How to create a String with the result of a drop down menu?

I've created a JFrame window with a JComboBox. I can select things but they don't do anything. I thought that event was a String, but it isn't. What is the solution for this?

import java.awt.FlowLayout;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;

import javax.swing.Icon;
import javax.swing.ImageIcon;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;

public class Gui extends JFrame {
    private JComboBox box;
    private JLabel picture;

    private static String[] filename = {"", "b.png", "x.png"};

    public Gui() {
        super("the title");
        setLayout(new FlowLayout());
        box = new JComboBox(filename);
        box.addItemListener(
                new ItemListener() {
                    public void itemStateChanged(ItemEvent event) {
                        if(event.getStateChange()==ItemEvent.SELECTED){
                            System.out.println("test");
                            if(event=="b.png") {
                                System.out.println("test2");
                            }
                        }
                    }
                });
        add(box);
    }
}

You need to get the selected item (which is a String in your case) and compare it to your string with equals:

if ("b.png".equals(event.getItem()))

Note that this is preferable to

event.getItem().equals("b.png")

since this can theoretically throw a NPE.

Also, use a generic type: JComboBox<String>, not the raw type.