User-Defined Classes in Java

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Questions and Answers

Why are user-defined classes used?

They enable representation of complex objects. (correct)

They provide unlimited data representation.

They are not used in programming.

They only use primitive data types.

What is a class?

Combines data and the methods that operate on the data.

A class is a ______ (think 'cookie cutter').

template

An object is an ______ of a class (think cookie).

instance Signup and view all the answers

That said, there are _ __ tied to the class instead of an object.

class variables Signup and view all the answers

A _____ of a class is a program that uses that class.

client Signup and view all the answers

Java can have ____ constructors.

many Signup and view all the answers

In Java, class/object variables must be ____.

declared Signup and view all the answers

In Java, everything has to have a type, including the of methods.

return value Signup and view all the answers

Match the following terms with their descriptions:

fields = Identifiers declared inside the class, but outside any method. constructors = Special-purpose methods called when an object is instantiated. methods = Code segments within a class that manipulate fields. Signup and view all the answers

What are instance variables?

Data specific to each instance of the object. Signup and view all the answers

What are class variables?

Static data shared by all objects of the class. Signup and view all the answers

Both types are available to ___ the methods.

access Signup and view all the answers

Each ____ in the class contains code that can manipulate one (or more) of the fields.

method Signup and view all the answers

What are members in the context of a class?

Fields and methods. Signup and view all the answers

What does an access modifier do?

Determines access rights for the class and its members. Signup and view all the answers

Methods of the same class and methods of other classes can be declared as _____

public Signup and view all the answers

Methods of the same class only can be declared as _____

private Signup and view all the answers

Methods of the same class, methods of subclasses, and methods of classes in the same package can be declared as _____

protected Signup and view all the answers

Methods in the same package only are declared with _____.

no access modifier Signup and view all the answers

____ variables are usually declared to be private.

Instance Signup and view all the answers

_____ that will be called by clients of the class are usually declared public.

Methods Signup and view all the answers

Methods that are _ called by other methods of the class, are declared private.

only, same Signup and view all the answers

You may only have one class declared as public per _____, and the file name must match the class name with the extension '.java'.

file Signup and view all the answers

What is encapsulation?

The concept of restricting access to class members. Signup and view all the answers

What is another term for information hiding?

Encapsulation Signup and view all the answers

Encapsulation is the idea that clients of the class do not ____ how the class is implemented.

know Signup and view all the answers

Encapsulation allows you to protect the ____ of the object's data.

integrity Signup and view all the answers

It allows you to change how the class is implemented without causing code that used the object to _____.

break Signup and view all the answers

How do we access private instance variables?

Using accessor and mutator methods. Signup and view all the answers

Can we add one to the current value this way? `ins.setValue(getValue() + 1)`

True Signup and view all the answers

What does Validation ensure?

It ensures that values cannot be set to unwanted values using mutators. Signup and view all the answers

Do not give the ______ the same name as the instance variable.

parameter Signup and view all the answers

What are constructors?

Special-purpose methods that are called only when an object is instantiated. Signup and view all the answers

What is an example of a class in Java that has ten different constructors?

Scanner Signup and view all the answers

In Java, the ____ is not passed as an explicit parameter.

object 'self' Signup and view all the answers

A constructor ______ (some or all) of the instance variables for the new object.

initializes Signup and view all the answers

Constructors do not have a ______.

return type Signup and view all the answers

Study Notes

User-Defined Classes Overview

User-defined classes enhance data representation beyond primitive data types like int, long, or double, allowing for more complex object modeling.
A class amalgamates related data and methods designed to operate on that data.

Class Concepts

A class acts as a template, akin to a cookie cutter, guiding the creation of objects.
An instance is a specific object created from a class, comparable to a cookie made from the cookie cutter.

Class Composition

Classes can include fields and methods, with fields storing data and methods performing actions on that data.
Class members encompass both fields (data variables) and methods (functions associated with the class).

Java Class Features

Java allows multiple constructors, providing variations in object instantiation.
Variables in Java must be explicitly declared, ensuring clear data structures.

Method and Return Value

Every method in Java should have an associated type, including the return value.
Constructor methods are special, serving to initialize fields when an object is created.

Access Modifiers

Access modifiers control visibility and accessibility of class components:
- public: accessible from outside the class.
- private: only accessible within the same class.
- protected: accessible within the same class, subclasses, and same package.
- No modifier: accessible within the same package.

Encapsulation and Data Integrity

Encapsulation promotes information hiding, ensuring class implementation details are not exposed to clients.
This protects data integrity and allows for internal changes without affecting external code relying on the object.

Accessing Private Variables

Private instance variables can be accessed through accessor (getter) and mutator (setter) methods, facilitating controlled data manipulation.

Constructors Role and Functionality

Constructors have no return type and are invoked when a new object is created, initializing instance variables.
Ensuring constructors do not use the same names as instance variables is critical for clarity.

Class Examples

The Scanner class in Java is an example featuring multiple constructors, illustrating flexibility in object creation.

Summary

Understanding user-defined classes, syntax, access control, and encapsulation is crucial for effective object-oriented programming in Java.

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Description

This quiz covers the essential concepts of user-defined classes in Java, including their structure, composition, and features. You will explore how classes are utilized as templates for creating objects and the role of methods and return values. Test your understanding of how Java enhances object-oriented programming through classes.