Java Cache : Static data loading

Application performance highly depends on how the data is loaded into memory and how it is reused during application processing. If our application does database query for each static \client data which is rarely changing then we need to consider putting this static data in memory\cache. The application performance increase drastically if we reuse this static data from memory. In the market we have lots of open source cache implementation available like ehCache, osCache[Terracota], Guava library etc. In some cases these open source implementation can be more complicated compare to application requirement. After java 1.5 concurrent collections it is become very handy to write own cache. In static data cache we always want to control memory foot print and we use LRU [ Least recently Used] idiom to do this. We fix the size of cache and clear the oldest cache data if the cache needs to be refreshed behind its size. Here is example of LRU cache which is backed by ConcurrentHashMap and controlled by ConcurrentLinkedQueue.

Cache simply means loading the data by some key and it should be in memory. If we are running our application in clustered mode then available implémentation is Coherence which can isolated interaction with DB and provided fail over and syncronization between multiple application cache.

Here is example how we can build LRU cache in java using concurrent package. This implementation will be fragile and blocking if we would have implemented cache using old synchronize idiom. Cache data is backed up by ConcurrentHashMap so that reading from cache is not blocked and writing to cache will have concurrent effect, it means only the buckets will be locked during write operation.

Caching Implementation without concurrency using wait() and notify()

Java 1.7 features

'Java latest changes' or' 'Java 1.7 changes' are covered in this topic. Java 1.7 [ Dolphin] is major update after java 1.5 tiger release. It is done on 2011-07-28 after around 5 years of java release 1.6 [Mustang]. These are major changes in java 1.7 release and most important is definitely 'Auto closeable' of resources.

1) Autocloseable Try Statement defining Resources – Java 1.7 introduces all new try-with-resources statement using which declaration and initialization of one or more resources can happen. But only the resources that implement the interface “java.lang.AutoCloseable” can be declared. Example:

      try (BufferedReader bufferedReader = new BufferedReader( FileReader(path)))

         {             return bufferedReader.readLine();  

         }

  In this code snippet, sampleBufferedReader instance is created within the try statement. Note that the example does not include a finally block that contains a code to close sampleBufferedReader as in Java 1.6 or earlier versions. Java 1.7 automatically closes the resources that are instantiated within the try-with-resources statement as shown above.

2) Catch Block Handling Multiple Exceptions – In Java 1.5 and Java 1.6, a catch block can handle only one type of exception. But in Java 1.7 and later versions, a single catch block can handle multiple exceptions. Here is an example showing catch blocks in Java 1.6:

     try {    }

      catch(SQLException exp1)

      {     throw exp1;    }

       catch(IOException exp2)

       {    throw exp2;   }

The same code snippet can be modified in Java 1.7 as:

      try  {….      }

      catch(SQLException | IOException |ArrayIndexOutofBoundsException exp1)

      {     throw exp1;   }

3) String Object as Expression in Switch Statement – So far only integral types are used as expressions in switch statement. But Java 1.7 permits usage of String object as a valid expression. Example:

   example:           

                            case "CASE1":

                                     System.out.println(“CASE1”);

                                     break;

4) JDBC in Java 1.7

JDBC contained in Java 1.7 / Java SE 7 is JDBC 4.1 that is newly getting introduced. JDBC 4.1 is more efficient when compared to JDBC 4.0.

5) Language Enhancements in JDBC 1.7

Java 1.7 introduces many language enhancements:

  Integral Types as Binary Literals – In Java 1.7 / Java SE 7, the integral types namely byte, short, int and long can also be expressed with the binary number system. To specify these integral types as binary literals, add the prefix 0B or 0b to number. For example, here is a byte literal represented as 8-bit binary number:

     byte sampleByte = (byte)0b01001101;

   

  Underscores Between Digits in Numeric Literal – In Java 1.7 and all later versions, “_” can be used in-between digits in any numeric literal. “_” can be used to group the digits similar to what “,” does when a bigger number is specified. But “_” can be specified only between digits and not in the beginning or end of the number. Example:

long NUMBER = 444_67_3459L;

In this example, the switch expression contains a string called sampleString.  The value of this string is matched with every case label and when the string content matches with case label then the corresponding case gets executed. 

  Automatic Type Inference during the Generic Instance Creation – In Java 1.7 while creating a generic instance, empty parameters namely <> can be specified instead of specifying the exact type arguments. However, this is permitted only in cases where the compiler can infer the appropriate type arguments. For example, in Java 1.7 you can specify:

      sampleMap = new HashMap<>();

Thus HashMap<> can be specified instead of HashMap>;. This <>; empty parameters of Java 1.7 are named as diamond operator.

6) Suppress Warnings - When declaring varargs method that includes parameterized types, if the body of the varargs method does not throw any exceptions like ClassCastException (which occurs due to improper handling of the varargs formal parameter) then the warnings can be suppressed in Java 1.7 by three different ways: 

(1) Add annotation @SafeVarargs to static method declarations and non constructor method declarations 

(2) Add annotation @SuppressWarnings({"unchecked", "varargs"}) to varargs method declaration 

(3) Directly use compiler option “-Xlint:varargs.

By suppressing the warnings in varargs method, occurrence of unchecked warnings can be prevented at compile time thus preventing Heap Pollution.

7) Java Virtual Machine Enhancements in Java 1.7

Java SE 7 / Java 1.7 newly introduce a JVM instruction called “invokedynamic” instruction.  Using “invokedynamic” instruction, the dynamic types programming language implementation becomes simpler. Thus Java 1.7 enables JVM support for the non-java languages.  

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Java Producer and Consumer Implementation using Blocking Queue

'Java Consumer Producer' example - This is one of frequently asked questions to senior core java developer. Java concurrency producer and consumer solution is demonstrated below. 
Java Concurrency Queuing options:
 The java concurrent executors and task holding queues can be configured in three ways-

Direct handoffs :  A good default choice for a work queue is a SynchronousQueue that hands off tasks to threads without otherwise holding them. Here, an attempt to queue a task will fail if no threads are immediately available to run it, so a new thread will be constructed. This policy avoids lockups when handling sets of requests that might have internal dependencies. Direct handoffs generally require unbounded maximumPoolSizes to avoid rejection of new submitted tasks. This in turn admits the possibility of unbounded thread growth when commands continue to arrive on average faster than they can be processed.

Unbounded queues : Using an unbounded queue (for example a LinkedBlockingQueue without a predefined capacity) will cause new tasks to wait in the queue when all corePoolSize threads are busy. Thus, no more than corePoolSize threads will ever be created. (And the value of the maximumPoolSize therefore doesn't have any effect.) This may be appropriate when each task is completely independent of others, so tasks cannot affect each others execution; for example, in a web page server. While this style of queuing can be useful in smoothing out transient bursts of requests, it admits the possibility of unbounded work queue growth when commands continue to arrive on average faster than they can be processed.

Bounded queues : A bounded queue (for example, an ArrayBlockingQueue) helps prevent resource exhaustion when used with finite maximumPoolSizes, but can be more difficult to tune and control. Queue sizes and maximum pool sizes may be traded off for each other: Using large queues and small pools minimizes CPU usage, OS resources, and context-switching overhead, but can lead to artificially low throughput. If tasks frequently block (for example if they are I/O bound), a system may be able to schedule time for more threads than you otherwise allow. Use of small queues generally requires larger pool sizes, which keeps CPUs busier but may encounter unacceptable scheduling overhead, which also decreases throughput.

   Example of PriorityBlockingQueue and see how the comparable is used with priority of task which depends on implementation of compare method in task.
From Java Docs-

The Blocking queue here is bounded with 11 as initial capacity as default constructor.  The queue is based on priority and least priority data is processed 
" An unbounded blocking queue that uses the same ordering rules as class PriorityQueue and supplies blocking retrieval operations. While this queue is logically unbounded, attempted additions may fail due to resource exhaustion (causing OutOfMemoryError). This class does not permit null elements. A priority queue relying on natural ordering also does not permit insertion of non-comparable objects (doing so results in ClassCastException).
This class and its iterator implement all of the optional methods of the Collection and Iterator interfaces. The Iterator provided in method iterator() is not guaranteed to traverse the elements of the PriorityBlockingQueue in any particular order. If you need ordered traversal, consider using Arrays.sort(pq.toArray()). Also, methoddrainTo can be used to remove some or all elements in priority order and place them in another collection."

Output
 Consumed Data [number=2, name=two]
 producer 0
 Consumed Data [number=10, name=ten]
 producer 1
 Consumed Data [number=20, name=twenty]
 producer 2
 Consumed Data [number=0, name=0]
 producer 3
 Consumed Data [number=1, name=1]
 producer 4
 Consumed Data [number=2, name=2]
 producer 5
 Consumed Data [number=3, name=3]

Consumer Producer Solution with Synchronisation:

This implementation with Synchronisation needs great care and it is more complicated in implementation:

Output 

The output will confim that there is no concurrency issue on putting data into same queue and wait() and notify() works perfectly fine.
 Got: 53613
 Put: 53614
 Got: 53614
 Put: 53615
 Got: 53615
 Put: 53616
 Got: 53616
 Put: 53617
 Got: 53617

Design Pattern in Java : Part 1

'Java investment bank interview' generally contains 'Java Design Pattern' questions. If you want to be a professional Java developer, you should know popular solutions for common\standard coding problems. Such solutions have been proved efficient and effective and always used by experienced developers. These solutions are described as so-called design patterns. Learning design patterns speeds up your experience accumulation in OOA/OOD. Once you grasped them, you would be benefit from them for all your life and jump up yourselves to be a master of designing and developing. Furthermore, you will be able to use these terms to communicate with your fellows more effectively.

Many programmers with many years of experience don't know how to use design patterns, but as an Object-Oriented programmer, you have to know them well, especially for new Java programmers. Actually, when you solved a coding problem, you have used one of the design pattern. 

        You may not use a popular name to describe it or may not choose an effective way to better intellectually control over what you built. Learning how the experienced developers to solve the coding problems and trying to use them in your project are a best way to earn your experience. We have mainly 3 type of design patterns and they are Creational, Structural and Behavioural. Behavioural pattern is explained in next blog - Design Pattern - part 2.

There are frequently asked question in design pattern, and these were asked to me in java interviews:
1) Explain Singleton Design pattern and How to solve Double Check locking?
2) What is Abstract Factory Pattern?
3) What is Adapter pattern?
4) What is Decorator Design Pattern?
5) Where we should use Facade in application?
6) What is Proxy and Composite?
7) What is Observer Pattern? 

Creational Patterns
[Details are from taken out from wiki description]

Factory:

The factory pattern \ factory method pattern is an object-oriented creational design pattern to implement the concept of factories and deals with the problem of creating objects (products) without specifying the exact class of object that will be created. The essence of this pattern is to "Define an interface for creating an object, but let the classes that implement the interface decide which class to instantiate. The Factory method lets a class defer instantiation to subclasses."

Creating an object often requires complex processes not appropriate to include within a composing object. The object's creation may lead to a significant duplication of code, may require information not accessible to the composing object, may not provide a sufficient level of abstraction, or may otherwise not be part of the composing object's concerns. The factory method design pattern handles these problems by defining a separate method for creating the objects, which subclasses can then override to specify the derived type of product that will be created.

The factory pattern relies on inheritance, as object creation is delegated to subclasses that implement the factory method to create objects. 

Example : Static factory method provided with Collections class like 

Collections.synchronizedMap( new HashMap()); 

Abstract Factory:

The abstract factory pattern is a software creational design pattern that provides a way to encapsulate a group of individual factories that have a common theme without specifying their concrete classes. In normal usage, the client software creates a concrete implementation of the abstract factory and then uses the generic interfaces to create the concrete objects that are part of the theme. The client does not know (or care) which concrete objects it gets from each of these internal factories, since it uses only the generic interfaces of their products. This pattern separates the details of implementation of a set of objects from their general usage and relies on object composition, as object creation is implemented in methods exposed in the factory interface.

An example of this would be an abstract factory class DocumentCreator that provides interfaces to create a number of products (e.g. createLetter() andcreateResume()). The system would have any number of derived concrete versions of the DocumentCreator class like FancyDocumentCreator or ModernDocumentCreator, each with a different implementation of createLetter() and createResume() that would create a corresponding object like FancyLetter or ModernResume. Each of these products is derived from a simple abstract class like Letter or Resume of which the client is aware. The client code would get an appropriate instance of the DocumentCreator and call its factory methods. Each of the resulting objects would be created from the same DocumentCreator implementation and would share a common theme (they would all be fancy or modern objects).

Example : The best example will be configuring Database connection factory which hide underline Database type. We can configure Oracle, DB2 or Sybase Connection Factory in application.

    > java.util.Calendar#getInstance()
    > java.util.ResourceBundle#getBundle()
    > java.text.NumberFormat#getInstance()




Singleton:

The Singleton pattern is a design pattern that restricts the instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system. The concept is sometimes generalized to systems that operate more efficiently when only one object exists, or that restrict the instantiation to a certain number of objects. The term comes from the mathematical concept of a singleton.

In the second edition of his book Effective Java, Joshua Bloch claims that "a single-element enum type is the best way to implement a singleton" for any Java that supports enums. The use of an enum is very easy to implement and has no drawbacks regarding serializable objects, which have to be circumvented in the other ways.

 Bill Pugh has written about the code issues underlying the Singleton pattern when implemented in Java. Pugh's efforts on the "Double-checked locking" idiom led to changes in the Java memory model in Java 5 and to what is generally regarded as the standard method to implement Singletons in Java. The technique known as the initialization on demand holder idiom, is as lazy as possible, and works in all known versions of Java. It takes advantage of language guarantees about class initialization, and will therefore work correctly in all Java-compliant compilers and virtual machines.

The nested class is referenced no earlier (and therefore loaded no earlier by the class loader) than the moment that getInstance() is called. Thus, this solution is thread-safe without requiring special language constructs (i.e. volatile or synchronized). More details on Singleton Pattern.
Example: Getting Runtime environment using Runtime class is example of Singleton design pattern -- java.lang.Runtime#getRuntime()

Builder:
The builder pattern is an object creation software design pattern. The intention is to abstract steps of construction of objects so that different implementations of these steps can construct different representations of objects. Often, the builder pattern is used to build products in accordance with the composite pattern.The intent of the Builder design pattern is to separate the construction of a complex object from its representation. By doing so, the same construction process can create different representations.

Builder

Abstract interface for creating objects (product).

Concrete Builder

     Provides implementation for Builder. It is an object able to construct other objects. Constructs and assembles parts to build the objects.
Example : Builder pattern is frequently used in Java classes where We need special purpose classes like StingBuilder and StringBuffer.

    > java.lang.StringBuilder#append() 
    > java.lang.StringBuffer#append() 
    > java.nio.ByteBuffer#put()

Prototype:

The prototype pattern is a creational design pattern used in software development when the type of objects to create is determined by a prototypical instance, which is cloned to produce new objects. This pattern is used to:

• avoid subclasses of an object creator in the client application, like the abstract factory pattern does.
• avoid the inherent cost of creating a new object in the standard way (e.g., using the 'new' keyword) when it is prohibitively expensive for a given application.

To implement the pattern, declare an abstract base class that specifies a pure virtual clone() method. Any class that needs a "polymorphic constructor" capability derives itself from the abstract base class, and implements the clone() operation.

The client, instead of writing code that invokes the "new" operator on a hard-coded class name, calls the clone() method on the prototype, calls a factory method with a parameter designating the particular concrete derived class desired, or invokes the clone() method through some mechanism provided by another design pattern.

Example: Best example will be Java.lang.Object#clone() method. Class has be implement Cloneable interface to use this.  

Design Pattern - part 2

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Java Interviews Frequently Asked Puzzles

Interview puzzles is the best way to check person's analytical skills. In investment banking job you need analytical skill to perform day to day job. It is not only applicable to traders , but some level of analytical skills is required by people who are developing these systems for understand business. Now a days it is become common practice to ask 1-2 puzzles during investment bank interview to check the candidate's analytical skills. The best way to clear this round is practice common puzzles and understand the logic to solve them. 

1) Fibonacci in Java:

The input will be number n and the output should be sum for 0 to n. for example

for n =4 the result should be 0+1+2+3+4 = 10

2) String Reverse

The input String "abcde" should return "edcba".

3) Reversing a linked list in Java

Here is example of revering the linked list in java using recursive function:

4) Find the missing number in Java

You have an array of numbers from 1 to 100 (both inclusive). The size of the array is 100. The numbers are randomly added to the array, but there is one random empty slot in the array. What is the quickest way to find that slot as well as the number that should be put in the slot?

Try it for practice. please suggest the answer .

[Trick sum of n numbers is n*(n+1)/2]

5) Write a substring function in Java

String test= "AA BB CC BB BB CC BB";

String[]{"BB", "CC", "AA"}

Result shd be BB=4; CC=2 and AA=1

Since B occurred 4 times C did 2 times and A only 1 time.

This basic problem can be asked in different ways like, You have multiple words in new paper and find out the frequency of words in one page of news paper?

6) Reverse a String in Java

Reverse the String by java function without recursion and with recursion

7) Find one string inside another in Java

We can use String.indexOf( subString ) and it will return the first index of substring;

for Last Index : lastIndexOf(String str)

8) Algo for finding largest number in Array of Integer

Easy one, Check the integer one by one and find the largest number.

9) Java Runtime method invocation question:

Example : Tell the output of this

Answer is : 20 .

10) Suppose you have a large file with lots of words. How would you find the unique words and their count? What kind of data structure u will use? What will be the time complexity and space complexity?

We need to take care of two things counting the words and second duplicate. the best performance will be using hash function.

11) A train is one mile long. It travels at the rate of one mile a minute through a tunnel which is also one mile long. Can you say how long it will take for the train to pass completely through the tunnel? 

Answer : 2 minutes

it will take two minutes if you count the time for it to completely pass through the tunnel. One minute to pass through the tunnel and another one minute to drag itself out of the tunnel completely so two minutes nice question though well logical.

12) Convert String = "98989" into an integer without using any library functions in java.

Give fastest way to do it and explain why your method is best.

// converting string to number using ascii code

13) Write a program to shuffle a deck of 52 cards and shuffle them equally to 4 players.

Answer this puzzle by comments.

Given n stairs, how many number of ways can you climb if u use either 1 or 2 at a time?

for example you have 4 stairs and you can climb like

1,1,1,1

1,1,2

1,2,1

2,1,1

2,2

so in overall 5 ways for 3 stairs.