LESSON 282 OF 671 · FUNCTIONS & CLASSES

Class Fields in Java

Lesson

TL ; DR:

A field is a variable that lives inside a class. Each object of that class has its own copy.
Declare a field with a type and a name, with public in front so other code can read and write it:
```
class Dog {
    public String name;
    public int age;
}
```
Access a field on an object with the dot operator: dog.name = "Rex";

Full lesson:

Right now your Dog class is just an empty shell — every Dog object you create is identical to every other one. We need a way to attach data to each object.

That's what fields are for. A field is a variable that belongs to a class — and each object created from that class gets its own copy of every field.

Declaring fields

class Dog {
    public String name;
    public int age;
}

public means "any code can read and write this field". You'll see private later for fields that should only be touched by the class itself.
String name; is just a variable declaration — same syntax you'd use anywhere else.

Reading and writing fields

Dog dog = new Dog();
dog.name = "Rex";        // write
dog.age = 4;
System.out.println(dog.name);   // read → prints "Rex"

Each object has its own slot for every field, so two dogs can have different names without interfering.

Default values

If you don't assign a field, Java fills it in for you:

Numbers default to 0
boolean defaults to false
References (String, arrays, other classes) default to null

Your task

Update the Dog class to have two fields: name (a String) and age (an int). Then in mainFunction, set them on the dog object so the pre-filled System.out.println prints:

Rex is 4 years old

STEP 1 OF 2

Guided

Ctrl + Enter to check