Object-Oriented Programming, Part 1

• Classes
• Methods
 Argument and return value
 Overloading
• Object Creation and Destruction
• Equality

OOP: Object-Oriented Programming, Part 1 1

 Classes in Java
• A class encapsulates a set of properties
 Some properties are hidden
 The remaining properties are the interface of the class

class ClassName { int x, y;

dataDeclaration Data declarations
char c;
constructors
methods
} Constructors

Methods

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 Example of a Class
public class Coin { // [Source Lewis and Loftus]
public static final int HEADS = 0;
public static final int TAILS = 1;
private int face;
public Coin () { // constructor
flip();
}
public void flip (){ // method “procedure”
face = (int) (Math.random() * 2);
}
public int getFace (){ // method “function”
return face;
}
public String toString(){ // method “function”
String faceName;
if (face == HEADS)
faceName = "Heads";
else
faceName = "Tails";
return faceName;
}
}
OOP: Object-Oriented Programming, Part 1 3
 Instance Variables
• An instance variable is a data declaration in a class. Every object
instantiated from the class has its own version of the instance
variables.
class Car {
private String make;
private String model;
private double price;
}

make: Ford make: Opel make: BMW

model: Taurus model: Kadett Model: M1
price: 100 price: 2500 price: 100

car1 car2 car3

OOP: Object-Oriented Programming, Part 1 4

 Methods in Java
• A method is a function or procedure that reads and/or modifies
the state of the class.
 A function returns a value (a procedure does not).
 A procedure has side-effects, e.g., change the state of an object.

char calc (int num1, int num2, String message)

method
Parameter list
name
The parameter list specifies the type
return and name of each parameter
type
The name of a parameter in the method
declaration is called a formal argument

OOP: Object-Oriented Programming, Part 1 5

 Methods in Java, cont.
• All methods have a return type
 void for procedures
 A primitive data type or a class for functions
• The return value
 Return stop the execution of a method and jumps out
 Return can be specified with or without an expression
• Parameter are pass-by-value
 Class parameter are passed as a reference

public double getPrice() { public double getError() {

return price; double a = 0;
} a++;
public void increaseCounter() { // compile-error
counter = counter + 1; }
//return;
}
OOP: Object-Oriented Programming, Part 1 6
 Method in Java, Example
public class Car{
// snip
/** Calculates the sales price of the car */
public int salesPrice(){
return (int)price;
}
/** Calculates the sales price of the car */
public int salesPrice(int overhead){
return (int)price + overhead;
}
/** Calculates the sales price of the car */
public double salesPrice(double overheadPercent){
return price + (overheadPercent * price);
}

/** Overwrites the toString method */

public String toString(){
return "make " + getMake() + " model "
+ getModel() + " price " + getPrice();
}
}
OOP: Object-Oriented Programming, Part 1 7
 Method in Java, Example, cont
• What is wrong here?

public class Car{

// snip
/** Calculates the integer sales price of the car */
public int salesPrice(){
return (int)price;
}
/** Calculates the double sales price of the car */
public double salesPrice(){
return (double)price;
}

public static void main(String[] args){

Car vw = new Car(“VW”, “Golf”, 1000);
vw.salesPrice();
}
}

OOP: Object-Oriented Programming, Part 1 8

 Scope
public int myFunction (){ // start scope 1
int x = 34;
// x is now available
{ // start scope 2
int y = 98;
// both x and y are available
// cannot redefine x here compile-time error
} // end scope 2
// now only x is available
// y is out-of-scope
return x;
} // end scope 1

• The redefinition of x in scope 2 is allowed in C/C++

OOP: Object-Oriented Programming, Part 1 9

 Object Creation in General
• Object can be created by
 Instantiating a class
 Copying an existing object

• Instantiating
 Static: Objects are constructed and destructed at the same time as the
surrounding object.
 Dynamic: Objects are created by executing a specific command.

• Copying
 Often called cloning

OOP: Object-Oriented Programming, Part 1 10

 Object Destruction in General
• Object can be destructed in two way.
 Explicit, e.g., by calling a special method or operator (C++).
 Implicit, when the object is no longer needed by the program (Java).

• Explicit
 An object in use can be destructed.
 Not handling destruction can cause memory leaks.
• Implicit
 Objects are destructed automatically by a garbage collector.
 There is a performance overhead in starting the garbage collector.
 There is a scheduling problem in when to start the garbage collector.

OOP: Object-Oriented Programming, Part 1 11

 Object Creation in Java
• Instantiazion: A process where storage is allocated for an
“empty” object.
• Initialization: A process where instances variables are assigned a
start value.

• Dynamic instantiazion in Java by calling the new operator.

• Static instantiazion is not supported in Java.
• Cloning implemented in Java via the method clone() in class
java.lang.Object.

• Initialization is done in constructors in Java

 Very similar to the way it is done in C++

OOP: Object-Oriented Programming, Part 1 12

 Object Destruction in Java
• Object destruction in Java is implicit an done via a garbage
collector.
 Can be called explicitely via System.gc().

• A special method finalize is called immediately before

garbage collection.
 Method in class Object, that can be redefined.
 Takes no parameters and returns void.
 Used for releasing resources, e.g., close file handles.
 Rarely necessary, e.g., “dead-conditions” for error dection purposes.

OOP: Object-Oriented Programming, Part 1 13

 Objects and References
• Variables of non-primitive types that are not initialized have the
special value null.
 Test: var1 == null
 Assignment: var2 = null
Object have identity but no name,
 i.e., not possible to identify an object O1 by the name of the variable
referring to O1.
• Aliasing: Many variables referring to the same object
var1
make: BMW
cylinders: 6
var2 model: M1
KW: 130
engine: ref
var3
car3 engine1
OOP: Object-Oriented Programming, Part 1 14
 Constructors in Java
• A constructor is a special method where the instance variables of
a newly created object are initialized with “reasonable” start
values.

• A class must have a constructor

 A default is provided implicitly (no-arg constructor).
• A constructor must have the same name as the class.
• A constructor has no return value.
 That's why it is as special method
• A constructor can be overloaded.
• A constructor can call other methods (but not vice-versa).
• A constructor can call other constructors (via this).

OOP: Object-Oriented Programming, Part 1 15

 Constructors in Java, cont.
• Every class should have a programmer defined constructor, that
explicitly guarantees correct initialization of new objects.

// redefined Coin class

public class Coin {
public static final int HEADS = 0;
public static final int TAILS = 1;
private int face;
// the constructor
public Coin () {
face = TAILS;
// method in object
flip();
// method on other object
otherObject.doMoreInitialization();
}
}

OOP: Object-Oriented Programming, Part 1 16

 Constructors and Cloning in Java
public class Car {
// instance variables
private String make;
private String model;
private double price;
/** The default constructor */
public Car() {
this("", "", 0.0); // must be the first thing
}
/** Construtor that assigns values to instance vars */
public Car(String make, String model, double price) {
this.make = make;
this.model = model;
this.price = price;
}

/** Cloning in Java overwrites the Object.clone() */

public Object clone() { // note the return type
return new Car(make, model, price);
}
}
OOP: Object-Oriented Programming, Part 1 17
 Constructor Initialization
public class Garage {
Car car1 = new Car();
static Car car2 = new Car(); // created on first access
}

public class Garage1 {

Car car1;
static Car car2;
// Explicit static initialization
static {
car2 = new Car();
}
}

OOP: Object-Oriented Programming, Part 1 18

 Constructor vs. Method
Similarities Dissimilarties
• Can take arguments • Has fixed name (same as the
 all pass-by-value class)
• Can be overloaded • No return value
• Access modifiers can be  “returns” a reference to object
specified (e.g., private or • Special call via new operator
public)  new Car()
• Can be final (covered later)  Cannot be called by methods
• Default constructor can by
synthesised by the system
• Cannot be declared static
 it is in fact a static method!

OOP: Object-Oriented Programming, Part 1 19

 Object Descrution in Java, cont.
class MemoryUsage{ /** Dummy class to take up mem */
int id; /** Id of object */
String name; /** Name of object */
MemoryUsage(int id){ /** Constructor */
this.id = id;
this.name = "Name: " + id;
}
/** Overwrite the finalize method */
public void finalize(){
System.out.println("Goodbye cruel world " + this.id);
}
}
public class Cleanup{
public static void main(String[] args){
for (int i = 0; i < 999; i++){
// allocate and discard
MemoryUsage m = new MemoryUsage(i);
if (i % 100 == 0){ System.gc(); }
}
}
}
OOP: Object-Oriented Programming, Part 1 20
 Value vs. Object
• A value is a data element without identity that cannot change
state.
• An object is an encapsulated data element with identity, state, and
behavior.

• An object can behave like value (or record). Is it a good idea?

• Values in Java are of the primitive type byte, short, int,

long, float, double, boolean, and char.
• Wrapper classes exists in Java for make the primitive type act as
objects.

OOP: Object-Oriented Programming, Part 1 21

 Strings in Java
• Strings in Java are of the class String.
• Objects of class String behave like values.

• Characteristics of Strings
 The notation "fly" instantiates the class String and initialize it with the
values "f"', "l", and "y".
 The class String has many different constructors.
 Values in a string cannot be modified (use StringBuffer instead).
 Class String redefines the method equals() from class Object.

OOP: Object-Oriented Programming, Part 1 22

 Equality
• Are the references a and b equal?

• Reference Equality
 Returns whether a and b points to the same object.
• Shallow Equality
 Returns whether a and b are structurally similar.
 One level of objects are compared.
• Deep Equality
 Returns where a and b have object-networks that are structurally similar.
 Multiple level of objects are compared recursively.

• Reference Equality  Shallow Equality  Deep Equality

OOP: Object-Oriented Programming, Part 1 23

 Equality Examples
var1
make: BMW
cylinders: 6
model: M1
KW: 130
engine: ref
var2
reference equal

make: BMW
var1 model: M1
engine: ref
cylinders: 6
KW: 130
make: BMW
var2 model: M1
engine: ref
shallow equal
OOP: Object-Oriented Programming, Part 1 24
 Equality Examples, cont.

make: BMW
cylinders: 6
var1 model: M1
KW: 130
engine: ref

make: BMW
cylinders: 6
var2 model: M1
KW: 130
engine: ref

deep equal
OOP: Object-Oriented Programming, Part 1 25
 Types of Equality in Java
• ==
 Equality on primitive data types
 8 == 7
 'b' == 'c'
 Reference equality on object references
 anotherPoint
 Strings are special
String s1 = "hello"; String s2 = "hello";
if (s1 == s2){
System.out.println(s1 + " equals" + s2);}

• equals
 Method on the class java.lang.Object.
 Default works like reference equality.
 Can be refined in subclass
 onePoint.equals(anotherPoint)

OOP: Object-Oriented Programming, Part 1 26

 equals example
public class Car {
// snip
/** Gets the make inst variable(helper function). */
public String getMake() {
return make;
}
// snip

/**
* Implements the equals method
* @see java.lang.Object#equals(java.lang.Object)
*/
public boolean equals(Object o) {
return o instanceof Car // is it a Car object?
&& ((Car) o).getMake() == this.make
&& ((Car) o).getModel() == this.model
&& ((Car) o).getPrice() == this.price;
// relies on “short circuiting”
}

OOP: Object-Oriented Programming, Part 1 27

 Summary
• Instance variables
• Strings are treated specially in Java
• Methods
 All computation should be done in methods
 Overloading is generally a good thing
• Initialization is critical for objects
 Java guarantees proper initialization using constructors
 Source of many errors in C
• Java helps clean-up with garbage collection
 Only memory is clean, close those file handles explicitly!
 No memory leaks, "show stopper" in a C/C++ project!
• Equality (three types of equality)

OOP: Object-Oriented Programming, Part 1 28

 Arrays in Java
• Not pointers like in C,
• Bounds checking at run-time
• int[] numbers; // equivalent
int number[];
• int[] numbers = {1, 2, 3, 4, 5, 6, 7};
 The size is fixed at compile-time!
• int[] numbers = new Integer[getSize()];
 The size is fixed at run-time!
 Cannot be resized

for (int i = 0; i < numbers.length; i++){

System.out.println(numbers[i]);
}

OOP: Object-Oriented Programming, Part 1 29