Java不可变序列String和可变序列StringBuilder、StringBuffer

String

String变量是不可变的，源码里面用了final修饰。

private final char value[];

String str = "Hello";

System.out.println(Integer.toHexString(str.hashCode()));

str = "World";  //字符串一旦变化就会重写生成一个对象

System.out.println(Integer.toHexString(str.hashCode()));

//输出

42628b2

4fe2b72

StringBuilder线程不安全，但效率高，常用。

StringBuffer线程安全，但效率低。

StringBuilder sb = new StringBuilder("Hello");

System.out.println(Integer.toHexString(sb.hashCode()));

sb.append(" World");    //追加

System.out.println(sb);

System.out.println(Integer.toHexString(sb.hashCode()));

//输出

45ee12a7

Hello World

45ee12a7

public class Test {

    public static void main(String[] args) {

        String str1 = "";

        long num1 = Runtime.getRuntime().freeMemory();

        long num2 = System.currentTimeMillis();

        for (int i = 0; i < 5000; i++) {

            str1 = str1 + i;

        }

        long num3 = Runtime.getRuntime().freeMemory();

        long num4 = System.currentTimeMillis();

        System.out.println("消耗内存:"+(num3-num1));

        System.out.println("消耗时间:"+(num4-num2));

        StringBuilder str2 = new StringBuilder("");

        long num5 = Runtime.getRuntime().freeMemory();

        long num6 = System.currentTimeMillis();

        for (int i = 0; i < 5000; i++) {

            str2.append(i);

        }

        long num7 = Runtime.getRuntime().freeMemory();

        long num8 = System.currentTimeMillis();

        System.out.println("消耗内存:"+(num7-num3));

        System.out.println("消耗时间:"+(num8-num4));

    }

}

//输出

消耗内存:-5895808

消耗时间:149

消耗内存:0

消耗时间:0