2021-10-16 Java中StringBuffer的基本使用方法

• StringBuffer构造函数
• 方法
• • 常用函数
  • 不常用函数
• 其他资料

参考资料：Oracle官方文档-StringBuffer

//Constructor and Description
StringBuffer()
//Constructs a string buffer with no characters in it and an initial capacity of 16 characters.
StringBuffer(CharSequence seq)
//Constructs a string buffer that contains the same characters as the specified CharSequence.
StringBuffer(int capacity)
//Constructs a string buffer with no characters in it and the specified initial capacity.
StringBuffer(String str)
//Constructs a string buffer initialized to the contents of the specified string.

StringBuffer sb = new StringBuffer();

//Appends the string representation of the argument to this sequence.
sb.append(123); // = sb.append(String.valueOf(123));
sb.append("abc");

//Returns the current capacity.
sb.capacity();
//Returns the length (character count).
sb.length(); 
//Returns a string representing the data in this sequence.
sb.toString();

//Returns the char value in this sequence at the specified index.
sb.charAt​(int index);

//Compares two StringBuffer instances lexicographically.
sb.compareTo​(new StringBuffer("123"));

//Removes the characters in a substring of this sequence.
sb.delete​(int start, int end)
//Removes the char at the specified position in this sequence.
sb.deleteCharAt​(int index)

//Causes this character sequence to be replaced by the reverse of the sequence.
sb.reverse();

//Ensures that the capacity is at least equal to the specified minimum.
void	ensureCapacity​(int minimumCapacity)	

//Characters are copied from this sequence into the destination character array dst.
void	getChars​(int srcBegin, int srcEnd, char[] dst, int dstBegin)	

//Returns the index within this string of the first occurrence of the specified substring.
int	indexOf​(String str)	

//Inserts the string representation of the argument into this sequence.
StringBuffer	insert​(int offset, boolean/char/char[]/int/double/...)
//Inserts the string representation of a subarray of the str array argument into this sequence.
StringBuffer	insert​(int index, char[] str, int offset, int len)		

//The character at the specified index is set to ch.
void	setCharAt​(int index, char ch)	
//Sets the length of the character sequence.
void	setLength​(int newLength)	

//Returns a new String that contains a subsequence of characters currently contained in this character sequence.
String	substring​(int start)
//Returns a new String that contains a subsequence of characters currently contained in this sequence.
String	substring​(int start, int end)	

• stringbuffer和stringbuilder哪个效率高,应该使用哪个？
  1. stringbuffer时线程安全的，因为他的append方法会有同步锁，而stringbuilder时线程不安全的
  2. 单线程的时候使用stringbuilder，在多线程的时候使用stringbuffer
• StringBuffer和String的优缺点比较
  1. String的值是不可变的，这就导致每次对String的操作都会生成新的String对象，不仅效率低下，而且大量浪费有限的内存空间。
  2. StringBuffer是可变类，和线程安全的字符串操作类，任何对它指向的字符串的操作都不会产生新的对象。
  3. 一般情况下,速度从快到慢:StringBuilder>StringBuffer>String,这种比较是相对的，不是绝对的。
  4. 总结
     （1）.如果要操作少量的数据用 = String
     （2）.单线程操作字符串缓冲区 下操作大量数据 = StringBuilder
     （3）.多线程操作字符串缓冲区 下操作大量数据 = StringBuffer