BigInteger 使用了多少空间？

BigInteger 对象通常使用多少字节的内存？

BigInteger 内部使用int[]来表示您使用的巨大数字。 因此它确实取决于您存储在其中的数字的大小. The int[]如果当前数量不能动态适应，则会增长。

要获取您的字节数BigInteger实例现在使用，您可以利用Instrumentation接口，尤其是getObjectSize(Object) http://docs.oracle.com/javase/7/docs/api/java/lang/instrument/Instrumentation.html#getObjectSize%28java.lang.Object%29.

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 http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/math/BigInteger.java，其中写着：

/**
 * 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;