Java action listener - same loop, different outcome

Question

Sorry in advance for a not very specific title - the problem was hard to describe. I essentialy have a very simple program, a window with an X, a number that tells what size is the font in said X, and two buttons - increase and decrease which clicked should change the font size and number by one. They're in a class which implement ActionListener

Not this is the actionPerformed. The if else is Ctrl+C, Ctrl+V, the only thing changed is ++ and --. The problem is that program once running has a correctly working decrease button (decreases by 1) but increase causes value and font to increase by two.

public void actionPerformed(ActionEvent e) { 
    if (e.getSource() == increase) {
        size.setText(++value+"");
        setFont(value);
    }
    else if (e.getSource() == decrease) {
        size.setText(--value+"");
        setFont(value);
    }
}

After staring at my code and trying diffrent ways of writing it - changing value in separate lines and so on, I still can't understand why this happens. Below is the whole program (quite short, as I said it's supposed to be a simple exercise on JFrame and its components):

class FontDisplay extends JFrame implements ActionListener{

JLabel size, letter;
JButton increase, decrease;
int value; //to get rid of converting from strings to ints when changing size

public FontDisplay() {
    value = 18;
    setLayout(new GridLayout(2,2));
    size = new JLabel("18");
    letter = new JLabel("X", JLabel.RIGHT);
    setFont(18);
    increase = new JButton("Increase");
    increase.addActionListener(this);
    decrease = new JButton("Decrease");
    decrease.addActionListener(this);

    add(letter);
    add(size);
    add(increase);
    add(decrease);
    increase.addActionListener(this);
}


public void actionPerformed(ActionEvent e) { 
    if (e.getSource() == increase) {
        size.setText(++value+"");
        setFont(value);

    }
    else if (e.getSource() == decrease) {
        value--;
        size.setText(value+"");
        setFont(value);
    }
}

private void setFont(int i) {
    Font font = new Font(letter.getFont().getFontName(), Font.PLAIN,i);
    letter.setFont(font);
}


public static void main(String[] args){
    FontDisplay t = new FontDisplay();
    t.setSize(200, 200);
    t.setTitle("Check Font Size");
    t.setVisible(true);
    t.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}

}