腾讯面试官问我Java中boolean类型占用多少个字节?我说一个,面试官让我回家等通知

本文首发于微信公众号:程序员乔戈里







什么是boolean类型,根据官方文档的描述

boolean: The boolean data type has only two possible values: true and false. Use this data type for simple flags that track true/false conditions. This data type represents one bit of information, but its "size" isn't something that's precisely defined.

谷歌翻译一下:

布尔类型:布尔数据类型只有两个可能的值:真和假。使用此数据类型为跟踪真/假条件的简单标记。这种数据类型就表示这一点信息,但是它的“大小”并不是精确定义



stackoverflow就有关于boolean占几个字节的讨论。
what-is-the-size-of-a-boolean-variable-in-java
其中有一个高赞回答:

 /**
 *出自公众号:程序员乔戈里
 */
class LotsOfBooleans
{
    boolean a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, aa, ab, ac, ad, ae, af;
    boolean b0, b1, b2, b3, b4, b5, b6, b7, b8, b9, ba, bb, bc, bd, be, bf;
    boolean c0, c1, c2, c3, c4, c5, c6, c7, c8, c9, ca, cb, cc, cd, ce, cf;
    boolean d0, d1, d2, d3, d4, d5, d6, d7, d8, d9, da, db, dc, dd, de, df;
    boolean e0, e1, e2, e3, e4, e5, e6, e7, e8, e9, ea, eb, ec, ed, ee, ef;
}

class LotsOfInts
{
    int a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, aa, ab, ac, ad, ae, af;
    int b0, b1, b2, b3, b4, b5, b6, b7, b8, b9, ba, bb, bc, bd, be, bf;
    int c0, c1, c2, c3, c4, c5, c6, c7, c8, c9, ca, cb, cc, cd, ce, cf;
    int d0, d1, d2, d3, d4, d5, d6, d7, d8, d9, da, db, dc, dd, de, df;
    int e0, e1, e2, e3, e4, e5, e6, e7, e8, e9, ea, eb, ec, ed, ee, ef;
}


public class Test
{
    private static final int SIZE = 100000;

    public static void main(String[] args) throws Exception
    {
        LotsOfBooleans[] first = new LotsOfBooleans[SIZE];
        LotsOfInts[] second = new LotsOfInts[SIZE];

        System.gc();
        long startMem = getMemory();

        for (int i=0; i < SIZE; i++)
        {
            first[i] = new LotsOfBooleans();
        }

        System.gc();
        long endMem = getMemory();

        System.out.println ("Size for LotsOfBooleans: " + (endMem-startMem));
        System.out.println ("Average size: " + ((endMem-startMem) / ((double)SIZE)));

        System.gc();
        startMem = getMemory();
        for (int i=0; i < SIZE; i++)
        {
            second[i] = new LotsOfInts();
        }
        System.gc();
        endMem = getMemory();

        System.out.println ("Size for LotsOfInts: " + (endMem-startMem));
        System.out.println ("Average size: " + ((endMem-startMem) / ((double)SIZE)));

        // Make sure nothing gets collected
        long total = 0;
        for (int i=0; i < SIZE; i++)
        {
            total += (first[i].a0 ? 1 : 0) + second[i].a0;
        }
        System.out.println(total);
    }

    private static long getMemory()
    {
        Runtime runtime = Runtime.getRuntime();
        return runtime.totalMemory() - runtime.freeMemory();
    }
}

运行结果

Size for LotsOfBooleans: 8257544
Average size: 82.57544
Size for LotsOfInts: 33599984
Average size: 335.99984




Java虚拟机规范一书提到 :

  • 在Java虚拟机中没有任何供 boolean值专用的字节码指令,Java语言表达式所操作的
    boolean值,在编译之后都使用Java虚拟机中的int数据类型来代替。
  • Java虚拟机直接支持 boolean类型的数组,虚拟机的 navarra指令参见第6章的newarray小节可以创建这种数组。boolean类型数组的访问与修改共用byte类型数组的baload和 bastore指令

  • 因为在虚拟机规范中说了,boolean值在编译之后都使用Java虚拟机中的int数据类型来代替,而int是4个字节,那么boolean值就是4个字节。
  • boolean类型数组的访问与修改共用byte类型数组的baload和 bastore指令,因为两者共用,只有两者字节一样才能通用呀,所以byte数组中一个byte是1个字节,那么boolean数组中boolean是1个字节。
  • 总结:boolean在数组情况下为1个字节,单个boolean为4个字节。



Java规范中,没有明确指出boolean的大小。在《Java虚拟机规范》给出了单个boolean占4个字节,和boolean数组1个字节的定义,具体 还要看虚拟机实现是否按照规范来所以1个字节、4个字节都是有可能的

References:
[1] 官方文档的描述: http://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html
[2] what-is-the-size-of-a-boolean-variable-in-java: https://stackoverflow.com/questions/383551/what-is-the-size-of-a-boolean-variable-in-java
[3] 所以1个字节、4个字节都是有可能的: https://blog.csdn.net/makingadream/article/details/53100237

如果是头条用户,可以在我的头条号程序员乔戈里后台回复 资源获取价值59998元的编程和考研资料
觉得文章不错的欢迎关注我的WX公众号:程序员乔戈里
我是BAT大厂后台开发工程师,,专注分享技术干货/编程资源/求职面试/成长感悟等,关注送5000G编程资源和自己整理的一份帮助不少人拿下java的offer的面经附答案,免费下载CSDN资源。

posted on 2019-12-08 10:46  程序员乔戈里  阅读(...)  评论(...编辑  收藏

统计