CODEWITHSUNDEEP
javaswing
user3226170
user3226170

Reputation: 175

getContentPane() requiring to be cast

I have created a class which extends JFrame, which by default will have a JPanel created with it. I have tried this:

public class Main extends JFrame {

JPanel pane;

private static final long serialVersionUID = 1L;

public Main() {

    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    int WIDTH = 500, HEIGHT = 500;
    setBackground(Color.LIGHT_GRAY);
    setSize(WIDTH, HEIGHT);
    setTitle("Window");
    setLocationRelativeTo(null);

    pane = getContentPane();
}

I'm using Eclipse, and it's giving an error, suggesting that I cast getContentPane()to return a JPanel, but to my understanding, this is what it should be returning already. I inserted System.out.println(getContentPane()); and it said in the console that it was returning a JPanel, so I'm a little confused as to why it needs to be cast. Is there something I'm missing here? Thanks.

Upvotes: 1

Views: 226

Answers (3)

user3437460
user3437460

Reputation: 17454

it's giving an error, suggesting that I cast getContentPane()to return a JPanel, but to my understanding, this is what it should be returning already

You get the error because you called:

getContentPane();

from a JFrame where the return type is a Container, and you are trying to assign a Container into a JPanel:

JPanel pane;
pane = getContentPane();

If you do not get the idea why the above is not allowed, think about this.

enter image description here

Since all Dogs are Animal, so in terms of code, you can always do this:

Animal animal = new Dog();  //Assign dog as Animal (O.K)

However if I have an Animal. Can I say it is definitely a Dog? 

Dog dog= new Animal();      //Assign animal as dog (NOT O.K)

The answer is no, unless you specifically tell me "trust me, this animal is a Dog. Let me assign this Dog object to a Dog variable.".

This idea is the same as casting in programming:

Animal animal = new Animal();
Dog dog = (Dog)animal;     //Tell Java this animal is indeed a Dog (O.K)

Now, your scenario of using the JPanel and Container is the same.

enter image description here

pane = getContentPane();           //Assign Container into JPanel (NOT O.K)
pane = (JPanel)getContentPane();   //Assign Container (casted as JPanel into JPanel) (O.K)

Ah ok. It's weird though cause I've seen in different code snippets around the web examples where it works without being cast. Check this out for example

The codes from your given link works because, the code only uses a .add() method from getContentPane().

Note: getContentPane() returns a Container object.

If you look at the documentation of Container, .add() is a method of Container class.

Since it is invoking a method which belongs to Container class. It will definitely work and no casting is needed.

Casting is needed unless you invoke a method from a subclass of the Container (such as a JPanel). For example, 

getContentPane.add(whatever);        //Casting NOT needed, add() is a method of Container
((JPanel)getContentPane()).getUI();  //Casting needed, getUI() is a method of JPanel

Upvotes: 3

MadProgrammer
MadProgrammer

Reputation: 347314

If you read the JavaDocs for JFrame#getContentPane you will see it returns an instance of Container, before you can assign it to a JPanel variable, you must cast it

In many cases you really should test to see if the return result is actually an instance (or is assignable to) a JPanel using instanceof in an if statement

Upvotes: 4

M.S.
M.S.

Reputation: 1091

Container is a parent in JPanel's inheritance hierarchy

getContentPane() returns a Container, which in this case, is a JPanel object. Therefore, a cast is required to cast the Container to its more specific child type, as shown below:

pane = (JPanel) getContentPane();

Upvotes: 6

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