Java Basic Interview Questions

1. Why is Java a platform independent language?

Java language was developed in such a way that it does not depend on any hardware or software due to the fact that the compiler compiles the code and then converts it to platform-independent byte code which can be run on multiple systems.

  • The only condition to run that byte code is for the machine to have a runtime environment (JRE) installed in it.

2. Why is Java not a pure object oriented language?

Java supports primitive data types – byte, boolean, char, short, int, float, long, and double and hence it is not a pure object-oriented language.

3. Pointers are used in C/ C++. Why does Java not make use of pointers?

Pointers are quite complicated and unsafe to use by beginner programmers. Java focuses on code simplicity, and the usage of pointers can make it challenging. Pointer utilization can also cause potential errors. Moreover, security is also compromised if pointers are used because the users can directly access memory with the help of pointers.

Thus, a certain level of abstraction is furnished by not including pointers in Java. Moreover, the usage of pointers can make the procedure of garbage collection quite slow and erroneous. Java makes use of references as these cannot be manipulated, unlike pointers.

4. What do you understand by an instance variable and a local variable?

Instance variables are those variables that are accessible by all the methods in the class. They are declared outside the methods and inside the class. These variables describe the properties of an object and remain bound to it at any cost.

All the objects of the class will have their copy of the variables for utilization. If any modification is done on these variables, then only that instance will be impacted by it, and all other class instances continue to remain unaffected.

Example:

class Athlete {
public String athleteName;
public double athleteSpeed;
public int athleteAge;
}

Local variables are those variables present within a block, function, or constructor and can be accessed only inside them. The utilization of the variable is restricted to the block scope. Whenever a local variable is declared inside a method, the other class methods don’t have any knowledge about the local variable.

Example:

public void athlete() {
String athleteName;
double athleteSpeed;
int athleteAge;
}

5. What do you mean by data encapsulation?

  • Data Encapsulation is an Object-Oriented Programming concept of hiding the data attributes and their behaviors in a single unit.
  • It helps developers to follow modularity while developing software by ensuring that each object is independent of other objects by having its own methods, attributes, and functionalities.
  • It is used for the security of the private properties of an object and hence serves the purpose of data hiding.

6. Tell us something about JIT compiler.

  • JIT stands for Just-In-Time and it is used for improving the performance during run time. It does the task of compiling parts of byte code having similar functionality at the same time thereby reducing the amount of compilation time for the code to run.
  • The compiler is nothing but a translator of source code to machine-executable code. But what is special about the JIT compiler? Let us see how it works:
    • First, the Java source code (.java) conversion to byte code (.class) occurs with the help of the javac compiler.
    • Then, the .class files are loaded at run time by JVM and with the help of an interpreter, these are converted to machine understandable code.
    • JIT compiler is a part of JVM. When the JIT compiler is enabled, the JVM analyzes the method calls in the .class files and compiles them to get more efficient and native code. It also ensures that the prioritized method calls are optimized.
    • Once the above step is done, the JVM executes the optimized code directly instead of interpreting the code again. This increases the performance and speed of the execution.

7. Can you tell the difference between equals() method and equality operator (==) in Java?

equals() ==
This is a method defined in the Object class. It is a binary operator in Java.
This method is used for checking the equality of contents between two objects as per the specified business logic.This operator is used for comparing addresses (or references), i.e checks if both the objects are pointing to the same memory location.

Note:

  • In the cases where the equals method is not overridden in a class, then the class uses the default implementation of the equals method that is closest to the parent class.
  • Object class is considered as the parent class of all the java classes. The implementation of the equals method in the Object class uses the == operator to compare two objects. This default implementation can be overridden as per the business logic.

8. How is an infinite loop declared in Java?

Infinite loops are those loops that run infinitely without any breaking conditions. Some examples of consciously declaring infinite loop is:

  • Using For Loop:
for (;;)
{
   // Business logic
   // Any break logic
}
  • Using while loop:
while(true){
   // Business logic
   // Any break logic
}
  • Using do-while loop:
do{
   // Business logic
   // Any break logic
}while(true);

9. Briefly explain the concept of constructor overloading

Constructor overloading is the process of creating multiple constructors in the class consisting of the same name with a difference in the constructor parameters. Depending upon the number of parameters and their corresponding types, distinguishing of the different types of constructors is done by the compiler.

class Hospital {
int variable1, variable2;
double variable3;
public Hospital(int doctors, int nurses) {
 variable1 = doctors;
 variable2 = nurses;
}
public Hospital(int doctors) {
 variable1 = doctors;
}
public Hospital(double salaries) {
 variable3 = salaries
}
}

Three constructors are defined here but they differ on the basis of parameter type and their numbers.

10. Comment on method overloading and overriding by citing relevant examples.

In Java, method overloading is made possible by introducing different methods in the same class consisting of the same name. Still, all the functions differ in the number or type of parameters. It takes place inside a class and enhances program readability.

The only difference in the return type of the method does not promote method overloading. The following example will furnish you with a clear picture of it.

class OverloadingHelp {
   public int findarea (int l, int b) {
           int var1;
           var1 = l * b;
           return var1;
   }
   public int findarea (int l, int b, int h) {
           int var2;
           var2 = l * b * h;
           return var2;
   }
}

Both the functions have the same name but differ in the number of arguments. The first method calculates the area of the rectangle, whereas the second method calculates the area of a cuboid.

Method overriding is the concept in which two methods having the same method signature are present in two different classes in which an inheritance relationship is present. A particular method implementation (already present in the base class) is possible for the derived class by using method overriding.
Let’s give a look at this example:

class HumanBeing {
       public int walk (int distance, int time) {
               int speed = distance / time;
               return speed;
       }
}
class Athlete extends HumanBeing {
       public int walk(int distance, int time) {
               int speed = distance / time;
               speed = speed * 2;
               return speed;
       }
}

Both class methods have the name walk and the same parameters, distance, and time. If the derived class method is called, then the base class method walk gets overridden by that of the derived class.

11. A single try block and multiple catch blocks can co-exist in a Java Program. Explain.

Yes, multiple catch blocks can exist but specific approaches should come prior to the general approach because only the first catch block satisfying the catch condition is executed. The given code illustrates the same:

public class MultipleCatch {
public static void main(String args[]) {
 try {
  int n = 1000, x = 0;
  int arr[] = new int[n];
  for (int i = 0; i <= n; i++) {
   arr[i] = i / x;
  }
 }
 catch (ArrayIndexOutOfBoundsException exception) {
  System.out.println("1st block = ArrayIndexOutOfBoundsException");
 }
 catch (ArithmeticException exception) {
  System.out.println("2nd block = ArithmeticException");
 }
 catch (Exception exception) {
  System.out.println("3rd block = Exception");
 }
}
}

Here, the second catch block will be executed because of division by 0 (i / x). In case x was greater than 0 then the first catch block will execute because for loop runs till i = n and array index are till n-1.

12. Explain the use of final keyword in variable, method and class.

In Java, the final keyword is used as defining something as constant /final and represents the non-access modifier.

  • final variable:
    • When a variable is declared as final in Java, the value can’t be modified once it has been assigned.
    • If any value has not been assigned to that variable, then it can be assigned only by the constructor of the class.
  • final method:
    • A method declared as final cannot be overridden by its children’s classes.
    • A constructor cannot be marked as final because whenever a class is inherited, the constructors are not inherited. Hence, marking it final doesn’t make sense. Java throws compilation error saying – modifier final not allowed here
  • final class:
    • No classes can be inherited from the class declared as final. But that final class can extend other classes for its usage.

13. Do final, finally and finalize keywords have the same function?

All three keywords have their own utility while programming.

Final: If any restriction is required for classes, variables, or methods, the final keyword comes in handy. Inheritance of a final class and overriding of a final method is restricted by the use of the final keyword. The variable value becomes fixed after incorporating the final keyword. Example:

final int a=100;
a = 0;  // error

The second statement will throw an error.

Finally: It is the block present in a program where all the codes written inside it get executed irrespective of handling of exceptions. Example:

try {
int variable = 5;
}
catch (Exception exception) {
System.out.println("Exception occurred");
}
finally {
System.out.println("Execution of finally block");
}

Finalize: Prior to the garbage collection of an object, the finalize method is called so that the clean-up activity is implemented. Example:

public static void main(String[] args) {
String example = new String("InterviewBit");
example = null;
System.gc(); // Garbage collector called
}
public void finalize() {
// Finalize called
} 

14. When can you use super keyword?

  • The super keyword is used to access hidden fields and overridden methods or attributes of the parent class.
  • Following are the cases when this keyword can be used:
    • Accessing data members of parent class when the member names of the class and its child subclasses are same.
    • To call the default and parameterized constructor of the parent class inside the child class.
    • Accessing the parent class methods when the child classes have overridden them.
  • The following example demonstrates all 3 cases when a super keyword is used.
public class Parent{
       protected int num = 1;
       
       Parent(){
           System.out.println("Parent class default constructor.");
       }
       
       Parent(String x){
           System.out.println("Parent class parameterised constructor.");
       }
       
       public void foo(){
           System.out.println("Parent class foo!");
       }
   }
   
   public class Child extends Parent{
       private int num = 2;
       
       Child(){
           System.out.println("Child class default Constructor");
           
           super();    // to call default parent constructor
           super("Call Parent");    // to call parameterised constructor.
       }
       
       void printNum(){
           System.out.println(num);
           System.out.println(super.num); //prints the value of num of parent class
       }
       
       @Override
       public void foo(){
           System.out.println("Parent class foo!");
           super.foo();    //Calls foo method of Parent class inside the Overriden foo method of Child class.
       }
   }

15. Can the static methods be overloaded?

Yes! There can be two or more static methods in a class with the same name but differing input parameters.

16. Can the static methods be overridden?

  • No! Declaration of static methods having the same signature can be done in the subclass but run time polymorphism can not take place in such cases.
  • Overriding or dynamic polymorphism occurs during the runtime, but the static methods are loaded and looked up at the compile time statically. Hence, these methods cant be overridden.

17. What is the main objective of garbage collection?

The main objective of this process is to free up the memory space occupied by the unnecessary and unreachable objects during the Java program execution by deleting those unreachable objects.

  • This ensures that the memory resource is used efficiently, but it provides no guarantee that there would be sufficient memory for the program execution.

18. What part of memory – Stack or Heap – is cleaned in garbage collection process?

Heap.

Java Intermediate Interview Questions

19. Apart from the security aspect, what are the reasons behind making strings immutable in Java?

A String is made immutable due to the following reasons:

  • String Pool: Designers of Java were aware of the fact that String data type is going to be majorly used by the programmers and developers. Thus, they wanted optimization from the beginning. They came up with the notion of using the String pool (a storage area in Java heap) to store the String literals. They intended to decrease the temporary String object with the help of sharing. An immutable class is needed to facilitate sharing. The sharing of the mutable structures between two unknown parties is not possible. Thus, immutable Java String helps in executing the concept of String Pool.
  • Multithreading: The safety of threads regarding the String objects is an important aspect in Java. No external synchronization is required if the String objects are immutable. Thus, a cleaner code can be written for sharing the String objects across different threads. The complex process of concurrency is facilitated by this method.
  • Collections: In the case of Hashtables and HashMaps, keys are String objects. If the String objects are not immutable, then it can get modified during the period when it resides in the HashMaps. Consequently, the retrieval of the desired data is not possible. Such changing states pose a lot of risks. Therefore, it is quite safe to make the string immutable.

20. How would you differentiate between a String, StringBuffer, and a StringBuilder?

  • Storage area: In string, the String pool serves as the storage area. For StringBuilder and StringBuffer, heap memory is the storage area.
  • Mutability: A String is immutable, whereas both the StringBuilder and StringBuffer are mutable.
  • Efficiency: It is quite slow to work with a String. However, StringBuilder is the fastest in performing operations. The speed of a StringBuffer is more than a String and less than a StringBuilder. (For example appending a character is fastest in StringBuilder and very slow in String because a new memory is required for the new String with appended character.)
  • Thread-safe: In the case of a threaded environment, StringBuilder and StringBuffer are used whereas a String is not used. However, StringBuilder is suitable for an environment with a single thread, and a StringBuffer is suitable for multiple threads.
    Syntax:
// String
String first = "InterviewBit";
String second = new String("InterviewBit");
// StringBuffer
StringBuffer third = new StringBuffer("InterviewBit");
// StringBuilder
StringBuilder fourth = new StringBuilder("InterviewBit");

21. Using relevant properties highlight the differences between interfaces and abstract classes.

  • Availability of methods: Only abstract methods are available in interfaces, whereas non-abstract methods can be present along with abstract methods in abstract classes.
  • Variable types: Static and final variables can only be declared in the case of interfaces, whereas abstract classes can also have non-static and non-final variables.
  • Inheritance: Multiple inheritances are facilitated by interfaces, whereas abstract classes do not promote multiple inheritances.
  • Data member accessibility: By default, the class data members of interfaces are of the public- type. Conversely, the class members for an abstract class can be protected or private also.
  • Implementation: With the help of an abstract class, the implementation of an interface is easily possible. However, the converse is not true;

Abstract class example:

public abstract class Athlete {
public abstract void walk();
}

Interface example:

public interface Walkable {
void walk();
}

22. In Java, static as well as private method overriding is possible. Comment on the statement.

The statement in the context is completely False. The static methods have no relevance with the objects, and these methods are of the class level. In the case of a child class, a static method with a method signature exactly like that of the parent class can exist without even throwing any compilation error.

The phenomenon mentioned here is popularly known as method hiding, and overriding is certainly not possible. Private method overriding is unimaginable because the visibility of the private method is restricted to the parent class only. As a result, only hiding can be facilitated and not overriding.

23. What makes a HashSet different from a TreeSet?

Although both HashSet and TreeSet are not synchronized and ensure that duplicates are not present, there are certain properties that distinguish a HashSet from a TreeSet.

  • Implementation: For a HashSet, the hash table is utilized for storing the elements in an unordered manner. However, TreeSet makes use of the red-black tree to store the elements in a sorted manner.
  • Complexity/ Performance: For adding, retrieving, and deleting elements, the time amortized complexity is O(1) for a HashSet. The time complexity for performing the same operations is a bit higher for TreeSet and is equal to O(log n). Overall, the performance of HashSet is faster in comparison to TreeSet.
  • Methods: hashCode() and equals() are the methods utilized by HashSet for making comparisons between the objects. Conversely, compareTo() and compare() methods are utilized by TreeSet to facilitate object comparisons.
  • Objects type: Heterogeneous and null objects can be stored with the help of HashSet. In the case of a TreeSet, runtime exception occurs while inserting heterogeneous objects or null objects.

24. Why is the character array preferred over string for storing confidential information?

In Java, a string is basically immutable i.e. it cannot be modified. After its declaration, it continues to stay in the string pool as long as it is not removed in the form of garbage. In other words, a string resides in the heap section of the memory for an unregulated and unspecified time interval after string value processing is executed.

As a result, vital information can be stolen for pursuing harmful activities by hackers if a memory dump is illegally accessed by them. Such risks can be eliminated by using mutable objects or structures like character arrays for storing any variable. After the work of the character array variable is done, the variable can be configured to blank at the same instant. Consequently, it helps in saving heap memory and also gives no chance to the hackers to extract vital data.

25. What are the differences between JVM, JRE and JDK in Java?

CriteriaJDK JREJVM
AbbreviationJava Development KitJava Runtime EnvironmentJava Virtual Machine
DefinitionJDK is a complete software development kit for developing Java applications. It comprises JRE, JavaDoc, compiler, debuggers, etc.JRE is a software package providing Java class libraries, JVM and all the required components to run the Java applications.JVM is a platform-dependent, abstract machine comprising of 3 specifications – document describing the JVM implementation requirements, computer program meeting the JVM requirements and instance object for executing the Java byte code and provide the runtime environment for execution.
Main PurposeJDK is mainly used for code development and execution.JRE is mainly used for environment creation to execute the code.JVM provides specifications for all the implementations to JRE.
Tools providedJDK provides tools like compiler, debuggers, etc for code developmentJRE provides libraries and classes required by JVM to run the program.JVM does not include any tools, but instead, it provides the specification for implementation.
SummaryJDK = (JRE) + Development toolsJRE = (JVM) + Libraries to execute the applicationJVM = Runtime environment to execute Java byte code.

26. What are the differences between HashMap and HashTable in Java?

HashMapHashTable
HashMap is not synchronized thereby making it better for non-threaded applications.HashTable is synchronized and hence it is suitable for threaded applications.
Allows only one null key but any number of null in the values.This does not allow null in both keys or values.
Supports order of insertion by making use of its subclass LinkedHashMap.Order of insertion is not guaranteed in HashTable.

27. What is the importance of reflection in Java?

  • The term reflection is used for describing the inspection capability of a code on other code either of itself or of its system and modify it during runtime.
  • Consider an example where we have an object of unknown type and we have a method ‘fooBar()’ which we need to call on the object. The static typing system of Java doesn’t allow this method invocation unless the type of the object is known beforehand. This can be achieved using reflection which allows the code to scan the object and identify if it has any method called “fooBar()” and only then call the method if needed.
Method methodOfFoo = fooObject.getClass().getMethod("fooBar", null);
methodOfFoo.invoke(fooObject, null);
  • Using reflection has its own cons:
    • Speed — Method invocations due to reflection are about three times slower than the direct method calls.
    • Type safety — When a method is invoked via its reference wrongly using reflection, invocation fails at runtime as it is not detected at compile/load time.
    • Traceability — Whenever a reflective method fails, it is very difficult to find the root cause of this failure due to a huge stack trace. One has to deep dive into the invoke() and proxy() method logs to identify the root cause.
  • Hence, it is advisable to follow solutions that don’t involve reflection and use this method as a last resort.

28. What are the different ways of threads usage?

  • We can define and implement a thread in java using two ways:
    • Extending the Thread class
class InterviewBitThreadExample extends Thread{  
   public void run(){  
       System.out.println("Thread runs...");  
   }  
   public static void main(String args[]){  
       InterviewBitThreadExample ib = new InterviewBitThreadExample();  
       ib.start();  
   }  
}
  • Implementing the Runnable interface
class InterviewBitThreadExample implements Runnable{  
   public void run(){  
       System.out.println("Thread runs...");  
   }  
   public static void main(String args[]){  
       Thread ib = new Thread(new InterviewBitThreadExample()); 
       ib.start();  
   }  
}
  • Implementing a thread using the method of Runnable interface is more preferred and advantageous as Java does not have support for multiple inheritances of classes.
  • start() method is used for creating a separate call stack for the thread execution. Once the call stack is created, JVM calls the run() method for executing the thread in that call stack.

29. What are the differences between constructor and method of a class in Java?

ConstructorMethod
Constructor is used for initializing the object state.Method is used for exposing the object’s behavior.
Constructor has no return type.Method should have a return type. Even if it does not return anything, return type is void.
Constructor gets invoked implicitly.Method has to be invoked on the object explicitly.
If the constructor is not defined, then a default constructor is provided by the java compiler.If a method is not defined, then the compiler does not provide it.
The constructor name should be equal to the class name.The name of the method can have any name or have a class name too.
A constructor cannot be marked as final because whenever a class is inherited, the constructors are not inherited. Hence, marking it final doesn’t make sense. Java throws compilation error saying – modifier final not allowed hereA method can be defined as final but it cannot be overridden in its subclasses.
Final variable instantiations are possible inside a constructor and the scope of this applies to the whole class and its objects.A final variable if initialised inside a method ensures that the variable cant be changed only within the scope of that method.

30. Java works as “pass by value” or “pass by reference” phenomenon?

Java always works as a “pass by value”. There is nothing called a “pass by reference” in Java. However, when the object is passed in any method, the address of the value is passed due to the nature of object handling in Java. When an object is passed, a copy of the reference is created by Java and that is passed to the method. The objects point to the same memory location. 2 cases might happen inside the method:

  • Case 1: When the object is pointed to another location: In this case, the changes made to that object do not get reflected the original object before it was passed to the method as the reference points to another location.

For example:

class InterviewBitTest{
   int num;
   InterviewBitTest(int x){ 
       num = x; 
   }
   InterviewBitTest(){ 
       num = 0; 
   }
}
class Driver {
   public static void main(String[] args)
   {
       //create a reference
       InterviewBitTest ibTestObj = new InterviewBitTest(20);
       //Pass the reference to updateObject Method
       updateObject(ibTestObj);
       //After the updateObject is executed, check for the value of num in the object.
       System.out.println(ibTestObj.num);
   }
   public static void updateObject(InterviewBitTest ibObj)
   {
       // Point the object to new reference
       ibObj = new InterviewBitTest();
       // Update the value 
       ibObj.num = 50;
   }
}
Output:
20
  • Case 2: When object references are not modified: In this case, since we have the copy of reference the main object pointing to the same memory location, any changes in the content of the object get reflected in the original object.

For example:

class InterviewBitTest{
   int num;
   InterviewBitTest(int x){ 
       num = x; 
   }
   InterviewBitTest(){ 
       num = 0; 
   }
}
class Driver{
   public static void main(String[] args)
   {
       //create a reference
       InterviewBitTest ibTestObj = new InterviewBitTest(20);
       //Pass the reference to updateObject Method
       updateObject(ibTestObj);
       //After the updateObject is executed, check for the value of num in the object.
       System.out.println(ibTestObj.num);
   }
   public static void updateObject(InterviewBitTest ibObj)
   {
       // no changes are made to point the ibObj to new location
       // Update the value of num
       ibObj.num = 50;
   }
}
Output:
50

31. Which among String or String Buffer should be preferred when there are lot of updates required to be done in the data?

StringBuffer is mutable and dynamic in nature whereas String is immutable. Every updation / modification of String creates a new String thereby overloading the string pool with unnecessary objects. Hence, in the cases of a lot of updates, it is always preferred to use StringBuffer as it will reduce the overhead of the creation of multiple String objects in the string pool.

32. How to not allow serialization of attributes of a class in Java?

  • In order to achieve this, the attribute can be declared along with the usage of transient keyword as shown below:
public class InterviewBitExample { 

   private transient String someInfo; 
   private String name;
   private int id;
   // :
   // Getters setters
   // :
} 
  • In the above example, all the fields except someInfo can be serialized.

33. What happens if the static modifier is not included in the main method signature in Java?

There wouldn’t be any compilation error. But then the program is run, since the JVM cant map the main method signature, the code throws “NoSuchMethodError” error at the runtime.

34. What happens if there are multiple main methods inside one class in Java?

The program can’t compile as the compiler says that the method has been already defined inside the class.

35. What do you understand by Object Cloning and how do you achieve it in Java?

  • It is the process of creating an exact copy of any object. In order to support this, a java class has to implement the Cloneable interface of java.lang package and override the clone() method provided by the Object class the syntax of which is:
protected Object clone() throws CloneNotSupportedException{
 return (Object)super.clone();
}
  • In case the Cloneable interface is not implemented and just the method is overridden, it results in CloneNotSupportedException in Java.

36. How does an exception propagate in the code?

When an exception occurs, first it searches to locate the matching catch block. In case, the matching catch block is located, then that block would be executed. Else, the exception propagates through the method call stack and goes into the caller method where the process of matching the catch block is performed. This propagation happens until the matching catch block is found. If the match is not found, then the program gets terminated in the main method.

37. Is it mandatory for a catch block to be followed after a try block?

No, it is not necessary for a catch block to be present after a try block. – A try block should be followed either by a catch block or by a finally block. If the exceptions likelihood is more, then they should be declared using the throws clause of the method.

38. Will the finally block get executed when the return statement is written at the end of try block and catch block as shown below?

public int someMethod(int i){
   try{
       //some statement
       return 1;
   }catch(Exception e){
       //some statement
       return 999;
   }finally{
       //finally block statements
   }
}

finally block will be executed irrespective of the exception or not. The only case where finally block is not executed is when it encounters ‘System.exit()’ method anywhere in try/catch block.

39. Can you call a constructor of a class inside the another constructor?

Yes, the concept can be termed as constructor chaining and can be achieved using this().

40. Contiguous memory locations are usually used for storing actual values in an array but not in ArrayList. Explain.

In the case of ArrayList, data storing in the form of primitive data types (like int, float, etc.) is not possible. The data members/objects present in the ArrayList have references to the objects which are located at various sites in the memory. Thus, storing of actual objects or non-primitive data types (like Integer, Double, etc.) takes place in various memory locations.

However, the same does not apply to the arrays. Object or primitive type values can be stored in arrays in contiguous memory locations, hence every element does not require any reference to the next element.

Additional Reading