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Monday, August 31, 2015

How to convert list to array in Java and vice-versa

There is no easy way to convert an array to list in Java, but you can easily convert a list into array by calling toArray() method, which List inherit from Collection interface. If you solely rely on core JDK, then only way to convert an array to list is looping over array and populating list one element at a time. But if you can use open source libraries like Google Guava or Apache Commons lang then there is many utility classes to convert list to array and vice-versa, as shown in this tutorial. If you are working on Java application, you will often need to convert between list and array. A list is nothing but a dynamic array which knows how to re-size itself when it gets full or get close to full. List uses load factor to decide when to re-size, default value of its is 0.75. When they re-size, list usually double their slots e.g. goes from 16 to 32 etc. You can find these nifty details in their implementation classes e.g. ArrayList is one of the popular list in Java which provides order and random access. BTW, if you want to truly master Java Collection framework, then you must read Java Generics and Collection book, written by Maurice Naftaline and one of the must read book to become expert on Java Collections framework.

Saturday, August 29, 2015

How to randomize elements in List in Java using shuffle method

java.util.Collections class provides shuffle() method which can be used to randomize object stored in a List in Java. Since List is an ordered collection and maintains the order on which objects are inserted into it, you may need to randomize elements if you need them in different order. Collections.shuffle() method uses default randomness to randomize element but you also has an overloaded version of shuffle() to provide an instance of  java.util.Random object, which can be used to randomize elements. Since this method except a List, you can also pass it to LinkedList, VectorCopyOnWriteArrayList and others, which doesn't implement RandomAccess method. In such cases, this method convert list to array before shuffling to avoid quadratic performance by shuffling sequential access list. Once shuffling is done it also converts back array to list. Shuffling has many usage e.g. shuffling deck of cards in a poker game simulation. You can also use shuffling to roll dice if you are developing any board game which requires dice e.g. Ludo.

Tuesday, August 25, 2015

2 ways to parse String to int in Java

Java provides Integer.parseInt() method to parse a String to an int value, but that's not the only way to convert a numeric String to int in Java. There is in fact a better way, which take advantage of parsing logic of parseInt() method as well as caching offered by Flyweight design pattern, which makes it more efficient and useful. Yes, you guessed it right, I am talking about Integer.valueOf() method, which implements Flyweight design pattern and maintains a cached pool of frequently used int values e.g. from -128 to 127. So every time you pass a numeric String which is in range of -128 to 127, Integer.valueOf() doesn't create a new Integer object but return the same value from cached pool. Only drawback is that Integer.valueOf() returns an Integer object and not an int primitive value like parseInt() method, but given auto-boxing is available in Java from JDK 5 onward, which automatically convert an Integer object to int value in Java. BTW, if you are learning Java and want to master fundamentals, I suggest you to take a look at Head First Java 2nd Edition, they explains the concept in the easiest way possible but also brings out important details.