Java Stacks With ArrayList

Question

Hey guys ive come stuck once again, I can manage to push items onto a stack and pop them off. I even got working a stack with open and close brackets. But when I come to do it with () or {} or [] brackets with my code I created it seems to ignore the if statement and goes to else any ideas why.

Main.java

public class DifferentBrackets {

    public static void main(String[] args) {
        Stack brackets = new Stack();
        Scanner k = new Scanner(System.in);
        System.out.println("* to Terminate or Enter bracket : ");
        String tempBrack = k.next();
        while (!tempBrack.equals("*")) {
            System.out.println("* to Terminate or Enter bracket : ");

            switch (tempBrack) {
                case "(":
                    brackets.push("(");
                    System.out.println(tempBrack + " is added");
                    break;

                case "{":
                    brackets.push("{");
                    System.out.println(tempBrack + " is added");
                    break;

                case "[":
                    brackets.push("[");
                    System.out.println(tempBrack + " is added");
                    break;

                case ")":

                    if (tempBrack.equals(brackets.arrayTop()))
                            {
                            System.out.println(brackets.pop() + " is popped");
                            }
                    else if(tempBrack.equals(brackets.arrayTop()))
                    {
                        System.out.println("Closed Bracket Doesnt Match Top Open Bracket");
                    }
                    else 
                    {
                        System.out.println("Closed Bracket Doesnt Match Top Open Bracket");
                    }
                    break;

//            case "}":
//                    if (tempBrack.equals(brackets.arrayTop()))
//                            {
//                            System.out.println(brackets.pop() + " is popped");
//                            }
//                    else if (!tempBrack.equals(brackets.arrayTop()))
//                    {
//                        System.out.println("Closed Bracket Doesnt Match Top Open Bracket");
//                    }
//
//                    break;

//                case "]":
//                    if (tempBrack.equals(brackets.arrayTop()))
//                            {
//                       if (!tempBrack.equals(brackets.arrayTop()))
//                    {
//                        System.out.println("Closed Bracket Doesnt Match Top Open Bracket");
//                    }
//                    break;
            }

            System.out.println("* to Terminate or Enter bracket : ");
            tempBrack = k.next();
        }

    }
}

Heres my Stack.java

import java.util.ArrayList;

public class Stack
{
    private ArrayList<String> a;

    public Stack()
    {
       a = new ArrayList(10);       //initial capacity of 10
    }

    public boolean isEmpty()
    {
         return a.isEmpty();
    }

    public String pop()          //pop integer element
    {
        String last;
        last = a.remove((a.size()- 1));
        return(last);      //underflow will crash
    }

    public void push(String x)      //push integer element
    {
        a.add(x);
    }

    public String arrayTop()
    {
        return(a.get(a.size() -1));
    }
}

What is your problem? I don't get it. Btw your else if is useless as it checkes the same condition as your if — Blub
– Blub, Commented Dec 15, 2013 at 14:43

Alexis C. · Accepted Answer · 2013-12-15 15:06:36Z

When you add a ")" you want to check if the top of the stack contains a "(", not a ")". But this is what you actually test here.

if (tempBrack.equals(brackets.arrayTop()))

is the same as (let's say your stack contains just "(" and you entered ")") :

if (")".equals("("))

which is obviously false.

You have to make the check in a way that when you're in a closing parenthesis/bracket, you have to check if the top of the stack contains an opening one.

case ")":

    if ("(".equals(brackets.arrayTop())){
        System.out.println(brackets.pop() + " is popped");
    }
    else {
        System.out.println("Closed Bracket Doesnt Match Top Open Bracket");
    }
    break;

The same logic applies for "{" and "[".

Note that :

The API already provides a Stack class, no need to reinvent the wheel, you should use this one.
Consider using a Stack of Characters, not Strings
You may end your program if a closing bracket doesn't match a top open one.

robbmj · Accepted Answer · 2013-12-15 15:22:41Z

Please see the comments for an explanation. To the OP, please read and understand ZouZou's answer.

import java.util.Scanner;

public class DifferentBrackets {

    public static void main(String[] args) {
        Stack brackets = new Stack();
        Scanner k = new Scanner(System.in);
        System.out.println("* to Terminate or Enter bracket : ");
        String tempBrack = k.next();

        while (!tempBrack.equals("*")) {

            // if the user added an opening bracket
            // push it on the stack
            if (isOpeningBracket(tempBrack)) {
                brackets.push(tempBrack);
            }
            else if (isClosingBracket(tempBrack)) {
                // first check to see of the element at the 
                // top of the stack is the mirror of the closing bracket
                if (brackets.arrayTop().equals(mirror(tempBrack))) {
                    // if it is pop it from the stack
                    System.out.println(brackets.pop() + " is popped");
                }
                else {
                    System.out.println("Closed Bracket Doesnt Match Top Open Bracket");
                }
            }
            else {
                System.out.println(tempBrack + " is an unsuported character");
            }
            tempBrack = k.next();
        }
        // if there is nothing on the stack after the user enters "*"
        // the user has entered balanced brackets  
        if (brackets.isEmpty()) {
            System.out.println("Congrats your brackets are balanced");
        }
        else {
            System.out.println("Sorry your brackets are not balanced");
        }
        k.close();
    }

    // find the closing bracket's mirror
    public static String mirror(String str) {
        switch (str) {
        case ")":
            return "(";
        case "]":
            return "[";
        case "}":
            return "{";
        default:
            return null;
        }
    }

    public static boolean isOpeningBracket(String str) {
        return str.equals("(") || str.equals("{") || str.equals("[");
    }

    public static boolean isClosingBracket(String str) {
        return str.equals(")") || str.equals("}") || str.equals("]");
    }
}

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