1. Creating Objects in a Few Simple Steps
Written by Dr. JJ Shepherd

2. Overview
These slides will go over the very basics of creating a class with an accompanying example.
The following steps are a suggested way to create classes, but may not be the most appropriate for all situations.
Whenever the enclosing symbol “<<>>” is used this indicates “Fill in the rest” or “substitute with something”.
Whenever the symbol “…” is used it indicates there is additional code that has been omitted

3. The Steps Define the Class Create the Instance Variables (attributes)
Make their scope private
Create the Constructors
Default
Parameterized for each attribute
Create the Accessors for every attribute
Create the Mutators for every attribute
Create the other Methods
Equals() and toString() are both good ones to have
Profit!

4. The Example Create a class called Bird
Bird has the following attributes
Name: A string representing the bird’s name
Weight: A decimal number indicating the bird’s weight
Age: A whole number indicating the bird’s age
Bird also has the following methods
Equals: A method that takes in another instance of Bird and returns true of false depending on if the attributes of one are equal to the other
toString: A method that takes nothing and returns a String composed of its attributes.

5. Define the Class
Classes are used to create instance of objects. They are a way of grouping together common attributes and actions into a single type.
Defining the class takes the following formula
public class <<Name of Class>> { }

6. Define the Class Example
public class Bird { }

7. Create Instance Variables
These are the properties / attributes that are important to each instance of the class.
Defining these take the following formula for each attribute
private <<type>> <<identifier>>;

8. Create Instance Example
public class Bird { private String name; private double weight; private int age; }

9. Create Constructors
Constructors are used to create the object in memory.
There are two types of constructors that are focused on in this course
Default
Parameterized

10. Create Default Constructor
The default constructor is used to create the object and set all the attributes to some default value.
They follow this formula
public <<Class’ Name>>() {
<<Set default values>> }

11. Create Default Constructor Example
public Bird() { this.name = “none”; this.weight = 0.0; this.age = 0; }

12. Create Parameterized Constructor
The parameterized (param) constructor is used to create the object and set all the attributes to values passed via a parameter.
Strong advisement for calling the classes Mutators (discussed later) to set the values while also checking for correctness.
They follow this formula
public <<Class’ Name>>(<<Param for each attribute>>) {
<<Set values using mutators>> }

13. Create Parameterized Constructor Example
public Bird(String aName, double aWeight, int anAge) { this.setName(aName); this.setWeight(aWeight); this.setAge(anAge); }

14. Create Accessors
Accessors give access to attributes outside of the class, and should be created for each attribute.
They follow this formula
public <<attribute type>> get<<attribute identifier>>() {
return this.<<attribute>>; }

15. Create Accessors Example
public String getName() { return this.name; } public double getWeight() return this.weight; public int getAge() return this.age;

16. Create Mutators
Mutators are used to set attribute values and should be created for each attribute.
They also check for potential errors in assigning values
They follow this formula
public void set<<attribute identifier>>(<<the value to be set>>) {
<<Check for errors>>
this.<<attribute>> = <<the value to be set>> }

17. Create Mutators Example
public void setName(String aName) { //We are not checking for errors on name’s this.name = aName; } public void setWeight(double aWeight) if(aWeight >= 0)//Assuming weights are non-negative this.weight = aWeight; public void setAge(int anAge) if(anAge >= 0)//Assuming ages are non-negative this.age = anAge;

18. Create Other Methods
Other methods are dependent on the object; as they represents the object’s actions.
Despite this there are two very common other methods that are not usually required, but are strongly recommended
toString
equals

19. Create toString
This method should return a string that represents the class and it’s attributes.
This is very useful for debugging as if the object is placed in a System.out.println() statement then it will print its toString’s String
They follow this formula
public String toString() {
return <<attribute 1>>+“ ”+… }

20. Create toString Example
public String toString() { return this.name+“ ”+ this.weight+“ ”+ this.age; }

21. Create equals
This is another useful method that will help determine if all of the attributes of one instance of an object is equal to the attributes of another.
Remember to use “==” for primitive types and “.equals” for object types
It follows this formula
public boolean equals(<<Another instance>>) {
return <<Another Instance>> != null &&
this.<<attribute>> <<either == or .equals) <<Another instance>>.get<<attribute>>()&& …; }

22. Create equals Example
public boolean equals(Bird aBird) { return aBird != null && this.name.equals(aBird.getName()) && this.weight == aBird.getWeight() && this.age == aBird.getAge(); }

23. Putting it all together Example
Here is the full definition of the Bird class example
public void setWeight(double aWeight)
public class Bird
if(aWeight >= 0)//Assuming weights are non-negative {
private String name;
this.weight = aWeight;
private double weight;
private int age;
public Bird()
public void setAge(int anAge)
this.name = "none";
if(anAge >= 0)//Assuming ages are non-negative
this.weight = 0.0;
this.age = 0;
this.age = anAge; }
public Bird(String aName, double aWeight, int anAge)
public String toString()
this.setName(aName);
this.setWeight(aWeight);
return this.name+" "+
this.setAge(anAge);
this.weight+" "+
this.age;
public String getName()
public boolean equals(Bird aBird)
return this.name;
return aBird != null &&
public double getWeight()
this.name.equals(aBird.getName()) &&
this.weight == aBird.getWeight() &&
return this.weight;
this.age == aBird.getAge();
public int getAge()
}//end class
return this.age;
public void setName(String aName)
//We are not checking for errors on name’s
this.name = aName;