StringBuilder 类是可修改的字符串类，内部由可变数组构成，如果需要频繁修改字符串，字符串追加等，最好使用此类
此类只能在单线程中使用，但效率较高，如果要在多线程中使用，用StringBuffer类，两类API完全相同
Constructor
StringBuilder(); Constructs a string builder with no characters in it and an initial capacity of 16 characters.
StringBuilder(String str); Constructs a string builder initialized to the contents of the specified string.
Method:
追加
StringBuilder apend(int i); // append are overloaded so as to accept data of any type.
Appends the string representation of the int argument to the sequence.
StringBuilder append( s, int start, int end)
Appends a subsequence of the specified CharSequence to this sequence
StringBuilder sb = new StringBuilder("h");sb.append("a"); // sb is hasb.append("_hahehe", 1, 3); // sb is haha 插入
StringBuilder insert(int offset, double d) // insert are overloaded so as to accept data of any type.
Inserts the string representation of the double argument into this sequence.
获取值
char charAt(int index)
Returns the char value in this sequence at the specified index.
删除
StringBuilder delete(int start, int end)
Removes the characters in a substring of this sequence
StringBuilder deleteCharAt(int index)
Removes the char at the specified position in this sequence.
查找
int indexOf(String str, int fromindex) //indexOf(String str) lastIndexOf 等同String
Returns the index within this string of the first occurrence of the specified substring, starting at the specified index.
长度
int length()
替换
StringBuilder replace(int start, int end, String str)
Replaces the characters in a substring of this sequence with characters in the specified String.
修改
void setCharAt(int index, char ch)
The character at the specified index is set to ch.
反转
StringBuilder reverse()
Causes this character sequence to be replaced by the reverse of the sequence
子字符串
String subString(int start, int end)
Returns a new String that contains a subsequence of characters currently contained in this sequence.
String toString()
Returns a string representing the data in this sequence.
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