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Introduction to Objects

class Example {
int value;
}
Example example = new Example();
System.out.println(example.value);
example.value = 10;
System.out.println(example.value);

A new era begins in CS 124. We leave the familiar world of primitive types and Strings behind and strike out to new horizons. This lesson begins our discussion of Java objects. Objects represent both a significant conceptual step forward, and dramatically improve our ability to work with data.

So let’s get started!

What Are Objects?
What Are Objects?

Java is an object-oriented language. What does that mean, exactly?

Wikipedia defines an object as:

In the class-based object-oriented programming paradigm, “object” refers to a particular instance of a class where the object can be a combination of variables, functions, and data structures.

Let’s examine this definition together.

Object Terminology: Class
Object Terminology: Class

A class definines how an entire group of objects behaves. For example, we might say that a person is a class of things where each has a name and an age. Let’s look at our first class definition together:

class Person {
String name;
int age;
}

Object Terminology: Instance
Object Terminology: Instance

Defining a class allows us to create instances of that class. Sometimes we use the term object and instance interchangeably, defining an object as an instance of a class.

If it helps understand the relationship between class and instance, here are some examples of this relationship between real-world things:

Now, let’s continue the example above using our Person class and create some instances.

class Person {
String name;
int age;
}

Creating Instances Using new
Creating Instances Using new

To create a new instance of a class we use the new keyword:

class Example {
int value = 0;
}
// Assignment of a new Example instance to the variable e
Example e = new Example();
// Assignment using local variable type inference
var another = new Example();

For now the syntax of new will seem somewhat mysterious. Why does it look like a function call? That will make more sense soon. But for now simply follow this template to create new class instances.

Accessing Fields Using Dot Notation
Accessing Fields Using Dot Notation

Objects allow us to create new Java types. These new types are in some ways just like the types that we’ve already been working with:

class Example {
int value;
}
int i = 0; // i is a variable of type int
Example e = new Example(); // e is a variable of type Example

But Java classes allow us to include any mixture of different types of data by declaring one or more fields:

// Example has one field named value of type int
class Example {
int value;
}
// I can access the field value using dot notation:
Example e = new Example();
e.value = 10;
System.out.println(e.value);
// Dog has two fields, one named name of type String and another named age of type double
class Dog {
String name;
double age;
}
// Again, I can access both Dog fields using dot notation
Dog d = new Dog();
d.name = "Chuchu";
d.age = 16.1;
System.out.println(d.name + " is " + d.age + " years old");

Be Patient!
Be Patient!

An important note: not everything about objects is going to make sense to you right away! That’s OK. We’re going to keep practicing, and things will become a bit more clear every day.

More Practice

Need more practice? Head over to the practice page.