Q1. What is the difference between an Inner Class and a Sub-Class?
Ans: An Inner class is a class which is nested within another class. An Inner class has access rights for the class which is nesting it and it can access all variables and methods defined in the outer class.
A sub-class is a class which inherits from another class called super class. Sub-class can access all public and protected methods and fields of its super class.
Q2. What are the various access specifiers for Java classes?
Ans: In Java, access specifiers are the keywords used before a class name which defines the access scope. The types of access specifiers for classes are:
- Public : Class, Method, Field is accessible from anywhere.
- Protected:Method, Field can be accessed from the same class to which they belong or from the sub-classes,and from the class of same package, but not from outside.
- Default: Method, Field, class can be accessed only from the same package and not from outside of it’s native package.
- Private: Method, Field can be accessed from the same class to which they belong.
Q3. What’s the purpose of Static methods and static variables?
Ans: When there is a requirement to share a method or a variable between multiple obiects of a class instead of creating separate copies for each object, we use static keyword to make a method or variable shared for all objects.
Q4. What is data encapsulation and what’s its significance?
Ans: Encapsulation is a concept in Object Oriented Programming for combining properties and methods in a single unit.
Encapsulation helps programmers to follow a modular approach for software development as each object has its own set of methods and variables and serves its functions independent of other objects. Encapsulation also serves data hiding purpose.
Q5. What is a singleton class? Give a practical example of its usage.
A singleton class in java can have only one instance and hence all its methods and variables belong to just one instance. Singleton class concept is useful for the situations when there is a need to limit the number of objects for a class.
The best example of singleton usage scenario is when there is a limit of having only one connection to a database due to some driver limitations or because of any licensing issues.
Q1. What is the difference between an Inner Class and a Sub-Class?
Ans: An Inner class is a class which is nested within another class. An Inner class has access rights for the class which is nesting it and it can access all variables and methods defined in the outer class.
A sub-class is a class which inherits from another class called super class. Sub-class can access all public and protected methods and fields of its super class.
Q2. What are the various access specifiers for Java classes?
Ans: In Java, access specifiers are the keywords used before a class name which defines the access scope. The types of access specifiers for classes are:
- Public : Class, Method, Field is accessible from anywhere.
- Protected:Method, Field can be accessed from the same class to which they belong or from the sub-classes,and from the class of same package, but not from outside.
- Default: Method, Field, class can be accessed only from the same package and not from outside of it’s native package.
- Private: Method, Field can be accessed from the same class to which they belong.
Q3. What’s the purpose of Static methods and static variables?
Ans: When there is a requirement to share a method or a variable between multiple obiects of a class instead of creating separate copies for each object, we use static keyword to make a method or variable shared for all objects.
Q4. What is data encapsulation and what’s its significance?
Ans: Encapsulation is a concept in Object Oriented Programming for combining properties and methods in a single unit.
Encapsulation helps programmers to follow a modular approach for software development as each object has its own set of methods and variables and serves its functions independent of other objects. Encapsulation also serves data hiding purpose.
Q5. What is a singleton class? Give a practical example of its usage.
A singleton class in java can have only one instance and hence all its methods and variables belong to just one instance. Singleton class concept is useful for the situations when there is a need to limit the number of objects for a class.
The best example of singleton usage scenario is when there is a limit of having only one connection to a database due to some driver limitations or because of any licensing issues.
system.out.printin(counter);
if (counter==4) {
break;
In the below example when counter reaches 4, loop jumps to next iteration and any statements after the continue keyword are skipped for current iteration.
for (counter=0;counter<10;counter++) system.out.printin(counter);
if (counter==4) {
continue;
system.out.printin(“This will not get printed when counter is 4”);
}
Q9. What is the difference between double and float variables in Java?
Ans: In java, float takes 4 bytes in memory while Double takes 8 bytes in memory. Float is single precision floating point decimal number while Double is double precision decimal number.
Q10. What is Final Keyword in Java? Give an example.
Ans: In java, a constant is declared using the keyword Final. Value can be assigned only once and after assignment, value of a constant can’t be changed.
In below example, a constant with the name const_val is declared and assigned avalue:
Private Final int const val=100
When a method is declared as final,it can NOT be overridden by the subclasses. This method are faster than any other method,because they are resolved at complied time.
When a class is declares as final,it cannot be subclassed. Example String, Integer and other wrapper classes.
Q11. What is ternary operator? Give an example.
Ans: Ternary operator, also called conditional operator is used to decide which value to assign to a variable based on a Boolean value evaluation. It’s denoted as ?
In the below example, if rank is 1, status is assigned a value of “Done” else “Pending”.
public class condition Test {
public static void main(String args|)) {
String status;
int rank = 3;
status = (rank == 1) ? “Done” : “Pending”;
System.out.printin(status);
Q12: How can you generate random numbers in Java?
Ans:
• Using Math.random() you can generate random numbers in the range greater than or equal to 0.1 and less than 1.0
Using Random class in package java.util
Q13. What is default switch case? Give example.
Ans: In a switch statement, default case is executed when no other switch condition matches.
Default case is an optional case.
It can be declared only once all other switch cases have been coded.
In the below example, when score is not 1 or 2, default case is used.
public class switchExample {
int score=4;
public static void main(String args)) {
switch (score) {
case 1:
system.out.printin(“Score is 1”); break;
case 2:
system.out.printin(“Score is 2”); break;
default:
system.out.printin(“Default Case”);
Q11. What is ternary operator? Give an example.
Ans: Ternary operator, also called conditional operator is used to decide which value to assign to a variable based on a Boolean value evaluation. It’s denoted as ?
In the below example, if rank is 1, status is assigned a value of “Done” else “Pending”.
public class condition Test {
public static void main(String args|)) {
String status;
int rank = 3;
status = (rank == 1) ? “Done” : “Pending”;
System.out.printin(status);
Q12: How can you generate random numbers in Java?
Ans:
• Using Math.random() you can generate random numbers in the range greater than or equal to 0.1 and less than 1.0
Using Random class in package java.util
Q13. What is default switch case? Give example.
Ans: In a switch statement, default case is executed when no other switch condition matches.
Default case is an optional case.
It can be declared only once all other switch cases have been coded.
In the below example, when score is not 1 or 2, default case is used.
public class switchExample {
int score=4;
public static void main(String args)) {
switch (score) {
case 1:
system.out.printin(“Score is 1”); break;
case 2:
system.out.printin(“Score is 2”); break;
default:
system.out.printin(“Default Case”);
Q20. Does Importing a package imports its sub-packages as well in Java?
Ans: In java, when a package is imported, its sub-packages aren’t imported and developer needs to import them separately if required.
For example, if a developer imports a package university.*, all classes in the package named university are loaded but no classes from the sub-package are loaded. To load the classes from its sub-package ( say department), developer has to import it explicitly as follows:
Import university.department.*
Q21. Can we declare the main method of our class as private?
Ans: In java, main method must be public static in order to run any application correctly. If main method is declared as private, developer won’t get any compilation error however, it will not get executed and will give a runtime error.
Q22. How can we pass argument to a function by reference instead of pass by value?
Ans: In java, we can pass argument to a function only by value and not by reference.
Q23. How an object is serialized in java?
Ans: In java, to convert an object into byte stream by serialization, an interface with the name Serializable is implemented by the class. All objects of a class implementing serializable interface get serialized and their state is saved in byte stream.
Q24. When we should use serialization?
Ans: Serialization is used when data needs to be transmitted over the network. Using serialization, object’s state is saved and converted into byte stream .The byte stream is transferred over the network and the object is re-created at destination.
Q25. Is it compulsory for a Try Block to be followed by a Catch Block in Java for Exception handling?
Ans: Try block needs to be followed by either Catch block or Finally block or both. Any exception thrown from try block needs to be either caught in the catch block or else any specific tasks to be performed before code abortion are put in the Finally block.
Q26. Is there any way to skip Finally block of exception even if some exception occurs in the exception block?
Ans: If an exception is raised in Try block, control passes to catch block if it exists otherwise to finally block. Finally block is always executed when an exception occurs and the only way to avoid execution of any statements in Finally block is by aborting the code forcibly by writing following line of code at the end of try block:
System.exit(0);
Q27. When the constructor of a class is invoked?
Ans: The constructor of a class is invoked every time an object is created with new keyword.
For example, in the following class two objects are created using new keyword and hence, constructor is invoked two times.
public class const example (
const example() {
system.out.printin(“Inside constructor”);
]
public static void main(String args|)) {
const _example c1=new const _example();
const_example c2=new const_example();
Q28. Can a class have multiple constructors?
Ans: Yes, a class can have multiple constructors with different parameters. Which constructor gets used for object creation depends on the arguments passed while creating the objects.
Q29. Can we override static methods of a class?
Ans: We cannot override static methods. Static methods belong to a class and not to individual objects and are resolved at the time of compilation (not at runtime.Even if we try to override static method,we will not get an complitaion error,nor the impact of overriding when running the code.
Q30. In the below example, what will be the output?
public class superclass {
public void displayResult() {
system.out.printin(“Printing from superclass”);
public class subclass extends superclass {
public void displayResult() {
system.out.printin(“Displaying from subClass”);
super.displayResult();
]
public static void main(String args|)) {
subclass obj=new subclass);
obj.displayResult();
Ans: Output will be:
Displaying from subclass
Displaying from superclass
Q31. Is String a data type in java?
Ans: String is not a primitive data type in java. When a string is created in java, it’s actually an object of Java.Lang.String class that gets created. After creation of this string object, all built-in methods of String class can be used on the string object.
Q32. In the below example, how many String Objects are created?
String s1=”I am Java Expert”;
String s2=”I am C Expert”;
String s3=”I am Java Expert”;
Ans: In the above example, two objects of Java. Lang.String class are created. s1 and s3 are references to same object.
Q33. Why Strings in Java are called as Immutable?
Ans: In java, string objects are called immutable as once value has been assigned to a string, it can’t be changed and if changed, a new object is created.
In below example, reference str refers to a string object having value “Value one”.
String str=”Value One”;
When a new value is assigned to it, a new String object gets created and the reference is moved to the new object.
str=”New Value”
Q34. What’s the difference between an array and Vector?
Ans: An array groups data of same primitive type and is static in nature while vectors are dynamic in nature and can hold data of different data types.
Q35. What is multi-threading?
Ans: Multi threading is a programming concept to run multiple tasks in a concurrent manner within a single program. Threads share same process stack and running in parallel. It helps in performance improvement of any program.
Q36. Why Runnable Interface is used in Java?
Ans: Runnable interface is used in java for implementing multi threaded applications.
Java.Lang.Runnable interface is implemented by a class to support multi threading.
Q37. What are the two ways of implementing multi-threading in Java?
Ans: Multi threaded applications can be developed in Java by using any of the following two methodologies:
1. By using Java.Lang. Runnable Interface. Classes implement this interface to enable multi threading. There is a Run() method in this interface which is implemented.