As suggested by @Reimeus , you need to add a Document Listener to each of your text fields. Since you are adding jButton1ActionPerformed() to your method, I have made a notification that listeners call this method whenever there are changes in your text fields.
Note Based on the comments, I updated my code to include only one DocumentListener , and also added a DocumentFilter to accept only numerical input. I also deleted the button that was present in the source code to perform the summation
Here is your updated code:
import javax.swing.event.DocumentEvent; import javax.swing.event.DocumentListener; import javax.swing.text.AbstractDocument; import javax.swing.text.AttributeSet; import javax.swing.text.BadLocationException; import javax.swing.text.DocumentFilter; public class Frame extends javax.swing.JFrame { public Frame() { initComponents(); } @SuppressWarnings("unchecked") // <editor-fold defaultstate="collapsed" desc="Generated Code"> private void initComponents() { n1 = new javax.swing.JTextField(); n2 = new javax.swing.JTextField(); n3 = new javax.swing.JTextField(); sum = new javax.swing.JTextField(); jLabel1 = new javax.swing.JLabel(); jLabel2 = new javax.swing.JLabel(); jLabel3 = new javax.swing.JLabel(); jLabel5 = new javax.swing.JLabel(); DocumentListener documentListener = new DocumentListener() { @Override public void removeUpdate(DocumentEvent e) { performSummation(null); } @Override public void insertUpdate(DocumentEvent e) { performSummation(null); } @Override public void changedUpdate(DocumentEvent e) { } }; DocumentFilter numericFilter = new DocumentFilter(){ @Override public void insertString(FilterBypass fb, int offset, String string, AttributeSet attr) throws BadLocationException { fb.insertString(offset, string.replaceAll("[^\\d]", ""), attr); } @Override public void replace(FilterBypass fb, int offset, int length, String text, AttributeSet attrs) throws BadLocationException { fb.replace(offset, length, text.replaceAll("[^\\d]", ""), attrs); } }; ((AbstractDocument) n1.getDocument()).setDocumentFilter(numericFilter); ((AbstractDocument) n2.getDocument()).setDocumentFilter(numericFilter); ((AbstractDocument) n3.getDocument()).setDocumentFilter(numericFilter); n1.getDocument().addDocumentListener(documentListener); n2.getDocument().addDocumentListener(documentListener); n3.getDocument().addDocumentListener(documentListener); setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE); jLabel1.setText("n1"); jLabel2.setText("n2"); jLabel3.setText("n3"); jLabel5.setText("result"); javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane()); getContentPane().setLayout(layout); layout.setHorizontalGroup( layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING) .addGroup(layout.createSequentialGroup() .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING) .addGroup(layout.createSequentialGroup() .addGap(15, 15, 15) .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.TRAILING) .addComponent(jLabel1) .addComponent(jLabel2) .addComponent(jLabel3))) .addGroup(layout.createSequentialGroup() .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE) .addComponent(jLabel5))) .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED) .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING) .addGroup(layout.createSequentialGroup() .addComponent(sum, javax.swing.GroupLayout.PREFERRED_SIZE, 59, javax.swing.GroupLayout.PREFERRED_SIZE) .addGap(30, 30, 30) .addContainerGap()) .addGroup(layout.createSequentialGroup() .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING) .addComponent(n1, javax.swing.GroupLayout.PREFERRED_SIZE, 50, javax.swing.GroupLayout.PREFERRED_SIZE) .addComponent(n2, javax.swing.GroupLayout.PREFERRED_SIZE, 50, javax.swing.GroupLayout.PREFERRED_SIZE) .addComponent(n3, javax.swing.GroupLayout.PREFERRED_SIZE, 50, javax.swing.GroupLayout.PREFERRED_SIZE)) .addGap(0, 0, Short.MAX_VALUE)))) ); layout.setVerticalGroup( layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING) .addGroup(layout.createSequentialGroup() .addGap(32, 32, 32) .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.TRAILING) .addGroup(layout.createSequentialGroup() .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE) .addComponent(n1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE) .addComponent(jLabel1)) .addGap(10, 10, 10) .addComponent(n2, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)) .addComponent(jLabel2)) .addGap(20, 20, 20) .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE) .addComponent(n3, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE) .addComponent(jLabel3)) .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED) .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE) .addComponent(sum, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE) .addComponent(jLabel5)) .addContainerGap(38, Short.MAX_VALUE)) ); pack(); }// </editor-fold> private void performSummation(java.awt.event.ActionEvent evt) { int total = 0; if(n1.getText().trim().length() > 0){ try{ total += Integer.parseInt(n1.getText()); }catch(NumberFormatException nbx){ } } if(n2.getText().trim().length() > 0){ try{ total += Integer.parseInt(n2.getText()); }catch(NumberFormatException nbx){ } } if(n3.getText().trim().length() > 0){ try{ total += Integer.parseInt(n3.getText()); }catch(NumberFormatException nbx){ } } sum.setText(""+total); } public static void main(String args[]) { /* Set the Nimbus look and feel */ //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) "> /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel. * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html */ try { for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) { if ("Nimbus".equals(info.getName())) { javax.swing.UIManager.setLookAndFeel(info.getClassName()); break; } } } catch (ClassNotFoundException ex) { java.util.logging.Logger.getLogger(Frame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex); } catch (InstantiationException ex) { java.util.logging.Logger.getLogger(Frame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex); } catch (IllegalAccessException ex) { java.util.logging.Logger.getLogger(Frame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex); } catch (javax.swing.UnsupportedLookAndFeelException ex) { java.util.logging.Logger.getLogger(Frame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex); } //</editor-fold> /* Create and display the form */ java.awt.EventQueue.invokeLater(new Runnable() { public void run() { new Frame().setVisible(true); } }); } // Variables declaration - do not modify private javax.swing.JLabel jLabel1; private javax.swing.JLabel jLabel2; private javax.swing.JLabel jLabel3; private javax.swing.JLabel jLabel5; private javax.swing.JTextField n1; private javax.swing.JTextField n2; private javax.swing.JTextField n3; private javax.swing.JTextField sum; // End of variables declaration }
A few pointers:
Never trust entries that come from the user. There is a possibility that the user may enter an incorrect value (say, something that is not integer). Thus, it is always recommended to check your input and check whether the input can really be analyzed. With that in mind, I updated your jButton1ActionPerformed() method
Another alternative is to use formatted text fields.