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boolean到底占几个字节

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boolean是4字节,boolean数组是1字节

1. 代码

https://stackoverflow.com/questions/383551/what-is-the-size-of-a-boolean-variable-in-java

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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];
        
        //clean
        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();
    }
}

/**
Size for LotsOfBooleans: 9092960
Average size: 90.9296
Size for LotsOfInts: 33599984
Average size: 335.99984
0
*/

2. 分析

结果int数组是boolean数组的4倍左右，int是4字节，boolean数组应是1字节

<<Java虚拟机规范>>:

  • 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。

    int是4字节，boolean就是4字节

  • Java虚拟机直接支持 boolean类型的数组,虚拟机的 navarra指令可以创建这种数组。boolean类型数组的访问与修改共用byte类型数组的baload和 bastore指令。

    两者字节相同时才能通用，byte是1字节，boolean数组也是1字节

but It depends on the virtual machine，上述只是规范，具体还是看虚拟机实现

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