String、StringBuilder、StringBuffer

所在包

java.lang

区别总结

String对象不可变验证（面试）

@Test
public void test1(){
    String str = "str, hello";
    StringBuilder sb = new StringBuilder(str);
    this.change(str, sb);
    System.out.println("change后，str = " + str);    
    System.out.println("change后，sb  = " + sb);        
}
public void change(String str, StringBuilder sb){
    str += "world";            
    sb.append("world");        
    System.out.println("change方法中，str = " + str);
    System.out.println("change方法中，sb  = " + sb);
    System.out.println("");
}
/*
打印结果：
change方法中，str = str, helloworld
change方法中，sb  = str, helloworld
change后，str = str, hello
change后，sb  = str, helloworld
*/

**

StringBuilder、StringBuffer详解

拼接字符的 效率 及 占用内存 验证

结论

注：在单线程情况下验证 效率： StringBuilder > StringBuffer > String

内存占用：StringBuilder = StringBuffer < String

代码验证

    public static void main(String[] args) {
        testString();
//        testStringBuilder();
//        testStringBuffer();
    }
    public static void testString(){
        String str = "";
        long s1 = System.currentTimeMillis();
        for (int i = 0; i < 100000; i++) {
            str += i;
        }
        System.out.println(str);
        long e1 = System.currentTimeMillis();
        long useMemory = Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();
        System.out.println("String用时(毫秒)：" + (e1 - s1) + ", 占用内存(字节)：" + useMemory);
    }
    public static void testStringBuilder(){
        long s2 = System.currentTimeMillis();
        StringBuilder sbuil = new StringBuilder();
        System.out.println(sbuil);
        for (int i = 0; i < 100000; i++) {
            sbuil.append(i);
        }
        long e2 = System.currentTimeMillis();
        long useMemory = Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();
        System.out.println("StringBuilder用时(毫秒)：" + (e2 - s2) + ", 占用内存(字节)：" + useMemory);
    }
    public static void testStringBuffer(){
        long s3 = System.currentTimeMillis();
        StringBuffer sbuff = new StringBuffer();
        System.out.println(sbuff);
        for (int i = 0; i < 100000; i++) {
            sbuff.append(i);
        }
        long e3 = System.currentTimeMillis();
        long useMemory = Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();
        System.out.println("StringBuffer 用时(毫秒)：" + (e3 - s3) + ", 占用内存(字节)：" + useMemory);
    }
/*
 结果（每次结果有差异）：
 String用时(毫秒)：36575, 占用内存(字节)：463297536
 StringBuilder用时(毫秒)：19, 占用内存(字节)：12519168
 StringBuffer 用时(毫秒)：34, 占用内存(字节)：12519168
 */