Interview Tips for Java Developer

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:

1. Public : Class, Method, Field is accessible from anywhere.

2. 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 the outside.

3. Default: Method, Field,class can be accessed only from the same package and not from outside of it’s native package.

4. 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 objects 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.

Q6. What are Loops in Java? What are three types of loops?

Ans: Looping is used in programming to execute a statement or a block of statement repeatedly. There are three types of loops in Java:

1) For Loops

For loops are used in Java to execute statements repeatedly for a given number of times. For loops are used when the number of times to execute the statements is known to programmer.

2) While Loops

While loops are used when certain statements need to be executed repeatedly until a condition is fulfilled. In while loops, condition is checked first before execution of statements.

3) Do While Loops

Do While Loops are the same as While loops with the only difference being that condition is checked after execution of block of statements. Hence, in case of do while loop, statements are executed at least once.

Q7. 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 a single precision floating point decimal number while Double is a double precision decimal number.

Q8. 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 the example below, 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 is faster than any other method, because it is resolved at complied time.

When a class is declared as final, it cannot be subclassed. Example String,Integer and other wrapper classes.

Q9. What’s the difference between an Abstract Class and Interface in Java?

Ans: The primary difference between an abstract class and interface is that an interface can only possess declaration of public static methods with no concrete implementation while an abstract class can have members with any access specifiers (public, private etc) with or without concrete implementation.

Another key difference in the use of abstract classes and interfaces is that a class which implements an interface must implement all the methods of the interface while a class which inherits from an abstract class doesn’t require implementation of all the methods of its super class.

A class can implement multiple interfaces but it can extend only one abstract class.

Q10. What are the performance implications of Interfaces over abstract classes?

Ans: Interfaces are slower in performance as compared to abstract classes as extra indirections are required for interfaces. Another key factor for developers to take into consideration is that any class can extend only one abstract class while a class can implement many interfaces.

Use of interfaces also puts an extra burden on the developers as any time an interface is implemented in a class; developer is forced to implement each and every method of interface.

Q11. 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.*

Q12. Can we declare the main method of our class as private?

Ans: In Java, the main method must be public static in order to run any application correctly. If the main method is declared private, the developer won’t get any compilation error . However, it will not get executed and will give a runtime error.

Q13. 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.

Q14. 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.

Q15. 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.

Q16. 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.

Q17. 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 methods, we will not get a compilation error, nor the impact of overriding when running the code.

Q18. 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.

Q19. 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.

Q20. 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.

Q21. Is it possible to define a method in Java class but provide it’s implementation in the code of another language like C?

Ans: Yes, we can do this by use of native methods. In case of native method based development, we define public static methods in our Java class without its implementation and then implementation is done in another language like C separately.

Q22. How are destructors defined in Java?

Ans: In Java, there are no destructors defined in the class as there is no need to do so. Java has its own garbage collection mechanism which does the job automatically by destroying the objects when no longer referenced.

Q23. Can a variable be local and static at the same time?

Ans: No, a variable can’t be static as well as local at the same time. Defining a local variable as static gives compilation error.

Q24. Can we have static methods in an Interface?

Ans: Static methods can’t be overridden in any class while any methods in an interface are by default abstract and are supposed to be implemented in the classes being implementing the interface. So it makes no sense to have static methods in an interface in Java.

Q25. In a class implementing an interface, can we change the value of any variable defined in the interface?

Ans: No, we can’t change the value of any variable of an interface in the implementing class as all variables defined in the interface are by default public, static and Final and final variables are like constants which can’t be changed later.

Q26. I want to re-reach and use an object once it has been garbage collected. How it’s possible?

Ans: Once an object has been destroyed by garbage collector, it no longer exists on the heap and it can’t be accessed again. There is no way to reference it again.

Q27. I want to control database connections in my program and want that only one thread should be able to make database connection at a time. How can I implement this logic?

Ans: This can be implemented by use of the concept of synchronization. Database related code can be placed in a method which hs synchronized keyword so that only one thread can access it at a time.

Q28. I want my class to be developed in such a way that no other class (even derived class) can create its objects. How can I do so?

Ans: If we declare the constructor of a class as private, it will not be accessible by any other class and hence, no other class will be able to instantiate it and formation of its object will be limited to itself only.

Q29. How are objects stored in Java?

Ans: In Java, each object when created gets a memory space from a heap. When an object is destroyed by a garbage collector, the space allocated to it from the heap is re-allocated to the heap and becomes available for any new objects.

Q30. How can we find the actual size of an object on the heap?

Ans: In Java, there is no way to find out the exact size of an object on the heap.