Multiple classes in single file

One Java file can consist of multiple classes with the restriction that only one of them can be public. As soon as you remove public keyword from your classes, you can combine them into a single Java file.

At the risk of spoon-feeding

Please read http://docs.oracle.com/javase/tutorial/java/javaOO/innerclasses.html

import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.text.DecimalFormat;

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


public class TheaterWindow extends JFrame
{
    private JPanel pnlAdultTicketPrice, pnlAdultTicketsSold, pnlChildTicketPrice, pnlChildTicketsSold,
        pnlCalculate, pnlMain;
    private JLabel lblAdultTicketPrice, lblAdultTicketsSold, lblChildTicketPrice, lblChildTicketsSold;
    private JTextField txtAdultTicketPrice, txtAdultTicketsSold, txtChildTicketPrice, txtChildTicketsSold;
    private JButton btnCalculate;

    public TheaterWindow()
    {
        // Sets window title
        setTitle("Theater");

        // Sets layout to BorderLayout
        setLayout(new GridLayout(5,1));

        // Specifies what happens when close button is clicked
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        // Builds the panels
        buildPanels();

        // Add the panels to the frame's content pane
        add(pnlAdultTicketPrice);
        add(pnlChildTicketPrice);
        add(pnlAdultTicketsSold);
        add(pnlChildTicketsSold);
        add(pnlCalculate);

        // Size the frame to fit all of the panels
        pack();

        // Display the window
        setVisible(true);
    }

    private void buildPanels()
    {
        // Creates labels to display instructions
        lblAdultTicketPrice = new JLabel("Adult ticket price");
        lblChildTicketPrice = new JLabel("Child ticket price");
        lblAdultTicketsSold = new JLabel("Adult tickets sold");
        lblChildTicketsSold = new JLabel("Child tickets sold");

        // Creates text fields that are 10 characters wide
        txtAdultTicketPrice = new JTextField(10);
        txtChildTicketPrice = new JTextField(10);
        txtAdultTicketsSold = new JTextField(10);
        txtChildTicketsSold = new JTextField(10);

        // Creates button with caption
        btnCalculate = new JButton("Calculate");

        // Adds action listener to button
        btnCalculate.addActionListener(new CalcButtonListener());

        // Creates panels
        pnlAdultTicketPrice = new JPanel();
        pnlChildTicketPrice = new JPanel();
        pnlAdultTicketsSold = new JPanel();
        pnlChildTicketsSold = new JPanel();
        pnlCalculate = new JPanel();
        pnlMain = new JPanel();

        // Adds elements to their proper panels
        pnlAdultTicketPrice.add(lblAdultTicketPrice);
        pnlAdultTicketPrice.add(txtAdultTicketPrice);
        pnlChildTicketPrice.add(lblChildTicketPrice);
        pnlChildTicketPrice.add(txtChildTicketPrice);
        pnlAdultTicketsSold.add(lblAdultTicketsSold);
        pnlAdultTicketsSold.add(txtAdultTicketsSold);
        pnlChildTicketsSold.add(lblChildTicketsSold);
        pnlChildTicketsSold.add(txtChildTicketsSold);
        pnlCalculate.add(btnCalculate);

        // Adds all of the above panels to a main panel
        pnlMain.add(pnlAdultTicketPrice);
        pnlMain.add(pnlChildTicketPrice);
        pnlMain.add(pnlAdultTicketsSold);
        pnlMain.add(pnlChildTicketsSold);
        pnlMain.add(pnlCalculate);
    }

    private class CalcButtonListener implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            // Creates object of Theater
            Theater theater = new Theater();

            // Sets the member variables of Theater to the user's input
            theater.setAdultTicketPrice(Double.parseDouble(txtAdultTicketPrice.getText()));
            theater.setChildTicketPrice(Double.parseDouble(txtChildTicketPrice.getText()));
            theater.setAdultTicketsSold(Integer.parseInt(txtAdultTicketsSold.getText()));
            theater.setChildTicketsSold(Integer.parseInt(txtChildTicketsSold.getText()));

            // Creates DecimalFormat object for rounding
            DecimalFormat dollar = new DecimalFormat("#.##");

            // Display the charges.
            JOptionPane.showMessageDialog(null, "Adult ticket gross: $" +
                    Double.valueOf(dollar.format(theater.getAdultGross())) + "\n" +
                    "Child ticket gross: $" + Double.valueOf(dollar.format(theater.getChildGross())) + "\n" +
                    "Adult ticket net: $" + Double.valueOf(dollar.format(theater.getAdultNet())) + "\n" +
                    "Child ticket net: $" + Double.valueOf(dollar.format(theater.getChildNet())) + "\n" +
                    "Total gross: $" + Double.valueOf(dollar.format(theater.getChildGross())) + "\n" +
                    "Total net: $" + Double.valueOf(dollar.format(theater.getTotalNet())));
        }
    }

    public class Theater
    {
        private double PERCENTAGE_KEPT = 0.20;

        private double adultTicketPrice, childTicketPrice;
        private int adultTicketsSold, childTicketsSold;

        public double getAdultGross()
        {
            return getAdultTicketPrice() * getAdultTicketsSold();
        }

        public double getAdultNet()
        {
            return PERCENTAGE_KEPT * getAdultGross();
        }

        public double getAdultTicketPrice()
        {
            return adultTicketPrice;
        }

        public int getAdultTicketsSold()
        {
            return adultTicketsSold;
        }

        public double getChildGross()
        {
            return getChildTicketPrice() * getChildTicketsSold();
        }

        public double getChildNet()
        {
            return PERCENTAGE_KEPT * getChildGross();
        }

        public double getChildTicketPrice()
        {
            return childTicketPrice;
        }

        public int getChildTicketsSold()
        {
            return childTicketsSold;
        }

        public double getTotalGross()
        {
            return getChildGross() + getAdultGross();
        }

        public double getTotalNet()
        {
            return getChildGross() + getChildNet();
        }

        public void setAdultTicketPrice(double adultTicketPrice)
        {
            this.adultTicketPrice = adultTicketPrice;
        }

        public void setAdultTicketsSold(int adultTicketsSold)
        {
            this.adultTicketsSold = adultTicketsSold;
        }

        public void setChildTicketPrice(double childTicketPrice)
        {
            this.childTicketPrice = childTicketPrice;
        }

        public void setChildTicketsSold(int childTicketsSold)
        {
            this.childTicketsSold = childTicketsSold;
        }
    }
}

Yes You can write your all classes in a single .java file, But you must have only one class public(if file name and class name same)

Example:

class A { }

class B { }

class C { }