Create and use an ArrayList from within a single method?

Question

Is it possible to create and use an ArrayList within a method? I would like to create a temporary stack in the public int min() method to track the smallest value. However, the compiler complains about the ArrayList add() method: Cannot resolve method 'add(int)' I presume that's because I'm trying to run it inside the same method that created the ArrayList?

import java.util.*;

public class MinStack {
private ArrayList<Integer> data;


public MinStack() {
    data = new ArrayList<Integer>();
}

public void push( int a ) {
    data.add( a );
}

public int pop() {
    if ( data.size() <= 0 ) {
        throw new IllegalStateException();
    }
    return data.remove( data.size() - 1 );
}

public int size() {
    return data.size();
}

public int min() {
    MinStack smalls = new MinStack();
    int elMin = (data.get((data.size() - 1)));
    smalls.add(elMin);
    while (!data.isEmpty()) {
        if (data.get(data.size() - 1) < elMin)
        elMin = data.get(data.size() - 1);
    }
    return elMin;
}

Why do you think there could be a restriction about using an ArrayList, or anything at all, “within a method”? Besides that, if all you want to use, is pushing and popping, you may use an ArrayDeque instead. — Holger
– Holger, Commented Aug 30, 2018 at 11:16

Ashishkumar Singh · Accepted Answer · 2018-08-29 04:44:44Z

3

You are not calling add method on an instance of Arraylist but instead you are calling it on an instance of MinStack class object which doesn't have add method.

You should be calling push method instead as it is defined in your class which will call add method of the arraylist. Or else define/rename(push) a method in MinStack class with name add

edited Aug 29, 2018 at 4:44

answered Aug 29, 2018 at 4:31

Ashishkumar Singh

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Comments

Aravinda Meewalaarachchi · Accepted Answer · 2018-08-29 09:13:15Z

2

You have used a improper design strategy to implement this class. Their are lots of flows in your implementation and here i am not gonna talk about those. Within you design approach, Instead of calling implemented behavior call push() in your class. You are trying to call a method which is not implement in this class called add(). If you want to use your implented behaviour within the class, Just call your method as smalls.push(elMin) instated of smalls.add(elMin).

answered Aug 29, 2018 at 9:13

Aravinda Meewalaarachchi

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DEVAS · Accepted Answer · 2018-08-29 04:30:52Z

1

smalls.add(elMin)

is looking for add method of MinStack class. You should call push method to insert elMin in data arrayList.

answered Aug 29, 2018 at 4:30

DEVAS

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Hearen · Accepted Answer · 2018-08-29 06:48:22Z

1

i think you can do this like below:

public int min() {

    if(data.isEmpty())
        throw new RuntimeException("the list is empty...");
    int elMin = data.get(0);
    for(int i : data) {
        if(i < elMin) {
            elMin = i;
        }
    }
    return elMin;
}

edited Aug 29, 2018 at 6:48

Hearen

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answered Aug 29, 2018 at 4:40

nnzhang

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user10288239 · Accepted Answer · 2018-08-29 06:18:33Z

0

Try this :

Call push method to insert elMin in ArrayList data.

answered Aug 29, 2018 at 6:18

user10288239

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