JPanel显示为一个小白框

Question

我正处于尝试创建Java2d图形绘制程序的早期阶段。我使用了流布局，并且使用了三个面板。前两行是按钮，组合框等，第三行是一个大的，空白的，白色的面板，将用于绘画。前两个面板显示得很漂亮，但paint面板显示为第二个按钮面板旁边的一个小白框。任何帮助都将不胜感激。

public class DrawingApp extends JFrame
{
    private final topButtonPanel topPanel = new topButtonPanel();
    private final bottomButtonPanel bottomPanel = new bottomButtonPanel();
    private final PaintPanel paintPanel = new PaintPanel();

    public DrawingApp()
    {
        super("Java 2D Drawings");
        setLayout(new FlowLayout());
        add(topPanel);
        add(bottomPanel);
        add(paintPanel);
    }

    public static void main(String[] args) 
    {
        DrawingApp frame = new DrawingApp();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(750,500);
        frame.setVisible(true); 
    }
}

public class topButtonPanel extends JPanel
{

    private final String[] names = {"Line", "Oval", "Rectangle"};

    private final JButton undo = new JButton("Undo");
    private final JButton clear = new JButton("Clear");
    private final JLabel shape = new JLabel("Shape:");
    private final JComboBox<String> shapesComboBox = new JComboBox(names);
    private final JCheckBox filled = new JCheckBox("Filled");

    public topButtonPanel()
    {
        super();
        setLayout(new FlowLayout());
        add(undo);
        add(clear);
        add(shape);
        shapesComboBox.setMaximumRowCount(3);
        add(shapesComboBox);
        add(filled);
    }
}


public class bottomButtonPanel extends JPanel
{

    private final JCheckBox useGradient = new JCheckBox("Use Gradient");
    private final JButton firstColor = new JButton("1st Color");
    private final JButton secondColor = new JButton("2nd Color");
    private final JLabel lineWidthLabel = new JLabel("Line Width:");
    private final JLabel dashLengthLabel = new JLabel("Dash Length:");
    private final JTextField lineWidthField = new JTextField(2);
    private final JTextField dashLengthField = new JTextField(2);
    private final JCheckBox filled = new JCheckBox("Dashed");

    public bottomButtonPanel()
    {
        super();
        setLayout(new FlowLayout());
        add(useGradient);
        add(firstColor);
        add(secondColor);
        add(lineWidthLabel);
        add(lineWidthField);
        add(dashLengthLabel);
        add(dashLengthField);
        add(filled);
    }
} 

public class PaintPanel extends JPanel
{

    public PaintPanel()
    {
        super();
        setBackground(Color.WHITE);
        setSize(700,400);
    }
}

Answer 1

基本上，这是对Swing API工作方式的误解。

Swing (在很大程度上)依赖于布局管理API，该API用于决定组件的大小(以及放置位置)

使用setSize是没有意义的，因为布局管理器会自己决定组件的大小，并相应地进行调整。

您可以向布局管理器建议您希望组件使用getPreferred/Minimum/MaximumSize的大小，例如

public class PaintPanel extends JPanel
{

    public PaintPanel()
    {
        super();
        setBackground(Color.WHITE);
    }

    public Dimension getPreferredSize() {
        return new Dimension(700, 400);
    }
}

请记住，布局管理器完全有权忽略这些值，因此您需要更好地了解这些管理器是如何工作的

有关详细信息，请参阅Laying Out Components Within a Container