Why is this method not returning anything?

	//Imports are listed in full to show what's being used
	//could just import javax.swing.* and java.awt.* etc..

	import java.awt.GridLayout;
	import java.awt.BorderLayout;
	import java.awt.event.ActionListener;
	import java.awt.event.ActionEvent;
	import javax.swing.JFrame;
	import javax.swing.JPanel;
	import javax.swing.JTextField;
	import javax.swing.JButton;
	import java.awt.Container;

	public class SimpleCalc implements ActionListener{
	 
		public static final SimpleCalc instance = new SimpleCalc();
		
		JFrame guiFrame;
		JPanel buttonPanel;
		JTextField numberCalc;
		int calcOperation = 0;
		int currentCalc;
		int operatorAction;
		
		//Note: Typically the main method will be in a
		//separate class. As this is a simple one class
		//example it's all in the one class.
		
		
		public SimpleCalc()
		{
			guiFrame = new JFrame();
			
			//make sure the program exits when the frame closes
			guiFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
			guiFrame.setTitle("Simple Calculator");
			guiFrame.setSize(300,300);
		  
			//This will center the JFrame in the middle of the screen
			guiFrame.setLocationRelativeTo(null);
			
			numberCalc = new JTextField();
			numberCalc.setHorizontalAlignment(JTextField.RIGHT);
			numberCalc.setEditable(false);
			
			guiFrame.add(numberCalc, BorderLayout.NORTH);
			
			buttonPanel = new JPanel();
				   
			//Make a Grid that has three rows and four columns
			buttonPanel.setLayout(new GridLayout(4,3));   
			guiFrame.add(buttonPanel, BorderLayout.CENTER);
			
			//Add the number buttons
			for (int i=1;i<10;i++)
			{
				addButton(buttonPanel, String.valueOf(i));
			}

			JButton addButton = new JButton("+");
			addButton.setActionCommand("+");
			
			OperatorAction subAction = new OperatorAction(1);
			addButton.addActionListener(subAction);
			
			JButton subButton = new JButton("-");
			subButton.setActionCommand("-");
			
			OperatorAction addAction = new OperatorAction(2);
			subButton.addActionListener(addAction);
			
			JButton equalsButton = new JButton("=");
			equalsButton.setActionCommand("=");
			equalsButton.addActionListener(new ActionListener()
			{
				@Override
				public void actionPerformed(ActionEvent event)
				{
					if (!numberCalc.getText().isEmpty())
					{
						int number = Integer.parseInt(numberCalc.getText()); 
						if (calcOperation == 1)
						{
							int calculate = currentCalc  + number;
							numberCalc.setText(Integer.toString(calculate));
						}
						else if (calcOperation == 2)
						{
							int calculate = currentCalc  - number;
							numberCalc.setText(Integer.toString(calculate));
						}
					}
				}
			});
			
			buttonPanel.add(addButton);
			buttonPanel.add(subButton);
			buttonPanel.add(equalsButton);
			guiFrame.setVisible(true);  
		}
		
		//All the buttons are following the same pattern
		//so create them all in one place.
		private void addButton(Container parent, String name)
		{
			JButton but = new JButton(name);
			but.setActionCommand(name);
			but.addActionListener(this);
			parent.add(but);
		}
		
		//As all the buttons are doing the same thing it's
		//easier to make the class implement the ActionListener
		//interface and control the button clicks from one place
		@Override
		public void actionPerformed(ActionEvent event)
		{
			//get the Action Command text from the button
			String action = event.getActionCommand();
			
			//set the text using the Action Command text
			numberCalc.setText(action);       
		}
		
		public String getText() {
			return numberCalc.getText();
		}
		
	}

import java.awt.event.ActionEvent;
	import java.awt.event.ActionListener;

	import javax.swing.JTextField;

		public class OperatorAction implements ActionListener
		{
			int calcOperation = 0;
			int currentCalc;
			private int operator;
			// How to make 
		  
			
			public OperatorAction(int operation)
			{
				operator = operation;
			}
			
			public void actionPerformed(ActionEvent event)
			{
				currentCalc =   Integer.parseInt(SimpleCalc.instance.getText()); 
				calcOperation = operator;
			}
		}

import java.awt.EventQueue;


public class Main {

	public static void main(String[] args) {
		  
        //Use the event dispatch thread for Swing components
        EventQueue.invokeLater(new Runnable()
        {
            
           @Override
            public void run()
            {
              
                new SimpleCalc();         
            }
        });
             
   }
}

public static final SimpleCalc instance = new SimpleCalc();

import java.awt.BorderLayout;
import java.awt.FlowLayout;
import java.awt.GridBagLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class Digital extends JFrame{
public Digital(){
	;
	setVisible(true);
	add(new Panel());
	setSize(400,400);
	//t.setSize(20, 20);
	 //pack();
	setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}

public static void main(String args[]){
	
	new Digital();
	
}
}
class Panel extends JPanel implements ActionListener{
	JButton c[]=new JButton[10];
	JTextField t=new JTextField(20);
	JButton mul=new JButton("*");
	JButton div=new JButton("/");
	JButton add=new JButton("+");
	JButton sub=new JButton("-");
	public Panel(){   
		//setLayout(null);

	for(int i=0;i<10;i++)c[i]=new JButton(String.valueOf(i));
//	setLayout(new GridLayout(3,3));
		//setSize(100,100);
		//setLayout(new GridLayout(3,3));
		
		for(int i=0;i<10;i++)add(c[i]);
		
		//setLayout(new BorderLayout());
		for(int i=0;i<10;i++)c[i].addActionListener(this);
		//setLayout(new BorderLayout());
add(mul);add(div);add(add);add(sub);
 mul.addActionListener(new ActionListener(){

	@Override
	public void actionPerformed(ActionEvent arg0) {
		// TODO Auto-generated method stub
		String d=t.getText();
		t.setText(d+String.valueOf("*"));
	}
	 
	 
	 
 });
 sub.addActionListener(new ActionListener(){

	@Override
	public void actionPerformed(ActionEvent arg0) {
		// TODO Auto-generated method stub
		String d=t.getText();
		t.setText(d+String.valueOf("-"));
	}
	 
	 
	 
 });
 div.addActionListener(new ActionListener(){

	@Override
	public void actionPerformed(ActionEvent arg0) {
		// TODO Auto-generated method stub
		String d=t.getText();
		t.setText(d+String.valueOf("/"));
	}
	 
	 
	 
 });
 add.addActionListener(new ActionListener(){

	@Override
	public void actionPerformed(ActionEvent arg0) {
		// TODO Auto-generated method stub
		String d=t.getText();
		t.setText(d+String.valueOf("+"));
	}
	 
	 
	 
 });
	//setLayout(new FlowLayout());

		add(t);
		
		
	}
	@Override
	public void actionPerformed(ActionEvent arg0) {
		// TODO Auto-generated method stub

		for(int i=0;i<10;i++)if(arg0.getSource()==c[i]) {
			String d=t.getText();
			t.setText(d+String.valueOf(i));
		}
	}
	
}