java基础复习4

String底层皆是字符数组
StringBuffer
可变字符串：内容改变，对象不变
案例：使用String，stringBUFFER，StringBuilder拼接三万次字符串，对比各自损耗的时间

public class CharSectionDemo {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		CharSectionDemo.stringTest();//1147ms
		CharSectionDemo.stringBufferTest();//2ms
		CharSectionDemo.stringBuilderTest();//1ms
	}
	public static void stringTest() {
		String str = "";
		long start = System.currentTimeMillis();
		for(int i = 0;i<30000;i++) {
			str +=i;
		}
		long end = System.currentTimeMillis();
		System.out.println(end-start);
	}
	public static void stringBuilderTest() {
		StringBuilder str = new StringBuilder("");
		long start = System.currentTimeMillis();
		for(int i = 0;i<30000;i++) {
			str.append(i);
		}
		long end = System.currentTimeMillis();
		System.out.println(end-start);
	}
	public static void stringBufferTest() {
		StringBuffer str = new StringBuffer();
		long start = System.currentTimeMillis();
		for(int i = 0;i<30000;i++) {
			str.append(i);
		}
		long end = System.currentTimeMillis();
		System.out.println(end-start);
	}

}

StringBuffer:线程安全，方法皆使用sychonized修饰
StringBuilder：线程不安全，但推荐使用

StringBuilder strBui = new StringBuilder(80);
		strBui.append("A");
		strBui.charAt(index);
		strBui.delete(start, end);
		strBui.deleteCharAt(index);