Questions and Answers
Object-oriented programming allows you to derive new classes from existing classes. This is called __________.
inheritance
Which of the following statements are true? (Choose more than one)
What is the output of the following code? public class Test1 { public static void main(String[] args) { ChildClass c = new ChildClass(); c.print(); } } class ParentClass { int id = 1; void print() { System.out.println(id); } } class ChildClass extends ParentClass { int id = 2; }
1
Analyze the following code: class Square extends GeometricObject { double length; Square(double length) { GeometricObject(length); } } What is true?
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Analyze the following code: public class A extends B { } class B { public B(String s) { } } (Choose more than one)
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Analyze the following code: public class Test extends A { public static void main(String[] args) { Test t = new Test(); t.print(); } } class A { String s; A(String s) { this.s = s; } public void print() { System.out.println(s); } } (Choose more than one)
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What is the output of running class C? class A { public A() { System.out.println("The default constructor of A is invoked"); } } class B extends A { public B() { System.out.println("The default constructor of B is invoked"); } } public class C { public static void main(String[] args) { B b = new B(); } }
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Which of the following is incorrect?
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Study Notes
Object-Oriented Programming Concepts
- Object-oriented programming supports the creation of new classes from existing classes, a process known as inheritance.
- A subclass is typically enhanced with more functionality and detail than its superclass.
Constructor Mechanics
- In Java, the statement
class A extends B
signifies thatA
is a subclass ofB
. - If a subclass does not define a constructor, the default constructor attempts to call its superclass’s default constructor. If the superclass lacks a default constructor, a compilation error occurs.
Code Behavior and Output
- In a code example with overlapping variable names in subclasses, the method in the superclass is utilized. Thus, when calling
print()
fromChildClass
, the output will reflect theParentClass
variable scope, displaying the parent's id. - The incorrect invocation of a superclass constructor may lead to compilation errors, emphasizing the importance of constructor chaining.
Common Compilation Errors
- Attempting to invoke an unknown or unsupported constructor results in a compile-time error. If constructor signatures do not match, the Java compiler will flag this as an issue.
- To resolve errors related to constructors, ensure that subclasses correctly call the superclass constructors using
super()
.
Constructor Characteristics
- Constructors cannot be declared as static; they must be instance methods.
- Private constructors can restrict instance creation, often used in singleton patterns.
- Constructors can call other constructors (overloaded ones) within the same class or through superclass constructors using
this()
orsuper()
respectively.
Execution Flow
- When an instance of a subclass is created, the superclass's constructor runs first, initializing inherited fields before the subclass's constructor executes. This results in cascading constructor calls.
General Misconceptions
- Some statements about constructors are incorrect; it is essential to understand their capabilities and limitations to avoid logical errors in Java programs.
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Description
Test your knowledge of object-oriented programming concepts from Chapter 11 of 'Intro to Java Programming, Ninth Edition'. Focus on key terms such as inheritance and subclass relationships. This quiz includes multiple-choice questions to reinforce your understanding.