Stack Array implementation of Java

Question 1

This is my array implementation on stack in Java. It involves push, pop, get and growth when limit of array size is reached.

I am a self-taught programmer starting to learn data structures and algorithms.

I am looking for

any recommendation on conventions
an honest opinion on how bad this piece of code if this is an interview.
anything wrong and room for improvement.

I wrote this code myself, not looking at any other similar questions on Code Review.

public class stackAlist{    int[] data;    int size;    static int growthmultipler = 1;    public stackAlist(){        data = new int[2*growthmultipler];        size = 0;    }    public void push(int value){        if(this.size == 0){            this.data[0] = value;            this.size += 1;            this.growthmultipler = 1;            }        else if(this.size == 2*growthmultipler){            growthmultipler += 1;            stackAlist newlist = new stackAlist();            newlist.data = new int[2*growthmultipler];            System.arraycopy(this.data, 0, newlist.data, 0, this.size);            this.data = newlist.data;            this.data[size] = value;            this.size += 1;        }        else{        this.data[size] = value;        this.size += 1;        }    }    public void pop(){        this.data[size-1] = 0;        this.size = this.size-1;    }    public void get(){        int i;        for(i =0; i < this.size; i++){            System.out.println(data[i]);        }    }    public static void main(String[] args){        stackAlist a = new stackAlist();        a.push(1);        a.push(2);        a.get();        a.pop();        a.get();        a.push(3);        a.push(4);        a.get();    }}

Question 2

First of all, you are using a static variable (growthmultipler) inside the instance of a class. This is fatal if you have more than one instance of this class. Because you always use 2*growthmultipler, you don't need it. Use +=2 instead.

You don't need to prealloc data in the constructor. It will be done if it's needed.

You don't need to handle "size == 0" separately. You only have to handle "size >= length".

You don't have to create a new class if you want to grow data.

In pop() you should test to not get negative (and throw an exception). And normally a pop function returns the value.

In summary it might look like this:

public class stackAlist{    int[] data;    int size;    public stackAlist(){        size = 0;        data = new int[size];    }    public void push(int value){        if(size>=data.length) {            int [] ndata = new int[data.length+2];            System.arraycopy(data, 0, ndata, 0, size);            data = ndata;        }        data[size] = value;        size += 1;    }    public int pop() {        int ret=0;        if(size>0) {            size -= 1;            ret = data[size];            data[size] = 0;        }        return ret;    }    .....

Question 3

Exceptions exist for a reason; the use of a "special" return value should bewell documented.

Question 4

I do think it should fail early, though.

Question 5

A couple things to add:

Classes should use CamelCase and should preferably have descriptive names.stackAlist would be better namedArrayStack.
You might want to have aStack interface thatArrayStack implements.
You might want to extract the resizing code into a separate method, and optionally make it public and/or take parameters.
In your pop method, there's no need to zero out the popped items.

Question 6

Thank you for all answers. I am kind of stuck at thinking about why I need a Stack interface that ArrayStack implements. Because I don't know why, I can't think of how to do it. An Interface should be used when there are multiple classes/data types involved. Are you saying that I should makeArrayStack generic so that it can accept all data types? Kinda confused here, please help, thanks.

Question 7

Generics might be good too. Also, having an interface isn't necessary, but it can allow you to decouple it from code that uses it.

Holger 3611 silver badge4 bronze badges · Accepted Answer · 2018-11-19 11:36:59Z

First of all, you are using a static variable (growthmultipler) inside the instance of a class. This is fatal if you have more than one instance of this class. Because you always use 2*growthmultipler, you don't need it. Use +=2 instead.

You don't need to prealloc data in the constructor. It will be done if it's needed.

You don't need to handle "size == 0" separately. You only have to handle "size >= length".

You don't have to create a new class if you want to grow data.

In pop() you should test to not get negative (and throw an exception). And normally a pop function returns the value.

In summary it might look like this:

public class stackAlist{    int[] data;    int size;    public stackAlist(){        size = 0;        data = new int[size];    }    public void push(int value){        if(size>=data.length) {            int [] ndata = new int[data.length+2];            System.arraycopy(data, 0, ndata, 0, size);            data = ndata;        }        data[size] = value;        size += 1;    }    public int pop() {        int ret=0;        if(size>0) {            size -= 1;            ret = data[size];            data[size] = 0;        }        return ret;    }    .....

Exceptions exist for a reason; the use of a "special" return value should bewell documented.

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Stack Array implementation of Java

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