Java容器

Question

我正在处理一些代码，并试图为JPanel添加边框。有人告诉我后，我终于明白了，但我不确定为什么我的第一行不起作用。有人可以给我一些见解吗？这是代码片段：

private Container currCntr;
currCntr = new JPanel();
currCntr.setBorder(new LineBorder(Color.BLACK));  //Doesn't work (cannot find symbol)
((JPanel)currCntr).setBorder(new LineBorder(Color.BLACK)); //Works

Answer 1

在第二行中，JPanel为implicitly upcasted到Container（JPanel的抽象超类），它没有setBorder方法。

后续修补程序将容器向下转发回JPanel，该setBorder具有private Container currCntr = new JPanel(); //JPanel upcasted to type Container //Container does not have a `setBorder` method currCntr.setBorder(new LineBorder(Color.BLACK)); //Downcasted to JPanel which has the setBorder method ((JPanel)currCntr).setBorder(new LineBorder(Color.BLACK)); 方法作为其公共方法之一。

{{1}}

Answer 2

您需要转为JPanel

((JPanel)currCntr).setBorder(new LineBorder(Color.BLACK));

Answer 3

Border border = BorderFactory.createLineBorder(Color.BLACK);
currCntr.setBorder(BorderFactory.createCompoundBorder(border, BorderFactory.createEmptyBorder(10, 10, 10, 10)));

//you can try createLineBorder, shuld be the same syntax.

这是因为您没有为行边框设置属性。请尝试设置属性。如厚度。