Java不可变序列String和可变序列StringBuilderStringBuffer

Posted 2022-06-13 chiweiming

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