Wednesday, 31 May 2017

Binary tree post order traversal in Java with example

Like many tree algorithms, the easiest way to implement post-order traversal is by using recursion. In fact, if you know how to write pre-order using recursion, you can use the same algorithm with bit of adjustment to implement post order traversal. All you need to do is instead of printing the value of node first, just call the recursive method with left subtree as shown in our example.

Unlike in-order traversal which prints all nodes of binary search tree in sorted order, post-order doesn't provide sorting but its useful while deleting nodes from binary tree, see a good book on data structure and algorithms e.g. Introduction to Algorithms by Thomas H. Cormen to learn more about different usage of post-order traversal in Computer Science and programming.

Monday, 29 May 2017

Java: Read Large XML by Using StAX

The following code shows how to read XML file in Java. It uses StAX API which reads xml files sequentially. If you want to read a large xml file, and get outofmemory error, you should be able to solve the problem by using the code below. The solution below read the xml file sequentially, and can process very large xml files, such as 10G or 20G. Therefore, it is a scalable solution!

Friday, 26 May 2017

java.util.ConcurrentModificationException

This post shows show to solve the problem of java.util.ConcurrentModificationException for ArrayList.

The error message looks like the following:

Exception in thread "main" java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
at java.util.ArrayList$Itr.next(Unknown Source)
...
...

Thursday, 25 May 2017

Constructors of Sub and Super Classes in Java?

This post summarizes a commonly asked question about Java constructors.

1. Why creating an object of the sub class invokes also the constructor of the super class?


class Super {
    String s;

    public Super(){
    System.out.println("Super");
    }
}

Thursday, 18 May 2017

Inheritance vs. Composition in Java

This article illustrates the concepts of inheritance vs. composition in Java. It first shows an example of inheritance, and then shows how to improve the inheritance design by using composition. How to choose between them is summarized at the end.
 

1. Inheritance


Let's suppose we have an Insect class. This class contains two methods: 1) move() and 2) attack().

Wednesday, 17 May 2017

JVM Statistics with jstat

I have written about several command-line tools provided with the Oracle and/or OpenJDK Java Development Kits (JDKs) in the past, but I’ve never written exclusively about the jstat tool. The Oracle JDK 9 Documentation Early Access states that jstat is used “to monitor Java Virtual Machine (JVM) statistics.” There is also a warning, “This command is experimental and unsupported.” Although I quoted the JDK 9 documentation, jstat has been a part of the Sun/Oracle JDK in some form (known at one time as jvmstat) in Java SE 8, Java SE 7, Java SE 6, and J2SE 5. Instrumentation for the HotSpot JVM was introduced with Java 1.4.1 (only enabled when -XX:+UsePerfData was set) and has provided “always-on instrumentation” since Java 1.4.2.

Tuesday, 16 May 2017

The Introduction of Java Memory Leaks

One of the most significant advantages of Java is its memory management. You simply create objects and Java Garbage Collector takes care of allocating and freeing memory. However, the situation is not as simple as that, because memory leaks frequently occur in Java applications.

Monday, 15 May 2017

The substring() Method in JDK 6 and JDK 7

The substring(int beginIndex, int endIndex) method in JDK 6 and JDK 7 are different. Knowing the difference can help you better use them. For simplicity reasons, in the following substring() represent the substring(int beginIndex, int endIndex) method.

Thursday, 11 May 2017

Introduction to JDBC

Java Database Connectivity(JDBC) is an Application Programming Interface(API) used to connect Java application with Database. JDBC is used to interact with various type of Database such as Oracle, MS Access, My SQL and SQL Server. JDBC can also be defined as the platform-independent interface between a relational database and Java programming. It allows java program to execute SQL statement and retrieve result from database.

JVM Architecture in Java

JVM (Java Virtual Machine) is an abstract machine. It is a specification that provides runtime environment in which java bytecode can be executed.

JVMs are available for many hardware and software platforms (i.e. JVM is platform dependent).

JVM Architecture , Java Tutorials

Wednesday, 10 May 2017

Java OOPs Concepts

In this page, we will learn about basics of OOPs. Object Oriented Programming is a paradigm that provides many concepts such as inheritance, data binding, polymorphism etc.

Simula is considered as the first object-oriented programming language. The programming paradigm where everything is represented as an object, is known as truly object-oriented programming language.

Smalltalk is considered as the first truly object-oriented programming language.

Thursday, 4 May 2017

JDBC from the Oracle Service Bus

There are different ways to do database calls from the Oracle Service Bus. In this blog post I look at several methods.

The methods looked at;
1 using an external webservice (without OSB)
2 using an external webservice proxied by the OSB
3 using the fn-bea:execute-sql function from the OSB
4 using the JCA DbAdapter from the OSB provided by Oracle as part of Oracle SOA Suite

Oracle Java, JDBC, Oracle Java Tutorials and Materials, Oracle Java Guide

Monday, 1 May 2017

Inheritance in Java

Inheritance in java is a mechanism in which one object acquires all the properties and behaviors of parent object.

The idea behind inheritance in java is that you can create new classes that are built upon existing classes. When you inherit from an existing class, you can reuse methods and fields of parent class, and you can add new methods and fields also.

Inheritance represents the IS-A relationship, also known as parent-child relationship.