Download Abstract classes

Lecture objectives
1
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Differences between Java and C#
Understand and use abstract classes
Casting in Java: why is it important?
Master exceptions
Create packages
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2
Java vs C#
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The More Things Change The More
They Stay The Same
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We Are All Objects
Keyword Jumble
Of Virtual Machines and Language Runtimes
Heap Based Classes and Garbage Collection
Arrays Can Be Jagged
No Global Methods
Interfaces, Yes. Multiple Inheritance, No.
Strings Are Immutable
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The More Things Change The More
They Stay The Same
4
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Unextendable Classes
Throwing and Catching Exceptions
Member Initialization at Definition and Static
Constructors
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The Same But Different
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Main Method
Inheritance Syntax
Run Time Type Identification (is operator)
Namespaces
Constructors, Destructors and Finalizers
Synchronizing Methods and Code Blocks
Access Modifiers
Reflection
Declaring Constants
Primitive Types
Wish you were here…
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Checked Exceptions
Cross Platform Portability (Write Once, Run
Anywhere)
Extensions
Dynamic Class Loading
Interfaces That Contain Fields
Anonymous Inner Classes
Static Imports Array Declarations
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7
Abstract classes
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Abstract Classes
8
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Syntax:
visibility abstract class className
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Difference from other classes:
• An abstract class cannot be instantiated
• An abstract class may declare abstract methods
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Includes abstract methods:
visibility abstract resultType methodName (parameterList);
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Example of an Abstract Class
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public abstract class Food {
public final String name;
private double calories;
// Actual methods
public double getCalories () {
return calories;
}
protected Food (String name, double calories) {
this.name
= name;
this.calories = calories;
}
// Abstract methods
public abstract double percentProtein();
public abstract double percentFat();
public abstract double percentCarbs();
}
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Interfaces, Abstract Classes, and Concrete
Classes
10
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Can an interface have:
–
–
–
•
Can an abstract class have:
–
–
–
•
abstract methods? (no body)
concrete methods? (with a body)
data fields
abstract methods? (no body)
concrete methods? (with a body)
data fields
Can a concrete class have:
–
–
–
abstract methods? (no body)
concrete methods? (with a body)
data fields
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Abstract Classes and Interfaces
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•
•
•
•
•
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Can an interface
– be instantiated?
– declare abstract methods?
Can an abstract class
– be instantiated?
– declare abstract methods?
Can an interface have constructors?
Can an abstract class have constructors?
Can an abstract class implement an interface?
Can an interface implement an abstract class?
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Inheriting from Interfaces vs Classes
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A class can extend 0 or 1 superclass
An interface cannot extend a class
A class or interface can implement 0 or more
interfaces
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Summary of Features of Actual Classes,
Abstract Classes, and Interfaces
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Class Object and Casting
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Class Object
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Object is the root of the class hierarchy
Every class has Object as a superclass
All classes inherit the methods of Object but may
override them
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Method toString
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You should always override toString method if
you want to print object state
If you do not override it:
will return a String
 Just not the String you want!
Example: ArrayBasedPD@ef08879
The name of the class, @, instance’s hash code
 Object.toString
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Operations Determined by Type of
Reference Variable
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Java is strongly typed
Object athing = new Integer(25);
 What
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is the type of the variable athing?
Operations Determined by Type of
Reference Variable (cont.)
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Why does the following generate a syntax error?
Integer aNum = aThing;
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Casting in a Class Hierarchy
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Casting does not change the object!

It creates an anonymous reference to the object
Integer aNum = (Integer) aThing;
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Using instanceof to Guard a
Casting Operation
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•
instanceof can guard against a
ClassCastException
Object obj = ...;
if (obj instanceof Integer) {
Integer i = (Integer) obj;
int val = i;
...;
} else {
...
}
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Method Object.equals
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Object.equals method has a parameter of
type Object
public boolean equals (Object other) {
... }
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Compares two objects to determine if they are
equal
A class must override equals in order to support
comparison
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Employee.equals()
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/** Determines whether the current object matches its
argument.
@param obj The object to be compared to the current
object
@return true if the objects have the same name and
address; otherwise, return false
*/
@Override
public boolean equals(Object obj) {
if (obj == this) return true;
if (obj == null) return false;
if (this.getClass() == obj.getClass()) {
Employee other = (Employee) obj;
return name.equals(other.name) &&
address.equals(other.address);
} else {
return false;
}
}
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A Java Inheritance Example—The Exception
Class Hierarchy
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Run-time Errors or Exceptions
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
Run-time errors
 occur
during program execution (i.e. at run-time)
 occur when the JVM detects an operation that it knows
to be incorrect
 cause the JVM to throw an exception
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Examples of run-time errors include
 division
by zero
 array index out of bounds
 number format error
 null pointer exception
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Class Throwable
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Throwable is the superclass of all exceptions
All exception classes inherit its methods
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Checked and Unchecked Exceptions
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Checked exceptions
normally not due to programmer error
 Must be explicitly caught
 all input/output errors are checked exceptions
 Extend class java.lang.Exception
 Examples: IOException, FileNotFoundException

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Checked and Unchecked Exceptions (cont.)
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Unchecked exceptions result from
programmer error (try to prevent them with defensive
programming).
 They do not have to be caught
 a serious external condition that is unrecoverable
 Extend class java.lang.RuntimeException
 Examples: NullPointerException,
ArrayIndexOutOfBoundsException

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Checked and unchecked exceptions are
functionally equivalent. There is nothing you
can do with checked exceptions that cannot
also be done with unchecked exceptions, and
vice versa.
Checked and Unchecked Exceptions (cont.)
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Class Error: due to serious external conditions
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Example: OutOfMemoryError
Unchecked exception
The class Exception and its subclasses can be handled
by a program
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RuntimeException and its subclasses are unchecked
Some Common Unchecked Exceptions
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ArithmeticException: division by zero, etc.
ArrayIndexOutOfBoundsException
NumberFormatException: converting a
“bad” string to a number
NullPointerException
NoSuchElementException: no more tokens
available
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Handling Exceptions
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Exception example on eclipse (java tutorial book)
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The try-catch Sequence
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The try-catch sequence resembles an if-thenelse statement
try {
// Execute the following statements until an
// exception is thrown
...
// Skip the catch blocks if no exceptions were thrown
} catch (ExceptionTypeA ex) {
// Execute this catch block if an exception of type
// ExceptionTypeA was thrown in the try block
...
} catch (ExceptionTypeB ex) {
// Execute this catch block if an exception of type
// ExceptionTypeB was thrown in the try block
...
}
Using try-catch
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
Let’s try this example
public static int getIntValue(Scanner scan) {
int nextInt = 0;
// next int value
boolean validInt = false; // flag for valid input
while(!validInt) {
try {
System.out.println("Enter number of kids: ");
nextInt = scan.nextInt();
validInt = true;
} catch (InputMismatchException ex) {
scan.nextLine();
// clear buffer
System.out.println("Bad data-enter an integer");
}
}
return nextInt;
}
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Packages and Visibility
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Packages
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A Java package is a group of cooperating classes
The Java API is organized as packages
Syntax:
package classPackage;
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Classes in the same package should be in the same
directory (folder)
The folder must have the same name as the package
Classes in the same folder must be in the same
package
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Packages and Visibility
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Packages need to be imported
For example,
x = Java.awt.Color.GREEN;
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If the package is imported, the packageName prefix is
not required.
import java.awt.Color;
...
x = Color.GREEN;
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The Default Package
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Files which do not specify a package are part of
the default package
If you do not declare packages, all of your classes
belong to the default package
Bad programming practice:
 All
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your classes belong to the default package!
Visibility Supports Encapsulation
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Visibility rules enforce encapsulation in Java
private: for members that should be invisible even
in subclasses
package: shields classes and members from classes
outside the package
protected: provides visibility to extenders of
classes in the package
public: provides visibility to all
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Visibility Supports Encapsulation
(cont.)
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Encapsulation insulates against change
Greater visibility means less encapsulation
So… use the most restrictive visibility possible to get the
job done!
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