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Inner Classes,
Anonymous Classes
Nested Classes
Classes that belong to a class or a
method
MELJUN CORTES, MBA,MPA,BSCS
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Definitions
Inner Class
Class defined within another class.
Instances have access to instance members of containingclass.
Method-Local Inner Class
Class defined within a method.
Only available within the method.
Anonymous Classes
Classes with no class name!
Defined during reference declaration.
Nested Class Class defined within another class.
Does not have access to instance members of containingclass.
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Why Create
Inner/Anonymous/Nested
Classes?
You have functionality that is specific to
only one other class, but does not belong
to that other classs responsibility.
Examples:
Iterators for Collections
Event listeners for GUI classes.
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Inner Classes
Instantiating an Inner Class
The outer class must exist first.
MyList myList = new MyList();
MyList.MyIterator iter= myList.new MyIterator );
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Method-Local Inner Classes
Instantiating a Method-Local Inner Class
Just instantiate as usual.
You can only instantiate within the method.
void init() {
class MyListener implements ActionListener {
...
}
ActionListener listener = new MyListener );this.addActionListner(listener);
...
}
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Anonymous Classes
Anonymous classes subclass an existing
type.
Runnable car = new Runnable() {
public void run() {
while(true) {
System.out.println(VROOM!);
}
}
};
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Anonymous Classes
The type of the reference must be of an
existing type.
Runnable car = new Runnable() {
public void run() {
while(true) {
System.out.println(VROOM!);}
}
};
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Anonymous Classes
You can create the class as a field of the class.
public class MyPanel extends JPanel {
private JButton b = new JButton();
private ActionListner l = new ActionListener() {public void actionPerformed(ActionEvent e) {
...
}
};
public MyPanel() {
b.addActionListner(l);
}
}
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Anonymous Classes
Or more likely, you create it within a method orconstructor body.
public class MyPanel extends JPanel {
private JButton b = new JButton();
public MyPanel() {ActionListner l = new ActionListener() {public void actionPerformed(ActionEvent e) {
...
}b.addActionListner(l);
}}
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Anonymous Classes
Or you might define it while passing it as parameter!
private JButton b = new JButton();
public MyPanel() {
b.addActionListner(new ActionListener() {public void actionPerformed(ActionEvent e) {
...
})
;
}
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Nested Classes
Defining an Nested Class
Just like defining an inner class, but it must be static.
public class MyList {...public MyIterator iterator() {
...}
public staticclass MyIterator {...}
}
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Nested Classes
Nested classes cannot access instance members.
public MyIterator iterator() {return new MyIterator(t s);
}
public staticclass MyIterator {private int pos = 0;private MyList
l st
;
private MyIterator MyList list) {
this.list = list;
}
public boolean hasNext() {
return pos < size;}public Object next() {
return l st.a[pos++];}
}
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Nested Classes
Instantiating an Nested Class
No instance of outer class needed
MyList.MyIterator iter= new MyList.MyIterator();
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Best Practice
Use nested classes instead of inner
classes.
Direct access to members of outer class often
gets confusing.
