JDK源码学习系列02----AbstractStringBuilder
因为看StringBuffer 和 StringBuilder 的源码时发现两者都继承了AbstractStringBuilder，并且很多方法都是直接super的父类AbstractStringBuilder的方法，所以还是决定先看AbstractStringBuilder的源码，然后再看StringBuffer 和 StringBuilder.
1.成员变量
AbstractStringBuilder和String一样，在其内部都是以字符数组的形式实现的。也就是String,StringBuffer以及StringBuilder在其内部都是以字符数组的形式实现的。
char value[];
int count;
2.构造函数
AbstractStringBuilder的构造函数中传入的capacity是指容量，实际长度是以leng中的count表示的，注意区分容量和实际长度。
AbstractStringBuilder() {
}
AbstractStringBuilder(int capacity) {
value = new char[capacity];
}
3.容量和长度
public int length() {
return count;
}
public int capacity() {
return value.length;
}
4.AbstractStringBuilder的扩容
public void ensureCapacity(int minimumCapacity) {
if (minimumCapacity > value.length) {//如果传入的容量大于原来的容量就扩容
expandCapacity(minimumCapacity);
}
}
void expandCapacity(int minimumCapacity) {
int newCapacity = (value.length + 1) * 2;//首先默认扩容为 （原容量+1）*2
if (newCapacity < 0) {
newCapacity = Integer.MAX_VALUE;
} else if (minimumCapacity > newCapacity) {//如果传入的容量大于 默认扩容量，则传入容量为新容量
newCapacity = minimumCapacity;
}
value = Arrays.copyOf(value, newCapacity);
}
5.字符串减少存储空间
如果实际长度小于容量，为了减少存储空间，就把容量缩小为刚好满足字符串长度。
public void trimToSize() {
if (count < value.length) {
value = Arrays.copyOf(value, count);
}
}
6.void setLength(int newLength)
public void setLength(int newLength) {
if (newLength < 0)
throw new StringIndexOutOfBoundsException(newLength);
if (newLength > value.length)//设置长度大于容量时先扩容
expandCapacity(newLength);
if (count < newLength) {//设置长度大于原来长度时，后面部分补以空白
for (; count < newLength; count++)
value[count] = '\0';
} else {
count = newLength;
}
}
7.char charAt(int index)
public char charAt(int index) {
if ((index < 0) || (index >= count))
throw new StringIndexOutOfBoundsException(index);
return value[index];
}
8.void getChars(int srcBegin, int srcEnd, char dst[], int dstBegin)
一定要注意参数情况的考虑，自己编程时一定要养成好习惯。主要是调用了System.arraycopy()的方法，以后具体分析。
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);
}
9.void setCharAt(int index, char ch)
方法的源码很简单，是直接替换该index的值，而不是后移之类的，因为是数组，不可变长度。
public void setCharAt(int index, char ch) {
if ((index < 0) || (index >= count))
throw new StringIndexOutOfBoundsException(index);
value[index] = ch;
}
10.append(String str)
对于StringBuffer和StringBuilder重要的append()方法的源码闪亮登场咯~~
public AbstractStringBuilder append(String str) {
if (str == null) str = "null";//若传入为null,则会在后面加上“null”
int len = str.length();
if (len == 0) return this;//传入长度为0，则返回本身
int newCount = count + len;
if (newCount > value.length)//传入后的长度大于容量就扩容
expandCapacity(newCount);
str.getChars(0, len, value, count);//用的是getChars()为value传值
count = newCount;
return this;
}
11.append(CharSequence s, int start, int end)
public AbstractStringBuilder append(CharSequence s, int start, int end) {
if (s == null)
s = "null";
if ((start < 0) || (end < 0) || (start > end) || (end > s.length()))//一定要注意情况考虑
throw new IndexOutOfBoundsException(
"start " + start + ", end " + end + ", s.length() "
+ s.length());
int len = end - start;
if (len == 0)
return this;
int newCount = count + len;
if (newCount > value.length)
expandCapacity(newCount);
for (int i=start; i<end; i++)//用的是charAt()为value循环赋值
value[count++] = s.charAt(i);//!!
count = newCount;
return this;
}
12.append(char str[])
若是字符数组，用的是System.arraycopy（）方法。
public AbstractStringBuilder append(char str[]) {
int newCount = count + str.length;
if (newCount > value.length)
expandCapacity(newCount);
System.arraycopy(str, 0, value, count, str.length);
count = newCount;
return this;
}
13.append(boolean b)
这个源码很简单
public AbstractStringBuilder append(boolean b) {
if (b) {
int newCount = count + 4;
if (newCount > value.length)
expandCapacity(newCount);
value[count++] = 't';
value[count++] = 'r';
value[count++] = 'u';
value[count++] = 'e';
} else {
int newCount = count + 5;
if (newCount > value.length)
expandCapacity(newCount);
value[count++] = 'f';
value[count++] = 'a';
value[count++] = 'l';
value[count++] = 's';
value[count++] = 'e';
}
return this;
}
14.append(int i)
public AbstractStringBuilder append(int i) {
if (i == Integer.MIN_VALUE) {
append("-2147483648");
return this;
}
int appendedLength = (i < 0) ? stringSizeOfInt(-i) + 1 : stringSizeOfInt(i);//!!!
int spaceNeeded = count + appendedLength;
if (spaceNeeded > value.length)
expandCapacity(spaceNeeded);
Integer.getChars(i, spaceNeeded, value);
count = spaceNeeded;
return this;
}
final static int [] sizeTable = { 9, 99, 999, 9999, 99999, 999999, 9999999,
99999999, 999999999, Integer.MAX_VALUE };
// Requires positive x
static int stringSizeOfInt(int x) {
for (int i=0; ; i++)
if (x <= sizeTable[i])
return i+1;
}
15.delete(int start, int end)
主要还是用的System.arraycopy()
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;
}
16.reverse()
这个是StringBuffer和StringBuilder常用到的方法，而String并没有这个牛逼的功能~~
public AbstractStringBuilder reverse() {
boolean hasSurrogate = false;
int n = count - 1;
for (int j = (n-1) >> 1; j >= 0; --j) {
char temp = value[j];
char temp2 = value[n - j];
if (!hasSurrogate) {
hasSurrogate = (temp >= Character.MIN_SURROGATE && temp <= Character.MAX_SURROGATE)
|| (temp2 >= Character.MIN_SURROGATE && temp2 <= Character.MAX_SURROGATE);
}
value[j] = temp2;
value[n - j] = temp;
}
if (hasSurrogate) {
// Reverse back all valid surrogate pairs
for (int i = 0; i < count - 1; i++) {
char c2 = value[i];
if (Character.isLowSurrogate(c2)) {
char c1 = value[i + 1];
if (Character.isHighSurrogate(c1)) {
value[i++] = c1;
value[i] = c2;
}
}
}
}
return this;
}
17. String toString()
这个只是一个抽象的方法，没有方法体。
public abstract String toString();