Abnormal ArrayList NullPointerException

Programming Questions

noncom

Abnormal ArrayList NullPointerException

in Programming Questions • 3 years ago

Here is the code:

(the code is far incomplete, but it is not the matter)

final int ACT_ADD = 1;
final int ACT_SUB = 2;
final int ACT_MUL = 3;
final int ACT_DIV = 4;
final int ACT_NON = 5; //in case if an element is not an operation but a simple number
//...
//and so on if needed
final int VARTYPE_INT = 1;
final int VARTYPE_FLOAT = 2;
final int STRUC_ELEMENT_A = 0;
final int STRUC_ELEMENT_B = 1;
final int STRUC_VARTYPE = 2;
final int STRUC_VALUE = 3;
final int STRUC_ACTION = 4;
class IntVal
{
int value;
IntVal(int val)
{
value = val;
}
}
class Element
{
ArrayList structure;
// [Element a][Element b][this.Element VARTYPE][this.Element value][Action]
Element(Element a, Element b, int vartype, int value, int action)
{
ArrayList structure = new ArrayList();
structure.add(a);
structure.add(b);
structure.add(new IntVal(vartype));
structure.add(new IntVal(value));
structure.add(new IntVal(action));
println( ((IntVal)structure.get(STRUC_ACTION)).value);
println("Evaluating: " + (Element)structure.get(1));
}
void Evaluate()
{
// println("Evaluating: " + (Element)structure.get(1));
int evalA = 0;
int evalB = 0;
IntVal actObj;
int act = 0;
actObj = (IntVal)this.structure.get(STRUC_ACTION);
act = actObj.value;
if(act != ACT_NON)
{
a = (Element) structure.get(STRUC_ELEMENT_A);
b = (Element) structure.get(STRUC_ELEMENT_B);
a.Evaluate();
b.Evaluate();
evalA = ( (IntVal)a.structure.get(STRUC_ELEMENT_A) ).value;
evalB = ( (IntVal)b.structure.get(STRUC_ELEMENT_B) ).value;
}
switch( ((IntVal)structure.get(STRUC_ACTION)).value)
{
case ACT_ADD:
this.structure.set( STRUC_VALUE, (int) (evalA + evalB) );
break;
case ACT_SUB:
this.structure.set( STRUC_VALUE, (int) (evalA - evalB) );
break;
case ACT_MUL:
this.structure.set( STRUC_VALUE, (int) (evalA * evalB) );
break;
case ACT_DIV:
this.structure.set( STRUC_VALUE, (int) (evalA / evalB) );
break;
case ACT_NON:
break;
}
}
}
Element five = new Element(null,null,VARTYPE_INT,5, ACT_NON);
Element six = new Element(null,null,VARTYPE_INT,6, ACT_NON);
Element addition = new Element(five,six,VARTYPE_INT,0, ACT_ADD);
void draw()
{
addition.Evaluate();
println( ((IntVal)addition.structure.get(STRUC_VALUE)).value);
}

If I compile it in this form, I get NPE on line 53 and on line 47(if uncommented). However, if i take away the ArrayList word marked red, the NPE only comes up on line 63. My question is: why am I getting this NPE? And most important is the question what don't I understand about ArrayList that makes it behave so strange to me?

Replies(5)

Jo Wood

Re: Abnormal ArrayList NullPointerException

3 years ago

You have declared 'structure' twice in your code. The first in line 29 is the one used by the draw() and Evaluate() methods. When you declare it for a second time in line 35, you are creating an entirely new object unconnected to the one in line 29. You are correct to remove the red highlighted word, so that the line 29 version of structure is initialised and populated in your Element() method.

One further point - all methods should, by convention, start with a lower case letter. This will make it easier to spot bugs as your code becomes more complex.

noncom

Re: Abnormal ArrayList NullPointerException

3 years ago

I see, you are right, I have cleaned up all the mess, but it still surprises me:

final int ACT_ADD = 1;
final int ACT_SUB = 2;
final int ACT_MUL = 3;
final int ACT_DIV = 4;
final int ACT_NON = 5; //in case if an element is not an operation but a simple number
//...
//and so on if needed
final int VARTYPE_INT = 1;
final int VARTYPE_FLOAT = 2;
final int STRUC_ELEMENT_A = 0;
final int STRUC_ELEMENT_B = 1;
final int STRUC_VARTYPE = 2;
final int STRUC_VALUE = 3;
final int STRUC_ACTION = 4;
class IntVal
{
int value;
IntVal(){}
IntVal(int val)
{
value = val;
}
}
class Element
{
ArrayList structure;
// [Element a][Element b][this.Element VARTYPE][this.Element value][Action]
Element(Element a, Element b, int vartype, int value, int action)
{
structure = new ArrayList();
structure.add(a);
structure.add(b);
structure.add(new IntVal(vartype));
structure.add(new IntVal(value));
structure.add(new IntVal(action));
println( ((IntVal)structure.get(STRUC_ACTION)).value);
println("Evaluating: " + (Element)structure.get(1));
}
void evaluate()
{
// println("Evaluating: " + (Element)structure.get(1));
int evalA = 0;
int evalB = 0;
IntVal actObj;
IntVal val_a = new IntVal();
IntVal val_b = new IntVal();
int act = 0;
Element a;
Element b;
actObj = (IntVal)this.structure.get(STRUC_ACTION);
act = actObj.value;
if(act != ACT_NON)
{
a = (Element) structure.get(STRUC_ELEMENT_A);
b = (Element) structure.get(STRUC_ELEMENT_B);
a.evaluate();
b.evaluate();
evalA = ( (IntVal)a.structure.get(STRUC_VALUE) ).value;
evalB = ( (IntVal)b.structure.get(STRUC_VALUE) ).value;
// val_a = (IntVal)a.structure.get(STRUC_VALUE);
// val_b = (IntVal)b.structure.get(STRUC_VALUE);
// evalA = val_a.value;
// evalB = val_b.value;
}
switch( ((IntVal)structure.get(STRUC_ACTION)).value)
{
case ACT_ADD:
this.structure.set( STRUC_VALUE, (int) (evalA + evalB) );
break;
case ACT_SUB:
this.structure.set( STRUC_VALUE, (int) (evalA - evalB) );
break;
case ACT_MUL:
this.structure.set( STRUC_VALUE, (int) (evalA * evalB) );
break;
case ACT_DIV:
this.structure.set( STRUC_VALUE, (int) (evalA / evalB) );
break;
case ACT_NON:
break;
}
}
}
Element five = new Element(null,null,VARTYPE_INT,5, ACT_NON);
Element six = new Element(null,null,VARTYPE_INT,6, ACT_NON);
Element addition = new Element(five,six,VARTYPE_INT,0, ACT_ADD);
void draw()
{
addition.evaluate();
println( "RES: "+ addition.structure.get(STRUC_VALUE));
// println( ((IntVal)addition.structure.get(STRUC_VALUE)).value);
}

The problem now is in the line 107 if I uncomment it. Says ClassCastException. Which is even more strangier - this expression perfectly works in the evaluate() method above! The new line 106 works fine, giving the expected RESult which is 11. The situation means that the compiler refuses to accept an expression with class casting in line 107 and happily takes in the line 106, the expression which it says to be inappropriate if I use it in class methods. Which is more strange, it somehow automatically extracts the variable 'value' from the IntVal object.. while in all other cases (see prints in class methods) it prints object instance reference pointer.. can anyone comment this?

PhiLho

Re: Abnormal ArrayList NullPointerException

3 years ago

Your code is quite strange. Why do you use an ArrayList to store elements of various types at fixed positions, then cast them back to some type (which can be wrong in some ways...)?
It should be much simpler to just define the right fields in your Element class, no? And it would allow to have native types, instead of having to wrap them (IntVal is just a primitive version of java.lang.Integer, BTW).

noncom

Re: Abnormal ArrayList NullPointerException

3 years ago

Phi.lho, I understand why are you saying this. I know, it looks strange but I just want to test some concept in programming about as much less pre-defined code as possible (the structure will get flexible later, without the pre-defined indices, it will be treated something like a program). And also to play with classes inter-relations and array lists to their maximum. You can say I want to create a programming medium of some kind and right now I am not concerned with speed optimization. Just want to see the paradigm in action and understand it's weak points. Like here I have stumbled upon this strange thing in line 106-107. Can you tell me anything about the ClassCastException there?(see my prev post).

The Element constructor is also just a placeholder, it will take an ArrayList as its args later. There will be not only the mathematical operations possible, but also other operations that are not quite mathematical. This code here is much-much of a draft.

I know that the IntVal is ridiculous but I had to go this way in order to get what I want.. because ArrayLists cannot store plain ints as far as I understand.

bjordal

Re: Re: Abnormal ArrayList NullPointerException

3 years ago

if you try this code inside draw:

println( addition.structure.get(STRUC_VALUE).getClass());

you will see that the object returned is of the type Integer. This is caused by the evaluate() method where you replace the 4th element in the arraylist with a int (that is then casted to a integer by java, i think)

this.structure.set( STRUC_VALUE, (int) (evalA + evalB) );

also,

because ArrayLists cannot store plain ints as far as I understand.

ArrayLists can't store ints but they can store Integer which is the int wrapper.

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