读 AbstractStringBuilder/StringBuilder/StringBuffer 源代码

1.  StringBuilder和StringBuffer都继承了AbstractStringBuilder。它们都是由可变的char[] value和int count组成。而AbstractStringBuilder中唯一的抽象方法是toString()。StringBuffer中所有的method都是synchronized的。

 

2.  StringBuilder和StringBuffer无参数的构造函数会分配大小为16的char array，而参数为String的构造函数也会比传入的String多分配16个char。

 

3.  AbstractStringBuilder.ensureCapacity(int minimumCapacity), 如果 minimumCapacity 大于length会取 minimumCapacity和 length*2 + 2中较大者作为新分配空间char array的大小。这个方法在char array扩展时都会被调用。

 

4.  trimToSize() 将char array缩小到count大小的空间。

 

5.  setLength(int newLength) 将 count变为newLength。如果newLength比原来的count大，扩展char array， 大于原来count部分的char用'\u0000' 填充。

 

6.  append(null) 的时候作为 append("null") 处理，与 String.valueOf() 同理。

 

7.  capacity()方法返回 count。

 

8.  System.arraycopy 可以用作在同一个array中移动部分数据，比如: System.arraycopy( array, 5 , array, 6, 10 ) 会将 array中的数据从第5个之后的10个数据往后移一位。

 

9.  reverse()方法是按CodePoint reverse的，但原来的char array中不成对的surrogate pairs，在reverse后很可能成为了成对的surrogate paris。因为不成对的surrogate pair会被当成两个BMP中的char处理。

 

10. StringBuilder 自己定义了 serialize 和 反 serialize方法：

/**
     * Save the state of the <tt>StringBuilder</tt> instance to a stream
     * (that is, serialize it).
     *
     * @serialData the number of characters currently stored in the string
     *             builder (<tt>int</tt>), followed by the characters in the
     *             string builder (<tt>char[]</tt>).   The length of the
     *             <tt>char</tt> array may be greater than the number of
     *             characters currently stored in the string builder, in which
     *             case extra characters are ignored.
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        s.defaultWriteObject();
        s.writeInt(count);
        s.writeObject(value);
    }

    /**
     * readObject is called to restore the state of the StringBuffer from
     * a stream.
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        s.defaultReadObject();
        count = s.readInt();
        value = (char[]) s.readObject();
    }

 

11. StringBuffer 自己写了 serialize 和 反 serialize方法：

 

 /**
     * Serializable fields for StringBuffer.
     *
     * @serialField value  char[]
     *              The backing character array of this StringBuffer.
     * @serialField count int
     *              The number of characters in this StringBuffer.
     * @serialField shared  boolean
     *              A flag indicating whether the backing array is shared.
     *              The value is ignored upon deserialization.
     */
    private static final java.io.ObjectStreamField[] serialPersistentFields =
    {
        new java.io.ObjectStreamField("value", char[].class),
        new java.io.ObjectStreamField("count", Integer.TYPE),
        new java.io.ObjectStreamField("shared", Boolean.TYPE),
    };

    /**
     * readObject is called to restore the state of the StringBuffer from
     * a stream.
     */
    private synchronized void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        java.io.ObjectOutputStream.PutField fields = s.putFields();
        fields.put("value", value);
        fields.put("count", count);
        fields.put("shared", false);
        s.writeFields();
    }

    /**
     * readObject is called to restore the state of the StringBuffer from
     * a stream.
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        java.io.ObjectInputStream.GetField fields = s.readFields();
        value = (char[])fields.get("value", null);
        count = fields.get("count", 0);
    }

 既然读入的时候忽略 shared，为什么还要写入呢？