一、概述
StringBuffer 和 StringBuilder 兩者都繼承了AbstractStringBuilder、AbstractStringBuilder和String一樣，
在其內部都是以字符數組的形式實現的。也就是String,StringBuffer以及StringBuilder在其內部都是以字符數組的形式實現的。
二、類聲明
abstract class AbstractStringBuilder implements Appendable, CharSequence {...}
實現了兩個接口，其中CharSequence這個字符序列的接口如下：
該接口規定了需要實現該字符序列的長度:length()；
可以取得下標爲index的的字符：charAt(int index)；
可以得到該字符序列的一個子字符序列： subSequence(int start, int end)；
規定了該字符序列的String版本（重寫了父類Object的toString()）：toString();
Appendable接口顧名思義，定義添加的’規則’：
append(CharSequence csq) throws IOException:如何添加一個字符序列
append(CharSequence csq, int start, int end) throws IOException：如何添加一個字符序列的一部分
append(char c) throws IOException:如何添加一個字符
三、成員屬性
char[] value; //value爲該字符序列的具體存儲，
int count; //count爲實際存儲的數量
注：
（和String中的value和count不同，String中的這兩者都是不可變的(final修飾)，
並且不能對value[]直接操作；而AbstractStringBuilder的兩者都是可變的,
並且也定義了getValues方法讓我們可以直接拿到value[],value實際上是個動態數組，和ArrayList的實現有很多相似的地方）
四、構造函數
//無參構造
AbstractStringBuilder() {
}
//AbstractStringBuilder的構造函數中傳入的capacity是指容量，實際長度是以leng中的count表示
AbstractStringBuilder(int capacity) {
value = new char[capacity];
}
五、成員方法
1.length():返回已經存儲的實際長度（就是count值）
@Override
public int length() {
return count;
}
2.capacity():這個單詞是’容量’的意思,得到目前該value數組的實際大小
public int capacity() {
return value.length;
}
3.ensureCapacity(int minimumCapacity):確保value數組的容量是否夠用，如果不夠用，調用expandCapacity(minimumCapacity)方法擴容，參數爲需要的容量
public void ensureCapacity(int minimumCapacity) {
if (minimumCapacity > value.length) {
expandCapacity(minimumCapacity);
}
}
4.expandCapacity(int minimumCapacity):對數組進行擴容，參數爲需要的容量
void expandCapacity(int minimumCapacity) {
int newCapacity = (value.length + 1) * 2;
if (newCapacity < 0) {
newCapacity = Integer.MAX_VALUE;
} else if (minimumCapacity > newCapacity) {
newCapacity = minimumCapacity;
}
value = Arrays.copyOf(value, newCapacity);
}
擴容的算法：
如果調用了該函數，說明容量不夠用了，先將當前容量+1的二倍(newCapacity)與需要的容量(minimumCapacity)比較。
如果比需要的容量大，那就將容量擴大到容量+1的二倍；如果比需要的容量小，那就直接擴大到需要的容量。
使用Arrays.copyOf()這個非常熟悉的方法來使數組容量動態擴大
5.trimToSize():如果value數組的容量有多餘的，那麼就把多餘的全部都釋放掉
public void trimToSize() {
if (count < value.length) {
value = Arrays.copyOf(value, count);
}
}
6.setLength(int newLength):強制增大實際長度count的大小，容量如果不夠就用 expandCapacity()擴大；將擴大的部分全部用’\0’（ASCII碼中的null）來初始化
public void setLength(int newLength) {
if (newLength < 0)
throw new StringIndexOutOfBoundsException(newLength);
if (newLength > value.length)
expandCapacity(newLength);
if (count < newLength) {
for (; count < newLength; count++)
value[count] = '\0';
} else {
count = newLength;
}
}
7.charAt(int index):得到下標爲index的字符
public char charAt(int index) {
if ((index < 0) || (index >= count))
throw new StringIndexOutOfBoundsException(index);
return value[index];
}
8.codePointAt(int index):得到代碼點
public int codePointAt(int index) {
if ((index < 0) || (index >= count)) {
throw new StringIndexOutOfBoundsException(index);
}
return Character.codePointAt(value, index);
}
9.getChars(int srcBegin, int srcEnd, char dst[],int dstBegin):將value[]的[srcBegin, srcEnd)拷貝到dst[]數組的desBegin開始處
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);
}
10.substring(int start)/substring(int start, int end):得到子字符串
public String substring(int start) {
return substring(start, count);
}
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);
}
11.subSequence(int start, int end):得到一個子字符序列
public CharSequence subSequence(int start, int end) {
return substring(start, end);
}
12.reverse():將value給倒序存放（注意改變的就是本value，而不是創建了一個新的AbstractStringBuilder然後value爲倒序）
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;
}
toString():唯一的一個抽象方法：toString()
public abstract String toString();
唯一的一個final方法：getValue()，得到value數組。可以對其直接操作
final char[] getValue() {
return value;
}
CRUD操作：
改：
13.setCharAt(int index, char ch)：直接設置下標爲index的字符爲ch
public void setCharAt(int index, char ch) {
if ((index < 0) || (index >= count))
throw new StringIndexOutOfBoundsException(index);
value[index] = ch;
}
14.replace(int start, int end, String str)：用字符串str替換掉value[]數組的[start,end)部分
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);
if (newCount > value.length)
expandCapacity(newCount);
System.arraycopy(value, end, value, start + len, count - end);
str.getChars(value, start);
count = newCount;
return this;
}
增(AbstractStringBuilder類及其子類中最重要的操作；Appendable接口的具體實現)
其中append都表示’追加’，insert都表示’插入’：
15.append(Object obj)：利用Object（或任何對象）的toString方法轉成字符串然後添加到該value[]中
public AbstractStringBuilder append(Object obj) {
return append(String.valueOf(obj));
}
16.append()的核心代碼：append(String str)/append(StringBuffer sb)/append(CharSequence s)。直接修改value[]，並且’添加’的意思爲鏈接到原value[]的實際count的後面
public AbstractStringBuilder append(String str) {
if (str == null) str = "null";
int len = str.length();
if (len == 0) return this;
int newCount = count + len;
if (newCount > value.length)
expandCapacity(newCount);
str.getChars(0, len, value, count);
count = newCount;
return this;
}
// Documentation in subclasses because of synchro difference
public AbstractStringBuilder append(StringBuffer sb) {
if (sb == null)
return append("null");
int len = sb.length();
int newCount = count + len;
if (newCount > value.length)
expandCapacity(newCount);
sb.getChars(0, len, value, count);
count = newCount;
return this;
}
// Documentation in subclasses because of synchro difference
public AbstractStringBuilder append(CharSequence s) {
if (s == null)
s = "null";
if (s instanceof String)
return this.append((String)s);
if (s instanceof StringBuffer)
return this.append((StringBuffer)s);
return this.append(s, 0, s.length());
}
同時注意返回的都是AbstractStringBuilder，意味着append方法可以連續無限調用，即AbstractStringBuilder對象.append(參數1).append(參數2).append(參數三)…………;
17.append(CharSequence s, int start, int end):添加字符序列s的部分序列，範圍是[start,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()))
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++)
value[count++] = s.charAt(i);
count = newCount;
return this;
}
18.append(char str[]):添加一個字符數組
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;
}
19.append(char str[], int offset, int len):添加一個字符數組的一部分，該部分的範圍是[offset,offset+len);
public AbstractStringBuilder append(char str[], int offset, int len) {
int newCount = count + len;
if (newCount > value.length)
expandCapacity(newCount);
System.arraycopy(str, offset, value, count, len);
count = newCount;
return this;
}
20.append(boolean b):添加布爾值。
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;
}
21.append(char c):添加一個字符
public AbstractStringBuilder append(char c) {
int newCount = count + 1;
if (newCount > value.length)
expandCapacity(newCount);
value[count++] = c;
return this;
}
22.append(int i):添加一個整數
public AbstractStringBuilder append(int i) {
if (i == Integer.MIN_VALUE) {
append("-2147483648");
return this;
}
int appendedLength = (i < 0) ? stringSizeOfInt(-i) + 1
: stringSizeOfInt(i); //stringSizeOfInt方法在下面，得到參數整數的位數。
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 };
static int stringSizeOfInt(int x) {
for (int i=0; ; i++)
if (x <= sizeTable[i])
return i+1;
}
23.append(long l):添加一個長整型的數據，原理同上一個append
public AbstractStringBuilder append(long l) {
if (l == Long.MIN_VALUE) {
append("-9223372036854775808");
return this;
}
int appendedLength = (l < 0) ? stringSizeOfLong(-l) + 1
: stringSizeOfLong(l);
int spaceNeeded = count + appendedLength;
if (spaceNeeded > value.length)
expandCapacity(spaceNeeded);
Long.getChars(l, spaceNeeded, value);
count = spaceNeeded;
return this;
}
// Requires positive x
static int stringSizeOfLong(long x) {
long p = 10;
for (int i=1; i<19; i++) {
if (x < p)
return i;
p = 10*p;
}
return 19;
}
24.append(float f)/append(double d):添加一個浮點數。
public AbstractStringBuilder append(float f) {
new FloatingDecimal(f).appendTo(this);
return this;
}
public AbstractStringBuilder append(double d) {
new FloatingDecimal(d).appendTo(this);
return this;
}
以上是append
以下是insert
25.insert(int index, char str[], int offset,int len):(insert的核心代碼)在value[]的下標爲index位置插入數組str的一部分，該部分的範圍爲：[offset,offset+len);
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);
int newCount = count + len;
if (newCount > value.length)
expandCapacity(newCount);
System.arraycopy(value, index, value, index + len, count - index);
System.arraycopy(str, offset, value, index, len);
count = newCount;
return this;
}
26.insert(int offset, Object obj)：在value[]的offset位置插入Object（或者說所有對象）的String版。
public AbstractStringBuilder insert(int offset, Object obj) {
return insert(offset, String.valueOf(obj));
}
27.insert(int offset, String str):在value[]的offset位置插入字符串
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();
int newCount = count + len;
if (newCount > value.length)
expandCapacity(newCount);
System.arraycopy(value, offset, value, offset + len, count - offset);
str.getChars(value, offset);
count = newCount;
return this;
}
28.insert(int offset, char str[]):在value[]的offset位置插入字符數組
public AbstractStringBuilder insert(int offset, char str[]) {
if ((offset < 0) || (offset > length()))
throw new StringIndexOutOfBoundsException(offset);
int len = str.length;
int newCount = count + len;
if (newCount > value.length)
expandCapacity(newCount);
System.arraycopy(value, offset, value, offset + len, count - offset);
System.arraycopy(str, 0, value, offset, len);
count = newCount;
return this;
}
29.insert(int dstOffset, CharSequence s)/insert(int dstOffset, CharSequence s,int start, int end):插入字符序列
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;
if (len == 0)
return this;
int newCount = count + len;
if (newCount > value.length)
expandCapacity(newCount);
System.arraycopy(value, dstOffset, value, dstOffset + len,
count - dstOffset);
for (int i=start; i<end; i++)
value[dstOffset++] = s.charAt(i);
count = newCount;
return this;
}
30.插入基本類型：除了char是直接插入外，其他都是先轉成String，然後調用編號爲28的insert方法
public AbstractStringBuilder insert(int offset, boolean b) {
return insert(offset, String.valueOf(b));
}
public AbstractStringBuilder insert(int offset, char c) {
int newCount = count + 1;
if (newCount > value.length)
expandCapacity(newCount);
System.arraycopy(value, offset, value, offset + 1, count - offset);
value[offset] = c;
count = newCount;
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));
}
刪:
31.delete(int start, int end):刪掉value數組的[start,end)部分，並將end後面的數據移到start位置
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;
}
32.deleteCharAt(int index):刪除下標爲index的數據，並將後面的數據前移一位
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;
}
查(其實CRUD前面那幾個方法中有一兩個也算查找)：
33.indexOf(String str)：在value[]中找字符串str，若能找到，返回第一個字符串的第一個字符的下標
public int indexOf(String str) {
return indexOf(str, 0);
}
34.從fromIndex開始，在value[]中找字符串str，若能找到，返回第一個字符串的第一個字符的下標
public int indexOf(String str, int fromIndex) {
return String.indexOf(value, 0, count,str.toCharArray(), 0, str.length(),fromIndex);
}
35.lastIndexOf(String str):從後往前找
public int lastIndexOf(String str) {
return lastIndexOf(str, count);
}
36.lastIndexOf(String str, int fromIndex):從後往前到fromIndex，找子串str
public int lastIndexOf(String str, int fromIndex) {
return String.lastIndexOf(value, 0, count,
str.toCharArray(), 0, str.length(), fromIndex);
}
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