Checking Balanced Parentheses (an Application of Stacks)

This lesson covers checking whether parentheses are balanced in Java using a stack, so you understand a practical application of the stack data structure. It's for anyone searching "Java balanced parentheses stack application."

Using a stack (last in, first out), you can check whether parentheses are balanced by pushing an opening bracket like ( or [ onto the stack, and checking whether it matches the top of the stack every time a closing bracket comes up.

The example uses a Map called pairs to look up the opening bracket that matches each closing one, and a Deque<Character> — pushing when an opening bracket comes up, and checking against the top of the stack when a closing bracket comes up. If the stack is empty once everything's processed, the parentheses are balanced.

A common early mistake is handling several types of brackets at once (parentheses, square brackets, and curly braces mixed together). Simply counting opens and closes isn't enough — you also need to confirm they're nested in the correct order.

A similar idea powers an editor's autocomplete or a code syntax checker under the hood, and plenty of similar tools exist for checking whether HTML tags are correctly opened and closed too.

📖 Reference code

import java.util.*;

public class Main {
    static boolean isBalanced(String str) {
        Deque<Character> stack = new ArrayDeque<>();
        Map<Character, Character> pairs = Map.of(')', '(', ']', '[', '}', '{');
        for (char ch : str.toCharArray()) {
            if (ch == '(' || ch == '[' || ch == '{') {
                stack.push(ch);
            } else if (ch == ')' || ch == ']' || ch == '}') {
                if (stack.isEmpty() || stack.pop() != pairs.get(ch)) return false;
            }
        }
        return stack.isEmpty();
    }

    public static void main(String[] args) {
        System.out.println(isBalanced("(a[b]{c})"));
        System.out.println(isBalanced("(a[b)]"));
    }
}

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