一、类签名
AbstractStringBuilder是StringBuilder和StringBuffer的父类，包含字符串操作的实现逻辑，子类根据各自需求对方法调用做同步处理，本身是线程不安全的。作为可变字符串的父类，AbstractStringBuilder借助字符数组的形式实现可变字符串，类中大部分方法是字符修改、插入、追加等操作。
abstract class AbstractStringBuilder implements Appendable, CharSequence
实现CharSequence，所以两个子类StringBuilder和StringBuffer的实例可以直接赋值给CharSequence。
二、数据成员
char[] value; // 保存字符串的数组
int count; // 记录字符长度
三、构造方法
// 必要的无参构造方法，用于序列化和子类
AbstractStringBuilder() { }
// 通过指定容量构造字符串数组
AbstractStringBuilder(int capacity) {
value = new char[capacity];
}
四、成员方法
4.1 长度相关
已保存在字符串数组的字符个数
@Override
public int length() {
return count;
}
字符串数组的空间长度，包含可插入新字符的空间。在同一时刻，总有length()小于等于capacity()
public int capacity() {
return value.length;
}
4.2 增加容量
保证字符数组大小至少要和具体的最小值相等。若当前容量小于参数具体值，则创建一个更大的数组，数值取下列之大者：
- minimumCapacity的具体大小；
- 原容量值的两倍
若minimumCapacity不可用，下列直接退出。本类有方法减少字符数组的长度
public void ensureCapacity(int minimumCapacity) {
if (minimumCapacity > 0)
ensureCapacityInternal(minimumCapacity);
}
// 包含参数检查，最小容量不得小于已有字符串数组长度，没有同步特性
private void ensureCapacityInternal(int minimumCapacity) {
// 仅minimumCapacity大于数组长度可用
if (minimumCapacity - value.length > 0)
expandCapacity(minimumCapacity);
}
// 容量扩增方法，没有大小检查或加入同步特性
void expandCapacity(int minimumCapacity) {
int newCapacity = value.length * 2 + 2;
if (newCapacity - minimumCapacity < 0)
newCapacity = minimumCapacity;
if (newCapacity < 0) {
if (minimumCapacity < 0) // 下溢
throw new OutOfMemoryError();
newCapacity = Integer.MAX_VALUE;
}
value = Arrays.copyOf(value, newCapacity);
}
4.3 裁剪、增加
可变字符数组有空余用于插入更多字符，可以通过裁剪剩余空间达到提高内存使用效率的目的。
如果内存空间不紧张或没有特殊要求，不要使用这个方法，因为向裁剪后的字符数组添加字符一定会引起扩容。而且裁剪也是一次数据内容拷贝的过程，超大数组的情况下可能会引起性能问题。
实现方式：把字符串拷贝到长度刚好合适的字符数组中返回，释放原数组空间
public void trimToSize() {
if (count < value.length) {
value = Arrays.copyOf(value, count);
}
}
通过添加字符'\0'，令字符串长度达到newLength，仅在newLength大于数组内字符串长度有效。
public void setLength(int newLength) {
if (newLength < 0)
throw new StringIndexOutOfBoundsException(newLength);
ensureCapacityInternal(newLength);
if (count < newLength) {
Arrays.fill(value, count, newLength, '\0');
}
count = newLength;
}
4.4 查找
返回指定索引值下的字符
@Override
public char charAt(int index) {
if ((index < 0) || (index >= count))
throw new StringIndexOutOfBoundsException(index);
return value[index];
}
返回指定索引下字符的编码值
public int codePointAt(int index) {
if ((index < 0) || (index >= count)) {
throw new StringIndexOutOfBoundsException(index);
}
return Character.codePointAtImpl(value, index, count);
}
返回指定索引前一位字符的编码值
public int codePointBefore(int index) {
int i = index - 1;
if ((i < 0) || (i >= count)) {
throw new StringIndexOutOfBoundsException(index);
}
return Character.codePointBeforeImpl(value, index, 0);
}
public int codePointCount(int beginIndex, int endIndex) {
if (beginIndex < 0 || endIndex > count || beginIndex > endIndex) {
throw new IndexOutOfBoundsException();
}
return Character.codePointCountImpl(value, beginIndex, endIndex-beginIndex);
}
public int offsetByCodePoints(int index, int codePointOffset) {
if (index < 0 || index > count) {
throw new IndexOutOfBoundsException();
}
return Character.offsetByCodePointsImpl(value, 0, count,
index, codePointOffset);
}
复制指定范围的字符串到给定的字符数组中
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);
}
4.5 append
public AbstractStringBuilder append(Object obj) {
return append(String.valueOf(obj));
}
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;
}
// 因为同步特性的差异，具体请看子类的文档说明(注:StringBuilder和StringBuffer)
public AbstractStringBuilder append(StringBuffer sb) {
if (sb == null)
return appendNull();
int len = sb.length();
ensureCapacityInternal(count + len);
sb.getChars(0, len, value, count);
count += len;
return this;
}
AbstractStringBuilder append(AbstractStringBuilder asb) {
if (asb == null)
return appendNull();
int len = asb.length();
ensureCapacityInternal(count + len);
asb.getChars(0, len, value, count);
count += len;
return this;
}
// 因为同步特性的差异，具体请看子类的文档说明(注:StringBuilder和StringBuffer)
@Override
public AbstractStringBuilder append(CharSequence s) {
if (s == null)
return appendNull();
if (s instanceof String)
return this.append((String)s);
if (s instanceof AbstractStringBuilder)
return this.append((AbstractStringBuilder)s);
return this.append(s, 0, s.length());
}
private AbstractStringBuilder appendNull() {
int c = count;
ensureCapacityInternal(c + 4);
final char[] value = this.value;
value[c++] = 'n';
value[c++] = 'u';
value[c++] = 'l';
value[c++] = 'l';
count = c;
return this;
}
@Override
public AbstractStringBuilder append(CharSequence s, int start, int end) {
if (s == null)
s = "null";
if ((start < 0) || (start > end) || (end > s.length()))
throw new IndexOutOfBoundsException(
"start " + start + ", end " + end + ", s.length() "
+ s.length());
int len = end - start;
ensureCapacityInternal(count + len);
for (int i = start, j = count; i < end; i++, j++)
value[j] = s.charAt(i);
count += len;
return this;
}
public AbstractStringBuilder append(char[] str) {
int len = str.length;
ensureCapacityInternal(count + len);
System.arraycopy(str, 0, value, count, len);
count += len;
return this;
}
public AbstractStringBuilder append(char str[], int offset, int len) {
if (len > 0)
ensureCapacityInternal(count + len);
System.arraycopy(str, offset, value, count, len);
count += len;
return this;
}
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;
}
@Override
public AbstractStringBuilder append(char c) {
ensureCapacityInternal(count + 1);
value[count++] = c;
return this;
}
public AbstractStringBuilder append(int i) {
if (i == Integer.MIN_VALUE) {
append("-2147483648");
return this;
}
int appendedLength = (i < 0) ? Integer.stringSize(-i) + 1
: Integer.stringSize(i);
int spaceNeeded = count + appendedLength;
ensureCapacityInternal(spaceNeeded);
Integer.getChars(i, spaceNeeded, value);
count = spaceNeeded;
return this;
}
public AbstractStringBuilder append(long l) {
if (l == Long.MIN_VALUE) {
append("-9223372036854775808");
return this;
}
int appendedLength = (l < 0) ? Long.stringSize(-l) + 1
: Long.stringSize(l);
int spaceNeeded = count + appendedLength;
ensureCapacityInternal(spaceNeeded);
Long.getChars(l, spaceNeeded, value);
count = spaceNeeded;
return this;
}
public AbstractStringBuilder append(float f) {
FloatingDecimal.appendTo(f,this);
return this;
}
eturn a reference to this object.
*/
public AbstractStringBuilder append(double d) {
FloatingDecimal.appendTo(d,this);
return this;
}
public AbstractStringBuilder appendCodePoint(int codePoint) {
final int count = this.count;
if (Character.isBmpCodePoint(codePoint)) {
ensureCapacityInternal(count + 1);
value[count] = (char) codePoint;
this.count = count + 1;
} else if (Character.isValidCodePoint(codePoint)) {
ensureCapacityInternal(count + 2);
Character.toSurrogates(codePoint, value, count);
this.count = count + 2;
} else {
throw new IllegalArgumentException();
}
return this;
}
4.6 移除字符
移除字符串
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;
}
移除一个字符
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;
}
4.7 替换
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;
}
public void setCharAt(int index, char ch) {
if ((index < 0) || (index >= count))
throw new StringIndexOutOfBoundsException(index);
value[index] = ch;
}
4.8 获取子序列
从指定起点获取后面字符序列，结果是String
public String substring(int start) {
return substring(start, count);
}
获取给定范围内的字符序列，结果是CharSequence
@Override
public CharSequence subSequence(int start, int end) {
return substring(start, end);
}
获取给定范围内的字符序列，结果是String
public String substring(int start, int end) {
if (start < 0)
throw new StringIndexOutOfBoundsException(start);
if (end > count)
throw new StringIndexOutOfBoundsException(end);
if (start > end)
throw new StringIndexOutOfBoundsException(end - start);
return new String(value, start, end - start);
}
4.9 insert
public AbstractStringBuilder insert(int index, char[] str, int offset,
int len)
{
if ((index < 0) || (index > length()))
throw new StringIndexOutOfBoundsException(index);
if ((offset < 0) || (len < 0) || (offset > str.length - len))
throw new StringIndexOutOfBoundsException(
"offset " + offset + ", len " + len + ", str.length "
+ str.length);
ensureCapacityInternal(count + len);
System.arraycopy(value, index, value, index + len, count - index);
System.arraycopy(str, offset, value, index, len);
count += len;
return this;
}
public AbstractStringBuilder insert(int offset, Object obj) {
return insert(offset, String.valueOf(obj));
}
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 AbstractStringBuilder insert(int offset, char[] str) {
if ((offset < 0) || (offset > length()))
throw new StringIndexOutOfBoundsException(offset);
int len = str.length;
ensureCapacityInternal(count + len);
System.arraycopy(value, offset, value, offset + len, count - offset);
System.arraycopy(str, 0, value, offset, len);
count += len;
return this;
}
public AbstractStringBuilder insert(int dstOffset, CharSequence s) {
if (s == null)
s = "null";
if (s instanceof String)
return this.insert(dstOffset, (String)s);
return this.insert(dstOffset, s, 0, s.length());
}
public AbstractStringBuilder insert(int dstOffset, CharSequence s,
int start, int end) {
if (s == null)
s = "null";
if ((dstOffset < 0) || (dstOffset > this.length()))
throw new IndexOutOfBoundsException("dstOffset "+dstOffset);
if ((start < 0) || (end < 0) || (start > end) || (end > s.length()))
throw new IndexOutOfBoundsException(
"start " + start + ", end " + end + ", s.length() "
+ s.length());
int len = end - start;
ensureCapacityInternal(count + len);
System.arraycopy(value, dstOffset, value, dstOffset + len,
count - dstOffset);
for (int i=start; i<end; i++)
value[dstOffset++] = s.charAt(i);
count += len;
return this;
}
public AbstractStringBuilder insert(int offset, boolean b) {
return insert(offset, String.valueOf(b));
}
public AbstractStringBuilder insert(int offset, char c) {
ensureCapacityInternal(count + 1);
System.arraycopy(value, offset, value, offset + 1, count - offset);
value[offset] = c;
count += 1;
return this;
}
public AbstractStringBuilder insert(int offset, int i) {
return insert(offset, String.valueOf(i));
}
public AbstractStringBuilder insert(int offset, long l) {
return insert(offset, String.valueOf(l));
}
public AbstractStringBuilder insert(int offset, float f) {
return insert(offset, String.valueOf(f));
}
public AbstractStringBuilder insert(int offset, double d) {
return insert(offset, String.valueOf(d));
}
4.10 查索引
public int indexOf(String str) {
return indexOf(str, 0);
}
public int indexOf(String str, int fromIndex) {
return String.indexOf(value, 0, count, str, fromIndex);
}
public int lastIndexOf(String str) {
return lastIndexOf(str, count);
}
public int lastIndexOf(String str, int fromIndex) {
return String.lastIndexOf(value, 0, count, str, fromIndex);
}
4.11 字符序列翻转
public AbstractStringBuilder reverse() {
boolean hasSurrogates = false;
int n = count - 1;
for (int j = (n-1) >> 1; j >= 0; j--) {
int k = n - j;
char cj = value[j];
char ck = value[k];
value[j] = ck;
value[k] = cj;
if (Character.isSurrogate(cj) ||
Character.isSurrogate(ck)) {
hasSurrogates = true;
}
}
if (hasSurrogates) {
reverseAllValidSurrogatePairs();
}
return this;
}
private void reverseAllValidSurrogatePairs() {
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;
}
}
}
}
4.12 其他
获取字符串序列
final char[] getValue() {
return value;
}
