基于String不可变字符与StringBuilder可变字符的效率问题

String result = "abc" + String.valueOf(1);
String result = "abc" + 1;

String result = new StringBuffer().append("abc").append(1).append("def").append(2).toString();

package 可变与不可变的陷阱以及测试效率;
public class TimeTest {
	public static void main(String[] args) {
		StringTime();
		StringBuilderTime();
	}
	public static void StringTime() {
		long num1 = Runtime.getRuntime().freeMemory(); // 获取系统内存剩余空间
		long time1 = System.currentTimeMillis();// 获取系统当前时间
		String str = "";
		for (int i = 0; i < 1000; i++) {
			str = str + i;
		}
		// System.out.println(str);//
		long num2 = Runtime.getRuntime().freeMemory(); // 获取系统内存剩余空间
		long time2 = System.currentTimeMillis();// 获取系统当前时间
		System.out.println("String所用内存空间" + (num1 - num2));// 所用内存空间
		System.out.println("String所用时间" + (time2 - time1));// 所用时间
	}
	public static void StringBuilderTime() {
		long num1 = Runtime.getRuntime().freeMemory(); // 获取系统内存剩余空间
		long time1 = System.currentTimeMillis();// 获取系统当前时间
		StringBuilder str = new StringBuilder("");
		for (int i = 0; i < 1000; i++) {
			str.append(i);
		}
		// System.out.println(str);
		long num2 = Runtime.getRuntime().freeMemory(); // 获取系统内存剩余空间
		long time2 = System.currentTimeMillis();// 获取系统当前时间
		System.out.println("StringBuilder所用内存空间" + (num1 - num2));// 所用内存空间
		System.out.println("StringBuilder所用时间" + (time2 - time1));// 所用时间
	}
}

//StringBuilder线程不安全
public class StringBuilderDemo {
	public static void main(String[] args) {
		long beginTime = System.currentTimeMillis();//开始计时
		StringBuilder builder = new StringBuilder();
		for(int i=0;i<80000000;i++) {
			builder.append(i);
		}
		long endtime = System.currentTimeMillis();//结束计时
		System.out.println("StringBuilder所用时间:"+(endtime-beginTime));
	}
}

//StringBuffer线程安全
public class StringBufferDemo {
	public static void main(String[] args) {
		long beginTime1 = System.currentTimeMillis();//开始计时
		StringBuffer buffer = new StringBuffer();
		for(int i=0;i<80000000;i++) {
			buffer.append(i);
		}
		long endtime1 = System.currentTimeMillis();//结束计时
		System.out.println("StringBuffer所用时间:"+(endtime1-beginTime1));
	}
}