All Flashcards
What does the following code output?
java
String str = "Hello";
String str2 = str;
str2 = "World";
System.out.println(str);
Hello
Identify the error in the following code:
java
public class Example {
int x;
public static void main(String[] args) {
Example obj = new Example;
System.out.println(obj.x);
}
}
Missing parentheses in the constructor call: new Example().
What does the following code output?
java
public class Test {
int num;
public Test(int num) {
this.num = num;
}
public static void main(String[] args) {
Test t1 = new Test(10);
Test t2 = t1;
t2.num = 20;
System.out.println(t1.num);
}
}
20
What does the following code output?
java
String str1 = new String("example");
String str2 = new String("example");
System.out.println(str1 == str2);
false
Identify the error in the following code:
java
public class MyClass {
private int x;
public void MyClass(int x) { // Intended to be a constructor
this.x = x;
}
public static void main(String[] args) {
MyClass obj = new MyClass(5);
System.out.println(obj.x);
}
}
The 'constructor' MyClass(int x) is actually a method because it has a return type (void is implied). This means the value of x is never properly initialized, and x will default to 0, leading to a compilation error as the constructor is never called.
What does the following code output?
java
public class Example {
public static void main(String[] args) {
String s1 = "Hello";
String s2 = "Hello";
System.out.println(s1 == s2);
}
}
true
What will be the output of this code?
java
public class Example {
public static void main(String[] args) {
String str = "Coding";
System.out.println(str.substring(2, 4));
}
}
di
What does the following code output?
java
public class Test {
int x = 5;
public static void main(String[] args) {
Test t1 = new Test();
Test t2 = new Test();
t1.x = 10;
System.out.println(t2.x);
}
}
5
What does the following code output?
java
public class Example {
public static void main(String[] args) {
String s = "Hello World";
System.out.println(s.indexOf("World"));
}
}
6
What is wrong with the following code?
java
public class Example {
public static void main(String[] args) {
Math m = new Math();
System.out.println(m.sqrt(25));
}
}
You cannot instantiate the Math class. All methods in the Math class are static, so you should call them using Math.sqrt(25).
What is the role of a class in object creation?
A class serves as a blueprint, defining the structure and behavior of objects.
What does the new keyword do?
It's used to create a new object (instance) of a class, calling the constructor.
Explain the difference between a class and an object.
A class is a blueprint, while an object is a specific instance of that blueprint.
What is the significance of objects being reference types?
Objects store memory locations rather than actual values, affecting how they are handled in memory and during assignment.
What is the purpose of a constructor?
To initialize the state of an object when it is created.
What are the key principles of OOP?
Abstraction, Encapsulation, Inheritance, and Polymorphism.
What is the role of methods in a class?
Methods define the behaviors or actions that an object can perform.
Why is understanding objects important in Java?
Because Java is an object-oriented language, and objects are fundamental building blocks.
What does it mean for an object to be an instance of a class?
It means the object is a specific realization or example created from the class's blueprint.
What happens when you assign one object to another?
You are copying the reference (memory address), not creating a new, independent object.
What is the definition of Object-Oriented Programming (OOP)?
A programming paradigm based on the concept of 'objects', which contain data in the form of fields (attributes) and code in the form of procedures (methods).
What is the definition of a Class?
A blueprint or template for creating objects. It defines the characteristics and behaviors that objects of that class will have.
What is the definition of an Object?
A specific instance of a class. It's a concrete entity created from a class blueprint.
What is the definition of a Constructor?
A special method used to create and initialize objects of a class.
What is the definition of a Method?
A function associated with an object, defining its behavior.
What is the definition of a Reference Type?
A data type that stores the memory address of an object, rather than the object's actual value.
What is the definition of Abstraction?
Showing only essential attributes and hiding unnecessary information.
What is the definition of Encapsulation?
Bundling of data and methods that operate on that data within a class, and restricting direct access to some of the object's components.
What is the definition of Inheritance?
A mechanism where a new class inherits properties and behaviors from an existing class.
What is the definition of Polymorphism?
The ability of an object to take on many forms.