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

Interview Questions and Answers - Order By Newest

Q491. Does an ArrayList allow elements of different types ? If not, Why the following code works List list1 = new ArrayList<>(); list1.add(1); list1.add("1");

Core Java

Ans. With Java 7 or Later. If you don't declare the list to be of specific type , it treats it as list of objects. int 1 is auto boxed to Integer and "1" is String and hence both are objects.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve arraylist list collections}

Q492. Are Strings objects in Java ?

Core Java

Ans. Yes, everything except primitive types are objects in Java.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve string}

Q493. Can we access private members of the parent class ? i.e Are private members of the class visible in the derived class ?

Core Java

Ans. No, If both Parent and Derived are outer classes.

public class Vehicle {
private static String manufacturingDate = "2016";
}

public class Car extends Vehicle{
public static void main(String args[]){
System.out.println(manufacturingDate); // error - The field Vehicle.manufacturingDate is not visible
}
}

Yes, If derived is the inner class of Parent.

public class Vehicle {
private static String manufacturingDate = "2016";
static public class Car extends Vehicle{
public static void main(String args[]){
System.out.println(manufacturingDate); // no problem
}
}
}

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve private members private methods private variables inheritance object oriented programming (oops) oops concepts members visibility inner classes nexted classes basic frequent}

Q494. Do we need to implement all interface methods if a class is implementing it ?

Core Java

Ans. Yes, a Class is supposed to define all abstract methods declared in the interface. With Java 8 , Interfaces can have default methods which need not be implemented by the implementing class.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve interfaces}

Q495. What is IdentityHashMap ?

Core Java

Ans. IdentityHashMap is a class that implements AbstractMap and provides a data structure with Elements having Key Value pair, just like HashMap. It is similar to HashMap except that it uses reference equality when comparing elements.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve IdentityHashMap map hashmap collections}

Q496. How and when the Buffer is cleared when using Buffered Writer Classes ?

Core Java

Ans. Buffer is cleared in 2 circumstances, i.e 1. naturally when the buffer is filled and 2. explicitly when the method flush is called ( for flushing the residual )

Ideally we just need to call flush once at the end of file writing so that the residual content should be dumped to file.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve BufferedWriter Writer PrintWriter File io flush file handling}

Q497. Where do we generally use comparable interface ?

Core Java

Ans. Implementing Comparable interface means that the elements of the class are comparable i.e the class provides the implementation of compareTo method that would help comparing the elements.

This is usually required if we are planning to sort elements of a collection, If compareTo method is not defined , the sorting class / method could never understand a way to compare its elements in order to sort them.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve comparable interface}

Try 1 Question(s) Test

Q498. Difference between multiple and multi level inheritance ?

Core Java

Ans. Multiple Inheritance refers to the concept of a class inheriting multiple classes. Example - Class C extends Class A ,Class B. This is not allowed in Java.

Multilevel Inheritance refers to the concept of Inheritance in a chain. Example - Class B extends Class A, Class C extends Class B. This is permitted in Java.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve inheritance object oriented programming (oops) oops concepts multiple inheritance object oriented programming (oops) oops concepts multi level inheritance object oriented programming (oops) oops concepts Basic frequent}

Q499. What is meant by size and index of a collection ?

Core Java

Ans. As the name suggest size is the size of collection / data structure i.e number of elements or memory utilized by the collection.

Index is the position of an element in a collection or data structure.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve collection index of collection size of collection Basic}

Q500. What is a classpath in Java ?

Core Java

Ans. Classpath is the parameter for JVM to look for java classes ( .class files ) that are to be looad by class loader

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve classpath Asked in 1 Companies basic}

Q501. What is the difference between = and == in Java ?

Core Java

Ans. = is the assignment operator that assigns the result of the expression on the right to the variable on the left, whereas

== is the operator to check object equality to see if the reference on left and right are pointing to the same object. For primitive types, its used to check if both variables holds the same value.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve = == assignment operator object equality difference between Basic}

Q502. How is == operator different for objects and primitive types ?

Core Java

Ans. For objects or references, == operator check if the reference on left and right points to the same object.

For primitive types or variables, == operator check if the variable on left and right holds the same value.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve == object equality operator Basic}

Q503. What are the benefits of Enums ?

Core Java

Ans. Enums in Java are used to declare predefined objects and then reuse them. they offer many benefits

1. Enum instance are type safe and thread safe.

2. Enum instances are singleton and hence reused.

3. If we use Enums with Switch , It makes sure that the passed argument is either of its instance and hence provides a safeguard.

4. If we use Enum with Sorted Collections we can sort the elements with a predefined priorities ( as per constant declaration in enum )

5. We can use Enum as a Factory by defining its constructor.

6. We can store related constant data within enum. For example - If we know the values for the map upfront, we can alternatively use an enum.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve Asked in 6 Companies}

Q504. Why Collection interface does not extend Cloneable interface ?

Core Java

Ans. Here is the list of classes that implements Collections Interface - http://www.buggybread.com/2015/02/java-collections-classes-that-implement.html

Having Collection interface to extend Cloneable interface would mean necessarily implement clone method by all implementing classes. As not all collection classes allow duplicate elements, it makes no sense to clone elements for them.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve cloneable collections collection interface cloning}

Q505. What happens to the Exception object after the exception handling is done ?

Core Java

Ans. As with all other objects its garbage collected after its usage unless its rethrown.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve exceptions exception handling rare}

Q506. What are the disadvantages of multithreading ?

Core Java

Ans. 1. Switching Overheads - Even though multi threading aims at improving performance by reducing the wait time and hence improving overall throughput, there is a cost of switching resources between threads and sometime this cost can surpass the benefits if there isnt much wait for IO or external communication.

2. Debugging is hard with multi threaded code.

3. Deadlock - Execution of multi threaded code many a times lead to deadlock due to shared resources.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve multithreading threads Asked in 4 Companies}

Q507. Which is the root interface of Collection classes in Java ?

Core Java

Ans. java.util.Collection

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve collection interface collections}

Q508. Write code for Binary tree traversal ?

Core Java

Ans. http://javabeat.net/binary-search-tree-traversal-java/

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve Asked in 1 Companies}

Q509. Can there be a Thread without a Process ?

Core Java

Ans. No.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve Thread Process Asked in 1 Companies Basic}

Q510. Can we have an inner class within Interface ?

Core Java

Ans. Yes, we can define an inner class within interface.Inside the interface, the inner class is implicitly public static.

So the following is legit in Java

public interface BuggyBreadInterface {
   void doSomething();

   public class BuggyBreadClass implements BuggyBreadInterface{
      void doSomething(){
         System.out.println("Do something");
      }
   }
}

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve inner classes inner class in interface nested classes}

Q511. Is it a valid identifier in Java ?

int 123Abc = 5;

Core Java

Ans. No, identifier in java cannot start with a numeral.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve identifiers}

Q512. What is the difference between Collections.emptyList() and creating new instance of List using new ?

Core Java

Ans. But Collections.emptyList() returns an Immutable list whereas new arraylist() creates a mutable list.

Advantage of getting an empty list using Collections.emptyList is that it returns a singleton list which can be shared among many references and hence made immutable. This is good fit for situations where we would like to initialize a list to basic minimum empty to avoid null pointer exception.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve collections Collections.emptyList immutable immutability immutability}

Very frequently asked.

Q513. What is the use of synchronized keyword ?

Core Java

Ans. Synchronize is used to achieve mutual exclusion i.e at one time, the segment of the code, method that has been declared synchronized should be executed by single thread only and hence the lock needs to be retrieved before executing the segment and then released.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve threads multithreading synchronized Asked in 5 Companies basic frequent}

Usually asked with Questions related to Generics.

Q514. What are Type Erasures in Java ?

Core Java

Ans. Type erasure applies to the use of generics. When generics are used, they're pre compiled into compile time checks and execution-time casts.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve type erasures generics Asked in 2 Companies intermediate rare}

Q515. Have you ever tried compressing data using Java ?

Core Java

Ans. Yes, we can use ZipInputStream class for compressing the FileInputstream object.

Sample Code for ZipInputStream

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve file handling FileInputstream ZipInputStream file handling}

Q516. Watching a directory for changes. Monitoring a directory.

Core Java

Ans. http://www.logicbig.com/tutorials/core-java-tutorial/java-nio/java-watch-service/

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve file handling monitoring directory}

Q517. How can one determine if JVM is 32-bit or 64-bit from Java Program ?

Core Java

Ans. There is a Java system property "sun.arch.data.model" that can tell if JVM is 32 bit of 64 bit

System.getProperty("sun.arch.data.model") can be used to get that property.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve jvm intermediate rare}

Q518. Is ++ or -- operation thread safe in Java ?

Core Java

Ans. No

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve rare}

Q519. How can we find the memory usage of JVM from Java code?

Core Java

Ans. Using Runtime object.

Runtime runtime = Runtime.getRuntime();

runtime.totalMemory();
runtime.freeMemory();

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve jvm memory management rare}

Q520. How can we catch an exception thrown by another thread ?

Core Java

Ans. http://stackoverflow.com/a/6548203/2789764

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve exception handling exception expert rare}

Search Interview Questions

Help us and Others Improve. Please let us know the questions asked in any of your previous interview.