JavaSE - day10 string buffer StringBuffer

Why introduce StingBuffer?

String is an immutable sequence of characters, which does not meet our needs, while StringBuffer is a mutable sequence of characters.

Difference between String and StringBuffer?

1. Mutability: String is an immutable sequence of characters, while StringBuffer is a mutable sequence of characters.

2. In memory: String will open up space in the in-memory constant pool when it is defined, which consumes more memory; StringBuffer (string buffer) will release memory after use.

Constructor in StringBuffer:

Constructor Summary
`StringBuffer()` Constructs a string buffer with no characters in it, with an initial capacity of 16 characters.
`StringBuffer(CharSequence seq)` public java.lang.StringBuilder(CharSequence seq) Constructs a string buffer containing the `CharSequence`same .
`StringBuffer(int capacity)` Constructs a string buffer with no characters but with the specified initial capacity.
`StringBuffer(String str)` Constructs a string buffer and initializes its contents to the specified string contents.

The acquisition function of StringBuffer:

int capacity()
Returns the current capacity. (If the capacity is exceeded, the system will automatically allocate (when storing strings, English))

int length()
Returns the length (number of characters).

Examples of get functions and constructors:

public class Demo {

	public static void main(String[] args) {
		StringBuffer sb =new StringBuffer();//Create a parameterless object sb
		System.out.println("sb:"+sb+".");//There is no output address value because toString is rewritten
		
		String s = new String("Helloworld");
// StringBuffer ssb = "helloworld";//String buffer has no such way of writing
		StringBuffer sb0 = new StringBuffer("helloworld");//Construct a string buffer and initialize its content to the specified string content.
		StringBuffer sb1 = new StringBuffer(s);//Construct a string buffer and initialize it to an object s of type String, namely "Helloworld"
		System.out.println(sb0);
		System.out.println(sb1);
		
		StringBuffer sb2 = new StringBuffer(20);//The capacity becomes 50
		System.out.println("length():"+sb2.length()+", no parameter Default capacity():"+sb.capacity()+", initialized to 20 capacity():"+sb2. capacity());
		
		
		StringBuffer sb3 = new StringBuffer(sb0);//The parameter should be of type CharSequence
		System.out.println(sb0);
		
	}

}

Added features of StringBuffer

`StringBuffer`	`append(boolean b)` Appends the string representation of the `boolean`parameter to the sequence.
`StringBuffer`	`append(char c)` Appends the string representation of the `char`parameter to this sequence.
`StringBuffer`	`append(char[] str)` Appends the string representation of the `char`array parameter to this sequence.
`StringBuffer`	`append(char[] str, int offset, int len)` Appends the string representation of the subarray of the `char`array parameter to this sequence.
`StringBuffer`	`append(CharSequence s)` `CharSequence`Appends the specified to the sequence.
`StringBuffer`	`append(CharSequence s, int start, int end)` `CharSequence`Appends the specified subsequence to this sequence.
`StringBuffer`	`append(double d)` Appends the string representation of the `double`parameter to this sequence.
`StringBuffer`	`append(float f)` Appends the string representation of the `float`parameter to this sequence.
`StringBuffer`	`append(int i)` Appends the string representation of the `int`parameter to this sequence.
`StringBuffer`	`append(long lng)` Appends the string representation of the `long`parameter to this sequence.
`StringBuffer`	`append(Object obj)` Append the string representation of the `Object`parameter .
`StringBuffer`	`append(String str)` Appends the specified string to this sequence of characters.
`StringBuffer`	`append(StringBuffer sb)` Appends the specified `StringBuffer` to this sequence.
`StringBuffer`	`appendCodePoint(int codePoint)` Appends the string representation of the `codePoint`parameter to this sequence.

The append method in the StringBuffer class seems to be able to append all types of data!

Add at specified location

`StringBuffer`	`insert(int offset, boolean b)` Insert the string representation of the `boolean`parameter into this sequence.
`StringBuffer`	`insert(int offset, char c)` Insert the string representation of the `char`parameter into this sequence.
`StringBuffer`	`insert(int offset, char[] str)` Inserts the string representation of the `char`array parameter into this sequence.
`StringBuffer`	`insert(int index, char[] str, int offset, int len)` `str`Inserts the string representation of a subarray of the array parameter into this sequence.
`StringBuffer`	`insert(int dstOffset, CharSequence s)` Insert the specified `CharSequence`into this sequence.
`StringBuffer`	`insert(int dstOffset, CharSequence s, int start, int end)` `CharSequence`Inserts the specified subsequence into this sequence.
`StringBuffer`	`insert(int offset, double d)` Insert the string representation of the `double`parameter into this sequence.
`StringBuffer`	`insert(int offset, float f)` Insert the string representation of the `float`parameter into this sequence.
`StringBuffer`	`insert(int offset, int i)` Insert the string representation of the `int`parameter into this sequence.
`StringBuffer`	`insert(int offset, long l)` Insert the string representation of the `long`parameter into this sequence.
`StringBuffer`	`insert(int offset, Object obj)` Inserts the string representation of the `Object`parameter into this sequence of characters.
`StringBuffer`	`insert(int offset, String str)` Insert a string into this sequence of characters.

Like the append method, the insert method can also insert almost any type! Returns the string buffer itself.

delete function

`StringBuffer`	`delete(int start, int end)` Removes characters from a substring of this sequence.
`StringBuffer`	`deleteCharAt(int index)` Removes the specified position of this sequence `char`.

One is to remove a string at a specified start and end, and the other is to remove a char at a specified position, both of which return StringBuffer itself.

Invert function

StringBuffer reverse()
将此字符序列用其反转形式取代。

就现在的观察来看，具有字符串反转方法的只有StringBuffer，String、Character、Integer等类中都没有reverse()这个方法。所以如果需要反转功能，就必须得先变成StringBuffer类，然后才能调用反转方法。

截取功能

`String`	`substring(int start)` 返回一个新的 `String`，它包含此字符序列当前所包含的字符子序列。
`String`	`substring(int start, int end)` 返回一个新的 `String`，它包含此序列当前所包含的字符子序列。

截取功能同String。

替换功能

StringBuffer replace(int start, int end, String str)
使用给定 String 中的字符替换此序列的子字符串中的字符。

格式不同于String，这个我们直接在形式参数里输入替代的值，被替代部分是用start--end这个区间锁定的。

StringBuffer,String,StringBuilder的区别?

StringBuffer和StringBuilder都是一个可变的字符序列,提供一个缓冲区.(两者都看做容器)
StringBuffer:线程安全的,同步的,执行效率低
StringBuilder:线程不安全的,不同步的,执行效率高,并且单线程中优先采用StringBuilder

StringBuffer 执行效率虽然低,但是由于String类型,并且他可变的字符序列,提供了缓冲区