1.Why Map interface doesn’t extend Collection interface?

• Set is unordered collection and does not allows duplicate elements.
• List is ordered collection allows duplicate elements.
• Where as Map is key-value pair.
• It is viewed as set of keys and collection of values.
• Map is a collection of key value pairs so by design they separated from collection interface.

2.What is difference between HashMap and Hashtable?

• Synchronization or Thread Safe 
• Null keys and null values 
• Iterating the values 
•  Default Capacity 

3.Differences between comparable and comparator?

• Comparable Interface is actually from java.lang package.
• It will have a method compareTo(Object obj)to sort objects
• Comparator Interface is actually from java.util package.
• It will have a method compare(Object obj1, Object obj2)to sort objects

4.How can we sort a list of Objects?

•  To sort the array of objects we will use  Arrays.sort() method.
• If we need to sort collection of object we will use Collections.sort().

5.What is difference between fail-fast and fail-safe?

• Fail fast is nothing but immediately report any failure. whenever a problem occurs fail fast system fails. 
• in java Fail fast iterator while iterating through collection of objects sometimes concurrent modification exception will come there are two reasons for this.
• If one thread is iterating a collection and another thread trying to modify the collection.
• And after remove() method call if we try to modify collection object

6. What is difference between Iterator ,ListIterator and Enumeration?

• Enumeration interface implemented in java 1.2 version.So Enumeration is legacy interface.
• Enumeration uses elements() method.
• Iterator is implemented on all Java collection classes.
• Iterator uses iterator() method.
• Iterator can traverse in forward direction only.
• ListIterator is implemented only for List type classes
• ListIterator uses listIterator() method.

What is difference between Iterator ,ListIterator and Enumeration?

7.What is difference between Set and List in Java?

• A set is a collection that allows unique elements.
• Set does not allow duplicate elements
• Set allows only one null value.
• Set having classes like :

• HashSet
• LinkedHashSet
• TreeSet
• List having index. and ordered  collection
• List allows n number of null values.
• List will display Insertion order with index.

• List having classes like :
• Vector
• ArrayList
• LinkedList

8.Differences between arraylist and vector?

• Vector was introduced in  first version of java . that's the reason only vector is legacy class.
• ArrayList was introduced in java version1.2, as part of java collections framework.
• Vector is  synchronized. 
• ArrayList is not synchronized.

9.What are the classes implementing List interface?

• ArrayList     
• LinkedList     
• Vector

10. Which all classes implement Set interface ?

• HashSet
• LinkedHashSet
• TreeSet

11.How to make a collection thread safe?

• Vector, Hashtable, Properties and Stack are synchronized classes, so they are thread-safe and can be used in multi-threaded environment.
• By using Collections.synchronizedList(list)) we can make list classes thread safe.
• By using 
  java.util.Collections.synchronizedSet()  we can make set classes thread safe.

12.Can a null element added to a TreeSet or HashSet?

• One null element can be added to hashset.
• TreeSet does not allow null values

13. Explain Collection’s interface hierarchy?

14.Which design pattern Iterator follows?

• Iterator design pattern

15.Which data structure HashSet implements

• Hashset implements hashmap internally.

16.Why doesn't Collection extend Cloneable and Serializable?

• List and Set and queue extends Collection interface.
• SortedMap extends Map interface.

17.What is the importance of hashCode() and equals() methods? How they are used in Java?

• equals() and hashcode() methods defined in "object" class. 
• If equals() method return true on comparing two objects then hashcode() of those two objects must be same.

18.What is difference between array & arraylist?

• Array is collection of similar type of objects and fixed in size.
• Arraylist is collection of homogeneous and heterogeneous elements.

19.What is the Properties class?

• Properties is a subclass of Hashtable. It is used to maintain lists of values in which the key and the value is String.

20.How to convert a string array to arraylist?

• ArrayList al=new ArrayList( Arrays.asList( new String[]{"java", "collection"} ) ); 
•  arrayList.toArray(); from list to array

1. Print prime numbers? 
2. Java Program Find Second highest number in an integer array 
3. Java Interview Program to find smallest and second smallest number in an array 
4. Java Coding Interview programming Questions : Java Test on HashMap  
5. How to check internet connection using java
6. Constructor chaining in java with example programs 
7. Top 10 Interview Programs and questions on method overriding in java
8. Swap two numbers without using third variable in java 
9. Find sum of digits in java 
10. How to create immutable class in java 
11. AtomicInteger in java 
12. Check Even or Odd without using modulus and division  
13. String Reverse Without using String API 
14. Find Biggest substring in between specified character
15. Check string is palindrome or not?
16. Reverse a number in java? 
17. Collections interview questions and programs

Static final variable java example:

• If we declare any variable as static means it is class level variable.
• If we declare any variable as static final its class level variable and its value can not be changed 
• final static variables are constants in java.
• We can access these final static variables directly without using object or class name.
• We can not change the value of final static variables in any object.

Static String:

• static Strings are class level strings.
• we can access these static Strings using class name.

• Lets discuss about what are the common functionalities everyone needs when developing a application and what are the classes we require mostly to rectify common problems.

Exceptions:

•  Exception classes are very important in every java project,
• Finding the places where the chances of occurring exception and placing in try and in catch block assigning to corresponding exception class or super class of all exceptions Exception.

1. Checked Exceptions
2. Unchecked Exceptions
3. Errors

Basic IO:

•  Basic input and output operations covered by java platform classes focuses on IO streams and serialization.

1.     Byte Streams are provided to handle I/O of raw binary data.
2.     Character Streams handles I/O of character data
3.     Buffered Streams optimize input and output by reducing the number of calls to the native API.
4.     Scanning and Formatting allows a program to read and write the formatted text.
5.     I/O from the Command Line describes the Standard Streams and the Console object.
6.     Data Streams will handle the binary I/O Strings and primitive data types
7.     Object Streams provided to handle binary I/O of all objects.

 Concurrency:

• Will Explains how to build applications which perform multiple tasks simultaneously.
• Java Platform having full support to concurrent programming 
• In JDK 5 Java introduces high level concurrent programming APIs.
• Like java.util.concurrent packages. 

  The Platform Environment 

• To examine and configure java environment java provides classes and libraries.
• JVM and Java class libraries
• Configuration Utilities describes APIs used to access configuration data supplied when the application is deployed, or by the application's user.
• System Utilities describes miscellaneous APIs defined in the System and Runtime classes.
• PATH and CLASSPATH describes environment variables used to configure JDK development tools and other applications.

• Here the List of  commonly used java classes

1. java.lang.Exception
2. java.lang.System
3. java.lang.String
4. java.util.ArrayList
5. java.util.HashMap
6. java.lang.Object
7. java.lang.Thread
8. java.lang.Class
9. java.util.Date
10. java.util.Iterator

• We can not override private methods in java.
• The basic inheritance principle is when we are extending a class we can access all non private members of a class so private members are private to that class only we can not access anywhere outside the class if we declare anything as private.
• Know more information about access specifiers here

• From the above program we can say super class private members are not accessible to sub class.
• lets see what will happen if we try to override a method of super class?
• Sorry if we are unable to access super class private methods then there should not be a questions of overriding that method right?
• Yes private methods of super class can not be overridden in sub class.
• Even if we try to override same method of super class that will became a sub class own method not overridden method.

• We can prove this by providing @override annotation in sub class method 

• Common coding interview question everybody facing in interviews is can we call sub class method using super class object? or can we call child class method using parent class? or can a super class call a subclass' method.
• The answer is No. but still we can say yes. so we need to what are all those scenarios of calling sub class method from super class and lets discuss about which is the actual possible case to call sub class method using super class object.
• Before that let me explain what is inheritance and how the super and sub classes related each other.

Inheritance:

•  Getting the properties from one class object to another class object is known as inheritance.
• Two types of inheritance
• Getting the properties from one class object to another with level wise and with some priorities is known as multilevel inheritance.
• Getting the properties from one class object to another class object with same priority is known as multiple inheritance
• Java does not supports multiple inheritance

Creating super class object and calling methods

• In above program super class and sub class is there and sub class extending super class.
• But we created object for super class so we can call only super class methods on that object.
• If we create sub class object we can call super class and sub class methods on that object now we can able to access super class A methods only.
• So by creating super class object we can call only super class methods

Creating super class object and calling methods

• Above program shows sub class object using super class methods and variables as we said in inheritance concept all super class members can accessed by the sub class means all super class members are available to sub class if sub class extending super class.
• So whenever we create object of sub class it will call sub class constructor and from sub class constructor super class constructor will be called so memory will be allocated to all super class non static member and sub class non static members.
• So we can call super class methods using sub class.
• B obj= new B();
• obj.print();
• So by creating sub class object we can call both super class methods and sub class methods.

 Assigning sub class reference to super class object.

• Yes we can assign sub class object to super class reference.
• So here Sub class object is created so memory allocated to all super class members
• so can we call all methods ?  No eventhough sub class object is created we can not call sub class methods because we assigned sub class object to super class reference.
• Then how its possible to call sub class methods?
• Yes its possible to call sub class methods using super class by type casting to sub class object .
• By type casting super class object to sub class object we can access all corresponding sub class and all super class methods on that reference.

 Assigning sub class reference to super class object.

 Assigning sub class reference to super class object. We can call sub class methods if overridden

Otherwise we need to typecast to call sub class methods

• Yes we can create a class as static. But class should be inner class or nested class.
• We know how to create static methods, static variables and static blocks.
• As Java Supports defining a class within a class we can create a static inner class inside a class.
• This inner static class inside a class can access static members of outer class even its private.

Java Program to create static inner class:

• In java there are two types of nested classes one is static and another one is non static nested classes.
• We saw static classes in java lets see what will be there in non static nested classes.

1. Member inner class
2. Anonymous inner class
3. Local inner class

• A class is defined within a class and outside of methods of that class known as member inner class.
• Anonymous inner class is a inner class which does not have a name and whose instance is created at the time creating class itself.
• A class which is defined inside a method of another class known as local inner class