All Flashcards
What is the purpose of the 'super(length, width, length, width);' line in the Rectangle constructor?
It calls the Quadrilateral constructor with the provided length and width values, effectively setting the sides of the rectangle.
java
public class Animal {
public Animal() {
System.out.println("Animal constructor");
}
}
public class Dog extends Animal {
public Dog() {
System.out.println("Dog constructor");
}
public static void main(String[] args) {
Dog d = new Dog();
}
}
What is the output of this code?
Animal constructor Dog constructor
java
public class A {
public A(int x) {
System.out.println("A: " + x);
}
}
public class B extends A {
public B() {
System.out.println("B");
}
public static void main(String[] args) {
B b = new B();
}
}
Identify the error in the following code.
The class A does not have a no-argument constructor. The class B's constructor needs to explicitly call A's constructor using super(value).
java
public class Parent {
public Parent(String name) {
System.out.println("Parent: " + name);
}
}
public class Child extends Parent {
public Child(String name) {
super(name);
System.out.println("Child: " + name);
}
public static void main(String[] args) {
Child c = new Child("Alice");
}
}
What is the output of this code?
Parent: Alice Child: Alice
java
public class Vehicle {
public Vehicle() {
System.out.println("Vehicle");
}
}
public class Car extends Vehicle {
// Complete the Car constructor to call the Vehicle constructor
public Car() {
// your code here
System.out.println("Car");
}
public static void main(String[] args) {
Car c = new Car();
}
}
Complete the Car constructor to call the Vehicle constructor.
super();
java
public class Base {
public Base(int x) {
System.out.println("Base: " + x);
}
}
public class Derived extends Base {
public Derived() {
super(5);
System.out.println("Derived");
}
public static void main(String[] args) {
Derived d = new Derived();
}
}
What is the output of this code?
Base: 5 Derived
java
public class Grandparent {
public Grandparent() {
System.out.println("Grandparent");
}
}
public class Parent extends Grandparent {
public Parent() {
System.out.println("Parent");
}
}
public class Child extends Parent {
public Child() {
System.out.println("Child");
}
public static void main(String[] args) {
Child c = new Child();
}
}
What is the output of this code?
Grandparent Parent Child
java
public class Device {
private String name;
public Device(String name) {
this.name = name;
}
}
public class Phone extends Device {
private String number;
public Phone(String name, String number) {
// Complete the constructor
this.number = number;
}
}
Complete the Phone constructor to call the Device constructor.
super(name);
java
public class Shape {
public Shape() {
System.out.println("Shape");
}
}
public class Circle extends Shape {
public Circle(int radius) {
System.out.println("Circle");
}
public static void main(String[] args) {
Circle c = new Circle(5);
}
}
What is the output of this code?
Shape Circle
java
public class Root {
public Root(int value) {
System.out.println("Root: " + value);
}
}
public class Branch extends Root {
public Branch() {
// Complete the constructor
System.out.println("Branch");
}
public static void main(String[] args) {
Branch b = new Branch();
}
}
Complete the Branch constructor to call the Root constructor with value 10.
super(10);
What is the definition of 'Superclass'?
A class from which other classes (subclasses) inherit attributes and methods.
What is the definition of 'Subclass'?
A class that inherits attributes and methods from another class (superclass).
What is the definition of the 'super' keyword?
A keyword used to call the superclass's constructor or access its members from the subclass.
What is the definition of 'Constructor'?
A special method used to initialize objects of a class.
What is the definition of 'Inheritance'?
A mechanism where a class acquires the properties (attributes and methods) of another class.
What is the definition of 'No-argument constructor'?
A constructor that takes no arguments.
What is the definition of 'Explicit constructor call'?
A constructor call made directly in the code, using the 'super' keyword.
What is the definition of 'Implicit constructor call'?
A constructor call automatically inserted by the compiler when no explicit call is made.
What is the definition of 'Object class'?
The ultimate superclass of all classes in Java.
What is the definition of 'Instance variable'?
A variable defined in a class (i.e., a member variable) for which every object of the class has its own copy.
Why are constructors not inherited?
Constructors are specific to the class they are defined in and are responsible for initializing objects of that class.
What is the purpose of the 'super' keyword in a subclass constructor?
To call the constructor of the superclass, allowing reuse of initialization logic.
What happens if a subclass constructor doesn't explicitly call a superclass constructor?
Java automatically inserts a call to the superclass's no-argument constructor.
Why is it important to call the superclass constructor in a subclass?
To ensure that the superclass's instance variables are properly initialized.
Explain the constructor call chain in Java.
Subclass constructor calls superclass constructor, which may call its superclass constructor, and so on, up to the Object class constructor.
What is the role of the Object class in the constructor call chain?
It is the ultimate superclass, and its constructor is always the first to be called.
How does the 'super' keyword promote code reuse?
It allows subclasses to leverage the initialization logic already defined in the superclass constructor.
What is the significance of calling 'super' as the first statement in a subclass constructor?
It ensures that the superclass is initialized before any subclass-specific initialization takes place.
Explain how instance variables are initialized in a subclass when using 'super'.
Instance variables of the superclass are initialized by the superclass constructor, and subclass-specific instance variables are initialized separately in the subclass constructor.
What is the impact of not properly initializing the superclass when creating a subclass?
It can lead to unexpected behavior or errors due to uninitialized or incorrectly initialized superclass members.