CODEWITHSUNDEEP
javanullpointerexceptionstackcharacter
ShadoWalkeR
ShadoWalkeR

Reputation: 123

Java Stack push method NullPointerException

I'm implementing a Character Buffer with two stacks. I am inserting an element with insert() method:

class Buffer
{
    private Stack<Character> stackLeft;
    private Stack<Character> stackRight;
    private int size;

    Buffer()
    {
        Stack<Character> stackLeft   = new Stack<Character>();
        Stack<Character> stackRight  = new Stack<Character>();
        size = 0;
    }

    public void insert(char c)
    {
        size ++;
        stackLeft.push(c); //This line returns NullPointerException
    }
}

Everytime it tries to push the c character variable, it throws Null pointer exception. Here's how I have called it from main:

public static void main(String[] args)
{
    Buffer tb = new Buffer();

    Character ch = new Character('a');
    tb.insert(ch);
    tb.insert(new Character('a'));
    tb.insert('a');
}

All of these return a null exception. Asserted that Character reaches inside the method fine. I used java.util.Stack; when it failed, I implemented my own. But it still failed.
What could I be doing wrong?
Any kind of help is appreciated!

Upvotes: 0

Views: 8857

Answers (3)

hankd
hankd

Reputation: 649

Buffer()
{
    Stack<Character> stackLeft   = new Stack<Character>();
    Stack<Character> stackRight  = new Stack<Character>();
    size = 0;
}

You need to change this to 

Buffer()
{
    stackLeft   = new Stack<Character>();
    stackRight  = new Stack<Character>();
    size = 0;
}

It thinks you are creating new local variables in the Buffer() constructor, instead of instantiating the class fields;

Upvotes: 1

Sashi Kant
Sashi Kant

Reputation: 13465

Its because the constructer Buffer() doesn't set the class variables, but create its own local variable(stackLeft, stackRight) and initializes that

Change this::

class Buffer
{
    private Stack<Character> stackLeft;
    private Stack<Character> stackRight;
    private int size;

    Buffer()
    {
        stackLeft   = new Stack<Character>();
        stackRight  = new Stack<Character>();
        size = 0;
    }

    public void insert(char c)
    {
        size ++;
        stackLeft.push(c); //This line returns NullPointerException
    }
}

Upvotes: 0

rgettman
rgettman

Reputation: 178263

With these lines in the constructor:

Stack<Character> stackLeft   = new Stack<Character>();
Stack<Character> stackRight  = new Stack<Character>();

You've created local variables named the same as the instance variables, so you haven't initialized the instance variables stackLeft and stackRight, so Java initializes them to null.

Remove the types so the names will resolve to the instance variables.

stackLeft   = new Stack<Character>();
stackRight  = new Stack<Character>();

Upvotes: 6

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