Помогите с кодом

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class prog1_ {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		okno nf=new okno();
	}
	}
class okno extends JFrame
{
	private JTextArea text;
	private double p1=0,p2=0;
	private int oper=0;
	private void btnclick(JButton btn)
	{
		String str=btn.getText();
		//JOptionPane.showMessageDialog(null, str);
		if (str =="EXIT")
		{
			System.exit(0);
		}
		else if (str=="c")
		{
			p1=0;p2=0;text.setText("");
		}
		else if(str=="+")
		{
			oper=1;
			p1 = Double.parseDouble(text.getText());
			text.setText("");
		}
		else if(str=="-")
		{
			oper=2;
			p1 = Double.parseDouble(text.getText());
			text.setText("");
		}
		else if(str=="*")
		{
			oper=3;
			p1 = Double.parseDouble(text.getText());
			text.setText("");
		}
		else if(str=="/")
		{
			oper=4;
			p1 = Double.parseDouble(text.getText());
			text.setText("");
		}
		else if(str=="=")
		{
			p2 = Double.parseDouble(text.getText());
			if(oper==1)
			{
				text.setText(""+(p1+p2));
			}
			else if(oper==2)
			{
				text.setText(""+(p1-p2));
			}
			else if(oper==3)
			{
				text.setText(""+(p1*p2));
			}
			else if(oper==4)
			{
				text.setText(""+(p1/p2));
			}
		}
		else
		{
			text.setText("" + text.getText() + str);

		}


		//JOptionPane.showMessageDialog(null, "Бутерброд");
	}
	public okno()
	{
		Container cont = getContentPane();
		JPanel pan = new JPanel();
		pan.setLayout(null);
		Font btnFont = new Font("serif",0,20);
		Font labFont = new Font("arial",1,30);
		Font textFont = new Font("arial",2,30);
		JButton[] btn = new JButton[17];
		for (int i=0;i<17;i++)
		{
			btn[i] = new JButton();

			btn[i].setSize(100,25);
			btn[i].setFont(btnFont);
			btn[i].setLocation(30,50 + 30*i);
			pan.add(btn[i]);

			btn[i].addActionListener(new ActionListener()
			{
			public void actionPerformed(ActionEvent e){
				btnclick((JButton)e.getSource());
			}});
		}
		for(int i=0;i<10;i++)btn[i].setText(""+i);
		btn[10].setText("+");
		btn[11].setText("-");
		btn[12].setText("/");
		btn[13].setText("*");
		btn[14].setText("=");
		btn[15].setText("c");
		btn[16].setText("EXIT");
		JLabel lab = new JLabel("Resultat: ");
		lab.setFont(labFont);
		lab.setBounds(130,0,300,50);
		pan.add(lab);
		text = new JTextArea();
		text.setFont(textFont);
		text.setBounds(300,10,300,35);
		text.setForeground(new Color(0,0,100));
		text.setBackground(Color.WHITE);
		pan.add(text);
		cont.add(pan);
		setBounds(0,0,800,600);
		setTitle ("Calculator");
		setVisible(true);
}
}