BigInteger抛出空指针异常

Question

I'm having trouble getting a BigInteger to add another BigInteger to it. Any suggestions The relevant code:

Declared in class:

private BigInteger mTotientSum; 

Done in constructor:

BigInteger mTotientSum = BigInteger.ZERO;

In relative method:

  BigInteger totientMultiplier = BigInteger.valueOf(mTotientMulitplier);
  for (long i = 1; i <= mMaxNum; i++)
  {
     BigInteger j = BigInteger.valueOf(i);
     System.out.println(j.toString());
     BigInteger num = j.multiply(totientMultiplier);
     System.out.println(num.toString());
     BigInteger totient = calculateTotient(num);
     System.out.println(totient);
     mTotientSum = mTotientSum.add(totient); //This is line 113
     System.out.println("Sum at" + i + " = " + mTotientSum.toString());
  }

The output I get is:

1
510510
17
16
16
Exception in thread "main" java.lang.NullPointerException
          at Totient.run(Totient.java:113) (Note that line 113 is noted above.)
      at Totient.main(Totient.java:131)

Answer 1

You're shadowing the variable in the constructor. By calling

BigInteger mTotientSum = BigInteger.ZERO;

you're only initializing a local mTotientSum variable and leaving the class field null.

Don't re-declare the variable in the constructor. Instead do:

mTotientSum = BigInteger.ZERO;

See the difference?