diff --git a/java/0020-valid-parentheses.java b/java/0020-valid-parentheses.java
index 5d54eb195..d41194111 100644
--- a/java/0020-valid-parentheses.java
+++ b/java/0020-valid-parentheses.java
@@ -28,15 +28,16 @@ public boolean isValid(String s) {
 class Solution {
     public boolean isValid(String s) {
         Stack<Character> brackets = new Stack<>();
-        Map<Character, Character> bracketLookup = new HashMap<>();
+        Map<Character, Character> bracketLookup = new HashMap<>(3);
 
         bracketLookup.put(')', '(');
         bracketLookup.put('}', '{');
         bracketLookup.put(']', '[');
 
-        for (char c : s.toCharArray()) {
+        for (int i = 0; i < s.length(); i++) {
+            char c = s.charAt(i);
             if (bracketLookup.containsKey(c)) {
-                if (brackets.size() != 0 && brackets.peek() == bracketLookup.get(c)) {
+                if (!brackets.isEmpty() && bracketLookup.get(c).equals(brackets.peek())) {
                     brackets.pop();
                 } else {
                     return false;
@@ -46,7 +47,6 @@ public boolean isValid(String s) {
             }
         }
 
-        if (brackets.size() == 0) return true;
-        return false;
+        return brackets.isEmpty();
     }
 }