Object and Classes

What is an Object and a Class?

An object is an thing (noun) that has certain characteristics and can perform certain tasks. A Class is the blueprint or definition for that object.

An example is a Person. A Person is Class. A person can have certain characteristics that distinguish it from another person. It can have blue eyes, it can be 30 years old, etc. A person can also perform certain tasks. It can walk, it can talk, etc.

A Java Object is an instance of a Java Class. In this instance “Bea” is a “Person”. “Bea” has brown eyes. This is a property (or data field) of “Bea”. “Bea” can speak in English. “Bea” can speak in Spanish. These are methods that “Bea” can perform.

Creating a Class!

Java is an object-oriented programming language, which means everything in Java is associated with an object and a class (the blueprint for the object).

For starters, the line of code that we used to print statements from activity 1 is actually a method call from a predefined class called System!

System.out.println("Hello World");
  1. System is a class that deals with user input and outputs.
  2. out is a variable in class System of the class/type PrintStream.
  3. println() is a method defined in the class PrintStream.

Another class we have interacted with in the previous exercises is String. The String class defines a set of rules on how a list of characters should behave.

With following line of code, we created a String object called name using the rules defined in the String class:

String name = "Patrick";

System, PrintStream, and String are predefined classes in Java. However, we are not limited to these predefined classes, and we can actually create our own data type by writing a class! This is useful for coders to create specific objects to have certain attributes and behaviors. Having access to these user-defined types allows us to build distinct programs.

Let’s learn about the different parts in a class below:


A class is a blueprint or prototype of a new type of object. In general, a class contains three important parts: (1) data fields/instance variables (2) constructor (3) methods.

A Class blueprint for an object contains instance varialbes/data fields which are data/attributes in the object, constructor which are methods that creates the obejct of the class, and methods, which are behaviors possible for the object.
data fields/instance variablesvariables an object of this class has access to that describe the objectPerson class has fields eyeColor, age, height.
constructormethod called to create an object from this ClassThere can be more than one constructor per class
methodsmethods an object of this class will be able to call to perform certain tasksPerson class has methods talk and walk.

For example:

public class SampleClass{
    // (1) data fields/instance variables
    private String name; // example

    // (2) constructor
    public SampleClass(){
        name = "example";
    // (3) methods
    public sampleMethod(){}

Try it Out 🐥!

Let’s make a Bird class to represent Patrick 🐥 and all of his Bird friends by following the steps below! Note: For this exercise, you will be working in Bird.java instead of Main.java.

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First, we start by defining the class name in the format public class name.

public class Bird{}

Second, let’s declare all the fields of the Bird class: species, name, hobby, age, loveMusic.

Each field is declared in the format: private data type name ;.

  1. We declare all 5 fields as private in the class Bird. This ensures these data can only be accessed within this class.

  2. Let’s identify the appropriate type for each field:

For example, to declare species as a private field of class Bird, you would put private String species; .

  1. Finish creating the other 4 fields of the Bird class!

Third, let’s create the constructor for the class Bird.

Usually, constructor is the method that initialializes value to all the fields in a class. It has the format public class name (parameter). Since we have 5 fields in this class, the constructor will take in 5 parameter/inputs.

public Bird(String speciesInput, String nameInput, String hobbyInput, int ageInput, boolean loveMusicInput){
    // constructor body

In the constructor body, we need to initialize all the instance variables, by assign each variables to its initial values:

species = speciesInput;
name = nameInput;
hobby = hobbyInput;
age = ageInput;
loveMusic = loveMusicinput;

Lastly, let’s create some methods for the class Bird.

We are gonna create 6 methods for this class!

Try to write out the first 5 methods with what you learned from the previous exercise about writing methods!

After, let's write the `toString()` method together!

toString() is a method that returns the String representation of the object.

We will return a String that include all fields of the Bird: name, age, species, hobby, loveMusic by calling the 5 methods you just implemented!

public String toString(){
    // initialize variable info to have empty String
    String info = "";

    // append information to the String in the format: 
    // Hi, my name is [name]. I am a [age] year old [species] who likes to [hobby].
    info = info + "Hi, my name is " + getName() + 
           ". I am a " + getAge() + " year old " + getSpecies() + 
           " who likes to " + getHobby() + ". ";

    // if the Bird likes music, append the String "I also like music very much!" to info
        info = info + "I also like music very much!";
        info = info + "However, I do not like music!";

    // return the full String
    return info;

If you correctly implemented the Bird class, you will see the following message when you click Run:

Congratulations! You correctly implemented the Bird Class :)

If there is still something incomplete or incorrect when you click Run:

  • You might get an error such as Main.java:3: error: cannot access Bird.
  • Or, you will see the message, Something is still not quite right!.

Creating Birds 🐥!

Congratulations! You just wrote you first Java class Bird class. Now let’s learn to write a program that uses Bird objects!

We defined the Bird class to have the following attributes (instance variables) and behaviors (methods): Bird Class blueprint for a Bird object contains instance variables/data fields which include String species, String name, String hobby, int age, and boolean loveMusic, constructor, which requires all 5 variables to be passed in, and methods, including String getSpecies(), String getName(), String getHobby(), int getAge(), boolean getLoveMusic(), and String toString().

To create a new object of a particular class, we call the constructor of that class in the format class name variable name = new constructor call.

Recall that the constructor of Bird class is the following:

public Bird(String speciesInput, String nameInput, String hobbyInput, int ageInput, boolean loveMusicInput);

Hence, we can create a Bird with these attributes (species - duck; name - Patrick; hobby - hangout with friends; age - 15; loveMusic - true) with this line of code.

Bird patrick = new Bird("duck", "Patrick", "hangout with friends", 15, true);

Try it out yourself and create multiple Birds of different species 🐦🐤🐔🐧!

Next, let's call the `toString()` method on these `Bird` objects we created to print the information of our friends. You can do either of the following:
  1. Print the return value of toString() method. (i.e. System.out.println(patrick.toString());)
  2. Directly print the Bird object, which prompt toString() to be called in the background. (i.e. System.out.println(patrick);).

Try it out and print out all the information of the bird friends you created 🐦🐤🐔🐧!

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