【StringBuffer】

String和StingBuffer都可以存储和操作字符串，不同的是，String是常量字符串，不可以更改；而StringBuffer是变量字符串，底层是char[]数组，方便修改和扩充。

1、StringBuffer的创建

//方法一：未指定缓冲区大小，则默认为16
StringBuffer buffer1 = new StringBuffer();

//方法二：指定缓冲区大小
StringBuffer buffer2 = new StringBuffer(3);

//方法三：缓冲区大小为字符串的长度加16
String s = "abcde";
StringBuffer buffer3 = new StringBuffer(s);

2、 末尾添加字符串

StringBuffer buffer = new StringBuffer("abc");
buffer.append("de");

3、 指定位置插入字符串

StringBuffer buffer = new StringBuffer("ade");
buffer.insert(1,"bc");

 4、删除指定位置字符

StringBuffer buffer = new StringBuffer("abcde");
buffer.deleteCharAt(1);

5、替换连续区域字符串

StringBuffer buffer = new StringBuffer("abcde");
buffer.replace(1,3,"12");

 6、字符串反转

StringBuffer buffer = new StringBuffer("abcde");
buffer.reverse();

 7、截取指定开始位置到末尾的字符串

StringBuffer buffer = new StringBuffer("abcdefghi");
System.out.println(buffer.substring(3));

 8、截取指定开始位置到结束位置的字符串

StringBuffer buffer = new StringBuffer("abcdefghi");
System.out.println(buffer.substring(3, 5));

 9、返回当前容量

String s = "abdc";
StringBuffer buffer = new StringBuffer(s);

//缓冲区容量
System.out.println(buffer.capacity());
//实际字符串长度
System.out.println(buffer.length());