import java.awt.*;
   import java.awt.event.*;
   import javax.swing.*; 
   import java.util.*;

public class FSM_OddAs_EvenBs extends JFrame implements ActionListener{
    private Font demoFont;
    private JTextArea inputStringArea;
    private JLabel resultLabel;
    private JButton actionButton;
    private String inputString;
    private LinkedList<Character> s;
    private boolean finalStateReached = false;
    
    public FSM_OddAs_EvenBs(){
      super( "Finite State Machine" );
         
         demoFont = new Font(Font.SANS_SERIF, Font.BOLD, 20);
         Container container = getContentPane();
         container.setLayout( new FlowLayout() );
      
         inputStringArea = new JTextArea("Please type your input string here.");
         inputStringArea.setRows(10);
         inputStringArea.setColumns(20);
         inputStringArea.setLineWrap(true);
         inputStringArea.setFont(demoFont);
         container.add( inputStringArea );
         
         resultLabel = new JLabel("                                ");
         resultLabel.setOpaque(true);
         resultLabel.setBackground(Color.yellow);
         resultLabel.setFont(demoFont);
         container.add( resultLabel );
      
         actionButton = new JButton( "Check Input String Syntax" );
         actionButton.setFont(demoFont);
         container.add( actionButton );
         actionButton.addActionListener( this );
         
         setSize( 460, 400 );
         setVisible( true );
    }
    
    public void actionPerformed( ActionEvent event ){
         String inputString = inputStringArea.getText();
         s = new LinkedList<Character>();
         for(int i=0; i<inputString.length(); i++)
            s.add(inputString.charAt(i));
         if(FSM())
            resultLabel.setText("String is in the language.");
         else
            resultLabel.setText("String is not in the language.");
    }
    
    private boolean FSM(){ //Character = char. Java does the conversion.
        finalStateReached=false;
        state1();  //start state
        return finalStateReached;
    }
    
    private void state1(){
       if(!s.isEmpty()){
            char c = s.remove();
            if(c=='a')
                state2();
            else if(c=='b')
                state3();
        }
    }
    private void state2(){
       if(!s.isEmpty()){  //can go out. different from the comments example
            char c = s.remove();
            if(c=='a')
                state1();
            else if(c=='b')
                state4();
        }
        else  //all chars checked.
            finalStateReached=true;
    }
    private void state3(){
      if(!s.isEmpty()){
            char c = s.remove();
            if(c=='a')
                state4();
            else if(c=='b')
                state1();
        }
    }
    private void state4(){
       if(!s.isEmpty()){
            char c = s.remove();
            if(c=='a')
                state3();
            else if(c=='b')
                state2();
        }
    }
    
    public static void main(String args[]){ 
        new FSM_OddAs_EvenBs(); 
    }
}