Wednesday, 17 January 2018

JDBC - How to get Row and Column Count from ResultSet in Java

One of the common problem in JDBC is that there is no way to get the total number of records returned by an SQL query. When you execute a Statement, PreparedStatement, or CallableStatement using execute()or executeQuery() they return ResultSet and it doesn't have any method to return the total number of records it is holding. The only way to find the total number of records is keeping the count while you are iterating over ResultSet while fetching the result. This way, you can print the total number of rows returned the SQL query but only after you have processed all records and not before, which may not be the right way and incur significant performance cost if the query returns a large number of rows.

Monday, 15 January 2018

Java Reflection, but much faster

What is the fastest way to read a getter from a Java class without knowing the class at compilation time? Java frameworks often do this. A lot. And it can directly influence their performance. So let’s benchmark different approaches, such as reflection, method handles and code generation.

Friday, 12 January 2018

Java 9 Module Services

Wiring and Finding

Java has a ServiceLoader class for long time. It was introduced in 1.6 but a similar technology was in use since around Java 1.2. Some software components used it, but the use was not widespread. It can be used to modularize the application (even more) and to provide a mean to extend an application using some kind of plug-ins that the application does not depend on compile time. Also, the configuration of these services is very simple: just put it on the class/module path. We will see the details.

Wednesday, 10 January 2018

Java Program to reverse an array in place? Fastest Example

It's easy to reverse an array if you have the luxury to use another array, but how would you reverse an array if a temporary buffer is not allowed? This is one of the testing array interview questions, which often proved tricky for Java programmers. Well, you can also reverse an array in place without using an additional buffer. If you know how to access array elements and how to loop over an array in Java using traditional for loop, you can easily solve this problem without using additional space. All you need to do is a loop over the array from start to the middle element and swap the first element to the last, second element to the second last etc. Once you reach the middle element, your array is already sorted and that too without using any additional space. You can even use this algorithm to reverse a String in Java as well. After all, a String is backed by character array.

Monday, 8 January 2018

How to implement Linear Search in Java? Example

Linear search is nothing but iterating over the array and comparing each element with target element to see if they are equal since we search the array sequential from start to end, this is also known as sequential search or linear search. It is very slow as compared to binary search because you have to compare each element with every other element and definitely not suitable for a large array. It's practically useful only in case of the small array up to 10 to 15 numbers. In the worst case, you need to check all elements to confirm if target element exists in an array or not.

Wednesday, 3 January 2018

Simple String Representation of Java Decimal Numbers without Scientific Notation

The primary types/objects used for decimal numbers in Java are float/Float, double/Double, and BigDecimal. Each of these has cases in which its “default” string representation is “computerized scientific notation.” This post demonstrates some simple approaches to provide a string representation of the decimal number in these cases without scientific notation.

Monday, 1 January 2018

Choosing Java Cryptographic Algorithms Part 1 – Hashing


I did all of the work for this post using the following major technologies. You may be able to do the same thing with different technologies or versions, but no guarantees.

◉ Java 1.8.0_152_x64
◉ NetBeans 8.2 (Build 201609300101)
◉ Maven 3.0.5 (Bundled with NetBeans)

Friday, 29 December 2017

Thread-safe Singleton in Java using Double Checked Locking Idiom

Singleton Pattern is one of the famous design patterns from the Gang of Four. Even though nowadays it is considered as an anti-pattern, it has served us well in the past. In Singleton pattern, a class has just one instance throughout its lifetime and that instance is shared between multiple clients. Singleton class has two responsibility, first to ensure that only instance of the class gets created and second, provide a method getInstance() so that everyone can get access to that single instance i.e. global access. One of the issue, faced by Singelton design pattern in the multi-threading program is to ensure that just one instance of the class gets created, even if multiple clients called getInstance() method same time. Many programmers solved this problem by making whole getInstance() method synchronized, which results in poor performance because every time a thread enters a synchronization method, it acquires the lock and while it's been inside the method, no other thread are allowed to enter, even if they are not creating instance and just accessing already created instance.

Wednesday, 27 December 2017

How to print leaf nodes of binary tree without recursion

You have learned how to print all leaf nodes of a binary tree in Java by using recursion and in this article, we'll solve the same problem without using recursion. Why should we do this? Well, it's a common pattern on programming job interview to solve the same problem using recursion and iteration. Since some problems are easy to solve using recursion e.g. tree based problems, tower of Hanoi, or Fibonacci series but their non-recursive solution is comparatively difficult, interviewer test candidates against this shift in the algorithm. If you have attended your computer science classes and enjoyed there, then you know that we can use Stack to convert a recursive algorithm to an iterative one. I'll use the same technique to print all leaf nodes of a binary tree without recursion.

Friday, 22 December 2017

How to pause Thread in Java using Sleep() and TimeUnit Example

There are multiple ways to pause or stop the execution of currently running thread, but putting the thread into sleep state using Thread.sleep() method is the right way to introduce pause. Some people would say, why not use wait and notify?. Using those methods just for pausing thread is not good. Those are the tools for a conditional wait and they don't depend on upon time. A thread blocked using wait() will remain to wait until the condition on which it is waiting is changed. Yes, you can put timeout there but the purpose of wait() method is different, they are designed for inter-thread communication in Java. By using sleep() method, you pause the current for some given time. You should never use sleep() in place of wait() and notify() and vice-versa. There is another reason why to wait and notify should not be used to pause the thread, a they need lock. You can only call them from a synchronized method or block and acquire and release a lock is not cheap.