Download Rules About Overridden Methods Notes

:: INT304 ::
Programming Syntax & Semantics
Rules About Overridden Methods
Notes
 Must have a return type that is identical to the
method it overrides
 Cannot be less accessible than the method it
overrides
 Cannot throw more exceptions than the method it
overrides
 If the keyword super is not explicitly used, the
superclass's default constructor is automatically
invoked.
 If a superclass defines constructors other than a
default constructor, the subclass cannot use the
default constructor of the superclass.
public class Employee{
// public Employee (String name){
public Employee (){
System.out.println(“Employee constructed”);
}
}
public class Manager extends Employee{
public Manager(){
System.out.println(“Manager constructed”);
}
}
public class Test {
public static void main(String args[]) {
Manager m1 = new Manager() ;
}
}
public class Parent {
int i = 10 ;
public void f1() {
System.out.println(“I am Parent”);
}
}
public class Child extends Parent{
String i = “I love you !!” ;
private void f1() {
System.out.println(“I am Child”);
}
}
public class UseBoth {
public static void main(String args[]){
Parent p1 = new Parent();
Parent p2 = new Child();
p1.method();
p2.method();
System.out.println(p2.i);
}
}
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:: INT304 ::
Programming Syntax & Semantics
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:: INT304 ::
Programming Syntax & Semantics
Grouping Classes
 Packages
~ Package declaration must be specified at the
beginning of the source file
~ Only one package declaration is permitted per
source file
Directory Layout and the CLASSPATH
Environment Variable
 Packages are stored in a directory tree
containing the package name.
package abc.financedept;
public class Employee {
...
// Class Employee of the Finance
// department for the ABC company
package abc.financedept;
public class Employee {
}
:: INT304 ::
Programming Syntax & Semantics
}
javac –d . Employee.java
...
 What is directory path for Employee.class ?
<current-directory>/abc/financedept
~ Package names are hierarchical and separated
by dots.
 The import Statement
 The roots of package trees to search for class
files are in the CLASSPATH
Example
~ Tells the compiler where to find classes to use
javac–d /java/mylib/mypackages Employee.java
Employee.class stored in
~ Must precede all class declarations
import abc.financedept.*;
public class Manager extends Employee{
String department;
/java/mylib/mypackages/abc/financedept
CLASSPATH = /java/mylib/mypackages
Employee[] subordinates;
}
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:: INT304 ::
Programming Syntax & Semantics
:: INT304 ::
Programming Syntax & Semantics
Static Initialize
The Final Keyword
 A class can contain code in a static block that
does not exist within a method body.
 Static block code executes only once, when the
class is loaded.
public class StaticInitDemo{
static int i=5;
static {
System.out.println(“Static code i= “+ i++);
}
}
public class Test{
public static void main(String args[]){
System.out.println(“Main code: I = “ +
}
 A final class cannot be subclassed.
 A final method cannot be overridden.
 A final variable is a constant.
Note:
If you mark a variable of reference type as final, that variable
cannot refer to any other object. It is however possible to change
the object’s contents, as only the reference itself is final
StaticInitDemo.i);
}
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:: INT304 ::
Programming Syntax & Semantics
Abstract Classes
:: INT304 ::
Programming Syntax & Semantics
Interfaces
 A class which declares the existence of methods
but not the implementation is called an abstract
class.
 A class can be declared as abstract by marking it
with the abstract keyword.
public abstract class Drawing{
public abstract void drawDot(int x, int y);
public void drawLine(int x1,int y1,int x2,int y2){
//draw using the drawDot() method repeatedly.
}
}
 An abstract class can contain non-abstract
methods and variables.
 An interface is a variation on the idea of an
abstract class.
 In an interface, all the methods are abstract.
 Multiple inheritance can be achieved by
implementing such interfaces.
 The syntax is
public interface Stack {
public static final int EMPTY = -1;
public void push(Object item) ;
public Object pop () ;
public int size() ;
}
Note:
We cannot create an instance from an abstract class using the
new operator, but an abstract class can be used as a data type.
Therefore, the following statement, which creates an array whose
elements are of Drawing type, is correct.
class PCDrawing extends Drawing {
public void drawDot(int x, int y) { }
}
Drawing dx = new PCDrawing() ;
Drawing d1[] = new Drawing[10] ;
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:: INT304 ::
Programming Syntax & Semantics
Interfaces
(2)
 Interfaces are useful for
~ Declaring methods that one or more classes are
expected to implement
~ Determining an object’s programming interface
without revealing the actual body of the class
~ Capturing similarities between unrelated classes
without forcing a class relationship
~ Describing “function-like” objects that can be
passed as parameters to methods invoked on
other objects
:: INT304 ::
Programming Syntax & Semantics
The Cloneable Interfaces
 Marker Interface: An empty interface.
 A marker interface does not contain constants or
methods, but it has a special meaning to the Java
system. The Java system requires a class to
implement the Cloneable interface to become
cloneable.
class Student implements Cloneable { ... }
...
...
Student st1 = new Student();
Student st2 = (Student) st1.clone();
public interface Comparable {
public int compareTo(Object o) ;
}
class Student implements Comparable {
public int compareTo(Object o){. . .}
}
...
...
Student st1 = new Student();
Student st2 = (Student) st1.clone();
boolean b1 = st1.compareTo(st2) ;
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:: INT304 ::
Programming Syntax & Semantics
Advanced Access Control
Modifier
:: INT304 ::
Programming Syntax & Semantics
Deprecation
Same Class Same Package Subclass Universe
public
yes
yes
yes
yes
protected
yes
yes
Yes
X
default
yes
Yes
X
X
private
Yes
X
X
X
 Deprecation is the obsoleting of class
constructors and method calls.
 Obsoleted methods and constructors are replaced
by methods with a more standardized naming
convention.
 When migrating code, compile the code with the
-deprecation flag:
javac –deprecation MyFile.java
pacakge p1
class C1
protected int x
class C3
C1 c1;
c1.x can be read or
modified
© Copyright Khaitong-2-5-0-9. 2003
pacakge p2
class C2 extends C1
x can be read or
modified in C2
class C4
C1 c1;
c1.x cannot be read
nor modified
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:: INT304 ::
Programming Syntax & Semantics
The == Operator Versus equals() Method
:: INT304 ::
Programming Syntax & Semantics
Inner Classes
 The equals() and == methods determine if
reference values refer to the same object.
 Allow a class definition to be placed inside
another class definition
 The equals() method is overridden in classes in
order to return true if the contents and type of two
separate objects match.
 Group classes that logically belong together
public class Student {
private int id ;
private String name ;
. . .
public boolean equals(Object stObj) throws
ClassMismatchException {
if ( !(stObj instanceof Student) )
throw new ClassMismatchException();
if (id == stObj.id && name.equals(stObj.name) )
return true;
return false;
}
}
 Have access to their enclosing class’s scope
import java.awt.*;
import java.awt.event.*;
public class MyFrame extends Frame {
Button myButton;
TextArea myTextArea;
int count;
public MyFrame() {
super("Inner Class Frame");
myButton = new Button("click me");
myTextArea = new TextArea();
add(myButton, BorderLayout.CENTER);
add(myTextArea, BorderLayout.NORTH);
ButtonListener bList = new ButtonListener();
myButton.addActionListener(bList);
}
class ButtonListener implements ActionListener {
public void actionPerformed (ActionEvent e) {
count++;
myTextArea.setText("clicked: "+ count+ "times");
}
} // end of innerclass ButtonListener
public static void main(String args[]) {
MyFrame f = new MyFrame();
f.setSize(300, 300);
f.setVisible(true);
}
}// end of class MyFrame
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:: INT304 ::
Programming Syntax & Semantics
Collection API
:: INT304 ::
Programming Syntax & Semantics
The Vector class
 A collection (or a container) is a single object
representing a group of objects, known as its
elements.
 The Vector or class provides methods for working
with dynamic arrays of varied element types.
 Collection classes Vector, Bits, Stack, Hashtable,
LinkedList, and so on are supported.
 The Collections API contains interfaces which
maintain objects as a
~ Collection
– A group of objects with no specific ordering
~ Set
– A group of objects with no duplication
~ List
– A group of ordered objects; duplication is
permitted
Synopsis
~ Each vector maintains a capacity and capacityIncrement.
~ As elements are added, storage for the vector increases in chunks up to
the size of the capacityIncrement variable.
~ Constructors
public Vector()
public Vector(int initialCapacity)
public Vector(int initialCapacity, int capacityIncrement)
~ Variables
protected int capacityIncrement
protected int elementCount
protected Object elementData[ ]
~ Methods
public final int size()
- Returns the number of elementsin the vector. (This is not the same
as the vector’s capacity.)
public final boolean contains (Object elem)
- Returns true if the specified object is a value of the collection.
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:: INT304 ::
Programming Syntax & Semantics
Example : Using the Vector
Synopsis
public final int indexOf (Object elem)
– Searches for the specified object starting from the first position, and
returns an index to it (or -1 if the element is not found). It uses the
object’s equals() method, so if this object does not override Object’s
equals() method, it will only compare object references, not object
contents.
public final synchronized Object elementAt (int index)
- Returns the element at the specified index. It throws
ArrayIndexOutOfBoundsException if index is invalid.
public final synchronized void setElementAt (Object obj, int index)
- Replaces the element at the specified index with the specified
object. It throws ArrayIndexOutOfBoundsException if indexis invalid.
public final synchronized void removeElementAt (int index)
- Deletes the element at the specified index. It throws
ArrayIndexOutOfBoundsException if index is invalid
public final synchronized void addElement (Object obj)
- Adds the specified object as the last element of the vector.
public final synchronized void insertElementAt (Object obj, int index)
- Inserts the specified object as an element at the specified index,
shifting up all elements with equal or greater index. It throws
ArrayIndexOutOfBoundsException if index is invalid
© Copyright Khaitong-2-5-0-9. 2003
:: INT304 ::
Programming Syntax & Semantics
Page: 17 of 19
import java.util.*;
public class MyVector extends Vector {
public MyVector() {
super(1, 1); //storage capacity & capacityIncrement
}
public void addInt(int i) {
addElement(new Student(i)); // addElement requires object arg
}
public void addFloat(float f) {
addElement(new Float(f));
}
public void addString(String s) {
addElement(s);
}
public void addCharArray(char a[]) {
addElement(a);
}
public void printVector() {
Object o;
int length = size(); //compare with capacity()
System.out.println("Number of vector elements is " + length +
" and they are: ");
for (int i = 0; i < length; i++) {
o = elementAt(i);
if (o instanceof char[ ]) {
// An array's toString() method does not print what we want.
System.out.println(String.copyValueOf((char[ ]) o));
}
else
System.out.println(o.toString());
}
}
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:: INT304 ::
Programming Syntax & Semantics
public static void main(String args[]) {
MyVector v = new MyVector();
int digit = 5;
float real = 3.14f;
char letters[] = {'a', 'b', 'c', 'd'};
String s = new String ("Hi there!");
v.addInt(digit);
v.addFloat(real);
v.addString(s);
v.addCharArray(letters);
v.printVector();
}
}
JPanel
DescPanel
- jtaTextDesc: JTextArea
- jlblTitle: JLabel
- jlblImage: JLabel
+ setTitle(title: String): void
+ setIcon(icon: ImageIcon): void
+ setTextDesc(text: String): void
+ getMinSize(): Dimension
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JFrame
1
TextAreaDemo
ActionListener
WindowListener
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