Object-Oriented Programming Concepts
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Questions and Answers

What represents an entity in the real world that can be distinctly identified?

An object

What is a construct that defines objects of the same type?

A class

An object is an instance of a __________.

class

The keyword __________ is required to declare a class.

<p>Class</p> Signup and view all the answers

What is invoked to create an object?

<p>A constructor</p> Signup and view all the answers

Which of the following statements are true? (Select all that apply)

<p>A default constructor is a no-arg constructor. (A), A default constructor is provided automatically if no constructors are explicitly declared in the class. (B)</p> Signup and view all the answers

Which of the following statements are true? (Select all that apply)

<p>Constructors are invoked using the new operator when an object is created. (A), Constructors do not have a return type, not even void. (B), Multiple constructors can be defined in a class. (C), Constructors must have the same name as the class itself. (D)</p> Signup and view all the answers

Analyze the following code and choose the correct statements:

public class Test {
 public static void main(String[] args) {
 A a = new A();
 a.print();
 }
}

class A {
 String s;

 A(String s) {
 this.s = s;}

 void print() {
 System.out.println(s);
 }
}

<p>The program would compile and run if you change A a = new A() to A a = new A(&quot;5&quot;). (A), The program has a compile error because class A does not have a default constructor. (B)</p> Signup and view all the answers

Analyze the following code and choose the correct statement:

class TempClass {
 int i;
 public void TempClass(int j) {
 int i = j;
 }
}

public class C {
 public static void main(String[] args) {
 TempClass temp = new TempClass(2);
 }
}

<p>The program has a compile error because TempClass does not have a constructor with an int argument. (A)</p> Signup and view all the answers

Given the declaration Circle x = new Circle(), which of the following statements is most accurate?

<p>x contains a reference to a Circle object.</p> Signup and view all the answers

What happens when Test test = null; is executed followed by System.out.println(test.x);?

<p>The program has a runtime NullPointerException because test is null while executing test.x.</p> Signup and view all the answers

The default value for data field of a boolean type, numeric type, and object type is __________, respectively.

<p>false, 0, null</p> Signup and view all the answers

Which of the following statements are true? (Select all that apply)

<p>Local variables do not have default values. (A), A variable of a reference type holds a reference to where an object is stored in the memory. (B), A variable of a primitive type holds a value of the primitive type. (C), Data fields have default values. (D)</p> Signup and view all the answers

Analyze the following code and find the reason for compile errors:

public class Test {
 public static void main(String[] args) {
 double radius;
 final double PI= 3.15169;
 double area = radius * radius * PI;
 System.out.println("Area is " + area);
 }
}

<p>The program has compile errors because the variable radius is not initialized. (A)</p> Signup and view all the answers

Analyze the following code and identify the issue:

public class Test {
 int x;

 public Test(String t) {
 System.out.println("Test");}

 public static void main(String[] args) {
 Test test = new Test();
 System.out.println(test.x);
 }
}

<p>The program has a compile error because Test does not have a default constructor. (A)</p> Signup and view all the answers

What is the outcome of compiling TestCircle.java and then Circle.java?

<p>Both compile fine.</p> Signup and view all the answers

Which of the following statements are correct? (Select all that apply)

<p>A data field in a class can be of an object type. (A), A reference variable references to an object. (B)</p> Signup and view all the answers

Which of the following code A or B, or both create an object of the Date class? (Select all that apply)

<p>public class Test { public Test() { new java.util.Date(); } } (A), public class Test { public Test() { java.util.Date date = new java.util.Date(); } } (B)</p> Signup and view all the answers

What can be placed in the blank line in the code?

public class Test {
 private int id;
 public void m1() {
 _____.id = 45;
 }
}```

<p>this</p> Signup and view all the answers

Analyze the following code and outline its behavior:

class Test {
 private double i;
 public Test(double i) {
 this.t();
 this.i = i;
 }
 public Test() {
 System.out.println("Default constructor");
 this(1);
 }
 public void t() {
 System.out.println("Invoking t");
 }
}```

<p>this.t() may be replaced by t(). (A), this(1) must be called before System.out.println(&quot;Default constructor&quot;). (B)</p> Signup and view all the answers

Analyze the following code and determine its behavior:

class Circle {
 private double radius;
 public Circle(double radius) {
 radius = radius;
 }
}```

<p>The program will compile, but you cannot create an object of Circle with a specified radius. (A), The object will always have radius 0. (B)</p> Signup and view all the answers

You can declare two variables with the same name in __________.

<p>different methods in a class</p> Signup and view all the answers

What is the output for the third statement in the main method from the following Java code?

public class Foo {
 static int i = 0;
 static int j = 0;
 public static void main(String[] args) {
 int i = 2;
 int k = 3;
 {
 int j = 3;
 System.out.println("i + j is " + i + j);
 }
 k = i + j;
 System.out.println("k is " + k);
 System.out.println("j is " + j);
 }
}```

<p>j is 0</p> Signup and view all the answers

What is the output for the second statement in the main method from the following Java code?

public class Foo {
 static int i = 0;
 static int j = 0;
 public static void main(String[] args) {
 int i = 2;
 int k = 3;
 {
 int j = 3;
 System.out.println("i + j is " + i + j);
 }
 k = i + j;
 System.out.println("k is " + k);
 System.out.println("j is " + j);
 }
}```

<p>k is 2</p> Signup and view all the answers

What is the output for the first statement in the main method from the following Java code?

public class Foo {
 static int i = 0;
 static int j = 0;
 public static void main(String[] args) {
 int i = 2;
 int k = 3;
 {
 int j = 3;
 System.out.println("i + j is " + i + j);
 }
 k = i + j;
 System.out.println("k is " + k);
 System.out.println("j is " + j);
 }
}```

<p>i + j is 23</p> Signup and view all the answers

Which of the following statements are true about an immutable object? (Select all that apply)

<p>All properties of an immutable object must be private. (A), The contents of an immutable object cannot be modified. (B), An immutable object contains no mutator methods. (C), A readable object type property in an immutable object must also be immutable. (D)</p> Signup and view all the answers

Assume java.util.Date[] dates = new java.util.Date, which of the following statements are true? (Select all that apply)

<p>dates = new java.util.Date is fine, which assigns a new array to dates. (A), dates is null. (B)</p> Signup and view all the answers

Given the declaration Circle[] x = new Circle, which of the following statements is most accurate?

<p>x contains a reference to an array and each element in the array can hold a reference to a Circle object.</p> Signup and view all the answers

What is the output of the following program:

import java.util.Date;
public class Test {
 public static void main(String[] args) {
 Date date = new Date(1234567);
 m1(date);
 System.out.print(date.getTime() + " ");
 m2(date);
 System.out.println(date.getTime());
 }
 public static void m1(Date date) {
 date = new Date(7654321);
 }
 public static void m2(Date date) {
 date.setTime(7654321);
 }
}```

<p>1234567 7654321</p> Signup and view all the answers

What is the value of times displayed?

public class Test {public static void main(String[] args) {
 Count myCount = new Count();
 int times = 0;
 for (int i=0; i

Signup and view all the answers

Study Notes

Object-Oriented Concepts

  • An object represents a real-world entity that can be distinctly identified.
  • A class is a construct that defines objects of the same type; it serves as a blueprint for creating objects.
  • An object is an instance of a class and is created using a constructor.
  • The keyword class is required to declare a class in Java.

Constructors

  • A constructor is invoked to create an object.
  • If no constructors are explicitly declared in a class, a default constructor is provided automatically.
  • Constructors do not have a return type and must share the same name as the class.
  • Multiple constructors can exist within a class.
  • Constructors are invoked using the new operator.

Code Analysis and Errors

  • Programs can fail to compile if a default constructor is absent when one is invoked.
  • Local variables do not receive default values; however, fields (data members) do have default values (e.g., boolean is false, numeric is 0, object type is null).
  • A program will compile if local variables are properly initialized, otherwise, it will result in compile-time errors.

Variable Scope and Behavior

  • You can declare variables with the same name in different methods within a class.
  • Variables within a block can overshadow class-level variables, leading to varying values based on scope.
  • Static variables retain their values across class instances and are shared among all instances.

Immutable Objects

  • An immutable object's properties cannot be modified once set.
  • All properties of immutable objects should be private, and they should not have mutator methods.
  • A readable property in an immutable object must also be immutable itself.

Arrays and References

  • When declaring an array of objects, such as Circle[] x, it indicates that x references an array where each element can point to a Circle object.
  • Assigning an array or creating an array must be done correctly to avoid null references.

Method Behaviors

  • When passing objects to methods, if the reference is reassigned, it does not affect the original object unless the object's properties are modified directly.
  • The newly assigned object inside a method does not alter the original object's reference.

Output Analysis

  • Understanding specific outputs from programming statements requires examining variable scopes, initializations, and method invocations.
  • Outputs reflect the status of static and instance variables as influenced by method executions and scope resolutions.

Code Compliance and Common Pitfalls

  • Incorrect constructor invocation leads to compile errors.
  • Failing to initialize variables before use will cause compile-time or runtime exceptions.
  • Ensure clarity in variable scope to avoid confusion surrounding local vs. class-level variables.

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Description

Test your understanding of object-oriented programming concepts with this quiz. Explore the definitions and functionalities of classes, objects, and constructors in Java. Answer questions that cover key principles and common errors associated with object creation.

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