Classes and objects in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a class in Java?

A class is a blueprint or template that defines the structure and behavior (fields and methods) that objects created from the class will have.

beginner

What is an object in Java?

An object is an instance of a class. It represents a real-world entity with state (data) and behavior (methods) defined by its class.

beginner

How do you create an object from a class in Java?

You use the 'new' keyword followed by the class constructor. For example: <br> <code>ClassName obj = new ClassName();</code>

intermediate

What is the purpose of a constructor in a class?

A constructor is a special method used to initialize new objects. It has the same name as the class and no return type.

beginner

Explain the difference between fields and methods in a class.

Fields are variables that hold data about the object. Methods are functions that define the behavior or actions the object can perform.

Which keyword is used to create an object in Java?

A new

B class

C object

D create

The 'new' keyword is used to create a new object instance from a class.

What does a constructor do in a Java class?

A Deletes objects

B Defines the class name

C Initializes new objects

D Calls methods

Constructors initialize new objects when they are created.

Which of the following is NOT a part of a class?

A Objects

B Methods

C Fields

D Constructors

Objects are instances created from classes, not part of the class itself.

How do you call a method named 'run' on an object 'car'?

A run.car();

B car.run();

C car->run();

D run(car);

You call a method on an object using dot notation: objectName.methodName();

What is the correct way to declare a class named 'Dog'?

A class = Dog {}

B Dog class {}

C new class Dog {}

D class Dog {}

The correct syntax is: class ClassName {}

Describe what a class and an object are in Java and how they relate to each other.

Think about how a cookie cutter (class) relates to cookies (objects).

Explain the role of constructors in creating objects and how they differ from regular methods.

Constructors prepare new objects when they are born.