Java – how do I call this class?

I have my main class to call a custom JFrame class called mainframe

public class App {

public static MainFrame mf;

public static void main(String[] args) {

    SwingUtilities.invokelater(new Runnable() {
        public void run() {
             MainFrame mf = new MainFrame(Workers); //jFrame
        }   
    }); 
}

public static void foo(String s){ //invoked by another class
    mf.validate();

}       
}

Calling MF's method outside of run () returns a null value How to call a method in MainFrame?

Solution

You only need to declare your class outside the scope of the method so that you can use it later

MainFrame mf;

SwingUtilities.invokelater(new Runnable() {
    public void run() {
        mf  = new MainFrame(Workers); //jFrame
    }   
});

It happens because of something called scope Your variable only lives in parentheses that declare it After that, the garbage collector will delete it automatically