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

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Very frequently asked. Usually asked in this format or as difference with interfaces / concrete classes.

Q841. What is an abstract class ?

Core Java

Ans. Abstract class is the class that is not supposed to be instantiated. The purpose of the class to only have extension to the derived class.

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Q842. What will be the output of following code

public class BuggyBread {
   static class A {
      A() {
         f();
      }

      public void f() {
         System.out.println("A ctor");
      }
   }

   static class B extends A {
      B() {
         f();
      }

      public void f() {
         System.out.println("B ctor");
      }
   }

   public static void main(String[] args) {
      B b = new B();
      b.f();
      A a = new A();
      a.f();
   }
}

Core Java

Ans. B ctor
B ctor
B ctor
A ctor
A ctor

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Q843. Can constructor be synchronized in Java ?

Core Java

Ans. No, constructors can not be synchronized in Java. In fact using the keyword synchronized with a constructor results in compilation error.

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Q844. Can a constructor throw an exception ?

Core Java

Ans. Yes

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Q845. Write code for LRU Cache

Core Java

Ans. https://www.geeksforgeeks.org/lru-cache-implementation/

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Q846. Create a Class Bank with the methods to deposit and Withdraw ?

Core Java

Ans. class Bank {
int balance;

Bank(){
balance = 0;
}

void deposit(int amount){
balance = balance amount;
}

int withdraw(int amount){
balance = balance - amount;
return balance;
}
}

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Q847. Why main method is declared static ?

Core Java

Ans. static is the keyword that makes it accessible even without creating any object and using class name only. Making it non static would like creation of object upfront before calling the method.

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Q848. What is the use of static keyword in Java ?

Core Java

Ans. static keyword is used to specify that the respective programming construct ( method , variable ) belongs to the class and not to its instance and is supposed to be shared by all instances of the class.

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Q849. How is hashcode calculated in java ?

or

What are the rules for hashcode calculation ?

Core Java

Ans. General contract of hashCode is:

1.Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer,

2.If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.

3.It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results.

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Q850. What is an exception and exception handling in Java ?

Core Java

Ans. An Exception in java is the occurrence during computation that is anomalous and is not expected.

Exception handling is the mechanism which is used to handle such situations.

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Q851. When is the situation when finally section won't execute ?

Core Java

Ans. If the process / app is abruptly killed or terminated.

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Q852. Why java doesn't support multiple Inheritance ?

or

Explain Java Diamond Problem.

Core Java

Ans. class A {
void test() {
System.out.println("test() method");
}
}

class B {
void test() {
System.out.println("test() method");
}
}

Suppose if Java allows multiple inheritance like this,

class C extends A, B {
}

A and B test() methods are inheriting to C class.

So which test() method C class will take? As A & B class test() methods are different , So here we would Facing Ambiguity.

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Q853. Does java supports operator overloading ?

Core Java

Ans. No

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Q854. Which operators in Java have highest precedence ?

Core Java

Ans. Braces, i.e () and [] have the highest precedence

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Q855. What are different types of variables a class can have ?

Core Java

Ans. 1. Method local variables - These are declared and defined within a method ( instance or static methods ) and their scope is limited to the method itself. They are destructed once the execution of method completes. They are stored in stack memory.

2. Instance variables - These are declared as non static variables as part of the class.They are initialized as part of object creation ( constructor ) and are destructed by java's garbage collection mechanism and hence stored in heap.

3. Static variables - These are declared with the static keyword and are part of the class. They are initialized at the time of class loading and are destructed by java's garbage collection mechanism and hence stored in heap.

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Q856. Name some of the Java Keywords ?

Core Java

Ans. Static , Final , Synchronized, private , public , protected, volatile, transient, super, this,import , abstract,native,default (effective java 8), new

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Q857. Explain some of the features of Java ?

Core Java

Ans. Object Oriented - Java is object oriented but isn't purely object oriented as we have primitives along with objects.

Platform Independent - As JVM provides the translation to the Machine code as per the underlying Operating System.

Interpreted as well as compiled - Java files are compiled as class files and then class files are interpreted by JVM.

Runs on a Virtual Machine - Which is JVM that acts as an independent sub environment.

Multi-threaded - As applications can run on single thread as well as multi thread.

Modularity - Through usage of Classes , methods and Interfaces.

Robust

Usage in variety of Application types - Web , Gaming, BigData.

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Q858. What is the difference between for and foreach loop in java ? Which one should be used in which cases ?

Core Java

Ans. 1. for loop in java is used with a counter as following

for(int counter=0;counter < 50;counter++){
System.out.println(list.get(counter));
}

for iterating and printing the contents of a collection

whereas foreach loop can be specified directly without the use of counter

for(String str:list){
System.out.println(list.get(counter));
}

2. for Each loop syntax is more clean if we have to iterate over the elements of a collection and we need not keep track of the record count

3. For is preferred when we need loops without the usage of collections or Array of objects and entirely primitives are being used

4. for loop is preferred if we need to keep track of record count and have to perform some action of the basis of that. For example - If we have to print something after every 5 records, With for each loop in such case, we will have to keep a separate counter.

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Q859. What is the difference between ServletConfig and ServletContext ?

Java EE

Ans. http://www.java4s.com/java-servlet-tutorials/difference-between-servletconfig-and-servletcontext-in-java/

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Q860. What is the difference between following code segments

1.

try {
for(int i=0;i<100;i++){
calculate(i);
}
} catch (Exception ex) {
System.out.println("Error while processing")
}

and

2.

for(int i=0;i<100;i++){
try {
calculate(i);
} catch (Exception ex) {
System.out.println("Error while processing")
}
}

Core Java

Ans. In first case the whole loop will terminate as soon as any exception happens in the method calculate ( assuming calculate method is not handling its exception and those are thrown back at the calling method )

In Second case exception will be caught for individual iteration and hence it wont break the loop and will continue for the next iteration.

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Q861. What do you look for when you do code review ?

Core Java

Ans. Modularity - First sign of good code is whether it has been segregated into methods and classes appropriately. I dont mind it in excess because I believe that is forward looking strategy as applications tends to expand and eventually become hard to read.

Self Explanatory - Variables and methods should be named in a way that the code should be self explanatory even without comments. Use of Constant variables to explain use of literal.

Proper Code Reuse - If there is anything being reused , it should be moved to parent classes / methods.

Proper composition calls - Composed hierarchy should not be access in just single line. One or two levels is ok but having multiple levels make it hard to read and debug.

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Q862. When should we use Abstract classes ?

Core Java

Ans. Abstract classes provide a mechanism of interfacing ( using abstract method ) as well as inheritance ( extending abstract class ). So they should be used in place of interfaces in case there is some code ( methods ) or object body ( member elements ) that can be reused with inheritance.

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Q863. Provide an implementation for Producer / Consumer using threads.

Core Java

Ans. https://www.tutorialspoint.com/javaexamples/thread_procon.htm

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Q864. Which type of memory is cleaned / recovered by garbage collection - stack or heap ?

Core Java

Ans. Heap memory.

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Q865. Why two types of memory - stack and heap are required by Java ?

Core Java

Ans. Because of the life cycle requirement for different type of values in java.

variables initialized and used in functions needs to be destructed with the execution of function and hence kept in stack. Same is applicable for the object references initialized within the method. If objects would have been created in stack, they wouldnt have been passed around across methods and hence they are created on heap.

So anything that is required beyond the scope of a method or function is kept on heap which is usually garbage collected by Java.

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Q866. How to make sure that only one instance is created in Singleton Pattern ?

Core Java

Ans. public class SingleTon {
private SingleTon() {
if (singleTon != null) {
throw new RuntimeException("cant not create the object");
}
}
public Object clone() throws CloneNotSupportedException {
throw new CloneNotSupportedException("can not be create");
}

static private volatile SingleTon singleTon;
public static SingleTon getInstance() {
SingleTon singleTon = this.sample;
if (singleTon == null) {
synchronized (this) {
singleTon = this.singleTon;
if (singleTon == null) {
singleTon = this.singleton = new SingleTon();
}
}
}
return singleTon;
}
}

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Q867. What is serialVersionUID ?

Core Java

Ans. Everytime an object is serialized the java serialization mechanism automatically computes a hash value by passing the meta information for the class. This id is used during deserialization to verify that the sender and receiver of a serialized object have loaded classes for that object that are compatible with respect to serialization

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Q868. What is the advantage of using static final or constant variables in Java ?

Core Java

Ans. Better Control - If the value is being used at multiple locations , that can be controlled better from single place. Any change would only require making single change.

Meaning , Aliasing and Better Readability - Sometimes its easy to read the value by its meaning or alias ( 0 as ZERO or 0 as NEUTRAL_VALUE ).

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Q869. What could be the problems due to the usage of static methods and elements ?

Core Java

Ans. static methods and static elements are shared by all objects of a class and hence could create problem.

Static methods are not synchronized by default and hence could create problem when accessed through multiple threads. Static elements are shareable by class and hence state of one object could be altered by another object.

Some limitations with Unit testing as not all mocking framework facilitate mocking them. Power mock allows but Mockito doesn't

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Q870. Explain some of the http status codes you know ?

Java EE

Ans. 500 is Internal Server Error

404 is resource not found

400 is Bad Request

403 is Forbidden

401 is Unauthorized

200 is OK

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