大数的功能实现可以不用做追究，知道用处就好的，感兴趣的可以看源码研究，构造器就不展示了
BigInteger：
public class BigInteger extends Number implements Comparable<BigInteger>{
final int signum;
final int[] mag;
private int bitCount;
private int bitLength;
private int lowestSetBit;
private int firstNonzeroIntNum;
final static long LONG_MASK = 0xffffffffL;
private static final int MAX_MAG_LENGTH = Integer.MAX_VALUE / Integer.SIZE + 1;
private static final int PRIME_SEARCH_BIT_LENGTH_LIMIT = 500000000;
private static final int KARATSUBA_THRESHOLD = 80;
private static final int TOOM_COOK_THRESHOLD = 240;
private static final int KARATSUBA_SQUARE_THRESHOLD = 128;
private static final int TOOM_COOK_SQUARE_THRESHOLD = 216;
static final int BURNIKEL_ZIEGLER_THRESHOLD = 80;
static final int BURNIKEL_ZIEGLER_OFFSET = 40;
private static final int SCHOENHAGE_BASE_CONVERSION_THRESHOLD = 20;
private static final int MULTIPLY_SQUARE_THRESHOLD = 20;
private static final int MONTGOMERY_INTRINSIC_THRESHOLD = 512;
//将val转为BigInteger类型
//0的BigInteger是ZERO
public static BigInteger valueOf(long val) {
if (val == 0)
return ZERO;
if (val > 0 && val <= MAX_CONSTANT)
return posConst[(int) val];
else if (val < 0 && val >= -MAX_CONSTANT)
return negConst[(int) -val];
return new BigInteger(val);
}
//数组加上val
public BigInteger add(BigInteger val) {
if (val.signum == 0)
return this;
if (signum == 0)
return val;
if (val.
