Simple Java Calculator using GU

Question

I'm trying to make a Simple Java Calculator. Right now I am stuck on how to convert the string that is entered by the user on the calculator to a double so that the operation can execute in the equals part of the program. InFixPostOut is supposed to be used as well.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.regex.Pattern;
import java.util.regex.Matcher;

class Calculator2 extends JFrame implements ActionListener { 

   private static final long serialVersionUID = 1L;
   JButton btn1 = new JButton("1");
   JButton btn2 = new JButton("2");
   JButton btn3 = new JButton("3");
   JButton btn_add = new JButton("+");
   JButton btn4 = new JButton("4");
   JButton btn5 = new JButton("5");
   JButton btn6 = new JButton("6");
   JButton btn_sub = new JButton("-");
   JButton btn7 = new JButton("7");
   JButton btn8 = new JButton("8");
   JButton btn9 = new JButton("9");
   JButton btn_mult = new JButton("*");
   JButton btn0 = new JButton("0");
   JButton btn_dot = new JButton(".");
   JButton btn_del = new JButton("DEL");
   JButton btn_div = new JButton("/");

   JButton btn_lpr = new JButton("(");
   JButton btn_rpr = new JButton(")");
   JButton btn_pow = new JButton("^");
   JButton btn_equ = new JButton("=");

   JTextArea txt = new JTextArea();
   String str_number = "";

   public Calculator2() {
      JFrame frame = new JFrame("Simple Java Caculator");
      frame.setSize(320,420);
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.setResizable(false);
      frame.setVisible(true);
      frame.setLayout(new GridLayout(2,1));

      JPanel HeadPanel = new JPanel();
      JPanel NumberPanel = new JPanel();
      JPanel LabelPanel = new JPanel();

      LabelPanel.setBackground(Color.LIGHT_GRAY);
      HeadPanel.setBackground(Color.LIGHT_GRAY);
      NumberPanel.setLayout(new GridLayout(5,4));
    //LabelPanel.setLayout(new BorderLayout());
      LabelPanel.setLayout(new GridLayout(1,1));

      NumberPanel.add(btn1);
      btn1.addActionListener(this);
      NumberPanel.add(btn2);
      btn2.addActionListener(this);
      NumberPanel.add(btn3);
      btn3.addActionListener(this);
      NumberPanel.add(btn_add);
      btn_add.addActionListener(this);

      NumberPanel.add(btn4);
      btn4.addActionListener(this);
      NumberPanel.add(btn5);

      btn5.addActionListener(this);
      NumberPanel.add(btn6);
      btn6.addActionListener(this);
      NumberPanel.add(btn_sub);
      btn_sub.addActionListener(this);

      NumberPanel.add(btn7);
      btn7.addActionListener(this);
      NumberPanel.add(btn8);
      btn8.addActionListener(this);
      NumberPanel.add(btn9);
      btn9.addActionListener(this);
      NumberPanel.add(btn_mult);
      btn_mult.addActionListener(this);

      NumberPanel.add(btn0);
      btn0.addActionListener(this);
      NumberPanel.add(btn_dot);
      btn_dot.addActionListener(this);
      NumberPanel.add(btn_del);
      btn_del.addActionListener(this);
      NumberPanel.add(btn_div);
      btn_div.addActionListener(this);
      LabelPanel.add(txt);

    //LabelPanel.add(btn_equ);
      NumberPanel.add(btn_lpr);
      btn_lpr.addActionListener(this);
      NumberPanel.add(btn_rpr);
      btn_rpr.addActionListener(this);
      NumberPanel.add(btn_pow);
      btn_pow.addActionListener(this);
      NumberPanel.add(btn_equ);
      btn_equ.addActionListener(this);

      txt.setEditable(false);
    //btn_del.setEnabled(false);
      HeadPanel.add(new JLabel("A Java Calculator"));
      frame.add(LabelPanel);
      frame.add(NumberPanel);
      frame.setVisible(true);
   }

   public void actionPerformed(ActionEvent e) {

      if(e.getSource()==btn1) {
         str_number+="1";
         txt.setText(str_number); }
      else if(e.getSource()==btn2) {
         str_number+="2";
         txt.setText(str_number); }
      else if(e.getSource()==btn3) {
         str_number+="3";
         txt.setText(str_number); }
      else if(e.getSource()==btn4) {
         str_number+="4";
         txt.setText(str_number); }
      else if(e.getSource()==btn5) {
         str_number+="5";
         txt.setText(str_number); }
      else if(e.getSource()==btn6) {
         str_number+="6";
         txt.setText(str_number); }
      else if(e.getSource()==btn7) {
         str_number+="7";
         txt.setText(str_number); }
      else if(e.getSource()==btn8) {
         str_number+="8";
         txt.setText(str_number); }
      else if(e.getSource()==btn9) {
         str_number+="9";
         txt.setText(str_number); }
      else if(e.getSource()==btn0) {
         str_number+="0";
         txt.setText(str_number); }
      else if(e.getSource()==btn_lpr) {
         str_number+="(";
         txt.setText(str_number); }
      else if(e.getSource()==btn_rpr) {
         str_number+=")";
         txt.setText(str_number); }
      else if(e.getSource()==btn_pow) {
         str_number+="^";
         txt.setText(str_number); }

      else if(e.getSource()==btn_add) {
         str_number+="+";
         txt.setText(str_number);}
      else if(e.getSource()==btn_sub) {
         str_number+="-";
         txt.setText(str_number);}
      else if(e.getSource()==btn_mult) {
         str_number+="*";
         txt.setText(str_number);}
      else if(e.getSource()==btn_div) {
         str_number+="/";
         txt.setText(str_number);}
      else if(e.getSource()==btn_equ) {
         System.out.println("you clicked equal sign!");
         str_number+="=";   
         txt.setText(str_number);
      // this is where I am stuck

      else if(e.getSource()==btn_dot) {
         System.out.println("you clicked dot button!");
         str_number+=".";   
         txt.setText(str_number);
      }
      else if(e.getSource()==btn_del) {
         System.out.println("you clicked DEL button!");
      }
   }

   public static void main(String[] args) {
      new Calculator2();

   }
}