标签：shtml JAVA 59073 tianya list groups http cn size

最近有朋友讨论说for循环里不要用list.size()，这样会导致每次都调用一次list.size()接口会耗性能，建议我们在外面使用一个变量来存储这个list.size()的值再放到循环里面，真的是这样吗？我做了个小测试，代码如下：

public class Test2 {
public static void main(String[] args) {
List list = new ArrayList();

    list.add("1");
    list.add("2");

    for (int i = 0; i < list.size(); i++) {
        System.out.println(i);
    }
}

}
这里将list.size()放在了for循环里，按朋友的说法，这里应该会调用多次。

接下来我查看了字节码发现，这里确实会调用多次list.size()方法，字节码如下：

具体字节码指令解析可查看：
接着查看了ArrayList调用size()方法的源码是直接返回一个int 型的 size变量的，而这个size变量是每次新增和删除都会改变这个size的值

结论：经过网友们热情的探讨与更正，for循环里会多次调用list.size()，list.size()方法会直接返回一个声明在类中的size全局变量，虽然这个size会直接保存在堆中，但每次发生函数调用确实会发生压栈出栈的性能开销。

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来源： https://blog.csdn.net/m0_51533776/article/details/112392886

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