All Flashcards
What does the following code output?
java
public class Example {
String message = "Hello";
public static void main(String[] args) {
Example obj = new Example();
System.out.println(obj.message);
}
}
Hello
Identify the error in the following code:
java
public class Test {
int x = 5;
public static void main(String[] args) {
System.out.println(x);
}
}
Non-static variable x cannot be referenced from a static context.
What does the following code output?
java
public class Point {
int x, y;
public Point(int x, int y) {
this.x = x;
this.y = y;
}
public static void main(String[] args) {
Point p = new Point(3, 4);
System.out.println(p.x + "," + p.y);
}
}
3,4
Identify the error in the following code:
java
public class Circle {
private double radius;
public void Circle(double r) { // Incorrect Constructor
radius = r;
}
}
The constructor name Circle should not have a void return type.
What does the following code output?
java
public class Counter {
int count = 0;
public Counter() {
count++;
}
public static void main(String[] args) {
Counter c1 = new Counter();
Counter c2 = new Counter();
System.out.println(c1.count);
}
}
1
Identify the error in the following code:
java
public class Rectangle {
private int length;
private int width;
public Rectangle(int l, int w) {
length = l;
width = w;
}
public static void main(String[] args) {
Rectangle rect = new Rectangle(); // Error here
}
}
The Rectangle class requires two integer arguments for its constructor, but the call in main provides none.
What does the following code output?
java
public class Example {
public static void main(String[] args) {
String str = new String("Java");
System.out.println(str.length());
}
}
4
Identify the error in the following code:
java
public class MyClass {
int x = 10;
public static void main(String[] args) {
MyClass obj;
System.out.println(obj.x); // Error here
}
}
The variable obj is declared but not initialized before being used, resulting in a NullPointerException.
What does the following code output?
java
public class Test {
int a = 5;
public static void main(String[] args) {
Test t1 = new Test();
Test t2 = new Test();
t2.a = 10;
System.out.println(t1.a);
}
}
5
Identify the error in the following code:
java
public class Example {
public static void main(String[] args) {
int num = null; // Error here
System.out.println(num);
}
}
Primitive type int cannot be assigned null. null is only for reference types.
What is a class?
A blueprint for creating objects, defining data (instance variables) and behaviors (methods).
What is an object?
A specific instance of a class, holding data and capable of performing actions.
What is a reference type?
A type that stores the memory location of data, rather than the data itself.
What are instance variables?
Variables that belong to a specific object and hold its data.
What is a constructor?
A special method used to create new objects and initialize their instance variables.
Define object instantiation.
The process of creating a new object from a class using the new keyword.
What is the dot operator used for?
Used to access instance variables and methods of an object.
What is a blueprint in the context of Objects and Classes?
A class serves as a blueprint, defining the structure and behavior of objects.
What is the 'new' keyword used for?
The 'new' keyword is used to create a new instance (object) of a class.
What is the purpose of the 'this' keyword?
Refers to the current object within a method or constructor.
What are the differences between a class and an object?
Class: Blueprint, defines structure and behavior | Object: Instance of a class, actual entity with data and behavior.
What are the differences between primitive types and reference types?
Primitive: Stores actual values directly | Reference: Stores memory address of the value.