Java Encapsulation and Immutable Classes

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Questions and Answers

What does encapsulation in Java primarily allow an object to do?

Require all fields to be public

Change its data freely

Expose all its internal features

Hide its data and methods (correct)

Which method is used to change the value of a private field in a class?

Accessor method

Setter method (correct)

Public method

Constructor

What must you remove from a class to make it immutable?

Setter methods (correct)

Public methods

Constructor

Private fields

Which of the following correctly describes a getter method?

It returns data from a private instance variable (C) Signup and view all the answers

What keyword is typically used to declare instance variables that are encapsulated?

private (A) Signup and view all the answers

Which statement about immutable classes is false?

They allow modifying the internal state after creation (C) Signup and view all the answers

What is the return type of a setter method?

void (C) Signup and view all the answers

Which part of a class typically encapsulates its state?

Instance variables (C) Signup and view all the answers

What is the primary purpose of encapsulation in Java?

To prevent direct access to object attributes (C) Signup and view all the answers

Which naming convention is correct for a getter method that retrieves a non-boolean property?

getPropertyName() (B) Signup and view all the answers

What does the 'extends' keyword signify in a class declaration?

The class is inheriting from a superclass (B) Signup and view all the answers

What type of class cannot be extended by any other class?

Final class (C) Signup and view all the answers

What is the default visibility of instance variables in a class without any access modifier?

Package-private (C) Signup and view all the answers

Which statement is true regarding the Object class in Java?

It is a superclass for all classes and has no parent class (B) Signup and view all the answers

In the context of inheritance, how is the subclass related to its superclass?

It inherits all public and protected members (D) Signup and view all the answers

Which of the following methods would be valid for accessing a private attribute named 'name'?

public String getName() { return name; } (D) Signup and view all the answers

What is the result of calling the constructor of the Student class?

Both Person and Student constructors execute, with Person first. (A) Signup and view all the answers

What does the keyword 'super' do in the context of constructors?

It calls the constructor of the parent class. (D) Signup and view all the answers

What is required for method overriding to occur in a subclass?

The method must have the same name and parameter list. (A) Signup and view all the answers

How must access level be defined for an overridden method compared to the parent method?

The overridden method must be at least as accessible. (A) Signup and view all the answers

Which of the following lines would properly instantiate a new Student object?

Student s = new Student(); (A) Signup and view all the answers

What will be printed as the output when a Student object is created?

Person Student (A) Signup and view all the answers

What happens if the parent constructor requires a parameter but the child constructor does not provide one?

A compilation error will occur. (A) Signup and view all the answers

What output will be produced if the Person constructor is modified to output its parameter?

It will print the name passed to the Person constructor. (C) Signup and view all the answers

What happens if the parent class does not define a no-argument constructor?

The Java compiler will throw an error. (A) Signup and view all the answers

What is the purpose of the super() method in the constructor?

To call the parent class constructor. (C) Signup and view all the answers

What will happen if a child class does not define any constructors?

The child class will have an invisible no-argument super(). (B) Signup and view all the answers

In what scenario is it not allowed to extend a constructor using super()?

When using multiple inheritance. (B) Signup and view all the answers

What will the compiler do if the first statement in a child class constructor is not a call to the parent constructor?

It will automatically insert a call to super(). (A) Signup and view all the answers

What happens if the child class constructor calls super() with an argument while the parent class lacks a matching constructor?

The compiler will throw an error. (D) Signup and view all the answers

What is the purpose of the `this()` keyword in the Student class?

To invoke the constructor of the Person class (D) Signup and view all the answers

What error will occur if a child class constructor does not explicitly call a parent class constructor?

Compilation error because the parent class lacks a no-argument constructor (C) Signup and view all the answers

What is indicated by the error regarding `super()` in the context of the Person class?

Person must have a no-argument constructor (B) Signup and view all the answers

How does Java handle the absence of a `super()` call in a child constructor?

By inserting a no-argument super() automatically (D) Signup and view all the answers

What will happen if the Person class is updated to include a no-argument constructor?

Child classes can be created without any errors (D) Signup and view all the answers

In which scenario will a compilation error occur during the instantiation of the Student class?

When super() is called after other statements (A) Signup and view all the answers

What would be the result of calling the constructor of the Person class directly from the Student constructor?

Ensures that Person class initialization logic is executed (B) Signup and view all the answers

Which statement is true regarding the constructors in the given context?

The first statement in a constructor must be a call to a superclass constructor (A) Signup and view all the answers

What happens if the Student class does not define any constructors?

An error will occur if the parent class has no no-argument constructor. (D) Signup and view all the answers

What is the effect of calling `super(name)` in the Student class constructor?

It initializes the parent's instance variables. (D) Signup and view all the answers

Which scenario is correct regarding the use of `super()` in multiple inheritance?

It can only call the constructor of the direct parent class. (D) Signup and view all the answers

What will happen if the Person class has a no-argument constructor and the Student class does not?

The no-argument constructor of Person will be used. (B) Signup and view all the answers

Under what condition will the compiler throw an error regarding constructors?

When the child class calls a non-existent constructor of the parent class. (A) Signup and view all the answers

What does the Java compiler do if the first statement in a child class constructor is not a call to the parent constructor?

The compiler automatically adds a call to the no-argument constructor of the parent class. (A) Signup and view all the answers

What occurs when a child class constructor does not provide arguments for a parent class constructor that requires them?

An error regarding constructor parameters will be thrown. (D) Signup and view all the answers

Why would a parent class throw an error if the child class constructor does not explicitly define any constructors?

Because the parent class constructor could require parameters. (B) Signup and view all the answers

Study Notes

Encapsulation

In Java, a class encapsulates both fields (state of an object) and methods (actions of an object).
Encapsulation means hiding data and methods within a class.
Accessing private fields can be achieved with public methods, such as getter (accessor) and setter (mutator) methods.
The benefits of encapsulation include reusable programs and restricted access to object features.
Setter methods begin with "set" and take an argument to set the field value, like public void setName(String name).
Getter methods begin with "get" (or "is" if the property is a boolean) and return the field value, like public String getName().

Immutable Classes

Immutable classes remain unchanged after an object is created.
To make a class immutable:
- Remove setter methods.
- Set the values in the constructor.
Example:
- A Student class with a private name field can be made immutable by only providing a constructor that receives the name and a getName() method.
- No setter method setName() should be provided.

Inheritance

Inheritance enables classes to acquire attributes and behaviors from a parent class (superclass, base class).
The subclass (derived class, child class) inherits from the superclass.
All Java classes inherit from the java.lang.Object class.
To prevent inheritance, mark a class with the final modifier.
Even with public access, private fields of a superclass cannot be accessed by its subclass directly.
Multiple Inheritance (extending from multiple classes) is not allowed in Java.

Constructors and Inheritance

Java invisibly adds a no-argument (super()) constructor to a class if one is not declared.
The super() call must be the first statement in the constructor (unless it's a call to another constructor in the same class using this()).
If the parent class does not have a no-argument constructor, the child class must provide a constructor with an explicit super() call to a parent constructor.
If a child class constructor doesn't have a super() call, the compiler will call the parent class's no-argument constructor, which might not exist leading to a compile error.

Method Overriding

You can redefine a method in a child class (method overriding), inheriting the definition from the parent class but changing its implementation.
To override a method, the method in the child class must have the same name, parameter list, and return type as the method in the parent class.
The overridden method in the child class must have at least as much accessibility as the method in the parent class.
Parent constructors are always executed before child constructors.

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Description

Explore the concepts of encapsulation and immutable classes in Java. This quiz covers the definition of encapsulation, the use of getter and setter methods, and the principles of constructing immutable classes. Test your knowledge on how these concepts enhance object-oriented programming.