Download Java Review Slides (updated!)

Java Programming Review
(Part I)
Enterprise Systems
Programming
Outline












Java programs and packages
Classes, fields, methods, objects
Naming conventions
Types, variables, parameters
Arrays
Inheritance, interfaces
Exceptions
Files and streams
Static fields and methods
Java core packages, collection classes
Generics, auto-boxing
Inner classes
Java programs


A Java program is a collection of classes
Each class is specified in a separate file



Compilation through the java development kit
(JDK)



Source file carries a .java extension
Compiled program carries a .class extension
Via the command line: javac ClassName.java
(produces ClassName.class)
Through an IDE such as Eclipse or JCreator
Execution requires a Java virtual machine
Java program execution

Standalone java programs execute through a run
time environment
(the Java VM)




Via the command-line: java ClassName
Requires ClassName.class
ClassName must have a main method
(entry point) but may make use of other java classes
Applets and Servlets


Virtual machine resides in browsers or servers
Java execution is automatic and triggered by events
Java packages


A package is a set of Java classes
Packages are Java’s way of organizing its classes
into modules or namespaces



Classes in a package share increased visibility
rules
Packages follow a hierarchical structure and
naming pattern


Manages complexity and naming conflicts
e.g., java.awt.event
Classes import classes from outside packages

e.g., import java.util.*;
Classes and filenames,
packages and folder names

The name of a Java class must match the name
of the source file that defines it


This Java source file must
be named MyClass.java
public class MyClass
{…
}
If it is part of a named package, it should reside
in a folder that follows the same hierarchy as its
package name
package myservlets;
public class WelcomeServlet
{…
}

WelcomeServlet.java should reside in a folder named
myservlets
Class definition in Java
BankAccount

Type (or class)

State/attributes (fields)

Behavior (methods)

Code for object creation
(usually) done outside
of this class
public class BankAccount
{
private double balance = 0;
BankAccount.java
public double getBalance()
{
return balance;
}
public void deposit( double amount )
{
balance = balance + amount;
}
…
}
A Class with a constructors



Constructor: special
method that handles
initialization
Constructor name must
match the name of class
In this example, creation
of bank account objects
may indicate an initial
balance
public class BankAccount
{
private double balance;
BankAccount.java
public BankAccount()
{
balance = 0;
}
public BankAccount( double initBal )
{
balance = initBal;
}
public double getBalance()
{
return balance;
}
public void deposit( double amount )
{
balance = balance + amount;
}
…
}
Object creation and method invocation


Object creation done
through the new keyword
Method invocation done
through the dot notation
object.method( params )
TestAccounts.java
public class TestAccounts
{
public static void main( String args[] )
{
BankAccount b = new BankAccount();
BankAccount x = new BankAccount( 5000.00 );
b.deposit( 1000.00 );
b.withdraw( 100.00 );
System.out.println( b.getBalance() );
x.deposit( 2000.00 );
b.deposit( 2000.00 );
System.out.println( b.getBalance() );
System.out.println( x.getBalance() );
}
}
Java naming conventions

Class names



Variable and method names




Start with a capital letter, capitalize first letters of
succeeding words
Examples: BankAccount, MyClass, WelcomeServlet
Start with a lowercase letter, capitalize first letters of
succeeding words
aka “camelCase”
Examples: balance, firstName, favoriteNumber
Following these conventions make your programs
easier to read!
Types in Java

Primitive types: one of the 8 built-in types in
Java



int, double, long, short, byte, float, char, boolean
Variables of primitive types hold values acceptable under
these types
Object types: classes


Some of these types are “built-in” (present in the Java
library) such as String or System, the rest are userdefined such as BankAccount or WelcomeServlet
Variables of object types hold references to objects
Variables for built-in types

Variables for built-in types
x
int x;
…
x = 5;
x
5
Variables for object types

Object (reference) type variables
x
BankAccount x;
…
x = new BankAccount( 1000 );
BankAccount Object
x
balance:
1000
Variables and assignment

Primitive variables


x
int x, y;
1000
x = 1000;
y = x;
y++;
// at this point, x = 1000, y = 1001
Object variables

y
BankAccount x, y;
x = new BankAccount( 1000 );
y = x;
y.deposit( 1 );
// at this point, x.getBalance() is 1001,
//
y.getBalance() is 1001
1001
x
balance:
1001
y
Parameter passing

BankAccount b = new BankAccount(100);
int x = 100;
changeAccount( b ); // update of b possible
changeNumber( x ); // update of x not possible
…
void changeAccount( BankAccount c )
{
c.deposit( 10 ); // b and c point to the same object
}
void changeNumber( int y )
{
y = y + 10; // y changes, but x does not
}

In the above code, since b is a reference to an object, that
object could be changed by the method.
The same does not hold for x. The value of x is copied into
y. Changes to y does not impact on x.

The null keyword

Set an object variables to null when you
want the variable to not refer to an object


Can be used to initialize object variables


null is a reserved word in Java
BankAccount x = null;
You may test for null first before you call
methods on an object variable

if ( x != null )
System.out.println( x.getBalance() );
The this keyword

When within a class and you wish to refer
to the current object, use the this
keyword


this is a reserved word in Java
Can use the dot notation within a class
when referring to fields/methods

public void deposit( double amount )
{
this.balance = this.balance + amount;
}
Arrays

Array variables are object references


type[] arrayVar;
Array creation done separately

arrayVar = new type[size];
Applicable for both primitive types and
object types
 Note: for an array of objects, array
creation creates object references


Individual objects need to be created
separately
Array of numbers
nums
Declare:
double[] nums;
Array of numbers
nums
Declare:
double[] nums;
Create:
nums = new double[8];
0
0.0
1
0.0
2
0.0
3
0.0
0.0
4
0.0
5
0.0
6
0.0
7
0.0
Array of numbers
nums
Declare:
double[] nums;
Create:
nums = new double[8];
Use:
nums[3] = 6.6;
0
0.0
1
0.0
2
0.0
3
0.0
6.6
4
0.0
5
0.0
6
0.0
7
0.0
Array of objects
BankAccount-type references
accounts
null
Declare:
BankAccount[] accounts;
Create array: accounts =
new BankAccount[5];
Create objects:
for ( i = 0; i < 5; i++ )
{ accounts[i] = new BankAccount(i * 10);
}
Use objects: e.g., accounts[3].getBalance();
0
null
1
null
2
null
3
null
4
null
BankAccount
balance
0
BankAccount
balance
10
BankAccount
balance
20
BankAccount
balance
30
(returns 30)
BankAccount
balance
40
Inheritance
Inheritance: an object-oriented
programming language feature that allows
for the definition of a class in terms of
another class
 In Java, use the extends keyword
 Promotes reusability of existing code

Example: CheckingAccount


Suppose we define CheckingAccount from scratch
Attributes



balance
number of checks drawn
Methods





deposit
withdraw
get balance
draw a check
…others
CheckingAccount
double balance
int numChecks
double getBalance()
void deposit( double amount )
void withdraw( double amount )
void drawCheck( double amount )
…others
Example: CheckingAccount
Resulting class is very similar to
BankAccount
 The same as BankAccount except for an
additional field and some methods
 Better to extend BankAccount instead

BankAccount revisited
public class BankAccount
{
private double balance = 0;
public double getBalance()
{
return balance;
}
public void deposit( double amount )
{
balance = balance + amount;
}
public void withdraw( double amount )
{
balance = balance - amount;
}
}
public class CheckingAccount
{
private double balance = 0;
private int numChecks = 0;
CheckingAccount.java
public int getBalance()
{
return balance;
Just like BankAccount
}
except for the code in
public void deposit( double amount )
bold
{
balance = balance + amount;
}
public void withdraw( double amount )
{
balance = balance - amount;
}
public void drawCheck( double amount )
{
balance = balance - amount; // or, withdraw( amount );
numChecks++;
}
public int numChecksDrawn()
{
return numChecks;
}
}
Using extends
public class CheckingAccount extends BankAccount
{
private int numChecks = 0;
CheckingAccount.java
public void drawCheck( double amount )
{
withdraw( amount ); // can’t do balance = balance – amount;
// because balance is private to BankAccount
numChecks++;
}
public int numChecksDrawn()
{
return numChecks;
}
}
Notice that (public) methods defined
in BankAccount (e.g., withdraw) can be
used within CheckingAccount
Using CheckingAccount objects
Can call methods defined
in BankAccount
CheckingAccount mary = new CheckingAccount();
mary.deposit( 1000 );
System.out.println( “Balance: ” + mary.getBalance() );
mary.drawCheck( 100 );
System.out.println( “Balance: ” + mary.getBalance() );
System.out.println( “Checks Drawn: ” +
mary.numChecksDrawn() );
… and methods defined in
CheckingAccount
Superclass variables, subclass objects



Checking accounts are bank accounts so it is
possible to have a BankAccount variable point to
a CheckingAccount object
But not the other way around
Superclass variables can refer to subclass
objects, not vice-versa


 BankAccount b1 = new CheckingAccount();
(note: only methods indicated in BankAccount may be
invoked through b1)
 CheckingAccount b2 = new BankAccount();
(not allowed because a bank account is not necessarily a
checking account)
Method overriding
A subclass may define a method already
in the superclass: this is called method
overriding
 The subclass method takes precedence
over the superclass method




Suppose class A extends class B and both
classes define a method m()
B x = new A();
x.m(); // calls the method defined in A, not B
If you call m() within class A, this pertains to
class A’s m(). To call B’s m(), use super.m()
Inheritance and constructors
public class BankAccount
{
private double balance;
public BankAccount()
{ balance = 0;
}
public BankAccount( double initBal )
{ balance = initBal;
}
public double getBalance()
{ return balance;
}
public void deposit( double amount )
{ balance = balance + amount;
}
public void withdraw( double amount )
{ balance = balance - amount;
}
public class CheckingAccount extends BankAccount
{
private int numChecks;
public CheckingAccount()
{ numChecks = 0;
}
public void drawCheck( double amount )
{ withdraw( amount );
numChecks++;
}
public int numChecksDrawn()
{ return numChecks;
}
}
CheckingAccount c =
new CheckingAccount();
}
Which of the constructors are called?
Inheritance and constructors
CheckingAccount = new CheckingAccount();


In the above statement, CheckingAccount’s
(default) constructor is called
Since CheckingAccount is a BankAccount, a
BankAccount constructor should also be called




Which one?
Answer: the default constructor
Note that BankAccount() is called before
CheckingAccount()
What if we want a particular constructor of a
superclass called?
Incorrect attempt
We want CheckingAccount c = new CheckingAccount( 1000 );
to create an account with an initial balance of 1000
public class CheckingAccount extends BankAccount
{
private int numChecks;
public CheckingAccount()
{ numChecks = 0;
}
public CheckingAccount( double startBal )
{ numChecks = 0;
}
public void drawCheck( double amount )
{ withdraw( amount );
numChecks++;
}
public int numChecksDrawn()
{ return numChecks;
}
}
This will still call
BankAccount( ),
not
BankAccount( 1000 )
Using super()
public class CheckingAccount extends BankAccount
{
private int numChecks;
public CheckingAccount()
{ numChecks = 0;
}
public CheckingAccount( double startBal )
{ super( startBal );
numChecks = 0;
}
public void drawCheck( double amount )
{ withdraw( amount );
numChecks++;
}
public int numChecksDrawn()
{ return numChecks;
}
}
•super( … ) indicates
which superclass
constructor will be called
•If not indicated, it
defaults to super( ) with
no parameters
•Call to super(…) should
be the first line in the
subclass’ constructor
Implicitly calls “super();”
or BankAccount( )
Calls a particular
constructor
BankAccount( int )
Interfaces in Java


Interface: collection of method signatures with no
bodies
Syntax
public interface InterfaceName
{
public type methodName1(…);
public type methodName2(…);
…
}

A java class may implement an interface
public class ClassName implements InterfaceName
{
// define methodName1, methodName2, ... here
}
Two types of inheritance

Class inheritance (extends)




public class A extends B { … }
Class A inherits fields and methods in class B
public methods of B can be invoked on A objects, unless
overridden in A
Interface inheritance (implements)



Also called implementation inheritance
public class X implements Y { … }
X must define all methods indicated in Y
Why use interfaces?



Interfaces enable and enforce “strict” sharing of
methods among classes
Recall that superclass variables can refer to
subclass objects
Suppose Y is an interface



Y var;
var can refer to an object of a class that implements Y
var.methodName( … ) calls the actual method defined
Example: Shape

A Shape is anything that can be drawn:
public interface Shape
{
public void draw();
}



Any class that implements Shape must
implement the draw() method
Suppose Block, Triangle, and LetterT implement
shape
We can have an array of Shapes with elements
referring to different kinds of objects

Use a loop to call draw() on all of the shapes
Example: Shape (and Block)
public class Block implements Shape
{
private int size;
public Block( int s )
{
size = s;
}
public void draw()
{
for( int i = 1; i <= size; i++ )
{
for( int j = 1; j <= size; j++ )
System.out.print( “*” );
System.out.println();
}
}
Because it implements Shape,
this class will not compile unless
draw() is defined
Example: array of Shapes
list
null
Shape[] list;
…
for ( int i = 0; i < 5; i++ )
{
list[i].draw( );
}
0
null
1
null
2
null
3
null
4
null
Block
object
Triangle
object
LetterT
object
Triangle
object
Block
object
Multiple inheritance

Multiple (class) inheritance is not
supported in Java
public class A extends B1, B2, B3 { … }
 will not compile

But a class may implement multiple
interfaces
public class X implements Y1, Y2, Y3 { … }
 will compile, but X must define methods in Y1, Y2, and Y3
About interfaces

Interfaces cannot be instantiated
 Shape s = new Shape();

Interfaces can be extended



public interface Y1 extends Y2 { … }
public class X implements Y1 { … }
X must define methods indicated in Y1 and Y2
Interfaces cannot declare instance fields and may
not have any method definitions

Use an abstract class instead