对某些Java语法感到困惑

Question

I am learning Java (slowly) from the ground up, but every now and again I peak at some "real" code and I found something that confused me.

The following line and ones like it:

JPanel panel = (JPanel) container.getContentPane();

My question is what is happening between (JPanel) and container.getContentPane() ? Its not like they are being multiplied right?

I know this is a basic part of the language and as I continue learning I'll get to this part, but I got really curious and wanted to know what it was right away. I didn't really know what to google to get the answer.

Answer 1

Its not like they are being multiplied right?

No. It means "get this thing and treat it as a JPanel." It's called type casting and that syntax is used in C++, C# and many other languages.

You have to make sure that the way that the classes are related to each other allows for casting. For example, this wouldn't work:

JPanel p = new JPanel();
JComponent c = (JComponent)p;
JButton b = (JButton)c;

JPanel is a JComponent and so is JButton , but JButton does not descend from JPanel thus you cannot cast between these objects. You can also cast from a child back to a parent, such as from JSpinner.DefaultEditor back to JPanel , but not from JPanel to JSpinner.DefaultEditor .

Answer 2

It's a cast expression (see JLS 15.16, Cast Expressions ).

It means "treat the results of the getContentPane() call as a JPanel ".

Casts can fail, causing a ClassCastException (see JLS 5.5, Casting Conversion ).

Answer 3

This is called a type cast. The method getContentPane() normally return a Container, but we want to get it in a JPanel so we'll be able to use JPanel methods. Of course the two types have to be compatible (JPanel is a specific implementation of Container (through JComponent) for example)

Answer 4

Yes this is how type casting is made in java. As you know java works with references. In your case you have a JPanel type reference and container.getContentPane() returns (correct me if I am wrong my Swing is a bit rusty) container, so since those two are incompatible you need to type cast to make them compatible. Hope that helps