os ：windows7 x64
jdk：jdk-8u131-windows-x64
ide：Eclipse Oxygen Release (4.7.0)


code：

package jizuiku1;

public class Demo100 {
	public static void main(String[] args) {
		StringBuffer sb = new StringBuffer();
		sb.append("cnblog");
		System.out.println(sb);
		System.out.println(sb.length());
		System.out.println(sb.capacity());
		
		System.out.println("------------------");
		
		sb.append("jizuiku jizuiku");
		System.out.println(sb);
		System.out.println(sb.length());
		System.out.println(sb.capacity());
		
		System.out.println("------------------");		
		
		sb.delete(0, sb.length()); //區間表示[0,sb.length)
		System.out.println(sb);
		System.out.println(sb.length());
		System.out.println(sb.capacity());
		System.out.println("------------------");
		
		sb.append("cnblog jizuiku jizuiku");
		System.out.println(sb);
		System.out.println(sb.length());
		System.out.println(sb.capacity());
		
		System.out.println("------------------");		
		
		sb.delete(0, sb.length()).trimToSize(); //區間表示[0,sb.length)
												//trimToSize 整理一下空間
		System.out.println(sb);
		System.out.println(sb.length());
		System.out.println(sb.capacity());
	}
}

result：

Java優秀，值得學習。
學習資源：API手冊+Java源碼+清凈的心地。

JavaSE8基礎 StringBuffer delete trimToSize 清空字符串緩沖區與整理緩沖區的空間