Array values return null constantly

Question

public void doMatrix(){
    double[][] arrayA = {{a11,a12,a13,a14},{a21,a22,a23,a24},{a31,a32,a33,a34},{a41,a42,a43,a44}};      
    double[][] arrayB = {{b1},{b2},{b3},{b4}};

    RealMatrix matrixA = MatrixUtils.createRealMatrix(arrayA);
    RealMatrix matrixB = MatrixUtils.createRealMatrix(arrayB);
    RealMatrix matrixInvA = new LUDecomposition(matrixA).getSolver().getInverse();
    RealMatrix matrixAmp = matrixInvA.multiply(matrixB);
    tryit = arrayA[2][2];

    }

The Value of tryit continues to return null when placed in a text box in my android main activity. Am I accessing the array wrong?.

a33 = -1*Zs1*(2*angleC1*(Sv1/Cv1)*Math.sqrt(1-Math.pow(angleC1*(Sv1/Cv1), 2))); — James Adams, Jul 23 '13 at 09:04
i have checked that the values of a11 - a44 contain the correct double values. (note they are large numbers) — James Adams, Jul 23 '13 at 09:05
We can assume that you store values in a11, a12, a13... prior to declare arrays, don't you? — Emilio, Jul 23 '13 at 09:05

Stephen C · Answer 1 · 2013-07-23T09:14:17.153

5

I'm afraid that you are confused.

If arrayA is declared to have type double[][] then it is impossible for arrayA[2][2] to be null.

The type of arrayA[2][2] is double, and null is not a member of that type. Not ever. The null value is only valid for reference types, and double is a primitive type, not a reference type.

Either the program is not as you are describing it, or something else is causing a different value to be placed into the text box.

(Now if arrayA was declared to be a Double[][], it would be a different matter.)

edited Jul 23 '13 at 09:14

answered Jul 23 '13 at 09:08

Stephen C

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Correct. Primitive types cannot be `null`. – Aritz Jul 23 '13 at 09:09
Agreed. If `a33` was `null` then the array initialization of `arrayA` would throw `NullPointerException` when unboxing `a33` – cyon Jul 23 '13 at 09:11
@cyon, `double` type cannot simply aquire `null` value. – Aritz Jul 23 '13 at 09:14
@XtremeBiker I know, but we do not see the declaration of `a33`. For all we know it could be a `Double`. – cyon Jul 23 '13 at 09:16
@cyon - what XtremeBiker is saying (correctly) is that the declaration and/or value of `a33` are irrelevant. – Stephen C Jul 23 '13 at 09:18

score 3 · Answer 2 · answered Jul 23 '13 at 09:09

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the code will work fine unless you have initialised all the variables. tryit maybe returning null as it is not initialised with a value.

answered Jul 23 '13 at 09:09

Abinash Sinha

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score 1 · Answer 3 · answered Jul 23 '13 at 09:07

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There is no issue in the syntax for accessing the array. arrayA[2][2] will return the value of the variable a33.You are getting null because probably the value of the variable a33 is null.

answered Jul 23 '13 at 09:07

MansoorShaikh

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Okay, i will rework through my code checking all previous syntax again, thankyou :) – James Adams Jul 23 '13 at 09:08

Array values return null constantly

3 Answers3