Knowledge Cafe

I will explain you what is java serialization, then provide you with a sample for serialization. Finally most importantly, lets explore what is inside a serialized object and what it means. That is internals of java serialization and how does it works. If you want to have your own implementation of java serialization, this article will provide you with a good platform to launch. What is Java Serialization? Primary purpose of java serialization is to write an object into a stream, so that it can be transported through a network and that object can be rebuilt again. When there are two different parties involved, you need a protocol to rebuild the exact same object again. Java serialization API just provides you that. Other ways you can leverage the feature of serialization is, you can use it to perform a deep copy. Why I used ‘primary purpose’ in the above definition is, sometimes people use java serialization as a replacement for database. Just a placeholder where you can persist an...

The  HashMap  is one of the most useful data structures in the Java programming language. Once you have a HashMap filled with data, you may want to iterate through its keys and values. Below are three different ways to iterate through a Java HashMap. Sample HashMap Here is our sample HashMap. The key is an Integer and the value is a String: HashMap <Integer, String> hm = new HashMap<Integer, String>(); hm.put(0, "zero"); hm.put(1, "one"); hm.put(2, "two"); In the above Java code, we first declare the HashMap. Then we add the values "zero", "one" and "two" with the keys 0, 1 and 2 respectively. Now that the HashMap has data, we can try to iterate over the keys and values. Iteration Example 1 for (int i=0; i < hm.size(); i++) { Integer key = hm.keySet().toArray()[i]; String val = hm.values().toArray()[i]; System.out.println("key,val: " + key + "," + val); } In this example, we use a  for  loop ...

There are two kinds of OUTER joins in SQL, LEFT OUTER join and RIGHT OUTER join. Main difference between RIGHT OUTER join and LEFT OUTER join, as there name suggest, is inclusion of non matched rows. Sine INNER join only include matching rows, where value of joining column is same, in final result set, but OUTER join extends that functionality and also include unmatched rows in final result. LEFT outer join includes unmatched rows from table written on left of join predicate. On the other hand RIGHT OUTER join, along with all matching rows, includes unmatched rows from right side of table. In short result of LEFT outer join is INNER JOIN + unmatched rows from LEFT table and RIGHT OUTER join is INNER JOIN + unmatched rows from right hand side table. Similar to difference between INNER join and OUTER join, difference between LEFT andRIGHT OUTER JOIN can be better understand by a simple example, which we will see in next section. LEFT and RIGHT OUTER Join Example in SQL In order to unders...

Difference between fail-safe and fail-fast Iterator is becoming favorite core java interview questions day by day, reason it touches concurrency a bit and interviewee can go deep on it to ask how fail-safe or fail-fast behavior is implemented .  How does a system react when there is a failure characterizes it as a fail fast or a fail safe system. This article is to discuss whether fail safe or fail fast is better. Then what it has to do with java. Concept of fail-safe iterator are relatively new in Java and first introduced with Concurrent Collections in Java 5 like ConcurrentHashMap and CopyOnWriteArrayList. fail-fast Iterators in Java As name suggest fail-fast Iterators fail as soon as they realized that structure of Collection has been changed since iteration has begun . Structural changes means adding, removing or updating any element from collection while one thread is Iterating over that collection. fail-fast behavior is implemented by keepinga modification count and if iterat...

Atomic is a toolkit of variable java.util.concurrent.atomic package classes, which assist in writing lock and wait-free algorithms with the Java language. When a data (typically a variable) can be accessed by several threads, you must synchronize the access to the data to ensure visibility and correctness. By example, if you have a simple counter: public class Counter { private int value; public int getValue(){ return value; } public int getNextValue(){ return value++; } public int getPreviousValue(){ return value--; } } This class works really well in single-threaded environment, but don’t work at all when several threads access the same Counter instance. You can solve the problem using synchronized method as below public class SynchronizedCounter { private int value; public synchronized int getValue(){ return value; } public synchronized int getNextValue(){ return value++; } public synchronize...

Race condition in Java  is a type of concurrency bug or issue which is introduced in your program because  parallel execution of your program by multiple threads at same time, Since Java is a multi-threaded programming language hence risk of Race condition is higher in Java which demands clear understanding of what causes a race condition and how to avoid that. Anyway Race conditions are just one of hazards or riskpresented by  use of multi-threading in Java just like deadlock in Java.  Race conditions  occurs when two thread operate on same object without proper synchronization and there operation interleaves on each other. Classical  example  of Race condition  is incrementing a counter since increment is not an atomic operation and can be further divided into three steps like read, update and write. if two threads tries to increment count at same time and if they read same value because of interleaving of read operation of one thread to update operation of another thread, one count ...

1) Comparator in Java is defined in  java.util  package while Comparable interface in Java is defined in  java.lang  package. 2) Comparator interface in Java has method  public int  compare (Object o1, Object o2)  which returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second. While Comparable interface has method  public int compareTo(Object o)  which returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object. 3) If you see then logical difference between these two is Comparator in Java compare two objects provided to him, while Comparable interface compares "this" reference with the object specified. 4) Comparable in Java is used to implement natural ordering of object. In Java API String, Date and wrapper classes implement Comparable interface. 5) If any class implement Comparable interface in Java then collection of that object e...