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();
}