Why the method can be called in this way?

Question

Hi all,
The following is JAVA SWING TREE custom data models program. There are two files in the program: one is TestFrame.java, and the other is MyDataModel.java. I only posted the important part on the Forum.My question is how method getChild(...) in MyDataModel.java was called in the program? Why I couldn't see the statement like tree.getChild(..) in TestFrame.java? After adding PRINT statements, I know the method getChild(..) was called automatically when object tree was initialized , but I don't know why. Thank you in advance.
======
TestFrame.java
// Imports
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.tree.*;
class TestFrame extends JFrame
// Instance attributes used in this example
private JPanel topPanel;
private JTree tree;
private JScrollPane scrollPane;
// Constructor of main frame
public TestFrame()
// Create a new tree control
MyDataModel treeModel = new MyDataModel( root );
tree = new JTree(treeModel);
=========
==========
MyDataModel.java
import javax.swing.tree.*;
class MyDataModel extends DefaultTreeModel
private DefaultMutableTreeNode root;
private String rootName = "";
public MyDataModel( TreeNode root )
super( root );
DefaultMutableTreeNode parentNode = (DefaultMutableTreeNode)root;
rootName = (String)parentNode.getUserObject();
System.out.println("rootName is: "+rootName);
public Object getChild( Object parent, int index )
DefaultMutableTreeNode parentNode = (DefaultMutableTreeNode)parent;
String parentName = (String)parentNode.getUserObject();
System.out.println("parentName is: "+parentName);
if( parentName.equals( rootName ) )
return super.getChild( parent, index );
else
return new DefaultMutableTreeNode( cardArray[index] );
}

Answer 1

This is the first time I saw this usage. I understood it now, but I was afraid I missed some important concepts on "interface as parameter in the constructor". Therefore, I am wondering where I can find some online document about it in order to understand the usage clearly.
For example, if there are a few method implementations of interface TreeModel in MyDataModel.java, which methods will it call after calling JTree constructor? all methods or only part of them?
Thanks