Core Java Interview Questions and Answers

1. What are the important features of Java 8 release?

   Java 8 has been released in March 2014, so it’s one of the hot topic in java interview questions. If you answer this question clearly, it will show that you like to keep yourself up-to-date with the latest technologies.
   Java 8 has been one of the biggest release after Java 5 annotations and generics. Some of the important features of Java 8 are:
   1. Interface changes with default and static methods
   2. Functional interfaces and Lambda Expressions
   3. Java Stream API for collection classes
   4. Java Date Time API
   I strongly recommend to go through above links to get proper understanding of each one of them, also read Java 8 Features.

1. What do you mean by platform independence of Java?

   Platform independence means that you can run the same Java Program in any Operating System. For example, you can write java program in Windows and run it in Mac OS.

1. What is JVM and is it platform independent?

   Java Virtual Machine (JVM) is the heart of java programming language. JVM is responsible for converting byte code into machine readable code. JVM is not platform independent, thats why you have different JVM for different operating systems. We can customize JVM with Java Options, such as allocating minimum and maximum memory to JVM. It’s called virtual because it provides an interface that doesn’t depend on the underlying OS.

1. What is the difference between JDK and JVM?

   Java Development Kit (JDK) is for development purpose and JVM is a part of it to execute the java programs.
   JDK provides all the tools, executables and binaries required to compile, debug and execute a Java Program. The execution part is handled by JVM to provide machine independence.

1. What is the difference between JVM and JRE?

   Java Runtime Environment (JRE) is the implementation of JVM. JRE consists of JVM and java binaries and other classes to execute any program successfully. JRE doesn’t contain any development tools like java compiler, debugger etc. If you want to execute any java program, you should have JRE installed.

1. Which class is the superclass of all classes?

   java.lang.Object is the root class for all the java classes and we don’t need to extend it.

1. Why Java doesn’t support multiple inheritance?

   Java doesn’t support multiple inheritance in classes because of “Diamond Problem”. To know more about diamond problem with example, read Multiple Inheritance in Java.
   However multiple inheritance is supported in interfaces. An interface can extend multiple interfaces because they just declare the methods and implementation will be present in the implementing class. So there is no issue of diamond problem with interfaces.

1. Why Java is not pure Object Oriented language?

   Java is not said to be pure object oriented because it support primitive types such as int, byte, short, long etc. I believe it brings simplicity to the language while writing our code. Obviously java could have wrapper objects for the primitive types but just for the representation, they would not have provided any benefit.
   As we know, for all the primitive types we have wrapper classes such as Integer, Long etc that provides some additional methods.

1. What is difference between path and classpath variables?

   PATH is an environment variable used by operating system to locate the executables. That’s why when we install Java or want any executable to be found by OS, we need to add the directory location in the PATH variable. If you work on Windows OS, read this post to learn how to setup PATH variable on Windows.
   Classpath is specific to java and used by java executables to locate class files. We can provide the classpath location while running java application and it can be a directory, ZIP files, JAR files etc.

1. What is the importance of main method in Java?

   main() method is the entry point of any standalone java application. The syntax of main method ispublic static void main(String args[]).
   main method is public and static so that java can access it without initializing the class. The input parameter is an array of String through which we can pass runtime arguments to the java program. Check this post to learn how to compile and run java program.

1. What is overloading and overriding in java?

   When we have more than one method with same name in a single class but the arguments are different, then it is called as method overloading.
   Overriding concept comes in picture with inheritance when we have two methods with same signature, one in parent class and another in child class. We can use @Override annotation in the child class overridden method to make sure if parent class method is changed, so as child class.

1. Can we overload main method?

   Yes, we can have multiple methods with name “main” in a single class. However if we run the class, java runtime environment will look for main method with syntax as public static void main(String args[]).

1. Can we have multiple public classes in a java source file?

   We can’t have more than one public class in a single java source file. A single source file can have multiple classes that are not public.

1. What is Java Package and which package is imported by default?

   Java package is the mechanism to organize the java classes by grouping them. The grouping logic can be based on functionality or modules based. A java class fully classified name contains package and class name. For example, java.lang.Object is the fully classified name of Object class that is part ofjava.lang package.
   java.lang package is imported by default and we don’t need to import any class from this package explicitly.

Complex Queries in Oracle

To fetch ALTERNATE records from a table. (EVEN NUMBERED    )

Select *from emp where rowid in (select decode(mod(rownum,2),0,rowid, null) from emp);

To select ALTERNATE records from a table. (ODD NUMBERED)

select * from emp where rowid in (select decode(mod(rownum,2),0,null ,rowid) from emp);

Find the 3rd MAX salary in the emp table.

select distinct sal from emp e1 where 3 = (select count(distinct sal) from emp e2 where e1.sal <= e2.sal);

Find the 3rd MIN salary in the emp table.

select distinct sal from emp e1 where 3 = (select count(distinct sal) from emp e2where e1.sal >= e2.sal);

Select FIRST n records from a table.

select * from emp where rownum <= &n;

Select LAST n records from a table.

select * from emp minus select * from emp where rownum <= (select count(*) - &n from emp);

List dept no., Dept name for all the departments in which there are no employees in the department.

select * from dept where deptno not in (select deptno from emp);
alternate solution:  select * from dept a where not exists (select * from emp b where a.deptno = b.deptno);
altertnate solution:  select empno,ename,b.deptno,dname from emp a, dept b where a.deptno(+) = b.deptno and empno is null;

How to get 3 Max salaries ?

Select distinct sal from emp a where 3 >= (select count(distinct sal) from emp b where a.sal <= b.sal) order by a.sal desc;

How to get 3 Min salaries ?

Select distinct sal from emp a  where 3 >= (select count(distinct sal) from emp b  where a.sal >= b.sal);

How to get nth max salaries ?

Select distinct hiredate from emp a where &n =  (select count(distinct sal) from emp b where a.sal >= b.sal);

Select DISTINCT RECORDS from emp table.

select * from emp a where  rowid = (select max(rowid) from emp b where  a.empno=b.empno);

How to delete duplicate rows in a table?

Delete from emp a where rowid != (select max(rowid) from emp b where  a.empno=b.empno);

Count of number of employees in  department  wise.

select count(EMPNO), b.deptno, dname from emp a, dept b  where a.deptno(+)=b.deptno  group by b.deptno,dname;

Suppose there is annual salary information provided by emp table. How to fetch monthly salary of each and every employee?

Select ename,sal/12 as monthlysal from emp;

Select all record from emp table where deptno =10 or 40.

Select * from emp where deptno=30 or deptno=10;

Select all record from emp table where deptno=30 and sal>1500.

Select * from emp where deptno=30 and sal>1500;

Select  all record  from emp where job not in SALESMAN  or CLERK.

Select * from emp where job not in ('SALESMAN','CLERK');

Select all record from emp where ename in 'BLAKE','SCOTT','KING'and'FORD'.

Select * from emp where ename in('JONES','BLAKE','SCOTT','KING','FORD');

Select all records where ename starts with ‘S’ and its lenth is 6 char.

Select * from emp where ename like's____';

Select all records where ename may be any no of  character but it should end with ‘R’.

Select * from emp where ename like'%R';

Count  MGR and their salary in emp table.

Select count(MGR),count(sal) from emp;

In emp table add comm+sal as total sal  .

Select ename,(sal+nvl(comm,0)) as totalsal from emp;

Select  any salary <3000 from emp table. 

Select * from emp  where sal> any(select sal from emp where sal<3000);

Select  all salary <3000 from emp table. 

Select * from emp  where sal> all(select sal from emp where sal<3000);

Select all the employee  group by deptno and sal in descending order.

Select ename,deptno,sal from emp order by deptno,sal desc;

How can I create an empty table emp1 with same structure as emp?

Create table emp1 as select * from emp where 1=2;

How to retrive record where sal between 1000 to 2000?

Select * from emp where sal>=1000 And  sal<2000

Select all records where dept no of both emp and dept table matches.

Select * from emp where exists(select * from dept where emp.deptno=dept.deptno)

If there are two tables emp1 and emp2, and both have common record. How can I fetch all the recods but common records only once?

(Select * from emp) Union (Select * from emp1)

How to fetch only common records from two tables emp and emp1?

(Select * from emp) Intersect (Select * from emp1)

How can I retrive all records of emp1 those should not present in emp2?

(Select * from emp) Minus (Select * from emp1)

Count the totalsa  deptno wise where more than 2 employees exist.

SELECT  deptno, sum(sal) As totalsal
FROM emp
GROUP BY deptno
HAVING COUNT(empno) > 2

Oracle Retrieving Top 5 Highly paid Employees

SQL > select rownum,empno,ename,job,sal from
(select rownum,empno,ename,job,sal from emp
order by sal desc )
where rownum <= 5;

What is Trigger and types of Triggers in Oracle

A set of pl/sql statements automatically executed whenever a dml statements are performed on a table. The act of executing a trigger is known as firing the trigger.
Two types of triggers
Row level trigger; this trigger will fire every row manipulated

Statement level trigger; this trigger will be fired only once for dml statements

Difference Between JOIN and UNION

Difference Between JOIN and UNION

Join	Union
1. Join the columns. 2. Duplicate are allowed 3.Combine the column based on condition.	1.Merge the row 2.Duplicates are not allowed 3. Combine the result of two select statements.

Javascript String Replace using while

<script>
var x = "<div><a onclick='hello()' href='#' </a>AAAAAAAAA<a onclick='hello()' href='#' </a>BBBBBBBBBBBBB<a onclick='hello()' href='#' </a>CCCCCCCCCC</div>";
document.write(x);
var start = x.indexOf('<a');
var end = x.indexOf('</a>') + 4;
var final = x.slice(start, end);
while (start > -1) {
x = x.replace(final, ' Welcome ');
start = x.indexOf('<a');
end = x.indexOf('</a>') + 4;
final = x.slice(start, end);
}
document.write(x);
</script>

Using jQuery to programmatically click an a link

<script>
$("#myAnchor").click(function(event)
{
var link = $(this);
var target = link.attr("target");
if($.trim(target).length > 0)
{
window.open(link.attr("href"), target);
}
else
{
window.location = link.attr("href");
}
event.preventDefault();
});
</script>
<html><a id=myAnchor href=index.php></a></html>