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Rarely asked as default methods have been introduced with Java 8.

Q811. Can we have a default method definition in the interface without specifying the keyword "default" ?

Core Java

Ans. No. Compiler complains that its an abstract method and hence shouldn't have the body.

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Q812. Can a class implement two Interfaces having default method with same name and signature ?

public interface DefaultMethodInterface {
default public void defaultMethod(){
System.out.println("DefaultMethodInterface");
}
}

public interface DefaultMethodInterface2 {
default public void defaultMethod(){
System.out.println("DefaultMethodInterface2");
}
}

public class HelloJava8 implements DefaultMethodInterface,DefaultMethodInterface2 {
public static void main(String[] args){
DefaultMethodInterface defMethIn = new HelloJava8();
defMethIn.defaultMethod();
}
}

Ans. No. Compiler gives error saying "Duplicate Default Methods"

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Q813. What If we make the method as abstract in another Interface ?

public interface DefaultMethodInterface {
default public void defaultMethod(){
System.out.println("DefaultMethodInterface");
}
}

public interface DefaultMethodInterface2 {
public void defaultMethod(){
System.out.println("DefaultMethodInterface2");
}
}

public class HelloJava8 implements DefaultMethodInterface,DefaultMethodInterface2 {
public static void main(String[] args){
DefaultMethodInterface defMethIn = new HelloJava8();
defMethIn.defaultMethod();
}
}

Ans. Even then the Compiler will give error saying that there is a conflict.

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Q814. What if we override the conflicting method in the Class ?

public interface DefaultMethodInterface {
default public void defaultMethod(){
System.out.println("DefaultMethodInterface");
}
}

public interface DefaultMethodInterface2 {
default public void defaultMethod(){
System.out.println("DefaultMethodInterface2");
}
}

public class HelloJava8 implements DefaultMethodInterface,DefaultMethodInterface2 {
public static void main(String[] args){
DefaultMethodInterface defMethIn = new HelloJava8();
defMethIn.defaultMethod();
}

public void defaultMethod(){
System.out.println("HelloJava8");
}
}

Ans. There won't be any error and upon execution the overriding class method will be executed.

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Q815. What will happen if there is a default method conflict as mentioned above and we have specified the same signature method in the base class instead of overriding in the existing class ?

Ans. There won't be any problem as the Base class method will have precedence over the Interface Default methods.

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Q816. If a method definition has been specified in Class , its Base Class , and the interface which the class is implementing, Which definition will be picked if we try to access it using Interface Reference and Class object ?

Ans. Class method definition is overriding both the definitions and hence will be picked.

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Q817. If a method definition has been specified in the Base Class and the interface which the class is implementing, Which definition will be picked if we try to access it using Interface Reference and Class object ?

Ans. Base Class Definition will have precedence over the Interface Default method definition.

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Q818. Can we access Interface static method using Interface references ?

Ans. No, only using Interface Name.

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Q819. Can we have default method with same name and signature in the derived Interface as the static method in base Interface and vice versa ?

Ans. Yes , we can do that as static methods are not accessible using references and hence cannot lead to conflict. We cannot do inverse as Default methods cannot be overridden with the static methods in derived interface.

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Q820. What does the following lambda expression means ?

helloJava8 ( x-> x%2 )

Ans. helloJava8 receives an Integer as argument and then returns the modulus of that Integer.

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Q821. What is the difference between namenode and datanode in Hadoop?

BigData

Ans. NameNode stores MetaData (No of Blocks, On Which Rack which DataNode is stored etc) whereas the DataNode stores the actual Data.

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Q822. Write a program to see if the number is perfect number or not ?

Core Java

Ans. http://www.c4learn.com/c-programs/program-to-check-whether-number-is.html

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Q823. Difference between TCP and UDP ?

Networking

Ans. http://www.cyberciti.biz/faq/key-differences-between-tcp-and-udp-protocols/

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Very frequently asked. Usually followed by questions related to private constructor and synchronized access. Frequently asked in JPMorgan and TCS (Based on 2 feedback)

Q824. Explain Singleton Design Pattern ?

Design

Ans. http://www.buggybread.com/2014/03/java-design-pattern-singleton-interview.html

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Q825. What is the use of Mockito.any ?

Ans. In case we need to verify that a method is being called with any argument and not a specific argument we can use Mockito.any(Class), Mockito.anyString, Mockito.anyLong etc.

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Q826. Name few Java Mocking frameworks ?

Ans. Mockito, PowerMock, EasyMock, JMock, JMockit

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Q827. Write an SQL to find all records having all upper case alphanumeric characters in a field ?

Ans. Select * from Table where field = upper(field)

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Q828. Write an SQL to find all records having all numeric characters in a field ?

Ans. Select * from Table where isnumeric(field) = 1;

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Q829. How can we protect an application from throwing a NullPointerException ?

Ans. By having Null Checks.

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Q830. What are the conventions regarding constant variables or static final variables ?

Core Java

Ans. They should be named all in upper case with underscore separating words.

Moreover the name should be such that it identifies the value. For example, if we are using constant for employee Id 123, it should be named something like EMPLOYEE_ID_123 and not EMPLOYEE_ID

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Q831. How do you define a class of constants in Java?

Ans. Make the class as final.
Disable the object construction by making constructor private.
Keep only Static Final Variables declared and Defined at the same time.

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Q832. What will the following code do ?

String dateStr = "2011 11 19";
DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
Date date = dateFormat.parse(dateStr);
System.out.println(date);

Ans. It will throw the ParseException as the date format doesn't abide with the format of the specified date.

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Q833. What will be the output of this code ?

Set mySet = new TreeSet();
mySet.add("4567");
mySet.add("5678");
mySet.add("6789");
for(String s: mySet){
System.out.println(s);
}

Ans. 4567
5678
6789

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Q834. public enum Day {
SUNDAY, MONDAY, TUESDAY, WEDNESDAY,
THURSDAY, FRIDAY, SATURDAY
}

public class Test {
Set mySet = new HashSet();
mySet.add(Day.MONDAY);
mySet.add(Day.SUNDAY);
mySet.add(Day.SATURDAY);

for(Day d: mySet){
System.out.println(d);
}
}

Ans. SUNDAY
MONDAY
SATURDAY

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Try 3 Question(s) Test

Q835. What will be the output of the following code ?

enum Day {
   MONDAY,TUESDAY,WEDNESDAY,THURSDAY,FRIDAY,SATURDAY,SUNDAY
}

public class BuggyBread1{
   public static void main (String args[]) {
      Set mySet = new HashSet();
      mySet.add(Day.SATURDAY);
      mySet.add(Day.WEDNESDAY);
      mySet.add(Day.FRIDAY);
      mySet.add(Day.WEDNESDAY);
      for(Day d: mySet){
         System.out.println(d);
      }
   }
}

Ans. FRIDAY , SATURDAY and WEDNESDAY will be printed but the order cannot be determined.

Only one FRIDAY will be printed as Set doesn't allow duplicates. Order cannot be determined as HashSet doesn't maintain elements in a particular order.

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Q836. public class BuggyBread1{

public static void main (String args[]) {
Set<String> mySet = new TreeSet<String>();
mySet.add(""1"");
mySet.add(""2"");
mySet.add(""111"");
for(String d: mySet){
System.out.println(d);
}
}
}

Core Java

Ans. 1
111
2

TreeSet maintains the elements in the ascending order which is identified by the compareTo method. compareTo method in String has been defined such that it results in the natural alphabetic Order. Here the elements in the TreeSet are of String and not of Integer. In String Natural Order, 111 comes before 2 as ascii of 1st character first determines the order.

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Q837. public class BuggyBread1{

public static void main (String args[]) {
Set<Integer> mySet = new TreeSet<Integer>();
mySet.add(1);
mySet.add(2);
mySet.add(111);
for(Integer d: mySet){
System.out.println(d);
}
}
}

Core Java

Ans. 1
2
111

TreeSet maintains the elements in the ascending order which is identified by the compareTo method. compareTo method in Integer has been defined such that it results in the natural numerical Order.

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Q838. Can we extend an Enum ?

Core Java

Ans. No. Enums are final by design.

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Q839. What would you do if you have to add a jar to the project using Maven ?

Maven

Ans. If its already there in Maven local repository, We can add that as a dependency in the project pom file with its Group Id, Artifact Id and version.

We can provide additional attribute SystemPath if its unable to locate the jar in the local repository.

If its not there in the local repository, we can install it first in the local repository and then can add it as dependency.

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Q840. Have you ever had problem getting your projects in eclipse refreshed after you made changes in the Pom files ?

Maven

Ans. Yes, It happens many times but I would usually perform mvn eclipse:eclipse and this would resolve the project refresh problems.

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