Bracket matching problem

Suppose you had a string which contains only '(' and ')' characters. Write a program to check whether it forms a valid expression (or) not.

For example

(()()()) is valid expression

()( is invalid expression.

Using a stack we can solve this problem.

1. For each character in the string

    If the symbol is '('

         push it on to the stack

   else

        If the stack is empty

            return false

       Pop top element

2. At the end if stack is empty, return true, else false.

import java.util.LinkedList;
import java.util.Objects;

public class BracketMatcher {

 public static boolean isValidExpression(String str) {
  Objects.nonNull(str);

  LinkedList<Character> list = new LinkedList<>();

  for (int i = 0; i < str.length(); i++) {
   if (str.charAt(i) == '(') {
    list.add('(');
   } else {
    if (list.isEmpty())
     return false;
    list.pop();
   }
  }

  if (list.isEmpty())
   return true;

  return false;
 }
}

Following is the junit test case for above program.

import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;

import org.junit.Test;

public class BracketMatcherTest {

 @Test
 public void test1() {
  assertTrue(BracketMatcher.isValidExpression("()()()()"));
  assertTrue(BracketMatcher.isValidExpression("((())())()()()"));

  assertFalse(BracketMatcher.isValidExpression("()()()())"));
  assertFalse(BracketMatcher.isValidExpression("(()()()()"));
 }
}