Found a NullPointerException

Question

I was making my first calculator in DrJava and it returned a NullPointerException. What am I doing wrong?

java.lang.NullPointerException

Here's the Java code:

import java.awt.GridLayout;
import java.awt.Font;
import java.awt.event.WindowListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JTextField;
import javax.swing.JLabel;


class calculadoraExercicio1 extends JFrame implements ActionListener
{
  JButton buttonDigit1, buttonDigit2, buttonDigit3, buttonDigit4, buttonDigit5, buttonDigit6, buttonDigit7, 
          buttonDigit8, buttonDigit9, buttonDigit0, buttonAddition, buttonSubtraction, buttonMultiplication, 
          buttonDivision, buttonClean, buttonEquals, buttonDecimal;
  JLabel mainCalculadora, labelNull1, labelNull2, labelNull3, labelNull4, labelNull5;
  calculadoraExercicio1()
  {
    setTitle("Calculadora");
    setSize(800,600);
    setResizable(false);
    setUndecorated(true);
    getContentPane().setLayout(new GridLayout());
    mainCalculadora = new JLabel("0");
    buttonDigit1 = new JButton("1");
    buttonDigit1.addActionListener(new ActionListener() {
      public+ void actionPerformed(ActionEvent e)
      {
        if (mainCalculadora.getText() != "0") {
        mainCalculadora.setText(mainCalculadora.getText() + "1");
        }
        else
        {
        mainCalculadora.setText("1");
        }
      }
    });
    buttonDigit1.setFont(new Font("Arial", Font.BOLD, 27));
    buttonDigit2 = new JButton("2");
    buttonDigit2.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e)
      {
        if (mainCalculadora.getText() != "0") {
        mainCalculadora.setText(mainCalculadora.getText() + "2");
        }
        else
        {
        mainCalculadora.setText("2");
        }
      }
    });
    buttonDigit2.setFont(new Font("Arial", Font.BOLD, 27));
    buttonDigit3 = new JButton("3");
    buttonDigit3.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e)
      {
        if (mainCalculadora.getText() != "0") {
        mainCalculadora.setText(mainCalculadora.getText() + "3");
        }
        else
        {
        mainCalculadora.setText("3");
        }
      }
    });
    buttonDigit3.setFont(new Font("Arial", Font.BOLD, 27));
    buttonDigit4 = new JButton("4");
    buttonDigit4.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e)
      {
        if (mainCalculadora.getText() != "0") {
        mainCalculadora.setText(mainCalculadora.getText() + "4");
        }
        else
        {
        mainCalculadora.setText("4");
        }
      }
    });
    buttonDigit4.setFont(new Font("Arial", Font.BOLD, 27));
    buttonDigit5 = new JButton("5");
    buttonDigit5.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e)
      {
        if (mainCalculadora.getText() != "0") {
        mainCalculadora.setText(mainCalculadora.getText() + "5");
        }
        else
        {
        mainCalculadora.setText("5");
        }
      }
    });
    buttonDigit5.setFont(new Font("Arial", Font.BOLD, 27));
    buttonDigit6 = new JButton("6");
    buttonDigit6.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e)
      {
        if (mainCalculadora.getText() != "0") {
        mainCalculadora.setText(mainCalculadora.getText() + "6");
        }
        else
        {
        mainCalculadora.setText("6");
        }
      }
    });
    buttonDigit6.setFont(new Font("Arial", Font.BOLD, 27));
    buttonDigit7 = new JButton("7");
    buttonDigit7.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e)
      {
        if (mainCalculadora.getText() != "0") {
        mainCalculadora.setText(mainCalculadora.getText() + "7");
        }
        else
        {
        mainCalculadora.setText("7");
        }
      }
    });
    buttonDigit7.setFont(new Font("Arial", Font.BOLD, 27));
    buttonDigit8 = new JButton("8");
    buttonDigit8.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e)
      {
        if (mainCalculadora.getText() != "0") {
        mainCalculadora.setText(mainCalculadora.getText() + "8");
        }
        else
        {
        mainCalculadora.setText("8");
        }
      }
    });
    buttonDigit8.setFont(new Font("Arial", Font.BOLD, 27));
    buttonDigit9 = new JButton("9");
    buttonDigit9.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e)
      {
        if (mainCalculadora.getText() != "0") {
        mainCalculadora.setText(mainCalculadora.getText() + "9");
        }
        else
        {
        mainCalculadora.setText("9");
        }
      }
    });
    buttonDigit9.setFont(new Font("Arial", Font.BOLD, 27));
    buttonDigit0 = new JButton("0");
    buttonDigit0.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e)
      {
        if (mainCalculadora.getText() != "0") {
          mainCalculadora.setText(mainCalculadora.getText() + "0");
        }
      }
    });
    buttonDigit0.setFont(new Font("Arial", Font.BOLD, 27));
    buttonDecimal = new JButton(",");
    buttonDecimal.addActionListener(this);
    buttonDecimal.setFont(new Font("Arial", Font.BOLD, 27));
    buttonAddition = new JButton("+");
    buttonAddition.addActionListener(this);
    buttonAddition.setFont(new Font("Arial", Font.BOLD, 27));
    buttonSubtraction = new JButton("-");
    buttonSubtraction.addActionListener(this);
    buttonSubtraction.setFont(new Font("Arial", Font.BOLD, 27));
    buttonMultiplication = new JButton("x");
    buttonMultiplication.addActionListener(this);
    buttonMultiplication.setFont(new Font("Arial", Font.BOLD, 27));
    buttonDivision = new JButton("÷");
    buttonDivision.addActionListener(this);
    buttonDivision.setFont(new Font("Arial", Font.BOLD, 27));

    getContentPane().add(labelNull1);
    getContentPane().add(mainCalculadora);
    getContentPane().add(labelNull2);
    getContentPane().add(labelNull3);
    getContentPane().add(labelNull4);
    getContentPane().add(buttonDigit1);
    getContentPane().add(buttonDigit2);
    getContentPane().add(buttonDigit3);
    getContentPane().add(buttonClean);
    getContentPane().add(buttonDecimal);
    getContentPane().add(buttonDigit4);
    getContentPane().add(buttonDigit5);
    getContentPane().add(buttonDigit6);
    getContentPane().add(buttonAddition);
    getContentPane().add(buttonSubtraction);
    getContentPane().add(buttonDigit7);
    getContentPane().add(buttonDigit8);
    getContentPane().add(buttonDigit9);
    getContentPane().add(buttonMultiplication);
    getContentPane().add(buttonDivision);
    getContentPane().add(buttonDigit0);
    getContentPane().add(labelNull5);
    getContentPane().add(buttonEquals);
  }
   public void actionPerformed(ActionEvent e)
  {
     String s = "";
     float r = 0;
     boolean d = false;
    if(e.getSource() == buttonClean)
    {
      mainCalculadora.setText("0");
    }
    if(e.getSource() == buttonDecimal)
    {
      if (d = false) {
      mainCalculadora.setText(mainCalculadora.getText() + ",");
      d = true;
      }
    }
    if(e.getSource() == buttonAddition)
    {
      s = "+";
    }
    if(e.getSource() == buttonSubtraction)
    {
      s = "-";
    }
    if(e.getSource() == buttonMultiplication)
    {
      s = "*";
    }
    if(e.getSource() == buttonDivision)
    {
      s = "/";
    }
    if(e.getSource() == buttonEquals)
    {
      if (s == "+") {
        r = r + Float.parseFloat(mainCalculadora.getText());
        mainCalculadora.setText(String.valueOf(r));
      }
      if (s == "-") {
        r = r - Float.parseFloat(mainCalculadora.getText());
        mainCalculadora.setText(String.valueOf(r));
      }
      if (s == "*") {
        r = r * Float.parseFloat(mainCalculadora.getText());
        mainCalculadora.setText(String.valueOf(r));
      }
      if (s == "/") {
        r = r / Float.parseFloat(mainCalculadora.getText());
        mainCalculadora.setText(String.valueOf(r));
      }
    }
  }
   public static void main (String args[])
  {
    JFrame Janela = new calculadoraExercicio1();
    Janela.show(); 
    WindowListener x = new WindowAdapter()
    {
      public void windowClosing(WindowEvent e)
      {
        System.exit(0);
      }
    };
    Janela.addWindowListener(x);
  }
}

I don't know what I'm doing wrong.

Also see [How do I compare strings in Java?](http://stackoverflow.com/questions/513832/how-do-i-compare-strings-in-java) — Pshemo, Sep 05 '16 at 18:29
You have to understand our frustration because this question has been asked tens of thousands of times for Java and has been answered tens of thousands of times right here on stackoverflow. So the fact that you're asking it means you haven't done due diligence before adding the question to stackoverflow. You shouldn't be copy/pasting 100 lines of code, you should trim this down to about 3 lines to reproduce the error simply. — Eric Leschinski, Sep 05 '16 at 18:38

Found a NullPointerException

0 Answers0