Question

10.6 LAB: Implement a stack using an array

Given main() complete the Stack class by writing the methods push() and pop(). The stack uses an array of size 5 to store elements. The command Push followed by a positive number pushes the number onto the stack. The command Pop pops the top element from the stack. Entering -1 exits the program.

Ex. If the input is

Push 1
Push 2
Push 3
Push 4
Push 5
Pop
-1

the output is

Stack contents (top to bottom):
1

Stack contents (top to bottom):
2
1

Stack contents (top to bottom):
3
2
1

Stack contents (top to bottom):
4
3
2
1

Stack contents (top to bottom):
5
4
3
2
1

Stack contents (top to bottom):
4
3
2
1

PushPopStack.java

import java.util.Scanner;

public class PushPopStack {

public static void main (String[] args) {
Scanner scnr = new Scanner(System.in);
Stack stack = new Stack(5);
String action; // Push or Pop   
int numInput; // Integer value to push

action = scnr.next();

while (!action.equals("-1")) {
if (action.equals("Push")) {
numInput = scnr.nextInt();
stack.push(numInput);
System.out.println("Stack contents (top to bottom):");
stack.printStack();
}
else if (action.equals("Pop")) {
stack.pop();
System.out.println("Stack contents (top to bottom):");
stack.printStack();
}

action = scnr.next();
}
}
}

Stack.java

public class Stack {
  
private int topElement;
int size;
int [] array;

public Stack(int arraySize) {
size = arraySize;
array = new int[size];
topElement = -1;
}

// TODO: Write a method to push a value into the stack

// TODO: Write a method to pop the top value from the stack   

public void printStack() {
int i;
  
for(i = topElement; i >= 0; --i) {
System.out.println(array[i]);
}
System.out.println();
}
}

Answer 1

public class Stack {

    private int topElement;
    int size;
    int[] array;

    public Stack(int arraySize) {
        size = arraySize;
        array = new int[size];
        topElement = -1;
    }

    public void push(int num) {
        if (topElement < size - 1) {
            array[topElement + 1] = num;
            topElement++;
        }
    }

    public int pop() {
        if (topElement > -1) {
            int result = array[topElement];
            topElement--;
            return result;
        } else {
            return -1;
        }
    }

    public void printStack() {
        int i;

        for (i = topElement; i >= 0; --i) {
            System.out.println(array[i]);
        }
        System.out.println();
    }
}