StringBuilder源码学习笔记

1、public final class StringBuilder extends AbstractStringBuilder
                        implements java.io.Serializable, CharSequence

2、几个构造函数
    默认构造函数创建一个长度为16的StringBuilder，调用父类的构造函数，父类AbstractStringBuilder维护了char[] value，int count。如果以String或者CharSequence对象作为参数，会创建长度为str.length() + 16的StringBuilder。

AbstractStringBuilder(int capacity) {
        value = new char[capacity];
}

public StringBuilder() {
    super(16);
}

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

public StringBuilder(String str) {
    super(str.length() + 16);
    append(str);
}

public StringBuilder(CharSequence seq) {
    this(seq.length() + 16);
    append(seq);
}

3、append方法，调用父类的append方法，如果参数的str是null，会直接append一个null字符串。内置许多重载的append方法。
@Override
public StringBuilder append(Object obj) {
    return append(String.valueOf(obj)); 
}
@Override
public StringBuilder append(String str) {
    super.append(str); 
    return this;
}

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;
}

private void ensureCapacityInternal(int minimumCapacity) {
    // overflow-conscious code
    if (minimumCapacity - value.length > 0) {
        value = Arrays.copyOf(value,
                newCapacity(minimumCapacity));
    }
}

public static char[] copyOf(char[] original, int newLength) {
    char[] copy = new char[newLength];
    System.arraycopy(original, 0, copy, 0,
                    Math.min(original.length, newLength));
    return copy;
}

4、StringBuilder扩容。扩容时会先生成一个newCapacity，为value.length*2+2，若其小于minCapacity，则直接扩容至minCapacity。一般情况下，newCapacity就是扩容之后的StringBuilder大小。
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;
    }

private int hugeCapacity(int minCapacity) {
    if (Integer.MAX_VALUE - minCapacity < 0) { // overflow
        throw new OutOfMemoryError();
    }
    return (minCapacity > MAX_ARRAY_SIZE)? minCapacity : MAX_ARRAY_SIZE;
}

5、delete方法。append和delete方法底层都是调用System.arraycopy方法。
@Override
public StringBuilder delete(int start, int end) {
    super.delete(start, end);
    return this;
}

public AbstractStringBuilder delete(int start, int end) {
    if (start < 0)
        throw new StringIndexOutOfBoundsException(start);
    if (end > count) 
        end = count;
    if (start > end)
        throw new StringIndexOutOfBoundsException();
    int len = end - start;
    if (len > 0) {
        System.arraycopy(value, start+len, value, start, count-end);
        count -= len;
    }
    return this;
}

@Override
public StringBuilder deleteCharAt(int index) {
    super.deleteCharAt(index);
    return this;
}

public AbstractStringBuilder deleteCharAt(int index) {
    if ((index < 0) || (index >= count))
        throw new StringIndexOutOfBoundsException(index);
    System.arraycopy(value, index+1, value, index, count-index-1);
    count--;
    return this;
}

6、insert方法。
@Override
public StringBuilder insert(int offset, String str) {
    super.insert(offset,  str);
    return this;
}

public AbstractStringBuilder insert(int offset, String str) {
    if ((offset < 0) || (offset > length()))
        throw new StringIndexOutOfBoundsException(offset);
    if (str == null)
        str = "null";
    int len = str.length();
    ensureCapacityInternal(count + len);
    System.arraycopy(value, offset, value, offset + len, count - offset);
    str.getChars(value, offset);
    count += len;
    return this;
}

public static native void arraycopy(Object src,  int  srcPos,
                                        Object dest, int destPos,
                                        int length);

7、toString方法，直接生成一个新的String对象。第二个是StringBuffer的toString方法，有一个String类型的toStringCache变量缓存了toString的内容。
@Override
public String toString() {
    // Create a copy, don't share the array
    return new String(value, 0, count);
}

@Override
public synchronized String toString() {
    if (toStringCache == null) {
        toStringCache = Arrays.copyOfRange(value, 0, count);
    }
    return new String(toStringCache, true);
}

8、序列化方法。
private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
    s.defaultWriteObject();
    s.writeInt(count);
    s.writeObject(value);
}

private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
    s.defaultReadObject();
    count = s.readInt();
    value = (char[]) s.readObject();
}