Illegalargumentexception when adding components to Jframe

Question

I’m doing a project of the game Sokoban, and I got to the part where I created 2 Jbuttons to select the level. Now when I try to run a program, it’s a mistake. I created both buttons and action in a class Buttons and named it that class in another class that starts the program. I used the following for the buttons:

public class Buttons extends JFrame implements ActionListener {
    private static final long serialVersionUID = 1L;

    public Buttons(){
        this.getContentPane().setLayout(new FlowLayout());
        JButton lvl1btn = new JButton("Level 1");
        JButton lvl2btn = new JButton("Level 2");
        lvl1btn.setBounds(700,0,100,25);
        lvl1btn.setBounds(800,0,100,25);
        lvl1btn.addActionListener(this);
        lvl2btn.addActionListener(this);
        lvl1btn.setActionCommand("Level 1");
        lvl2btn.setActionCommand("Level 2");
    }


    @Override
    public void actionPerformed(ActionEvent e) {
        String action = e.getActionCommand();
        if(action.equals("Level 1")){
            Board board = new Board();
            board.createWorldOne();
        }else if(action.equals("Level 2")){
            Board board = new Board();
            board.createWorldTwo();
        }
    }

And I called it that:

public final class SokobanStart extends JFrame {

public SokobanStart() {
    InitUI();
}

public void InitUI() {
    Board board = new Board();
    Buttons buttons = new Buttons();
    add(board,buttons);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setSize(board.getBoardWidth(),board.getBoardHeight());
    setLocationRelativeTo(null);
    setTitle("Sokoban Game!");
}


public static void main(String[] args) {
    SokobanStart sokobanStart = new SokobanStart();
    sokobanStart.setVisible(true);
}

}

And from the following thread:

Exception in thread "main" java.lang.Illegalargumentexception: cannot add to layout: Constraint must be a string (or null) at java.desktop/java.awt.Borderlayout.addLayoutComponent(Borderlayout.java:431) java.desktop/javax.swing.Jrootpane$1.addLayoutComponent(Jrootpane.java:507) at java.desktop/java.awt.Container.addImpl(Container.java:1148) at java.desktop/java.awt.Container.add(Container.java:1025) at java.desktop/javax.swing.Jframe.addImpl(Jframe.java:553) at java.desktop/java.awt.Container.add(Container.java:993) At Sokoban.SokobanStart.Initui(Sokobanstart.java:16) At Sokoban.SokobanStart.(Sokobanstart.java:10) At Sokoban.SokobanStart.main(Sokobanstart.java:25)

Answer 1

This line here is cause of error:

add(board,buttons);

You didn’t put in a complete code, so I’m guessing this class Board be some other component you have created.

If you look at the class documentation JFrame, you will see that none of the methods add that it inherits gets 2 components at the same time, that line is wrong. If you want to add board and Buttons in the JFrame, need to be one at a time, you need to break this addition:

add(board);
add(buttons);

Only it would still make a mistake because the class Buttons is a Jframe and you cannot add a Jframe within another. You will have to transform these classes Buttons and Board in JPanel if you want to add them this way.

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It is worth mentioning the recommendation below:

Avoid using absolute layout, unless it is of extreme necessity and you know the consequences of it, because absolute layout makes it difficult to maintain the screen and makes your application look different depending on the monitor and resolution that is running.

There are several layouts so you don’t have to worry about positioning or manually organizing components. Not to mention that using layouts makes your code easier to maintain than inserting a lot of setbounds, and if you need to change the position of any component, in the absolute layout, you will have to reposition all manually.

And it’s also worth noting that all graphic applications involving the swing API should be dispatched to the EDT.

I recommend for study: Creating a GUI With JFC/Swing