A Java class that returns true or false if parenthesis syntax is valid. This assignment was inspired by a programming interview question I received from a technical interview for a developer internship.

ParenthesisSyntax.java

public class ParenthesisSyntax {

	// returns if the parenthesis syntax is valid
	public static boolean isParanthesSyntaxValid(String paranthesis) {
		return isFirstLastValid(paranthesis) && areSequencesValid(paranthesis);
	}

	// return if the first or last character is invalid syntax
	private static boolean isFirstLastValid(String paranthesis) {
		final int PARANTHESIS_LENGTH = paranthesis.length();

		// return first character as valid '(' and last character as valid if ')'
		return paranthesis.charAt(0) == '(' || paranthesis.charAt(PARANTHESIS_LENGTH - 1) == ')';
	}

	// return if the parenthesis sequences are valid
	private static boolean areSequencesValid(String paranthesis) {
		final int PARANTHESIS_LENGTH = paranthesis.length();

		int open = 0;
		int closed = 0;
		boolean searchOpen = true;
		for(int i = 0; i < PARANTHESIS_LENGTH; i++) {
			if(paranthesis.charAt(i) == '(' && searchOpen) {
				open++;
			} else if(paranthesis.charAt(i) == '(') {
				// a parenthesis sequence finished, check parenthesis count
				if(open != closed) {
					return false;
				}
				// search for open parenthesis again
				searchOpen = true;
				open++;
			} else if(paranthesis.charAt(i) == ')') {
				closed++;
				searchOpen = false;
			}
		}
		// fence-post solution
		return closed == open;
	}
}