to implement NFA for language L={aba^n U abab^n | n >= 0 }

LanguageNFA.java

//to implement NFA for language L={aba^n U abab^n | n >= 0 }
import java.io.*;
class LanguageNFA
{
        static void checkLang(String str)
        {
                char foo;
                int state = 0;
                for(int i = 0 ; i < str.length() ; i++)
                {
                        foo = str.charAt(i);
                        if(state == 0)
                        {
                                if(foo == 'a')
                                        state = 1;
                                else
                                        state = -1;
                        }
                        else if(state == 1)
                        {
                                if(foo == 'b')
                                        state = 2;
                                else
                                        state = -1;
                        }
                        else if(state == 2)
                        {
                                if(foo == 'a' && i != str.length()-1)
                                {
                                        if(str.charAt(i+1) == 'a')
                                                state = 3;
                                        if(str.charAt(i+1) == 'b')
                                                state = 4;
                                }
                                else if(foo == 'a')
                                	state = 3;
                                else
                                        state = -1;
                        }
                        else if(state == 3)
                        {
                        	if(foo == 'a')
                        		state = 3;
                        	else
                        		state = -1;
                        }
                        else if(state == 4)
                        {
                        	if(foo == 'b')
                        		state = 4;
                        	else
                        		state = -1;
                        }
                        if(state != -1)
                        	System.out.println("State: "+state);
                }
                if(state == 2 || state == 3 || state == 4)
                        System.out.println("Valid language");
                else
                        System.out.println("Invalid language");
        }
        public static void main(String [] args)throws IOException
        {
                BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
                String inp;
                System.out.println("Enter a string: ");
                inp = br.readLine();
                checkLang(inp);
        }
}