Valid Parentheses

Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.

public class Solution {
    public boolean isValid(String s) {
        Stack<Character> stack = new Stack<Character>();
        char[] c = s.toCharArray();
        for(int i=0;i<c.length;i++){
            if(stack.isEmpty() || !is(stack.peek(),c[i])){
                stack.push(c[i]);
            }
            else{
                stack.pop();
            }
        }
        return stack.isEmpty();
    }
    public boolean is(char a,char b){
        if(a=='(') return b==')';
        if(a=='{') return b=='}';
        if(a=='[') return b==']';
        return false;
    }
}