java学习笔记-String,StringBuffer,StringBuilder浅析

参考来源

String:定长的字符串,它的内容是不可以改变的,正因为如此,编译器可以让字符串共享.

StringBuffer:可变长的字符串缓冲区,多线程操作是安全的,并且在字符串连接操作上提供了性能和效率都优于String类的"+"的append()方法,因此如果需要大量的频繁的进行字符连接操作时,优先采用StringBuffer的append()方法.如果只是简单的字符串连接可以采用String的"+"来提高代码的可读性.

StringBuilder:是StringBuilder的一个等价类,区别在于StringBuilder是非线程安全的,但正因为如此少了很多同步的操作,在效率上要高于StringBuilder,因此如果不涉及多线程操作,可以优先考虑StringBuilder来提高方法的执行效率.

public class StringTest {
	/**
	 * main method
	 * 
	 * @param args
	 */
	public static void main(String[] args) {
		String s = "test";
		// String test
		String str = "";
		long str_start = System.currentTimeMillis();
		for (int i = 0; i < 10000; i++) {
			str += s;
		}
		long str_end = System.currentTimeMillis();
		System.out.println("String need time is : " + (str_end - str_start));

		// StringBuffer test
		StringBuffer s_buffer = new StringBuffer("");
		long buf_start = System.currentTimeMillis();
		for (int i = 0; i < 1000000; i++) {
			s_buffer.append(s);
		}
		long buf_end = System.currentTimeMillis();
		System.out.println("StringBuffer need time is :"
				+ (buf_end - buf_start));

		// StringBuilder testF
		StringBuilder s_builder = new StringBuilder("");
		long bui_start = System.currentTimeMillis();
		for (int i = 0; i < 1000000; i++) {
			s_builder.append(s);
		}
		long bui_end = System.currentTimeMillis();
		System.out.println("StringBuilder need time is : "
				+ (bui_end - bui_start));
	}
}

运行结果：

String need time is : 83485
StringBuffer need time is :15
StringBuilder need time is : 0

string中"+"实现字符串连接分析:

public class PlusTest {

	public static void main(String[] args) {
		String a = "a";
		String b = "b";
		String c = a + b;
		String d = c + 1;
	}
}

将该段代码编译与反编译（反编译参考）：

public class PlusTest
{

    public PlusTest()
    {
    }

    public static void main(String args[])
    {
        String a = "a";
        String b = "b";
        String c = (new StringBuilder(String.valueOf(a))).append(b).toString();
        String d = (new StringBuilder(String.valueOf(c))).append(1).toString();
    }
}

通过反编译的结果可以看出,"+"实现字符串连接的过程中,实际上是借助了StringBuilder类的append方法,每拼接一次都要实例化一个StringBuilder对象,效率低是必然的.