具有构造函数的newInstance（）类

Question

The following code is returning null:

    private JComponent setupComponent(Class<? extends JComponent> c, Object... constructor) {

        try {
            return c.getConstructor(new Class[] { c.getClass() }).newInstance(constructor);
        }
        catch (Exception e) { }

        return null;
    }

I am calling it here:

    JTextField userText = (JTextField) setupComponent(JTextField.class, "Test");

Why is it returning null and how can I fix it?

Answer 1

You need to use the static version of class.

   private JComponent setupComponent(Class<? extends JComponent> c, Object... constructor) {

        try {
            return c.getConstructor(new Class[] { c}).newInstance(constructor);
        }
        catch (Exception e) { }

        return null;
    }

Answer 2

It's returning null as its silently failing in the exception block. Display the stacktrace to show the source of the exception.

The main problem is that you need to use matching class type arguments which correspond to the constructor arguments passed in rather than the type of the specific JComponent itself to target the correct constructor. ie JTextField(String)

private JComponent setupComponent
                 (Class<? extends JComponent> c, Object... constructor) {

    // build matching class args    
    Class<?>[] classArgs = new Class[constructor.length];
    for (int i = 0; i < constructor.length; i++) {
        classArgs[i] = constructor[i].getClass();
    }

    try {
        return c.getConstructor( classArgs ).newInstance(constructor);
    } catch (Exception e) {
        e.printStackTrace(); // add this
    }

    return null;
}