CODEWITHSUNDEEP
javaoop
Yonshoku
Yonshoku

Reputation: 117

Why can a method be called without instance?

I'm sorry, maybe it's simple, but I can't explain this. There is following code(swing):

public class Sandbox2 extends Frame implements ActionListener {  
    JTextField tf; JLabel l; JButton b;  

    Sandbox2() {  
        tf=new JTextField();  
        ...   

        //there is what i can`t understand
        add(b);add(tf);add(l);    
        setSize(400,400);  
        setLayout(null);  
        setVisible(true);  
    }

    public void actionPerformed(ActionEvent e) {  
        ... 
    }  

    public static void main(String[] args) {  
        new Sandbox2();  
    } 
}

Firstly I create a button, text field and others, after that I add it to a frame. But how it works if all methods, add(), setSize()... were called without frame instance?

I understand that it due with a Frame inheritance, but how?

Upvotes: 0

Views: 284

Answers (3)

fabfas
fabfas

Reputation: 2228

The class SandBox2extends JFrame meansSandBox2 is-a JFrame class. The SandBox2class inherits the following methods from the super class JFrame. Hence can be called like add(b)

add(b);
add(tf);
add(l);    
setSize(400,400);  
setLayout(null);  
setVisible(true);

As per the Doc As a convenience add and its variants, remove and setLayout have been overridden to forward to the contentPane as necessary. This means you can write: frame.add(child);

SandBox2 uses inheritance by which class allows to inherit the features(fields and methods) of another class. The subclass can add its own fields and methods in addition to the superclass fields and methods.

Upvotes: 0

chrylis -cautiouslyoptimistic-
chrylis -cautiouslyoptimistic-

Reputation: 77187

All of these calls are happening inside the context of an instance. If you just call setSize, then that is the same as saying this.setSize. (If you tried to call setSize in a static method, which specifically means that it is not linked to a specific instance, then you would get an error.)

Upvotes: 4

Jason
Jason

Reputation: 11832

When you construct an instance of a class that extends another class, the parent constructor is called first, setting up anything that it needs, before your local constructor is called (either implicitly, or by calling super() in the first line of your constructor).

So, since Sandbox2 is-a JFrame (extends the class), the JFrame stuff will be ready by the time the code in your Sandbox2 constructor is executed.

Upvotes: 0

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