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- Question
What is the difference between an Interface and an Abstract class ?
- Correct Answer
- 1Main difference is methods of a Java interface are implicitly abstract and cannot have implementations A Java abstract class can have instance methods that implements a default behavior 2Variables declared in a Java interface is by default final An abstract class may contain non-final variables 3Members of a Java interface are public by default A Java abstract class can have the usual flavors of class members like private, protected, etc 4Java interface should be implemented using keyword ?implements?; A Java abstract class should be extended using keyword ?extends? 5An interface can extend another Java interface only, an abstract class can extend another Java class and implement multiple Java interfaces 6A Java class can implement multiple interfaces but it can extend only one abstract class 7Interface is absolutely abstract and cannot be instantiated; A Java abstract class also cannot be instantiated, but can be invoked if a main() exists 8In comparison with java abstract classes, java interfaces are slow as it requires extra indirection
- 1. Can you access non static variable in static context ? Discuss
- 2. What is autoboxing and unboxing ? Discuss
- 3. What is the output of this program ? class main_arguments { public static void main(String [ ] args) { String [][] argument = new String[2][2]; int x; argument[0] = args; x = argument[0].length; for (int y = 0; y < x; y++) System.out.print(" " + argument[0][y]); } }
Options- A. 1 1
- B. 1 0
- C. 1 0 3
- D. 1 2 3 Discuss
- 4. What?s the difference between Enumeration and Iterator interfaces ? Discuss
- 5. What do you know about the big-O notation and can you give some examples with respect to different data structures ? Discuss
- 6. Which of the following statements are equal for a variable declared in the interface ? 1. int X=10 2. public int X=10 3. public static final int X=10
Options- A. Only 1 and 2 are equal
- B. Only 2 and 3 are equal
- C. All are unequal
- D. All are equal Discuss
- 7. Inside an interface when should the variables be initialized?
Options- A. Globally
- B. Whenever required
- C. Inside a function
- D. During the time of declaration Discuss
- 8. Which of the following method declarations are allowed inside interface?
Options- A. public void m1()
- B. private void m1()
- C. protected void m1()
- D. abstract public void m1() Discuss
- 9. Why interface method is always public? Discuss
- 10. Which among the following is a scope resolution operator?
Options- A. :
- B. ::
- C. :?
- D. None Discuss
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Correct Answer: A static variable in Java belongs to its class and its value remains the same for all its instances A static variable is initialized when the class is loaded by the JVM If your code tries to access a non-static variable, without any instance, the compiler will complain, because those variables are not created yet and they are not associated with any instance
Correct Answer: Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes For example, converting an int to an Integer, a double to a Double, and so on If the conversion goes the other way, this is called unboxing
Correct Answer: 1 2 3
Explanation:
In argument[0] = args;, the reference variable arg[0], which was referring to an array with two elements, is reassigned to an array (args) with three elements.
Output:
$ javac main_arguments.java
$ java main_arguments
1 2 3
Correct Answer: Enumeration is twice as fast as compared to an Iterator and uses very less memory However, the Iterator is much safer compared to Enumeration, because other threads are not able to modify the collection object that is currently traversed by the iterator Also, Iteratorsallow the caller to remove elements from the underlying collection, something which is not possible with Enumerations
Correct Answer: The Big-O notation simply describes how well an algorithm scales or performs in the worst case scenario as the number of elements in a data structure increases The Big-O notation can also be used to describe other behavior such as memory consumption Since the collection classes are actually data structures, we usually use the Big-O notation to chose the best implementation to use, based on time, memory and performance Big-O notation can give a good indication about performance for large amounts of data
Correct Answer: All are equal
Explanation:
All the statements are equal because inside an interface a variable declared will be public static final by default
Correct Answer: During the time of declaration
Explanation:
They should be initialized during the time of declaration itself otherwise it is a compilation eror
Correct Answer: abstract public void m1()
Explanation:
Inside an interface every method will be by default abstract and public
Correct Answer: To make this method available to every implementation class
Correct Answer: ::
Explanation:
:: is the scope resolution operator
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