Why does my BigInteger.add() get a NullPointerException?
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03-06-2021 - |
Pergunta
I'm trying to make a Pascal Triangle print to a hundred rows, but Java's int seems to return negative values. I'm trying to use BigInteger, but I get NullPointerException whenever I add two numbers! I think I initialized them. Here is my code:
import java.math.BigInteger;
public class Pascal {
public static void main(String[] args) {
BigInteger[][] p = new BigInteger[100][];
p[0] = new BigInteger[3];
p[0][1] = BigInteger.ONE;
for (int i = 1; i <= N; i++) {
p[i] = new BigInteger[i + 3];
for(int j = 0; j < p[i].length; j++){
p[i][j] = new BigInteger("0");
}
for (int j = 1; j < p[i].length - 1; j++)
p[i][j] = p[i-1][j-1].add(p[i-1][j]); //NPE!
}
for (int i = 0; i <= N; i++) {
for (int j = 1; j < p[i].length - 1; j++) {
System.out.print(p[i][j] + " ");
}
System.out.println();
}
}
}
Solução
Consider the very first iteration of:
for (int j = 1; j < p[i].length - 1; j++)
p[i][j] = p[i-1][j-1].add(p[i-1][j]); //NPE!
The p[i-1][j-1]
dereferences p[0][0]
, which as far as I can see has not been initialized.
Outras dicas
These lines are concerning:
p[0][1] = BigInteger.ONE;
for (int i = 1; i <= N; i++)
You are initialising [0][1]
but not [0][0]
and you are starting your loops at index 1
not 0
(arrays are indexed at 0).
Consider what should go into position [0][0]
and how the array loops should start accordingly.
Declaring a reference name and type isn't sufficient; you have to allocate memory for the the reference to point to.
It's a classic with arrays. I see a lot of people who declare an array like this and wonder why they get NullPointerException when they try to use it:
int numValues = 10;
Integer [] values = new Integer[numValues]; // All ten of these references initially point to null
// You have to point them to something
for (int i = 0; i < numValues; ++i) {
values[i] = new Integer(i);
}