Question

Is it possible to write generic class with one constructor which explicitly defines type of its class?

Here is my attempt to do that:

import javax.swing.JComponent;
import javax.swing.JLabel;

public class ComponentWrapper<T extends JComponent> {

    private T component;

    public ComponentWrapper(String title) {
        this(new JLabel(title));  // <-- compilation error
    }

    public ComponentWrapper(T component) {
        this.component = component;
    }

    public T getComponent() {
        return component;
    }

    public static void main(String[] args) {
        JButton button = new ComponentWrapper<JButton>(new JButton()).getComponent();
        // now I would like to getComponent without need to cast it to JLabel explicitly
        JLabel label = new ComponentWrapper<JLabel>("title").getComponent();
    }

}

Answer 1

Your current code can easily lead to invalid states (e.g. ComponentWrapper<SomeComponentThatIsNotAJLabel> that wraps a JLabel), and that’s likely why the compiler stops you there. You should use a static method instead in this case:

public static ComponentWrapper<JLabel> wrapLabel(final String title) {
    return new ComponentWrapper<JLabel>(new JLabel(title));
}

Which would be much safer in many ways.