Core JAVA

Classes and Objects
1. Describe OOPs concepts.

The four main concepts are involved in OOP:
1. Abstraction
2. Encapsulation
3. Inheritance
4. Polymorphism

Abstraction
 Abstraction is hiding of information from others. In case of Java, its hiding implementation of a method/ object. For example if an interface is available to outer world, so end user won't know about implementing class details. This is just a kind of abstraction. Abstraction could be anywhere in java, wherever we hide some information, we call it abstraction.
Encapsulation
 Encapsulation is combining data and method along with implementation hiding within the class. Implementation hiding is done with the help of access modifiers (public, protected, package, private).
Inheritance
 Inheritance is the capability of a class to use the properties and methods of another class while adding its own functionality. Although inheritance may sometimes imply (especially in Java, where the keyword for inheritance is extends) that you are going to add new methods to the interface, that's not necessarily true. The second and more important way to differentiate your new class is to change the behavior of an existing base-class method. This is referred to as overriding that method.
Polymorphism
 Polymorphism is something like one name many forms. Polymorphism is also known as dynamic binding or late binding or run-time binding.

2. What is an abstract class?
You create an abstract class when you want to manipulate a set of classes through this common interface. All derived-class methods that match the signature of the base-class declaration will be called using the dynamic binding mechanism.
A class containing abstract methods is called an abstract class. Class does not contain implementation for Abstract methods. If a class contains one or more abstract methods, the class itself must be qualified as abstract. (Otherwise, the compiler gives you an error message.)

3. What is an interface?
The interface keyword takes the abstract concept one step further. You could think of it as a "pure" abstract class. It allows the creator to establish the form for a class: method names, argument lists, and return types, but no method bodies. An interface can also contain fields, but these are implicitly static and final. An interface provides only a form, but no implementation. An interface says, "This is what all classes that implement this particular interface will look like."

4. What is Polymorphism in java?
The meaning of Polymorphism is something like one name many forms. Polymorphism is also known as dynamic binding or late binding or run-time binding. In java polymorphism exists in three distinct forms

1. Method overloading

Method overloading can be done on the basis of number of argument in a method , or order of arguments or class of the function. Overloading can't be done on the basis of return type. This is because sometime methods are called only for their side effect and their return value might be ignored. E.g.

 public class Test {
 // First method
 public void display(){}
 // Second method
 public void display(String msg){}
 // Third method: Not allowed
 public String display(){}
 }
     

In the above example first and second display methods are overloaded on the basis of number of arguments while we can't define third display method with same arguments and different return type as compiler won't be able to differentiate that which method is called.
2. Method overriding through inheritance

overriding methods in extended class which are defined in base class. E.g.

 public class ExtendedTest extends Test {
 // overriding Method
 public void display(String msg){}
 public static void main(String[] args) {
     ExtendedTest eTest = new ExtendedTest();
     eTest.display("ExtendedTest");
     Test test = new Test();
     test.display("Test");
 }
 }
     

That means the method "display" in the "ExtendedTest" class is given more preference than " display " in the "Test" class for any " ExtendedTest " object. This is what is called overriding the methods. If there is no "display" method defined in "ExtendedTest" class then object from "ExtendedTest" will use method defined in "Test" class. That means compiler doesn't have information at compile time that which method to call. This decision is takes at run time. That is why this is called late binding or runtime binding.

3. Method overriding through the Java interface

This could be the case when we have one interface implemented by more than one java class. Let's assume that there is an interface ITest and classes TestA & TestB both implement this interface. Any statement like E.g.

 ITest test1 = new TestA();
 //Or
 ITest test2 = new TestB();
     
can't be resolved at compile time. This could be resolved at runtime.

5. What is a static in java?

A class is the blueprint from which individual objects are created. We won't get anything until we create an object of that class with new and at that point we get space of data storage and handle (method) for this data.
There are two cases which don't fit in above situation. One we want only one storage of data for every object or even if no object is created. The second situation is that if we want a method that is not associated with any object. Method is directly associated with the class and operate on class data (static) only.
We can achieve both by static keyword. When we say variable or method static that means it is related with class and every instance will access same copy.
If a method is declared static then we can use only static data member inside it. Static method in super class can be overridden by static method in sub-class only. However we can't override a static method with non-static method. Static method is implicitly final.
It is not recommended to use instance name for accessing static data or method since this gives a feeling that data or method is associated with object and not with the class. But that is not the case.

6. What are transient variables?

During serialization, there might be a particular variable that you don't want Java's serialization mechanism to automatically save and restore. This is commonly the case if that variable represents sensitive information that you don't want to serialize. Even if that information is private in the object, once it has been serialized, it's possible for someone to access it by reading a file or intercepting a network transmission.
To control this, you can turn off serialization on a field-by-field basis using the transient keyword. This keyword indicates that the value of this variable doesn't have to be serialized. When the object will be de-serialized, this variable will be initialized with a default value of its data type.


String
1. Describe String in Java?

Strings are immutable. A string can't be altered once created. Applying a method on string won't change string's value; rather it will create a new string. String is also final, and so can't be sub classed. When you assign one String variable to another, no copy is made. Even when you take a substring there is no new String created.
Creating String : String can be created by assigning string literals as shown below. The text between double quotes is a string literal. By assigning a string literal, you can avoid using the new keyword. In fact, this special shortcut syntax was designed to improve String performance. Each JVM only keeps one copy of each string literal. In the following code firstStr and secondStr points to same string reference. thirdStr creates a new String in String pool.

String firstStr = "I am a String Literal";
String secondStr = "I am a String Literal";
// Points to the same object as firstString
String thirdStr = new String("I am a String Literal");
// By using constructor, It will create a new Object in JVM

Note: You almost always want to initialize String references with literals to avoid creating unnecessary objects in the JVM. This can really add up if you are creating hundreds of identical Strings.

2. What are the differences between the == operator and the equals() method?

Equals method compare values of the receiver and sender object. It returns true if the values are same. While '==' operator is a fundamental operator, it compares reference of the sender and receiver object. E.g.

String s1 = "Hello";
String s2 = "Hello";
String s3 = new String("Hello");
System.out.println(s1 == s2); // true
System.out.println(s1 == s3);// false
System.out.println(s1.equals(s2));// true
System.out.println(s1.equals(s3));// true

From the example, we can see that reference and value for s1 & s2 are same. At the same time s3 is a newly created instance which has same value as of s1 and s2 but reference is different.

3. What is String.intern()?

String.intern returns a canonical representation for the string object. A pool of strings, initially empty, is maintained privately by the class String. All literal strings and string-valued constant expressions are stored in this pool.

When the intern method is invoked, if the pool already contains a string equal to this String object as determined by the equals(Object) method, then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned.

It follows that for any two strings s and t, s.intern() == t.intern() is true if and only if s.equals(t) is true. By using intern one can get better performance as compared to equals method in String class. equals does the character by character that you probably want.

There are some disadvantages of intern. If we start using intern with every string then pool size will increase and could lead to memory problem. Secondly in case of bigger pool, finding a string could be more costly that having character by character comparison.

4. Which one of the following is more efficient?

 a. String str1 = "Hello " + "Guest";
 b. String str2 = (new StringBuffer().append("Hello ")).append("Guest")
                  .toString();

str1 is resolved at compile time, while str2 is resolved at runtime time with an extra StringBuffer and String. The version that can be resolved at compile time is more efficient. It avoids the overhead of creating a String and an extra StringBuffer, as well as avoiding the runtime cost of several method calls.

5. What will be output of the following program?

public static void main(String[] args) {
  String s1 = new String("Hello");
  change(s1);
  System.out.println(s1);
}

public static void change(String s1) {
  s1 += " Guest";
}
     

The output of the program will be "Hello" only. " Guest" won't be appended to the original string.
This is due to immutable nature of string. In the change method, appending string to s1 won't change previous string, rather it will create a new string which will be "Hello Guest" and will be pointing to some other memory location and not to the String location which is passed to the 'change' method.

Containers
1. What do you know about Containers?

There are two types of container library available in java.
 1. Collection: Collection category only holds one item in each location. This includes List and Set Interfaces.
 a. List (ArrayList, Vector, LinkedList): A collection of elements, duplicates allowed.
 b. Set (HashSet, LinkedHashSet, TreeSet): A collection of unique elements.

 2. Map: The Map holds key-value pairs, rather like a mini database. We can look up on the basis of key. This includes Map interface.
 a. Map (HashMap, HashTable, LinkedHashMap, TreeMap): A mapping of keys to values.

2. What are the differences between Vector and ArrayList?

The two classes are very similar, however there are few differences, which can give one a clear idea about their usages.Vector is synchronized. Content in vector is thread-safe. ArrayList is not synchronized hence its content is not thread-safe. Synchronization is not at free of cost. It affects vector's performance. So if we don't require thread-safe data, we should use ArrayList.
Internally, both the ArrayList and vector hold data in Array. When we add a new element into ArrayList or Vector, and the object will need to expand its internal array if it runs out of memory. Vector doubles the size of Array by default, while the ArrayList increases its size by 50 percent. Vector does have slight advantage since they allow to reset increment size.Addition and deletion operation in both classes takes same time.
Addition at the end of container can be performed in constant time O(1). Deletion and Addition are more expansive when the element is added/removed from middle, it takes O(n-i). Here n is number of elements in the container and i is index of the element.

3. What are the differences between ArrayList and LinkedList?

Both classes implement List interface. Order is the most important feature of a List; it promises to maintain elements in a particular sequence. Following are the main differences
1. An ArrayList implemented with Array, allowed fast random access to an element, while LinkedList are optimal for sequential access and are relatively slow for random access.
2. ArrayList is slow when inserting or removing element from middle of the List while LinkedList provide inexpensive insertion and deletion from the middle of the List.

4. Difference between HashTable and HashMap?
 The HashTable is almost equal to HashMap except following points.
1. HashTable is synchronized, while HashMap is not.
2. HashTable doesn't permit null as key while in HashMap it is allowed.
3. HashTable guarantee that order of the table will remain constant over time while in HashMap it is not.


Garbage Collector
1. What is Garbage Collection ?

Memory for the new object created in java is allocated in heap at runtime. When an object is no longer referenced by the program, the heap space it occupies must be recycled so that the space is available for subsequent new objects.
Garbage collection is the process of automatically freeing objects that are no longer referenced by the program.
During object cleaning, free memory in the heap may not be continuous, free memory may be between live objects. It's job of GC to fragment heap memory.
There are many advantages of having garbage collector
1. It increases productivity. One need not worry about memory issues.
2. It ensures program integrity. Garbage collection is an important part of Java's security strategy. Java programmers are unable to accidentally (or purposely) crash the Java virtual machine by incorrectly freeing memory.

A potential disadvantage of a garbage-collected heap is that it adds an overhead that can affect program performance. The Java virtual machine has to keep track of which objects are being referenced by the executing program, and finalize and free unreferenced objects on the fly. This activity will likely require more CPU time than would have been required if the program explicitly freed unnecessary memory. In addition, programmers in a garbage-collected environment have less control over the scheduling of CPU time devoted to freeing objects that are no longer needed.

2. How you can force the garbage collection?
 We can't have forced garbage collection. System.gc( ) does not necessarily force a synchronous garbage collection. Instead, the gc( ) call is really a hint to the runtime that now is a good time to run the garbage collector. The runtime decides whether to execute the garbage collection at that time and what type of garbage collection to run.

3. What are the advantages and disadvantages of reference counting in garbage collection over tracing algorithm?
??
Both classes implement List interface. Order is the most important feature of a List; it promises to maintain elements in a particular sequence. Following are the main differences
1. An ArrayList implemented with Array, allowed fast random access to an element, while LinkedList are optimal for sequential access and are relatively slow for random access.
2. ArrayList is slow when inserting or removing element from middle of the List while LinkedList provide inexpensive insertion and deletion from the middle of the List.


Thread

1. What is a thread?
A thread is an independent path of execution within a process. Many threads can run concurrently within a process. A process is a program having its own address space.

2. What is the difference between multithreading & multitasking?

In multithreaded program, more than one thread is running concurrently. It is not equivalent to starting a program twice at the same time. If we start two or more programs at a time, this will require separate address space for both data and code, making it more CPU intensive operation. This type of program execution is called multitasking.
In multithreaded environment, sharing of data is done across the threads, which makes it less CPU intensive but brings up some other issues which may even corrupt data if not handled carefully.
Multitasking is a heavyweight task that require own address space. Communication between different processes is very expensive. Also context switching from one process to another is costly since they are running in different address spaces.
Multithreading is lightweight. Communication between two threads is not that expensive as it is in case of processes. Context switching is also less expensive as both share same address space.

3. What is synchronization?

You can think of a single threaded program which is accessing some resource at a time. You are not worried by the fact that if some other thread is accessing same resource at the same time.
With multithreaded environment there will be possibility that two or more threads are trying to access same resource at the same time. This may lead to some error. Colliding over resource must be prevented to avoid any such error.

Consider the bathroom in your house; multiple people (threads) may each want to have exclusive use of the bathroom (the shared resource). To access the bathroom, a person knocks on the door to see if it's available. If so, they enter and lock the door. Any other thread that wants to use the bathroom is "blocked" from using it, so that thread waits at the door until the bathroom is available.
In Java, to solve this problem of thread collision, all the multithreaded program should synchronize access to shared resource. This is achieved by putting a locked clause around a piece of shared code so that one thread at a time may pass through that piece of code. Serializing access of shared resource is called synchronization. Synchronization is Java is achieved by "synchronized " keyword.


Patterns
1. What are the different types of Java patterns?

Let's first understand what a design pattern is.
When a problem occurs repetitively in all application across programmers, the solution to that problem is defined as a design pattern.
Most of design patterns are language independent strategies to common object oriented problems.

Typically java has three design patterns.
1. Creational Design Pattern
 As the name suggests, this pattern is all about instantiated a class and how object is managed across the program to reduce duplicate objects and unnecessary coding which might be caused by using the basic form of object creation. This pattern is again subdivided in to class creational patterns and object creational patterns. Common examples of such patterns are -
 a. Factory
  Delegates the work of creating the object instance to derived classes of the interface.
 b. Abstract Factory
  Used for creating groups of dependant instances of classes without specifying their concrete classes. It's the centralize decision of what factory to instantiate.
c. Builder

It's used to separate the instantiation of a complex object from its representation so that same construction process can be used for different representations.
d. Singleton

Ensures at every point in the application, there is only and only one instance of that class and also provides a global access point to the same.
e. Prototype

When the typical way of instantiating an object (i.e. "new") is costly. In this pattern new instances are created by copying the prototype of that class.
f. Lazy initiation

Supply when demanded. Instead of already having a created object, create only when it's required.
g. Objects pool

Recycling the objects when they are not in used, rather than releasing them and created new ones when they are required.
2. Structural Design Patterns

These are the patterns used for realizing the relationship between different entities. It defines ways to compose an object to obtain new functionality. Common examples of such patterns are:
a. Adapter/Wrapper

Its used to convert an interface into the one that client expects to see. That is, it adapts the interface to an expected class. It makes incompatible interfaces work together by wrapping them with its own interface.
b. Composite

It's used for defining a whole hierarchy tree structure of objects where each object has the same interface. This allows client to treat a group of different objects in the same way.
c. Bridge

It separates an abstraction from its implementation so that two can vary differently. That is the implemented class can be launched independently and work as its own system.
d. Decorator

Used for adding functionality to the objects at runtime. It's an alternative for sub classing. Sub classing ads functionality to an object at compile time where as decorating adds functionality at the runtime.
e. Facade

Providing an additional layer to existing subsystems to hide complexities. It makes an entry system to the sub systems.
f. Flyweight

When data is shared between numbers of objects to prevent memory consumptions. It's not possible to share all the object state data; therefore it's commonly put in external data structures to populate object's property at runtime (On the Fly).
g. Proxy

Using a mirror substitute of original object if creation of original object is time taking or memory consuming (when that memory consuming operation is not needed for this object to perform). This is often used in as a placeholder typically used in conjunction which flyweight pattern where one instance and multiple proxy are created all pointing to same reference.
2. What is singleton pattern? Write an example singleton class?

A singleton class is a class which has only one instance per application. Rather per JVM. This design pattern is extensively used when user needs exactly one instance. For example facade objects, state objects or for global variables (as it permits lazy allocation and initialization which may not happen in all languages).

To make a class singleton, its constructor is made private or protected so that it can not be instantiated. Instead, a getInstance () method is written to return a new instance of that class if it doesn't exist or return the reference of that object if it already exists. If it's a multithreaded environment make sure to synchronize the getInstance() menthod.
Conventional way of writing a singleton class is :

public class Singleton {
private final static Singleton INSTANCE = null;
// to prohibit instantiation
private Singleton() {}

public static Singleton getInstance() {
 if(instance==null) {
  // lazy initialisation
  INSTANCE = new Singleton();
     }
 return INSTANCE;
 }
}
            

You can synchronise on getInstance() method for a multi threaded environment.

Another thread-safe Java programming language lazy-loaded solution is:

public class Singleton {
 private Singleton() {}

 // lazy initialisation
 private static class SingletonHolder {
  private final static Singleton INSTANCE = new Singleton();
 }

 public static Singleton getInstance() {
  return SingletonHolder.INSTANCE;
 }
}
         

Remember a singleton class is one instance per JVM. So if you have your application running in a cluster of servers. Every server will have its own JVM, therefore its own instance of singleton class. In this case, purpose of making a class singleton is not fulfilled. Hence this pattern should be used with care.