Objects and Classes Quiz

Questions and Answers

What is an anonymous object in terms of Java object creation?

An object that retains a specific reference.

An object that cannot be accessed after creation.

An object created and not assigned to a variable. (correct)

An object created without a class definition.

Which statement correctly describes an instance variable?

It refers to methods that belong to all instances equally.

It is static and shared across all instances of a class.

It is a data field associated with a specific instance of a class. (correct)

It can be accessed without creating an object.

How is a method invoked on an object in Java?

Through a constructor call.

By simply writing the method name.

Using the object member access operator. (correct)

Using square brackets.

What happens when a reference type data field is not referencing any object?

It holds a special value called null. Signup and view all the answers

Which of the following is true about instance methods?

They must be invoked on a specific instance of a class. Signup and view all the answers

What is the purpose of the dot operator in Java?

To reference members of an object. Signup and view all the answers

What would be the output of System.out.println('Area is ' + new Circle(5).getArea()) if getArea returns the area of a Circle?

The area calculated will be displayed in a string format. Signup and view all the answers

What does the 'new' keyword indicate when used in `new Circle();`?

It initializes an object and allocates memory for it. Signup and view all the answers

What is the typical order of operations when creating and using an object in OOP?

Define class, create object, reference methods and fields. Signup and view all the answers

Study Notes

Object Creation and Constructors

A constructor is a method used to initialize an object, invoked with the new operator.
Constructors must share the same name as their class and do not have a return type, not even void.
Overloading is allowed, enabling multiple constructors with different parameters within the same class.
A default constructor is provided by Java if no constructors are explicitly defined, allowing instantiation without arguments.

Accessing Objects

Objects are accessed using reference variables, which hold the memory address of the object instead of the object itself.
The dot (.) operator is used to access the object’s data fields and methods, denoting member access.
A class is a reference type, meaning a variable of that class can reference an instance; e.g., Circle myCircle; defines a reference variable for a Circle object.

Object vs. Reference Variable

An object reference variable points to an object in memory, while the object is the actual instance created.
It is common to refer to a reference variable as if it were the object for simplicity.

Arrays as Objects

In Java, arrays are treated as objects created using the new operator.
An array variable holds a reference to the created array object, similar to other object types.

Accessing Data Fields and Methods

Data fields and methods of an object are accessed using the dot operator:
- For data fields: objectRefVar.dataField
- For methods: objectRefVar.method(arguments)
Example: myCircle.radius accesses the radius field, while myCircle.getArea() invokes the getArea method.

Instance Variables and Methods

Instance variables depend on the specific object instance, while instance methods are invoked on that object.
The object on which an instance method is called is termed the calling object.

Anonymous Objects

Objects can be created without variable assignment, known as anonymous objects.
Example: System.out.println("Area is " + new Circle(5).getArea()); creates and immediately uses a Circle object without referencing it.

Reference Data Fields and Null Values

Data fields can also be of reference types, like String, which is a predefined Java class.
If a reference field contains no object reference, it holds the value null, indicating the absence of an object.

Description

This quiz tests your understanding of objects and classes in programming. Topics include constructing objects using constructors, accessing objects via reference variables, and using the object member access operator. Assess your knowledge of data fields and reference types through a series of questions.