BigInteger使用多少空间？

Question

BigInteger对象一般使用多少字节的内存？

Answer 1

BigInteger在内部使用a int[]来表示您使用的巨大数字.因此,它实际上取决于您存储在其中的数字的大小.int[]如果当前数字不适合动态,则会增长.

要获取BigInteger实例当前使用的字节数,您可以使用该Instrumentation接口,尤其是getObjectSize(Object).

import java.lang.instrument.Instrumentation;

public class ObjectSizeFetcher {
    private static Instrumentation instrumentation;

    public static void premain(String args, Instrumentation inst) {
        instrumentation = inst;
    }

    public static long getObjectSize(Object o) {
        return instrumentation.getObjectSize(o);
    }
}

为了说服自己,请看一下源代码,它说:

/**
 * The magnitude of this BigInteger, in <i>big-endian</i> order: the
 * zeroth element of this array is the most-significant int of the
 * magnitude.  The magnitude must be "minimal" in that the most-significant
 * int ({@code mag[0]}) must be non-zero.  This is necessary to
 * ensure that there is exactly one representation for each BigInteger
 * value.  Note that this implies that the BigInteger zero has a
 * zero-length mag array.
 */
final int[] mag;

Answer 2

下面这个帖子:

BigInteger:
  int bitCount +4 bytes
  int bitLength +4 bytes
  int firstNonzeroIntNum +4 bytes
  int lowestSetBit +4 bytes
  int signum +4 bytes
  int[] mag +?

这总共是20个字节+整数数组.长度为N的整数数组的大小为4N + 24(数组开销+ 4字节/整数).

总共这会产生4N + 44个字节,具体取决于您的数字有多大.不要忘记对象的引用也使用内存.

编辑:16个额外字节作为对象开销,使其达到4N + 60字节.为此添加填充(每个对象使用8个字节的倍数),我们得到额外的4个字节.

这导致4N + 64字节.