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.
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What is invoked to create an object?
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Which of the following statements are true? (Select all that apply)
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Which of the following statements are true? (Select all that apply)
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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);
}
}
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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);
}
}
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Given the declaration Circle x = new Circle(), which of the following statements is most accurate?
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What happens when Test test = null; is executed followed by System.out.println(test.x);?
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The default value for data field of a boolean type, numeric type, and object type is __________, respectively.
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Which of the following statements are true? (Select all that apply)
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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);
}
}
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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);
}
}
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What is the outcome of compiling TestCircle.java and then Circle.java?
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Which of the following statements are correct? (Select all that apply)
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Which of the following code A or B, or both create an object of the Date class? (Select all that apply)
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What can be placed in the blank line in the code?
public class Test {
private int id;
public void m1() {
_____.id = 45;
}
}```
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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");
}
}```
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Analyze the following code and determine its behavior:
class Circle {
private double radius;
public Circle(double radius) {
radius = radius;
}
}```
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You can declare two variables with the same name in __________.
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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);
}
}```
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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);
}
}```
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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);
}
}```
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Which of the following statements are true about an immutable object? (Select all that apply)
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Assume java.util.Date[] dates = new java.util.Date, which of the following statements are true? (Select all that apply)
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Given the declaration Circle[] x = new Circle, which of the following statements is most accurate?
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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);
}
}```
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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
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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 thatxreferences an array where each element can point to aCircleobject. - 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.
