Classes & Objects: Blueprint vs Instance

A class defines the structure. An object is a real thing built from that structure using the new keyword.

The Blueprint (Class Definition)

Class = Blueprint

public class Pet

Fields (what data each Pet holds)

private String name

private String species

private int age

Constructor (how to build a Pet)

public Pet(String name, String species, int age) {
  this.name = name;
  this.species = species;
  this.age = age;
}

Methods (what a Pet can do)

String getName()

void setName(String name)

String getSpecies()

void setSpecies(String species)

int getAge()

void setAge(int age)

new creates a fresh object in memory from this blueprint

Tip: The class itself doesn't hold data. It only describes what data an object will have. Each object gets its own separate copy.

Create Objects (Instances)

Name

Species

Age

No objects created yet. Use the form above to create a Pet.

Modify Selected Object

Use a setter to change one value. Notice: only this object changes. Others stay the same.

Field

New Value

📜

Class = Structure

A class defines what data and methods an object will have. It's a template, not a real thing.

📦

Object = Instance

An object is a specific instance created from a class. It holds actual values in its fields.

🔗

Many from One

You can create many objects from one class. Each is independent with its own data.

🔒

Each Object is Separate

Changing one object's field with a setter does not affect any other object. They each have their own memory.

Click on an object card to select it, then modify it with a setter. Watch how only that object changes.