英文:

How to apply Java Swing in a different class?

问题

我有两个不同的类，

主窗口：

```java
package opdracht4;

public class Opdracht4 {

	public static void main(String[] args) {
		new App();
	}
}

而这个类是

package opdracht4;

import javax.swing.*;

public class App() {
	JFrame app = new JFrame();
	JPanel paneel = new JPanel();
	
	paneel.setBorder(BorderFactory.createEmptyBorder(30, 30, 10, 30));
}

我不太确定我在这里做错了什么，因为我在Java方面不是很擅长，所以有谁能告诉我第二个类中第5行是什么问题吗？

<details>
<summary>英文:</summary>

I have 2 different classes,

Main window:

package opdracht4;

public class Opdracht4 {

public static void main(String[] args) {
	new App();
}

}

And this class

package opdracht4;

import javax.swing.*;

public class App() {
JFrame app = new JFrame();
JPanel paneel = new JPanel();

paneel.setBorder(BorderFactory.createEmptyBorder(30, 30, 10, 30));

}

I'm not quite sure what I am doing wrong here, as I'm not the best in Java, so could anyone tell me whats wrong at line 5 in the second class?

</details>


# 答案1
**得分**: 2

paneel.setBorder(BorderFactory.createEmptyBorder(30, 30, 10, 30));这行代码位于任何方法之外，您需要向类App添加一个构造函数，例如：

public App(){
    paneel.setBorder(BorderFactory.createEmptyBorder(30, 30, 10, 30));
    app.setVisible(true);// 用于显示JFrame
}

<details>
<summary>英文:</summary>

The line `paneel.setBorder(BorderFactory.createEmptyBorder(30, 30, 10, 30));` is out of any method, you need to add a constructor to the class `App`, for example:

public App(){
paneel.setBorder(BorderFactory.createEmptyBorder(30, 30, 10, 30));
app.setVisible(true);// to display the JFrame
}

</details>