Java By Source

Whenever we install Java using JDK installer it creates two folders in installation directory one for JDK and one for JRE. However, JDK folder also contains one JRE folder itself and both have the same directory structure. JDK Structure Let’s assume JDK is installed at \jdk1.7.0 , below are some of the most important directories and their explanation jdk1.7.0     db     include     src.zip     bin          java*          javac*          javap*          javah*          javadoc*     lib          tools.jar          dt.jar     jre    ...

According to Java standards and common practices we should declare every class in its own source file. And even if we declare multiple classes in the single source file (.java) still each class will have its own class file after compilation. But the fact is that we can declare more than one class in a single source file with below constraints, Each source file should contain only one public class and the name of that public class should be similar to the name of the source file. If you are declaring the main method in your source file then main should lie in that public class If there is no public class in the source file then main method can lie in any class and we can give any name to the source file. If you are not following 1st constraint then you will receive a compilation error saying “ The public type A must be defined in its own file ”.  While if you are not following the second constraint you will receive an error “ Error: Could not find or load main class User ” after ...

In Java, we generally create objects using the new keyword or we use some DI framework e.g. Spring to create an object which internally use Java Reflection API to do so. In this Article, we are going to study the reflective ways to create objects. There are two methods present in Reflection API which we can use to create objects Class.newInstance() → Inside java.lang package Constructor.newInstance() → Inside java.lang.reflect package However there are total 5 ways create objects in Java, if you are not aware of them please go through this article 5 Different ways to create objects in Java with Example . Both Class.newInstance() and java.lang.reflect.Constructor.newInstance() are known as reflective methods because these two uses reflection API to create the object. Both are not static and we can call earlier one on a class level object while latter one needs constructor level object which we can get by using the class level object. Class.newInstance() The Class class is th...

While being a Java developer we usually create lots of objects daily, but we always use the new or dependency management systems e.g. Spring to create these objects. However, there are more ways to create objects which we are going to study in this article. There are total 5 core ways to create objects in Java which are explained below with their example followed by bytecode of the line which is creating the object. However, lots of Apis are out there are which creates objects for us but these Apis will also are using one of these 5 core ways indirectly e.g. Spring BeanFactory. If you will execute program given in end, you will see method 1, 2, 3 uses the constructor to create the object while 4, 5 doesn’t call the constructor to create the object. 1. Using the new keyword It is the most common and regular way to create an object and actually very simple one also. By using this method we can call whichever constructor we want to call (no-arg constructor as well as parametrise...

Plain Old Java Object or POJO is just an ordinary Java object, The term was originally coined by Martin Fowler, Rebecca Parsons, and Josh Mackenzie in September 2000. According to Martin Fowler The term was coined while Rebecca Parsons, Josh MacKenzie and I were preparing for a talk at a conference in September 2000. In the talk we were pointing out the many benefits of encoding business logic into regular java objects rather than using Entity Beans. We wondered why people were so against using regular objects in their systems and concluded that it was because simple objects lacked a fancy name. So we gave them one, and it’s caught on very nicely. Generally, a POJO is not bound to any restriction and any Java object can be called a POJO but there are some directions. A well-defined POJO should follow below directions. Each variable in a POJO should be declared as private. private long id; private String value = ""; private Collection<Term> children = Co...

As being a Java programmer we all know what is a reference, But let’s go through this again which it will help us to understand our current topic which is “Types of References in Java”. All types in Java lies under two categories Primitive Types: There are 8 primitive types (byte, short, int, long, float, double, char and boolean) in Java which holds their values directly in form of bits.  Reference Types: All types other than primitive lies under the category of reference types e.g. Classes, Interfaces, Enums, Arrays etc.  We can define a reference similar as primitive as Employee emp = new Employee(); Here “new Employee()” is actually the object which gets space in heap, and “emp” is called reference variable which is holding the object, holding means “emp” variable stores the address of the object in form of bits. As long as “emp” is holding this object it is not eligible for garbage collection. Because it is still in use and is reachable from “emp” reference. Now if we a...