如何更改JFrame的背景颜色（我是大一学生）

Question

Alright so my question here is how am I supposed to change the JFrame background color when my if statements check my booleans to give me either "Strong", "Fair" etc. frame.setBackground(Color.green) would be what I needed but I dont think it wants me to access that object. (PS I know the code is kind of sloppy).

Thanks guys.

public class PasswordJFrame extends JFrame implements ActionListener {

private JLabel title;
private JTextField input;
private JButton rate;
private JLabel rating;
private JButton reset;
private String password;
private boolean upperlower = false;
private boolean symbol = false;
private boolean number = false;


public PasswordJFrame(){

    Container pane = this.getContentPane();
    pane.setLayout(new FlowLayout());
    title = new JLabel("Enter Your Password:");
    input = new JTextField(15);
    rate = new JButton("Rate my password");
    rating = new JLabel("");
    reset = new JButton("Reset");

    pane.add(title);
    pane.add(input);
    pane.add(rate);
    pane.add(rating);
    pane.add(reset);
    rate.addActionListener(this);
    reset.addActionListener(this);

}

public void actionPerformed(ActionEvent e) {

    Object source = e.getSource();
    password = input.getText();

    if (source == rate){

        for(int i = 0; i<password.length() - 1; i++){

            switch(input.getText().charAt(i)){

                case '@':
                symbol = true;
                break;
                case '$':
                symbol = true;
                break;
                case '*':
                symbol = true;
                break;
                case '+':
                symbol = true;
                break;
                case '%':
                symbol = true;
                break;
                case '&':
                symbol = true;
                break;
                case '0':
                number = true;
                break;
                case '1':
                number = true;
                break;
                case '2':
                number = true;
                break;
                case '3':
                number = true;
                break;
                case '4':
                number = true;
                break;
                case '5':
                number = true;
                break;
                case '6':
                number = true;
                break;
                case '7':
                number = true;
                break;
                case '8':
                number = true;
                break;
                case '9':
                number = true;
                break;  
            }

            if(!password.equals(password.toLowerCase()) && !password.equals(password.toUpperCase())){       
                upperlower = true;
            }
        }

        if(upperlower && symbol && number){
            rating.setText("Strong");
            frame.setBackground(Color.green);
        }
        else if (upperlower && number){
            rating.setText("Fair");
            frame.setBackground(Color.yellow);
        }
        else if (upperlower){
            rating.setText("Weak");
            frame.setBackground(Color.red);
        }
        else{
            rating.setText("Awful");
            frame.setBackground(Color.black);
        }

    }
}

public static void main(String[] args){

    PasswordJFrame frame = new PasswordJFrame();
    frame.setSize(300,300);
    frame.setResizable(false);
    frame.setVisible(true);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);


}

}

Answer 1

The frame contains a content pane where you add all your components. So you need to set the background of the content pane:

frame.getContentPane().setBackground(...);

See the section from the Swing tutorial on Using Top Level Containers for more information and diagrams showing the content pane relationship to the frame.