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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.

6 ways to convert char to String in Java - Examples

If you have a char value e.g. 'a' and you want to convert it into equivalent String e.g. "a" then you can use any of the following 6 methods to convert a primitive char value into String in Java :

1) String concatenation
2) String.valueOf()
3) Character.toString()
4) Character wrapper class + toString
5) String constructor with char array
6) String.valueOf(char [])

In this article, we will see examples of each approach and learn a little bit more about it. Actually, there is lot of overlap between each method as some of them internally calls String.valueOf(), which eventually calls to a String constructor which accepts char array and creates a String object containing primitive char value with length 1. Once you know, how they work internally, it easy to decide which one is more efficient for purpose.

How to use BigInteger class in Java? Large Factorial Example

When you calculate factorial of a relatively higher number most of the data type in Java goes out of their limit. For example, you cannot use int or long variable to store factorial of a number greater than 50. In those scenarios where int and long are not big enough to represent an integral value, you can use java.math.BigInteger class. BigInteger variable can represent any integral number, there is no theoretical limit, but it allocates only enough memory required to hold all the bits of data it is asked to hold. There is not many time you need this class but its good to know about it and that's why I am giving one scenario which is perfectly suitable for using BigInteger class. In this article, you will learn how to calculate factorial of large number using BigInteger object. We will use same formula, we have used to calculate normal factorials as discussed in my previous post about factorials.

How to sort HashSet in Java? Example

Somebody asked me recently, how do you sort an HashSet? For lists, we use the Collections.sort(List) method, but there is nothing for Set. If I have an HashSet then how would I go about sorting it? The answer is you cannot sort an HashSet, why? because HashSet is an unordered collection. When you insert an element in HashSet than you lose the order guarantee. You cannot do reordering or sorting in Set because it does not have random access methods (ie, .get() an element at a given index), which is basically required for sort algorithms. Though you can sort the HashSet by first converting HashSet to List and then sorting it. Also, some of Set implementation may keep the order intact e.g. LinkedHashSet maintains insertion order, which means you can sort LinkedHashSet but not HashSet. Alternatively, you can also use TreeSet to keep elements in the sorted order from the start.

Why you shouldn't use == with float and double in Java?

In this article, you are going to learn why you shouldn't use == with float and double in Java? Especially for checking loop termination condition. Java programmers often make the mistake of using floating point number in a loop and checking condition with the == operator, in the worst case this could create an infinite loop, causing your Java application to hung.


For example, following code will not work as you expect :

for(double balance = 10; balance!=0; balance-=0.1) {
   System.out.println(balance);
}