Java error - Square root calculation

Question

I was doing some basic exercises in Java and one of them asked:

In this example, we will use the method sqrt class Math to extract the square root of the number that is typed in a text box, a 'Text field' component for the user to enter the desired number, and a 'Label' component to display the result, i.e., the square root of that number.

Then I did everything right, only it gave a problem, when I type the 1 letter:

Exception in thread "AWT-EventQueue-0" java.lang.NumberFormatException: empty String
    at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1842)
    at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110)
    at java.lang.Double.parseDouble(Double.java:538)
    at Apresentacao.TelaRaiz.jtNumeroKeyPressed(TelaRaiz.java:67)
    at Apresentacao.TelaRaiz.access$000(TelaRaiz.java:12)
    at Apresentacao.TelaRaiz$1.keyPressed(TelaRaiz.java:38)
    at java.awt.Component.processKeyEvent(Component.java:6491)
    at javax.swing.JComponent.processKeyEvent(JComponent.java:2832)
    at java.awt.Component.processEvent(Component.java:6310)
    at java.awt.Container.processEvent(Container.java:2236)
    at java.awt.Component.dispatchEventImpl(Component.java:4889)
    at java.awt.Container.dispatchEventImpl(Container.java:2294)
    at java.awt.Component.dispatchEvent(Component.java:4711)
    at java.awt.KeyboardFocusManager.redispatchEvent(KeyboardFocusManager.java:1954)
    at java.awt.DefaultKeyboardFocusManager.dispatchKeyEvent(DefaultKeyboardFocusManager.java:806)
    at java.awt.DefaultKeyboardFocusManager.preDispatchKeyEvent(DefaultKeyboardFocusManager.java:1074)
    at java.awt.DefaultKeyboardFocusManager.typeAheadAssertions(DefaultKeyboardFocusManager.java:945)
    at java.awt.DefaultKeyboardFocusManager.dispatchEvent(DefaultKeyboardFocusManager.java:771)
    at java.awt.Component.dispatchEventImpl(Component.java:4760)
    at java.awt.Container.dispatchEventImpl(Container.java:2294)
    at java.awt.Window.dispatchEventImpl(Window.java:2746)
    at java.awt.Component.dispatchEvent(Component.java:4711)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
    at java.awt.EventQueue.access$500(EventQueue.java:97)
    at java.awt.EventQueue$3.run(EventQueue.java:709)
    at java.awt.EventQueue$3.run(EventQueue.java:703)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:90)
    at java.awt.EventQueue$4.run(EventQueue.java:731)
    at java.awt.EventQueue$4.run(EventQueue.java:729)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

It runs everything and even with error it calculates the root, but only when I type the second letter, or when I type the first letter and hit ENTER. Why are you making this mistake? I wanted when I typed it to capture and show the result.

Excerpt from Error:

private void jtNumeroKeyPressed(java.awt.event.KeyEvent evt) {                                    
        int i = (int)(Double.parseDouble(jtNumero.getText()));
        jlResultado.setText("A raiz quadrada de: "+ i + " é: " + Math.sqrt(i));
    }

Answer 1

The error occurs because an empty String is being passed to the parse method. Try to validate before capturing the field value:

private void jtNumeroKeyPressed(java.awt.event.KeyEvent evt){
    String valor = jtNumero.getText().trim();

    if(valor.isEmpty()){
        //exibe algo se o campo estiver vazio
    } else {
        int i = (int)(Double.parseDouble(valor));
        jlResultado.setText("A raiz quadrada de: "+ i + " é: " + Math.sqrt(i));
    }
}

Answer 2

The problem is that you are using the event KeyPressed. This event captures the pressed key, but at this point the text attribute of the component still has the old value. Use the event KeyReleased, because it is triggered after the key is released and the text is already with the definitive content.

private void jtNumeroKeyReleased(java.awt.event.KeyEvent evt) {                                        
    if ("".equals(jtNumero.getText())){return;}
    int i = (int)(Double.parseDouble(jtNumero.getText()));
    jlResultado.setText("A raiz quadrada de: "+ i + " é: " + Math.sqrt(i));
}                                       

Answer 3

Try to do so:

private void jtNumeroKeyPressed(java.awt.event.KeyEvent evt) {
    try {                     
        double i = Double.parseDouble(jtNumero.getText());
        jlResultado.setText("A raiz quadrada de " + i + " é " + Math.sqrt(i) + ".");
    } catch (NumberFormatException e) {
        jlResultado.setText("Por favor, digite um número válido.");
    }
}

Here, the catch is capturing the exception if the entered value is not numerical. Also, the entered value must be a double, and not a int. Thus, the square root of 1.21 will be 1.1, and not 1.