity} argument is nonpositive, this
     * method takes no action and simply returns.
     * Note that subsequent operations on this object can reduce the
     * actual capacity below that requested here.
     *
     * @param   minimumCapacity   the minimum desired capacity.
     */
    public void ensureCapacity(int minimumCapacity) {
        if (minimumCapacity > 0)
            ensureCapacityInternal(minimumCapacity);
    }

调用的ensureCapacityInternal

   

 /**
     * For positive values of {@code minimumCapacity}, this method
     * behaves like {@code ensureCapacity}, however it is never
     * synchronized.
     * If {@code minimumCapacity} is non positive due to numeric
     * overflow, this method throws {@code OutOfMemoryError}.
     */
    private void ensureCapacityInternal(int minimumCapacity) {
        // overflow-conscious code
        if (minimumCapacity - value.length > 0) {
            value = Arrays.copyOf(value,
                    newCapacity(minimumCapacity));
        }
    }

如何要扩大得容量比起当前得数据得大小要大，则拷贝数据。

 

   private int newCapacity(int minCapacity) {
        // overflow-conscious code
        int newCapacity = (value.length << 1) + 2;
        if (newCapacity - minCapacity < 0) {
            newCapacity = minCapacity;
        }
        return (newCapacity <= 0 || MAX_ARRAY_SIZE - newCapacity < 0)
            ? hugeCapacity(minCapacity)
            : newCapacity;
    }

新得大小是当前得大小得两倍 + 2，如果比要扩容得大小要小，就设置扩容得大小为传进来得大小，让newCapacity 跟Integer.MAX_VALUE对比，如果比其大，则报OutOfMemoryError 异常。

append（）函数可以穿很多种数据类型，所以有各个类型得重载，所以这里看下传String得实现。

    

/**
     * Appends the specified string to this character sequence.
     * <p>
     * The characters of the {@code String} argument are appended, in
     * order, increasing the length of this sequence by the length of the
     * argument. If {@code str} is {@code null}, then the four
     * characters {@code "null"} are appended.
     * <p>
     * Let <i>n</i> be the length of this character sequence just prior to
     * execution of the {@code append} method. Then the character at
     * index <i>k</i> in the new character sequence is equal to the character
     * at index <i>k</i> in the old character sequence, if <i>k</i> is less
     * than <i>n</i>; otherwise, it is equal to the character at index
     * <i>k-n</i> in the argument {@code str}.
     *
     * @param   str   a string.
     * @return  a reference to this object.
     */
    public AbstractStringBuilder append(String str) {
        if (str == null)
            return appendNull();
        int len = str.length();
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }

首先先确保append 上str后得容量足够，然后调用getChars ，接着更新当前得大小。看下getChars 得实现。

   

 /**
     * Characters are copied from this sequence into the
     * destination character array {@code dst}. The first character to
     * be copied is at index {@code srcBegin}; the last character to
     * be copied is at index {@code srcEnd-1}. The total number of
     * characters to be copied is {@code srcEnd-srcBegin}. The
     * characters are copied into the subarray of {@code dst} starting
     * at index {@code dstBegin} and ending at index:
     * <pre>{@code
     * dstbegin + (srcEnd-srcBegin) - 1
     * }</pre>
     *
     * @param      srcBegin   start copying at this offset.
     * @param      srcEnd     stop copying at this offset.
     * @param      dst        the array to copy the data into.
     * @param      dstBegin   offset into {@code dst}.
     * @throws     IndexOutOfBoundsException  if any of the following is true:
     *             <ul>
     *             <li>{@code srcBegin} is negative
     *             <li>{@code dstBegin} is negative
     *             <li>the {@code srcBegin} argument is greater than
     *             the {@code srcEnd} argument.
     *             <li>{@code srcEnd} is greater than
     *             {@code this.length()}.
     *             <li>{@code dstBegin+srcEnd-srcBegin} is greater than
     *             {@code dst.length}
     *             </ul>
     */
    public void getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin)
    {
        if (srcBegin < 0)
            throw new StringIndexOutOfBoundsException(srcBegin);
        if ((srcEnd < 0) || (srcEnd > count))
            throw new StringIndexOutOfBoundsException(srcEnd);
        if (srcBegin > srcEnd)
            throw new StringIndexOutOfBoundsException("srcBegin > srcEnd");
        System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);
    }

调用系统得arraycopy 数组拷贝函数来实现数据得拷贝。

  

  /**
     * Appends the string representation of the {@code boolean}
     * argument to the sequence.
     * <p>
     * The overall effect is exactly as if the argument were converted
     * to a string by the method {@link String#valueOf(boolean)},
     * and the characters of that string were then
     * {@link #append(String) appended} to this character sequence.
     *
     * @param   b   a {@code boolean}.
     * @return  a reference to this object.
     */
    public AbstractStringBuilder append(boolean b) {
        if (b) {
            ensureCapacityInternal(count + 4);
            value[count++] = 't';
            value[count++] = 'r';
            value[count++] = 'u';
            value[count++] = 'e';
        } else {
            ensureCapacityInternal(count + 5);
            value[count++] = 'f';
            value[count++] = 'a';
            value[count++] = 'l';
            value[count++] = 's';
            value[count++] = 'e';
        }
        return this;
    }

append bool类型得数据，还是比较容易看懂得。

看下replace 函数得实现

 

 /**
     * Replaces the characters in a substring of this sequence
     * with characters in the specified {@code String}. The substring
     * begins at the specified {@code start} and extends to the character
     * at index {@code end - 1} or to the end of the
     * sequence if no such character exists. First the
     * characters in the substring are removed and then the specified
     * {@code String} is inserted at {@code start}. (This
     * sequence will be lengthened to accommodate the
     * specified String if necessary.)
     *
     * @param      start    The beginning index, inclusive.
     * @param      end      The ending index, exclusive.
     * @param      str   String that will replace previous contents.
     * @return     This object.
     * @throws     StringIndexOutOfBoundsException  if {@code start}
     *             is negative, greater than {@code length()}, or
     *             greater than {@code end}.
     */
    public AbstractStringBuilder replace(int start, int end, String str) {
        if (start < 0)
            throw new StringIndexOutOfBoundsException(start);
        if (start > count)
            throw new StringIndexOutOfBoundsException("start > length()");
        if (start > end)
            throw new StringIndexOutOfBoundsException("start > end");

        if (end > count)
            end = count;
        int len = str.length();
        int newCount = count + len - (end - start);
        ensureCapacityInternal(newCount);

        System.arraycopy(value, end, value, start + len, count - end);
        str.getChars(value, start);
        count = newCount;
        return this;
    }

里面得实现得步骤跟append 函数得实现流程差不多，先判断 start ，end 是否满足，然后确保容量满足，接着调用System.arraycopy来实现数据拷贝。

 

AbstractStringBuilder是非线程安全得，里面得函数得实现没有用到synchronized 线程安全，但是AbstractStringBuilder 得运行速度比起线程安全要快。