Java Interview Questions TechM

1) how to remove duplicates in an ArrayList?

Ans:

You can create a LinkedHashSet from the list. The LinkedHashSet will contain each element only once, and in the same order as the List.

list = new ArrayList<String>(new LinkedHashSet<String>(list))

example:

       ArrayList ss = new ArrayList<>();

ss.add("a");

       ss.add("t");

       ss.add("d");

       ss.add("i");

       ss.add("s");

       ss.add("a");

       System.out.println(ss);

       ArrayList ss2 = new ArrayList<>(new LinkedHashSet(ss));

       System.out.println(ss2);

2) why do we override hashCode() method.

Ans:

"If two objects are equal using Object class equals method, then thehashcode method should give the same value for these two objects." So, if in our class we override equals we should override hashcode method also to fallow this rule.

3) write a thread safe singleton?

Ans:

public class Dev21Singleton {

            private static Dev21Singleton instance = null;

            protected Dev21Singleton() {

            }

            // Lazy Initialization (If required then only)

            public static Dev21Singleton getInstance() {

                        if (instance == null) {

                                    // Thread Safe. Might be costly operation in some case

                                    synchronized (Dev21Singleton.class) {

                                                if (instance == null) {

                                                            instance = new Dev21Singleton();

                                                }

                                    }

                        }

                        return instance;

            }

}

4) Questions about abstract class constructor, private variables in abstract class etc.

5) why can’t we create instance of an abstract class? what stops us? have you ever tried
   creating one?

Ans:

It's because an abstract class isn't complete. One or more parts of its public interface are not implemented. You've said what they're going to be - what they're called, what their name is, what they return - but no implementation has been provided, so nobody would be able to call them.

Compilers prevent you from instantiating such incomplete objects on the basis that if you do instantiate an object you're going to want to use its entire public interface (this assumption is related to the idea that an object should do a minimal set of things, so that the public interface is not too large to manage). It's also a useful safety net. The compiler says "hang on, this is just part of a class, you need to provide the rest of it".

6) MVC questions -- like how to pass textbox val back to controller etc etc.

7) data annotation, 1 to many relationship in entity framework, primary and composite key in entity framework.