Initialize array of objects(which has an array) in Java

Question

I am relatively new to java, and this is only the second time I have ever tried to design a program that has an array of class objects. I am having trouble because when I go into debug mode, it is showing me that the arrays are being built correctly. However, when I try to build a string or even print to the console, I am getting a null pointer exception.

I am trying to make an array of Slot class objects.

public class C {
int slotNum;
Slot[] s;


public C(int byteSize, int slotNum){
    this.slotNum = slotNum;

    Slot[] s = new Slot[slotNum]; //create an array of slots

    for(int i = 0; i < slotNum; i++){ //for every slot and instantiate
        s[i] = new Slot(byteSize); 
    }
    display();
}

And the Slot class has its own array, block[], constructed by a value passed in through its constructor.

public class Slot {

boolean v;
short block[];

public Slot(int byteSize){
    valid = false;

    //Instantiate each element in block[] to 0
    short block[] = new short[byteSize];
    for(int i = 0; i < block.length; i++){
        block[i] = 0;
    }
}

Later in the program I then try to print to the console or build a string and it always breaks when I try to use any element in the slot[] in the C class.

Have I instantiated and initialized my 2 different arrays correctly?

No you are just initializing the local arrays that you make in the constuctor — 3kings
– 3kings, Commented Oct 18, 2015 at 20:14

3kings · Accepted Answer · 2015-10-18 20:16:43Z

1

public class C {
int slotNum;
Slot[] s;


public C(int byteSize, int slotNum){
this.slotNum = slotNum;

s = new Slot[slotNum]; //create an array of slots

for(int i = 0; i < slotNum; i++){ //for every slot and instantiate
    s[i] = new Slot(byteSize); 
}
display();
}

Try something like these instead

public class Slot {

boolean v;
short block[];

public Slot(int byteSize){
valid = false;

//Instantiate each element in block[] to 0
block = new short[byteSize];
for(int i = 0; i < block.length; i++){
    block[i] = 0;
}
}

answered Oct 18, 2015 at 20:16

3kings

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1 Comment

L. Morrell Over a year ago

It worked... I can't believe something so simple tripped me up for so long. Thank you!

Ravindra babu · Accepted Answer · 2015-10-18 20:18:25Z

1

Replace

short block[] = new short[byteSize];

with

block = new short[byteSize];

answered Oct 18, 2015 at 20:18

Ravindra babu

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Comments

Kumar Abhinav · Accepted Answer · 2015-10-18 20:19:37Z

0

Always use a this operator to initialize in a constructor so that you may differentiate clearly between local variables and instance variables

public Slot(int byteSize){
    valid = false;

    //Instantiate each element in block[] to 0
    this.block = new short[byteSize];
    for(int i = 0; i < block.length; i++){
        block[i] = 0;
    }
}

answered Oct 18, 2015 at 20:19

Kumar Abhinav

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