Some class, preferably an inner class of B if you're going to allow input only from JPanel B, has to implement ActionListener. That ActionListener has to be added to the JTextField:
myInputTextField.addActionListener(theActionListenerIAmGoingToWrite).
That ActionListener has to define actionPerformed() something like this :
String text = inputTextField.getText();
Integer inputInteger = Integer.getInteger(text).toInt();
graph.doStuffWithIntegerInput(inputInteger);
The design questions to be answered are :
Is the ActionListener an anonymous inner class? If you're only going to use it with a specific instances of graph and B which know about each other, then go this way. If so, it has to get a reference to the Graph instance from its containing class, which implies B has such an instance to offer it.
Is the action of the ActionListener reusable elsewhere? Will the "enter numbers" widget appear elsewhere on other JPanels? If so make it a stand alone class. You still need a reference to graph so either graph in as a constrcutor parameter and/or have a setGraph method because somehow it needs to know about the graph instance so it can send it a message.
done!
edit 2
OK so let's map what you have to an MVC framework. So your view consists of the Graph class and your class B and your textFields.
Now contained within your graph class, you also have the data you want to graph. That's the start, finish, x , y variables you've defined. To be a good citizen of MVC-land, you would have to pull these out and contain them within a separate object b/c they constitute the model of your graph. A model is, very basically, the set of facts you wish to present, irrespective of how they are going to be presented, while the view is all the code concerned with painting stuff to the screen.
So this model object would be a simple data object with getters and setters for all the variables you've defined and would be passed into the Graph object either at the constructor or through a setter on the Graph object.
By creating this GraphData object, you've separated the view, the Graph from it's model the GraphData object.
With a more complicated Graph, you'd actually specify in a GraphModel what "kind" of Graph object you want to create, what the axis names would be, what kind of line you want to draw, stuff like that, but that's overkill for what you have.
OK so now you have your Graph and GraphData objects and some way to get them together.
Next let's look at your class B. Since it's a JPanel, a view, it's essentially an aggregator and presenter of its contained Views. Those would be the Graph and the JTextFields. So you add them as you have, specifying the layout.
So a hypothetical refactoring goes like this:
Create main program. In that main create: JTextFields, ActionListener, JPanel (B), Graph and GraphModel.
Associate GraphModel with Graph.
Associate Graph with JPanel.
Associate ActionListener with JJtextField
Associate JTextField with JPanel
As far as writing ActionListener goes just make it its own class, that's the easiest to write and understand. So thus:
public class MyTextFieldListener implements ActionListener
{
private Graph graph;
public MyTextFieldListener(Graph graph)
{
this.graph = graph;
}// construtor
public void actionPerformed (ActionEvent ae)
{
String text = ((JTextField)ae.getSource()).getText();
Integer inputInteger = Integer.getInteger().(text).toInt();
graph.doStuffWithIntegerInput(inputInteger);
} // actionPerformed
}// class
Now a complication could be that you can't update the graph until you have info from all four textfields. There are different ways to change this code to accommodate that but this is the basic idea of where you need to go.
HTH
