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StringBuffer 和 StringBuilder 的区别

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1. StringBuffer 和 StringBuilder

在 Java 中 String 是字符串常量，通过 String 声明的对象放在内存中是不变的，每次对该变量的拼接、截取等类似操作，都会产生一个新的对象。
而 StringBuilder 和 StringBuffer 都是字符串变量，常用于字符串拼接、截取等操作，该对象会发生相应变化，并不会产生一个新的对象；

StringBuilder 和 StringBuffer 的主要区别是：StringBuffer 是线程安全的，而 StringBuilder 是非线程安全的；

所以，通常来说，StringBuilder 执行速度相对 StringBuffer 要快，但是多线程情况下，应使用 StringBuffer 来保证数据的安全。

JDK 文档介绍 StringBuilder 如下:

A mutable sequence of characters. This class provides an API compatible with StringBuffer, but with no guarantee of synchronization. This class is designed for use as a drop-in replacement for StringBuffer in places where the string buffer was being used by a single thread (as is generally the case). Where possible, it is recommended that this class be used in preference to StringBuffer as it will be faster under most implementations.

StringBuilder 和 StringBuffer 速度测试代码：

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public static void main(String[] args) {
        int times = 20000000;

        {
            StringBuilder stringBuilder = new StringBuilder("");
            String helloWorld = "Hello World";
            Long begin = System.currentTimeMillis();
            for (int i = 0; i < times; i++) {
                stringBuilder.append(helloWorld);
            }
            System.out.println("StringBuilder Waste Time: " + (System.currentTimeMillis() - begin));
        }
        {
            StringBuffer stringBuffer = new StringBuffer("");
            String helloWorld = "Hello World";
            Long begin = System.currentTimeMillis();
            for (int i = 0; i < times; i++) {
                stringBuffer.append(helloWorld);
            }
            System.out.println("StringBuffer Waste Time: " + (System.currentTimeMillis() - begin));
        }

    }

运行结果如下：

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StringBuilder Waste Time: 853
StringBuffer Waste Time: 1273

Thanks! 😊