Importent Core Java interview questions with Answers(2)

11) Is final keyword allowed in method signature? Public void handson(final String s) { --- }


A) Yes. Always write code and cross check the theoretical concepts

12) What is static import?

A) Its introduce in jdk1.5
with static import, we can import static members from a class. Syntax is as follows
import static java.lang.Math.*. now, all static methods can be used without class name prefix.

13) Which collection object is used for sorting strings?

A)TreeSet. SortedSet is interface. TreeSet is a class which implements the SortedSet interface

14) When to use singleton pattern?

A) Few scenarios

1) If a class does not have any instance variables, we don’t need to create an instance of new object every time it's needed by other object

2) If a class has a final static variable only, that means the variables are read only, we don’t need to create an instance of new object every time it's needed by other object

3) if the class has instance variables and must be shared with lots of objects, we can still consider singleton. But, take care that synchronization does not create performance issues.

Few singletons examples in java and related frameworks

1) Runtime class in core java

2) Action class in struts 1x is singleton. Action class in struts 2x is not singleton. In strust2, new action class is created for every request.

3) Spring beans are singletons. They are singletons in the container

Note: Think about threading issues when dealing with Singleton classes.

15) Which collections retain the insertion order? Let's say, we have added "Hyderabad", "chennai", "blore", "mumbai" in that order to List and Set? When you iterate, would the data be retrieved in the same insertion order?

A) ArrayList will return the data in the insertion order.

HashSet does not return the data in the insertion order.
We should use LinkedHashSet to retrieve the data in insertion order.
Set s = new LinkedHashSet();

// List s = new ArrayList();

Iterator itr = s.iterator();

while ( itr.hasNext() ){
--------
}

16) What is the difference between final, finally, finalize?

A)final is keyword used for the following purposes
1) if it is used with class, that class cannot be inherited
2) if it is used with method, that method cannot be overridden
3) if it is used with variable, the value of the variable cannot be changed.
finally is used in try/catch/finally concept
finally is used for clean up activities like closing the jdbc connections.

Note: If the interviewer asks you to write jdbc code, always write finally block and close the connection.

finalize method is in Object class. Garbage collector invokes finalize (), if present, before reclaiming the memory for the object

17) What is the difference between throw and throws?

A) Throw is used to throw an exception. It is used to throw
user defined or custom Exceptions.

for example

public String handson(){
if( a == null)
throw new Exception("Hands-on training");
}
return "Krishna TCUBE";
}
throws is used in method signature. It denotes that the method throws checked exceptions
public void handson(String Tcube) throws SQLException{
-----
}

18) In try/catch/finally concept, is catch block necessary?

A) With the try block either catch or finally is sufficient.
Both can be used also

19) What is transient keyword?

A) The data in the transient variable is not serialized.

20) Very general interview question

DriverManager, Connection, Statement, ResultSet. Which are classes, which are interfaces

A) DriverManager is class, remaining are interface

21) Collection and Collections. Which is interface and which is class?

A) Collections is class in util package, Collection is interface in util package