StringBuilder、StringBuffer的API

常用的API，StringBuilder、StringBuffer的API是完全一致的

（1）StringBuffer append(xx)：拼接，追加

（2）StringBuffer insert(int index, xx)：在[index]位置插入xx

（3）StringBuffer delete(int start, int end)：删除[start,end)之间字符

StringBuffer deleteCharAt(int index)：删除[index]位置字符

（4）void setCharAt(int index, xx)：替换[index]位置字符

（5）StringBuffer reverse()：反转

（6）void setLength(int newLength) ：设置当前字符序列长度为newLength

（7）StringBuffer replace(int start, int end, String str)：替换[start,end)范围的字符序列为str

（8）int indexOf(String str)：在当前字符序列中查询str的第一次出现下标

​ int indexOf(String str, int fromIndex)：在当前字符序列[fromIndex,最后]中查询str的第一次出现下标

​ int lastIndexOf(String str)：在当前字符序列中查询str的最后一次出现下标

​ int lastIndexOf(String str, int fromIndex)：在当前字符序列[fromIndex,最后]中查询str的最后一次出现下标

（9）String substring(int start)：截取当前字符序列[start,最后]

（10）String substring(int start, int end)：截取当前字符序列[start,end)

（11）String toString()：返回此序列中数据的字符串表示形式

	@Test
	public void test6(){
		StringBuilder s = new StringBuilder("helloworld");
		s.setLength(30);
		System.out.println(s);
	}
	@Test
	public void test5(){
		StringBuilder s = new StringBuilder("helloworld");
		s.setCharAt(2, 'a');
		System.out.println(s);
	}
	
	
	@Test
	public void test4(){
		StringBuilder s = new StringBuilder("helloworld");
		s.reverse();
		System.out.println(s);
	}
	
	@Test
	public void test3(){
		StringBuilder s = new StringBuilder("helloworld");
		s.delete(1, 3);
		s.deleteCharAt(4);
		System.out.println(s);
	}
	
	
	@Test
	public void test2(){
		StringBuilder s = new StringBuilder("helloworld");
		s.insert(5, "java");
		s.insert(5, "H5");
		System.out.println(s);
	}
	
	@Test
	public void test1(){
		StringBuilder s = new StringBuilder();
		s.append("hello").append(true).append('a').append(12).append("atguigu");
		System.out.println(s);
		System.out.println(s.length());
	}

### 10.6.3 效率测试

/*
 * Runtime：JVM运行时环境
 * Runtime是一个单例的实现
 */
public class TestTime {
	public static void main(String[] args) {
//		testStringBuilder();
		testStringBuffer();
//		testString();
	}
	public static void testString(){
		long start = System.currentTimeMillis();
		String s = new String("0");
		for(int i=1;i<=10000;i++){
			s += i;
		}
		long end = System.currentTimeMillis();
		System.out.println("String拼接+用时："+(end-start));//444
		
		long memory = Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();
        System.out.println("String拼接+memory占用内存: " + memory);//53185144字节
	}
	public static void testStringBuilder(){
		long start = System.currentTimeMillis();
		StringBuilder s = new StringBuilder("0");
		for(int i=1;i<=10000;i++){
			s.append(i);
		}
		long end = System.currentTimeMillis();
		System.out.println("StringBuilder拼接+用时："+(end-start));//4
		long memory = Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();
        System.out.println("StringBuilder拼接+memory占用内存: " + memory);//1950488
	}
	public static void testStringBuffer(){
		long start = System.currentTimeMillis();
		StringBuffer s = new StringBuffer("0");
		for(int i=1;i<=10000;i++){
			s.append(i);
		}
		long end = System.currentTimeMillis();
		System.out.println("StringBuffer拼接+用时："+(end-start));//7
		long memory = Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();
        System.out.println("StringBuffer拼接+memory占用内存: " + memory);//1950488
	}
}