问题

我试图用一个名为PhotoComponent的自定义JComponent替换我的Jlabels。到目前为止，我已经按照我的教程视频中所述的所有步骤创建了这个组件。但是，尝试用这两个错误替换所需的Jlabels：

构造函数PhotoComponent()未定义
对于类型new ActionListener(){}，方法currentPic(ImageIcon)未定义

这是我的自定义组件的代码：

import javax.swing.ImageIcon;
import javax.swing.JComponent;

public class PhotoComponent extends JComponent {
    private ImageIcon pic;

    public PhotoComponent(){

    }

    public PhotoComponent(ImageIcon p){
        pic=p;
        this.setSize(pic.getIconWidth(),pic.getIconHeight());
    }

    public void paintComponent(Graphics g){
        pic.paintIcon(this, g, 0, 0);
    }
}

    The constructor PhotoComponent() is undefined
    The method currentPic(ImageIcon) is undefined for the type new ActionListener(){}

import javax.swing.ImageIcon;
import javax.swing.JComponent;

public class PhotoComponent extends JComponent {
    private ImageIcon pic;

    public PhotoComponent(){

    }

    public PhotoComponent(ImageIcon p){
        pic=p;
        this.setSize(pic.getIconWidth(),pic.getIconHeight());
    }

    public void PaintComponent(Graphics g){
        pic.paintIcon(this, g, 0, 0);
    }
}

答案1

得分: 3

方法 `currentPic` 不存在，你已经将 `currentPic` 定义为 `PhotoComponent` 的一个实例，因此我假设你想调用它的方法。

如果你想要创建一个新的 `PhotoComponent` 实例，你应该使用：

    currentPic = new PhotoComponent(picList.get(pos))

但是就我个人而言，我不会这样做，我会在 `PhotoComponent` 中定义一个 `setIcon` 方法，这样我就可以简单地更改它显示的图像

```java
public class PhotoComponent extends JComponent {

    private ImageIcon pic;

    public PhotoComponent() {

    }

    public PhotoComponent(ImageIcon p) {
        pic = p;
        this.setSize(pic.getIconWidth(), pic.getIconHeight());
    }

    public void setIcon(ImageIcon icon) {
        pic = icon;
        repaint();
    }

    public void PaintComponent(Graphics g) {
        pic.paintIcon(this, g, 0, 0);
    }
}

然后，你需要将所有类似 currentPic(picList.get(pos)); 的调用更改为 currentPic.setIcon(picList.get(pos));

然而，PhotoComponent 并没有正确地设置，因为它实际上不会绘制任何内容。

首先，应该避免使用 setSize，因为许多组件将会忽略它并使用大小提示，即 preferred/minimum/maximum 大小。

另一个问题是 PaintComponent 定义错误。记住，Java 是大小写敏感的，这意味着如果你想要覆盖一个方法，你需要拼写正确。

你应该将它替换为类似以下的内容...

@Override
protected void paintComponent(Graphics g) {
    super.paintComponent(g);
    pic.paintIcon(this, g, 0, 0);
}

因此，考虑到上述两点，PhotoComponent 应该更像是...

public class PhotoComponent extends JComponent {

    private ImageIcon pic;

    public PhotoComponent() {

    }

    public PhotoComponent(ImageIcon p) {
        pic = p;
    }

    public void setIcon(ImageIcon icon) {
        pic = icon;
        revalidate();
        repaint();
    }

    @Override
    public Dimension getPreferredSize() {
        if (pic == null) {
            return new Dimension(0, 0);
        }
        return new Dimension(pic.getIconWidth(), pic.getIconHeight());
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        pic.paintIcon(this, g, 0, 0);
    }
}

另外，不要使用 mainPic.add(currentPic);，这不是你应该使用 JScrollPane 的方式... 实际上，你只需要在初始化界面时调用一次 mainPic.setViewportView(currentPic);

我建议你阅读并书签下面的资源：

• 使用 JFC/Swing 创建 GUI
• 如何使用滚动窗格
• 执行自定义绘图

<details>
<summary>英文:</summary>

The method `currentPic` doesn&#39;t exist, you&#39;ve defined `currentPic` as an instance of `PhotoComponent`, so I&#39;m assuming you want to call a method on it.  

If you want to create a new instance of `PhotoComponent`, then you should be using 
    
    currentPic = new PhotoComponent(picList.get(pos))

But personally, I wouldn&#39;t be doing this, instead I&#39;d have a `setIcon` method defined in `PhotoComponent` so I could simply change the image it displays


    public class PhotoComponent extends JComponent {

        private ImageIcon pic;

        public PhotoComponent() {

        }

        public PhotoComponent(ImageIcon p) {
            pic = p;
            this.setSize(pic.getIconWidth(), pic.getIconHeight());
        }
        
        public void setIcon(ImageIcon icon) {
            pic = icon;
            repaint();
        }

        public void PaintComponent(Graphics g) {
            pic.paintIcon(this, g, 0, 0);
        }
    }

Then, you&#39;d change all the calls like `currentPic(picList.get(pos));` to `currentPic.setIcon(picList.get(pos));`

How ever, `PhotoComponent` is not really set up properly, as it won&#39;t actually paint anything

First, `setSize` should be avoided, as many components will simply ignore it and use the sizing hints, `preferred`/`minimum`/`maximum` size, instead.

Another issues is `PaintComponent` is defined incorrectly.  Remember, Java is case sensitive, this means if you want to override a method, you need to spell it correctly.

You should replace it with something more like...

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        pic.paintIcon(this, g, 0, 0);
    }

So, taking the above to comments into account, `PhotoComponent` should look something more like...


    public class PhotoComponent extends JComponent {

        private ImageIcon pic;

        public PhotoComponent() {

        }

        public PhotoComponent(ImageIcon p) {
            pic = p;
        }

        public void setIcon(ImageIcon icon) {
            pic = icon;
            revalidate()
            repaint();
        }

        @Override
        public Dimension getPreferredSize() {
            if (pic == null) {
                return new Dimension(0, 0);
            }
            return new Dimension(pic.getIconWidth(), pic.getIconHeight());
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            pic.paintIcon(this, g, 0, 0);
        }
    }


Also, done use `mainPic.add(currentPic);`, this isn&#39;t how you should be using a `JScrollPane` ... and in fact, you only need to call `mainPic.setViewportView(currentPic);` once, when you initialise the UI

I would recommend that you read and book mark

* [Creating a GUI With JFC/Swing](https://docs.oracle.com/javase/tutorial/uiswing/index.html)
* [How to Use Scroll Panes](https://docs.oracle.com/javase/tutorial/uiswing/components/scrollpane.html)
* [Performing Custom Painting](https://docs.oracle.com/javase/tutorial/uiswing/painting/index.html)


</details>