In Java, which of the following statements is/are True? S1: The ‘final’ keyword applied to a class definition prevents the class from being extended through derivation. S2: A class can only inherit one class but can implement multiple interfaces. S3: Java permits a class to replace the implementation of a method that it has inherited. It is called method overloading.
S1 and S2 only
Let's analyze each statement regarding Java programming concepts like the final keyword, inheritance, interfaces, and method overriding vs. overloading.
Understanding Java Statements: Final Keyword, Inheritance, and Method Concepts
The question asks us to determine the truthfulness of three statements about Java. We will examine each statement individually.
Analyzing Statement S1: Final Keyword on a Class
Statement S1 says: The ‘final’ keyword applied to a class definition prevents the class from being extended through derivation.
- In Java, when the final keyword is applied to a class declaration (e.g., final class MyClass {...}), it signifies that this class cannot be subclassed or inherited by any other class.
- Attempting to extend a final class will result in a compile-time error.
- This is a core use case for the final keyword in Java, often used for security reasons, performance optimizations, or to ensure the integrity of the class's behavior.
Based on the definition of the final keyword applied to a class in Java, Statement S1 is True.
Analyzing Statement S2: Class Inheritance and Interface Implementation
Statement S2 says: A class can only inherit one class but can implement multiple interfaces.
- Java supports single inheritance for classes. This means a class can directly inherit from only one parent class using the extends keyword. For example, class Child extends Parent {...} is allowed, but class Child extends Parent1, Parent2 {...} is not.
- However, Java allows a class to implement multiple interfaces. An interface defines a contract (method signatures), and a class can agree to fulfill the contracts of several interfaces. This is achieved using the implements keyword, followed by a comma-separated list of interfaces (e.g., class MyClass implements Interface1, Interface2 {...}).
- This design choice (single inheritance for classes, multiple implementation for interfaces) helps avoid the complexities and ambiguities associated with multiple inheritance of implementation (like the "diamond problem") while still providing the benefits of polymorphism through interfaces.
Based on Java's rules for inheritance and interface implementation, Statement S2 is True.
Analyzing Statement S3: Method Overloading vs. Method Overriding
Statement S3 says: Java permits a class to replace the implementation of a method that it has inherited. It is called method overloading.
- The first part of the statement is true: Java does permit a subclass to provide a specific implementation for a method that is already provided by one of its parent classes. This mechanism allows subclasses to specialize or modify the behavior inherited from their superclasses.
- This concept is specifically known as method overriding. A method in a subclass overrides a method in its superclass if it has the same name, parameters (signature), and return type (or a covariant return type).
- The second part of the statement is incorrect: This process is called method overriding, not method overloading. Method overloading, on the other hand, occurs when multiple methods within the same class have the same name but different parameters. Overloading allows a class to have multiple methods with the same name performing similar operations but handling different types or numbers of arguments.
Since Statement S3 incorrectly identifies method overriding as method overloading, Statement S3 is False.
Summary of Statement Analysis
Let's summarize our findings:
- Statement S1: True
- Statement S2: True
- Statement S3: False
We are looking for the option that reflects S1 and S2 are true, and S3 is false.
| Statement | Truth Value | Explanation |
|---|---|---|
| S1: Final keyword on class prevents extension | True | A final class cannot be inherited. |
| S2: Single class inheritance, multiple interface implementation | True | Java classes support single inheritance and can implement any number of interfaces. |
| S3: Replacing inherited method is overloading | False | Replacing an inherited method's implementation is called method overriding, not overloading. |
Based on this analysis, only statements S1 and S2 are true.
Final Answer Selection
Comparing our findings with the given options:
- Option 1: S1 and S2 only (Matches our findings)
- Option 2: S1 and S3 only (S3 is false)
- Option 3: S2 and S3 only (S3 is false)
- Option 4: All of S1, S2 and S3 (S3 is false)
Therefore, the correct option is the one stating that S1 and S2 are true only.
Revision Table: Java Concepts
| Concept | Description | Keyword |
|---|---|---|
| Final Class | A class declared with the final keyword. Cannot be subclassed. | final |
| Inheritance | Mechanism where one class acquires the properties (fields and methods) of another class. Single inheritance for classes in Java. | extends |
| Interface | A blueprint of a class. It has static constants and abstract methods. A class can implement multiple interfaces. | interface, implements |
| Method Overriding | Providing a specific implementation of a method that is already defined in the parent class, within the subclass. Same method signature. | Inheritance, Polymorphism |
| Method Overloading | Defining multiple methods in the same class with the same name but different parameter lists. | Polymorphism |
Additional Information: Deep Dive into Related Java Topics
Java Inheritance Types
Java supports several types of inheritance through classes and interfaces:
- Single Inheritance: A class inherits from one parent class (e.g., Class B extends Class A).
- Multiple Inheritance: A class inherits from multiple parent classes (Not directly supported for classes in Java to avoid complexity).
- Hierarchical Inheritance: Multiple subclasses inherit from a single parent class (e.g., Class B extends Class A, Class C extends Class A).
- Multilevel Inheritance: A class inherits from a derived class (e.g., Class C extends Class B, Class B extends Class A).
- Hybrid Inheritance: A mix of two or more types (Can be achieved in Java using classes and interfaces).
While multiple inheritance of implementation is not allowed for classes, multiple inheritance of type is achieved through implementing multiple interfaces.
Interfaces vs. Abstract Classes
Both interfaces and abstract classes are used to achieve abstraction and polymorphism in Java, but they have key differences:
- Interfaces: Can only contain abstract methods (before Java 8), default and static methods (since Java 8), and private methods (since Java 9). Variables are implicitly public static final. A class can implement multiple interfaces.
- Abstract Classes: Can contain both abstract and concrete (implemented) methods. Variables can be of any access modifier. A class can extend only one abstract class.
Interfaces define a contract for what a class must do, while abstract classes provide a common base for related classes, potentially including some shared behavior.
Method Overriding vs. Method Overloading Explained
It's crucial to distinguish between method overriding and method overloading:
- Method Overriding: Relates to polymorphism and inheritance. A subclass provides its own implementation of a method that is already defined in its superclass. The method signature (name and parameter types) must be the same. The return type must be the same or covariant.
- Method Overloading: Relates to polymorphism within a single class or across the inheritance hierarchy. Multiple methods have the same name but different parameter lists (different number, types, or order of parameters). The return type can be the same or different, but overloading is determined solely by the method signature (parameters).
Overriding occurs at runtime (runtime polymorphism), while overloading is resolved at compile time (compile-time polymorphism).
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When an applet begins, which of the following sequence methods is CORRECT?
1) start()
2) paint()
3) init()
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class test
{public static void main (String args [ ])
{
System.out.println(20 + 30 + "Java");
System.out.println("Java" + 20 + 30);
}
}
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