How Do I Test A Java Aplication

import java.applet.Applet;
import java.awt.event.*;
import java.awt.*;

//intializes the Applet and the ActionListener
public class Problem1 extends Applet implements ActionListener
{
  // creates space for the labels and textfields
  Label lblPrice = new Label ("Product Price: ");
  Label lblQuantity = new Label ("Quantity: ");
  TextField txtPrice = new TextField(25);
  TextField txtQuantity = new TextField(10);
  TextArea txaOutput = new TextArea("Calculation Result" + "\n", 10,30);
  Button btnCalculate = new Button ("Calculate Price");
  // initializes the objects to be created and link it with an ActionListener
  public void init()
  {
    DesignLayout();
    txtPrice.requestFocus();
    txtPrice.addActionListener(this);
    txtQuantity.addActionListener(this);
    btnCalculate.addActionListener(this);
  }
  // tells what the ActionListener to do
  public void actionPerformed(ActionEvent evt)
  {
      float Price = Float.valueOf(txtPrice.getText()).floatValue();
      float Quantity = Float.valueOf(txtQuantity.getText()).floatValue();
      float Total;
      Total = Price*Quantity;
      txaOutput.append("\n" + Price + "\t" + Quantity + "\t" + Total);
      ClearTextFields();
  }
    // shows how things are placed using the DesignLayout() tool
    public void DesignLayout()
    {
      GridBagLayout gridbag = new GridBagLayout();
      GridBagConstraints constraints = new GridBagConstraints();
      setLayout(gridbag);
      constraints.insets = new Insets(5,5,5,5);

      //Row 1 Label
      constraints.weightx = 1.0;
      constraints.anchor = GridBagConstraints.WEST;
      constraints.fill = GridBagConstraints.NONE;
      gridbag.setConstraints(lblPrice, constraints);
      add (lblPrice);

      //Row 1 Text

      constraints.weightx = 3.0;
      constraints.gridwidth = GridBagConstraints.REMAINDER;
      constraints.anchor = GridBagConstraints.EAST;
      gridbag.setConstraints(txtPrice, constraints);
      add(txtPrice);

      //Row 2 Label

      constraints.weightx = 0;
      constraints.anchor = GridBagConstraints.WEST;
      constraints.fill = GridBagConstraints.NONE;
      gridbag.setConstraints(lblQuantity, constraints);
      add(lblQuantity);

      //Row 2 Text
      constraints.gridwidth = GridBagConstraints.REMAINDER;
      constraints.anchor = GridBagConstraints.EAST;
      gridbag.setConstraints(txtQuantity, constraints);
      add(txtQuantity);

      //Row 3 Button
      constraints.gridwidth = GridBagConstraints.REMAINDER;
      constraints.gridx = 0;
      constraints.anchor = GridBagConstraints.EAST;
      gridbag.setConstraints(btnCalculate, constraints);
      add(btnCalculate);

      //Row 4 Text Area
      constraints.gridx = 0;
      constraints.gridwidth = 2;
      constraints.anchor = GridBagConstraints.CENTER;
      gridbag.setConstraints(txaOutput, constraints);      
      txaOutput.setEditable(false);
      add(txaOutput);
    }
   // this method sets ClearTextFields to clear all the text after each query
    public void ClearTextFields()
    {
      txtPrice.setText("");
      txtQuantity.setText("");
    }
  }