Questions and Answers
What is stored in the object obj in the following lines of code? box obj;
Which of these keywords is used to create a class?
Which of the following is a valid declaration of an object of class Box?
Which of these operators is used to allocate memory for an object?
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Which of these statements is incorrect?
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What is the output of this program? class main_class { public static void main(String args[]) { int x = 9; if (x == 9) { int x = 8; System.out.println(x); } } }
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Which of the following statements is correct?
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What is the output of this program? class box { int width; int height; int length; } class mainclass { public static void main(String args[]) { box obj = new box(); obj.width = 10; obj.height = 2; obj.length = 10; int y = obj.width * obj.height * obj.length; System.out.print(y); } }
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What is the output of this program? class mainclass { public static void main(String args[]) { box obj1 = new box(); box obj2 = new box(); obj1.height = 1; obj1.length = 2; obj1.width = 1; obj2 = obj1; System.out.println(obj2.height); } }
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What is the output of this program? class box { int width; int height; int length; } class mainclass { public static void main(String args[]) { box obj = new box(); System.out.println(obj); } }
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What is the return type of a method that does not return any value?
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What is the process of defining more than one method in a class differentiated by method signature?
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Which of the following is a method having the same name as that of its class?
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Which method can be defined only once in a program?
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Which of these statements is incorrect?
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What is the output of this program? class box { int width; int height; int length; int volume; void volume(int height, int length, int width) { volume = width*height*length; } } class Prameterized_method{ public static void main(String args[]) { box obj = new box(); obj.height = 1; obj.length = 5; obj.width = 5; obj.volume(3,2,1); System.out.println(obj.volume); } }
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What is the output of this program? class equality { int x; int y; boolean isequal(){ return(x == y); } } class Output { public static void main(String args[]) { equality obj = new equality(); obj.x = 5; obj.y = 5; System.out.println(obj.isequal()); } }
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What is the output of this program? class box { int width; int height; int length; int volume; void volume() { volume = width*height*length; } } class Output { public static void main(String args[]) { box obj = new box(); obj.height = 1; obj.length = 5; obj.width = 5; obj.volume(); System.out.println(obj.volume); } }
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What is the output of this program? class Output { static void main(String args[]) { int x , y = 1; x = 10; if (x != 10 && x / 0 == 0) System.out.println(y); else System.out.println(++y); } }
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What is the output of this program? class area { int width; int length; int volume; area() { width=5; length=6; } void volume() { volume = width*length*height; } } class cons_method { public static void main(String args[]) { area obj = new area(); obj.volume(); System.out.println(obj.volume); } }
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Which of these classes is the superclass of every class in Java?
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Which of these keywords can be used to prevent inheritance of a class?
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Which of these keywords cannot be used for a class that has been declared final?
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Which of these classes relies upon its subclasses for complete implementation of its methods?
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Study Notes
Class and Object Fundamentals
- Memory allocation for an object occurs with the
newoperator; declaring a reference (e.g.,box obj;) does not allocate memory, leading to a pointer to NULL. - The keyword
classis used to define a class in Java.
Object Declaration
- The correct syntax for declaring an object of the Box class is
Box obj = new Box();.
Memory Allocation Operator
- The operator
newis used to dynamically allocate memory for an object and returns a reference to its address in memory.
Main Method Requirements
- Not every class is required to contain a
main()method; only one publicmain()method is permitted in a Java program.
Variable Naming Rules
- Two variables with the same name cannot coexist in a class; attempting to do so results in a compilation error.
Access Modifiers
- Public methods are accessible from any class in the hierarchy, allowing broader access than methods restricted to subclasses.
Program Outputs
- In an example program, a Box object's dimensions lead to an output of
200when multiplied (width: 10, height: 2, length: 10). - Assigning one object to another copies all elements, thus returning the expected value if both objects are of the same type.
Object Reference Behavior
- When an object is printed directly, it displays its memory address rather than its properties, resulting in a "Garbage value" output.
Method Return Types
- A method that does not return a value must have a return type of
void.
Function Overloading
- Function overloading allows multiple methods in a class to share the same name, provided they differ in parameters or return types.
Constructors
- A constructor is a special method that initializes an object upon creation and carries the same name as the class.
Single Main Method Rule
- The
main()method serves as the program's execution entry point and can only be defined once in any Java program.
Memory Allocation for Methods
- All objects of a class share a single copy of the methods, as they utilize memory for variables but not for methods themselves.
Checking for Equality
- An equality check method can return a boolean indicating if two properties are equal, which can be demonstrated in a simple program.
Abstract and Final Classes
- A class declared as
finalcannot be subclassed, and thus no methods can be abstract. Abstract classes, however, require subclasses for complete implementation of their methods.
Superclass in Java
- The
Objectclass is the root superclass for all classes in Java, providing foundational methods.
Inheritance Restrictions
- The
finalkeyword is used to prevent inheritance from a class, ensuring that no subclass can extend the functionality of that class.
Abstract Class Functionality
- An abstract class lacks a complete implementation and relies on its subclasses for defining concrete methods, differing from regular classes which can be fully defined.
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Description
This quiz covers the essential concepts of classes and objects in Java. Learn about memory allocation, object declaration, and access modifiers. Perfect for beginners seeking to understand Java's object-oriented programming principles.
