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Basic Java Interview Questions

Q1. Is Java platform independent?

Answer Yes. Java is a platform independent language. We can write java code on one platform and run it on another platform. For e.g. we can write and compile the code on windows and can run the generated bytecode on Linux or any other supported platform. This is one of the main features of java.

 

Q2. What all memory areas are allocated by JVM?

Answer Classloader, Class area, Heap, Stack, Program Counter Register and Native Method Stack

 

 

 

 

Q3. Java vs. C ++?

Answer

Here are the few differences between Java and C++:

  • Platform dependency – C++ is platform dependent while java is platform independent

  • No goto support – Java doesn’t support goto statement while C++ does.

  • Multiple inheritance – C++ supports multiple inheritance while java does not.

  • Multithreading – C++ does not have in-build thread support, on the other hand java supports multithreading

  • Virtual keyword – C++ has virtual keyword, it determines if a member function of a class can be overridden in its child class. In java there is no concept of virtual keyword.

Q4. Explain public static void main(String args[])

Answer 
Here public is an access modifier, which means that this method is accessible by any class.

static – static keyword tells that this method can be accessed without creating the instance of the class. 

void – this main method returns no value.

main – It is the name of the method.

  – The args is an array of String type. This contains the command line arguments that we can pass while running the program.

 

Q5. What is javac ?
Answer The javac is a compiler that compiles the source code of your program and generates bytecode. In simple words javac produces the java byte code from the source code written *.java file. JVM executes the bytecode to run the program.

Q6. What is class?
Answer A class is a blueprint or template or prototype from which you can create the object of that class. A class has set of properties and methods that are common to its objects.

 

Q7. What is the base class of all classes?
Answer java.lang.Object is the base class (super class) of all classes in java.

 

Q8. What is a wrapper class in Java?
Answer A wrapper class converts the primitive data type such as int, byte, char, boolean etc. to the objects of their respective classes such as Integer, Byte, Character, Boolean etc. 

 

 

Q9. What is a path and classPath in Java?
Answer Path specifies the location of .exe files. Classpath specifies the location of bytecode (.class files).


Q10. 
Different Data types in Java.
Answer 
byte – 8 bit

short – 16 bit

char – 16 bit Unicode

int – 32 bit (whole number)

float – 32 bit (real number)

long – 64 bit (Single precision)

double – 64 bit (double precision)

 

Q11. What is Unicode?
Answer Java uses Unicode to represent the characters. Unicode defines a fully international character set that can represent all of the characters found in human languages.

Q12. What are Literals?
Answer 
Any constant value that is assigned to a variable is called literal in Java. For example –

// Here 101 is a literal int num = 101

 

Q13. Dynamic Initialization?
Answer Dynamic initialization is process in which initialization value of a variable isn’t known at compile-time. It’s computed at runtime to initialize the variable.

 

Q14. What is Type casting in Java?
Answer When we assign a value of one data type to the different data type then these two data types may not be compatible and needs a conversion. If the data types are compatible (for example assigning int value to long) then java does automatic conversion and does not require casting. However if the data types are not compatible then they need to be casted for conversion.

For example:

//here in the brackets we have mentioned long keyword, this is casting double num = 10001.99; long num2 = (long)num;

 

Q15. What is an Array?
Answer An array is a collection (group) of fixed number of items. Array is a homogeneous data structure which means we can store multiple values of same type in an array but it can’t contain multiple values of different types. For example an array of int type can only hold integer values.


Q16. 
What is BREAK statement in java?
Answer The break statement is used to break the flow sequence in Java.

break statement is generally used with switch case data structure to come out of the statement once a case is executed.

It can be used to come out of the loop in Java

Q17. Arrays can be defined in different ways. Write them down.
Answer 
int arr[]; int[] arr;

OOPs Interview Questions

Q18. Four main principles of OOPS Concepts?
Answer

Inheritance

Polymorphism

Data Encapsulation

Abstraction


Q19. What is inheritance?
Answer The process by which one class acquires the properties and functionalities of another class is called inheritance. Inheritance brings reusability of code in a java application.

Q20. Does Java support Multiple Inheritance?
Answer When a class extends more than one classes then it is called multiple inheritance. Java doesn’t support multiple inheritance whereas C++ supports it, this is one of the difference between java and C++.


Q21. 
What is Polymorphism and what are the types of it?
Answer Polymorphism is the ability of an object to take many forms. The most common use of polymorphism in OOPs is to have more than one method with the same name in a single class. There are two types of polymorphism: static polymorphism and dynamic polymorphism. Refer these guides to understand the polymorphism concept in detail: 1) Java Polymorphism 2) Types of Polymorphism

 

 

Q22. What is method overriding in Java?
Answer When a sub class (child class) overrides the method of super class(parent class) then it is called overriding. To override a method, the signature of method in child class must match with the method signature in parent class.

 

 

 

 

Q23. Can we override a static method?
Answer No, we cannot override a static method in Java.

 

Q24. What is method overloading?
Answer When a class has more than one methods with the same name but different number, sequence or types of arguments then it is known as method overloading. 


Q25. 
Does Java support operator overloading?
Answer Operator overloading is not supported in Java.

Q26. Can we overload a method by just changing the return type and without changing the signature of method?
Answer No, We cannot do this. To overload a method, the method signature must be different, return type doesn’t play any role in method overloading.

Q27. Is it possible to overload main() method of a class?
Answer Yes, we can overload main() method in Java.


Q28. 
What is static and dynamic binding in Java?
Answer Binding refers to the linking of method call to its body. A binding that happens at compile time is known as static binding while binding at runtime is known as dynamic binding.

 

 

Q29. What is Encapsulation?
Answer Wrapping of the data and code together is known as encapsulation.

 

Q30. What is an abstract class in Java?
Answer An abstract class is a class which can’t be instantiated (we cannot create the object of abstract class), we can only extend such classes. It provides the generalised form that will be shared by all of its subclasses, leaving it to each subclass to fill in the details. We can achieve partial abstraction using abstract classes, to achieve full abstraction we use interfaces.

 

Q31. What is Interface in java?
Answer An interface is used for achieving full abstraction. A class implements an interface, thereby inheriting the abstract methods of the interface.


Q32. 
What is the difference between abstract class and interface?
Answer 
1) abstract class can have abstract and non-abstract methods. An interface can only have abstract methods.

2) An abstract class can have static methods but an interface cannot have static methods.

3) abstract class can have constructors but an interface cannot have constructors.

 

 

Q33. Name the access modifiers that can be applied to the inner classes?
Answer public ,private , abstract, final, protected.

 

Q34. What is a constructor in Java?
Answer Constructor is used for creating an instance of a class, they are invoked when an instance of class gets created. Constructor name and class name should be same and it doesn’t have a return type.

 

Q35. Can we inherit the constructors?
Answer No, we cannot inherit constructors.


Q36. 
Can we mark constructors final?
Answer No, Constructor cannot be declared final.

Q37. What is default and parameterized constructors?
Answer 
Default: Constructors with no arguments are known as default constructors, when you don’t declare any constructor in a class, compiler creates a default one automatically.

Parameterized: Constructor with arguments are known as parameterized constructors.

Q38. Can a constructor call another constructor?
Answer Yes. A constructor can call the another constructor of same class using this keyword. For e.g. this() calls the default constructor.

Note: this() must be the first statement in the calling constructor.


Q39. 
Can a constructor call the constructor of parent class?
Answer Yes. In fact it happens by default. A child class constructor always calls the parent class constructor. However we can still call it using super keyword. For e.g. super() can be used for calling super class default constructor.

Note: super() must be the first statement in a constructor.

 

Q40. THIS keyword?
Answer The this keyword is a reference to the current object.

 

 

 

Q41. Can this keyword be assigned null value?
Answer No, this keyword cannot have null values assigned to it.


Q42. 
Explain ways to pass the arguments in Java?
Answer In java, arguments can be passed as call by value – Java only supports call by value, there is no concept of call by reference in Java.

Note: super() must be the first statement in a constructor.

 

Q43. What is static variable in java?
Answer Static variables are also known as class level variables. A static variable is same for all the objects of that particular class in which it is declared.

 

Q44. What is static block?
Answer A static block gets executed at the time of class loading. They are used for initializing static variables.


Q45. 
What is a static method?
Answer Static methods can be called directly without creating the instance (Object) of the class. A static method can access all the static variables of a class directly but it cannot access non-static variables without creating instance of class.

 

Q46. Use of final keyword in Java?
Answer 
Final methods – These methods cannot be overridden by any other method.

Final variable – Constants, the value of these variable can’t be changed, its fixed.

Final class – Such classes cannot be inherited by other classes. These type of classes will be used when application required security or someone don’t want that particular class.


Q47. 
What is a Object class?
Answer 
This is a special class defined by java; all other classes are subclasses of object class. Object class is superclass of all other classes. Object class has the following methods

objectClone () – to creates a new object that is same as the object being cloned.

boolean equals(Object obj) – determines whether one object is equal to another.

finalize() – Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. A subclass overrides the finalize method to dispose of system resources or to perform other cleanup.

toString () – Returns a string representation of the object.

 


Q48. 
What are Packages in Java?
Answer A Package can be defined as a grouping of related types (classes, interfaces, enumerations and annotations).

 

Q49. What is the difference between import java.util.Date and java.util.* ?
Answer The star form (java.util.* ) includes all the classes of that package and that may increase the compilation time – especially if you import several packages. However it doesn’t have any effect run-time performance.

 

Q50. Garbage collection in java?
Answer Since objects are dynamically allocated by using the new operator, java handles the de-allocation of the memory automatically, when no references to an object exist for a long time. This whole process is called garbage collection. The whole purpose of Garbage collection is efficient memory management.

 


Q51. 
Use of finalize() method in java?
Answer finalize() method is used to free the allocated resource.

 

 

Q52. How many times does the garbage collector calls the finalize() method for an object?
Answer The garbage collector calls the finalize() method only once for an object.

 

Q53. What are two different ways to call garbage collector?
Answer System.gc() OR Runtime.getRuntime().gc().

 

Q54. Can the Garbage Collection be forced by any means?
Answer No, its not possible. you cannot force garbage collection. you can call system.gc() methods for garbage collection but it does not guarantee that garbage collection would be done.

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