Core Java - Interview Questions and Answers for 'Core Java' | Search Interview Question

Core Java - Interview Questions and Answers for 'Core Java' - 1261 question(s) found - Order By Rating

Q1. Why cannot we initialize a final field within the constructor if the initialization is within try block ?

Ans. What if the initialization throws an exception. In that case , it will let it move forward without initializing the final field. So it's a way to enforce that either the field is initialized or it fails completely.

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Q2. Can we initialize final fields within the constructor ? Why ?

Core Java

Ans. Yes, Because final fields needs to be initialized before the construction of the object completes. Not necessarily at the time of class loading.

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Q3. What are the main features of Java?

Core Java

Ans. a) Object Oriented
b) Simple
c) Platform Independent
d) Secured
e) Robust
f) Portable
g) Multithreaded
h) Distributed

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Q4. What are the fundamental principles of object oriented programming? / What are the OOPs concepts?

Core Java

Ans. a) Inheritance
b) Abstraction
c) Polymorphism
d) Encapsulation

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Q5. What do you mean by inheritance in java?

Core Java

Ans. Inheritance is one of the key principle of object oriented programming. Through inheritance, one class can inherit the properties of another class. The class from which properties are inherited is called super class and the class to which properties are inherited is called sub class.

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Q6. What are the differences between static and non-static methods?

Core Java

Ans. Static method is common to all instances of a class. Static methods are stored in the class memory. Whereas non-static methods are stored in the object memory. Each instance of a class will have their own copy of non-static methods.

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Q7. What are the different ways of creating threads in java?

Core Java

Ans. There are two ways to create the threads in java
a) By extending java.lang.Thread class.
b) By implementing java.lang.Runnable interface.

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Q8. What are Loops in Java? What are three types of loops?

Core Java

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 number of times to execute the statements is known to programmer.
2) While Loops
While loop is 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 Loop is same as While loop with only difference that condition is checked after execution of block of statements. Hence in case of do while loop, statements are executed at least once.

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Q9. What is the difference between double and float variables in Java?

Core 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.

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Q10. Can main() method in Java can return any data?

Core Java

Ans. In java, main() method can't return any data and hence, it's always declared with a void return type.

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Q11. What's the difference between an array and Vector?

Core Java

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.

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Q12. How to swap two numbers without using a third variable?

Core Java

Ans. int a = 10;
int b = 20;

b = b + a; // now b is sum of both the numbers
a = b - a; // b - a = (b + a) - a = b (a is swapped)
b = b - a; // (b + a) - b = a (b is swapped)

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Q13. Explain method overridding.

Core Java

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Q14. Write a program to print next largest palindrome number?

Core Java

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Q15. String processing in Java

Core Java

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Q16. What is concurrent interface ?

Core Java

Ans. ConcurrentMap is an interface and it is a member of the Java Collections Framework. It represents a Map that is capable of handling concurrent access to it without affecting the consistency of entries in a map. ConcurrentMap interface present in java.util.concurrent package.

HashMap operations are not synchronized, while Hashtable provides synchronization. Though Hashtable is thread-safe, it is not very efficient. To solve this issue, the Java Collections Framework introduced ConcurrentMap in Java 1.5.

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Q17. What are the benefits of Java over other Programming Languages ?

Core Java

Ans. 1. JVM that makes it platform independent and facilitates portability.
2. Garbage Collector that protects from Memory Leaks.
3. Only usage of references and not pointer arithmetic and hence protects the application from memory issues like dangling pointers, memory leaks etc.
4. Support for OOPs that makes application development and maintenance easy.
5. Facilitates library development through use of interface / abstract classes.
6. Facilitates creation of scalable applications.
7. Huge repository of libraries and pool of people with java skill.

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Q18. What is the different between collection and Stream Api ?

Core Java

Ans. data under collection are actually stored in memory so that they can be retrieved when needed whereas data in streams are not stored and hence we need to construct it again when needed.

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Q19. Interface vs Abstraction

Core Java

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Q20. How can we create an arraylist of unique values ?

Core Java

Ans. We can put the value in a set to enforce uniqueness and then dum those value into an arraylist.

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Q21. Why wait(), notify(), notifyAll() methods are in Object class instead of Thread class?

Core Java

Ans. wait() and notify() methods are defined in Object class rather than Thread class.If wait() and notify() were on the Thread instead then each thread would have to know the status of every other thread and there is no way to know thread1 that thread2 was waiting for any resource to access.Hence, notify, wait, notifyAll methods are defined in object class in Java.

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Q22. Can the value of a static variable be changed ?

Core Java

Ans. Yes the value of a static variable can be changed. It's the final keyword that restrict the modification of value not static.

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Q23. What are the advantages and disadvantages of Lambda Expression ?

Core Java

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Q24. Write a program to make an array act as a set.

Core Java

Ans. public Set convertArrayToList(T array[]) {
Set set = new HashSet<>(Arrays.asList(array));
return set;
}

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Q25. What is the use of defining equals , compareTo and hashcode methods in a class ? Where are they used ?

Core Java

Ans. equals, compareTo and hashcode are of use when the objects are used within collections.

Equals helps with collections that helps maintaining only unique objects ( like Set )

compare and compareTo helps with collections that helps maintaining objects in order ( TreeSet, TreeMap etc )

hascode helps with collections that facilitates hash searching ( like hashSet, hashMap etc )

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Q26. Write a Program to draw a fish ?

Core Java

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Q27. Should we do anything to avoid memory leaks in Java ?

Core Java

Ans. Java has a intrinsic way of checking leaks and reclaiming memory called garbage collection. Moreover Java doesn't support pointer arithmetics , so there is very low comparative chance of memory leaks, compared to C,C++

Though there may be tools that may audit code to make sure that we are not leaving much for the garbage collection to work but I have never heard anyone doing anything for memory leaks in Java.

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Q28. Explain Collections in Java

Core Java

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Q29. Is String mutable or immutable in Java ?

Core Java

Ans. String is Immutable because of String Constant Pool.

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Q30. What are different types of Polymorphism in Java ?

Core Java

Ans. run time polymorphism
compile time polymorphism

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