Java Generics is a feature introduced from JDK 5. It allows us to use type parameter when defining class and interface. It is extensively used in Java Collection framework. The type erasure concept is one of the most confusing part about Generics. This article illustrates what it is and how to use it. 1. A … Read more
A good programmer does not only know how to program a task, but also knows why it is done that way and how to do it efficiently. Indeed, we can find almost any code by using Google, knowing why it is done that way is much more difficult than knowing how to do it, especially … Read more
Comparable and Comparator are two interfaces provided by Java Core API. From their names, we can tell they may be used for comparing stuff in some way. But what exactly are they and what is the difference between them? The following are two examples for answering this question. The simple examples compare two HDTV’s size. … Read more
This shows how a host sends a IP-PDU(Protocol Data Unit) to a host in another network. Mainly about how an IP-PDU goes through Network and Link Layers. It demonstrate how switch table, ARP table, routing table and forwarding table works with each other in the whole process. Click to download the diagram. This diagram is … Read more
Topics covered in Network class include application layer, transport layer, network layer, link layer.
If you took operating system course in college, you might remember that monitor is an important concept of synchronization in operating systems. It is also used in Java synchronization. This post uses an analogy to explain the basic idea of “monitor”. 1. What is a Monitor? A monitor can be considered as a building which … Read more
This example belongs to Eclipse JDT Tutorial Series. First of all, Eclipse JDT ASTParser CAN work in a standard Java application. This privides the convenience of simple parsing of a large amount of Java class files. Suppose you want to parse 100 .java files which are under a directory. You would not be able to … Read more
How to read file content into a string? The following is the Java code to do that. To make it work, the filePath need to be changed. public static String readFileToString() throws IOException { File dirs = new File("."); String filePath = dirs.getCanonicalPath() + File.separator+"src"+File.separator+"TestRead.java"; StringBuilder fileData = new StringBuilder(1000);//Constructs a string buffer with … Read more
If you want to develop Java Desktop application, you probably have two choices of libraries: SWT vs. Swing. Where there is pros and cons for both of them, here is a comparison between them. Recall the role of Standard Widget Tootlkit (SWT) in eclipse platform below. JFace is a high-level user interface(UI) widget library built … Read more
As you may already know that once an object is created, it’s just a series of bytes in the heap. You may be curious about how exactly Java objects look like in the memory? 1. Fields Here is an example of an object layout for class “Base”(B). This class does not have any method. We … Read more
Java access level contains two parts: 1) access level for classes and 2) access level for members. For class access level, the keyword can be public or no explicit modifier(package-private). For member access level, the keyword can be public, protected, package-private (no explicit modifier), or private. The following table summarizes the access level of different … Read more
If a method is private, it means that it can not be accessed from any class other than itself. This is the access control mechanism provided by Java. When it is used appropriately, it can produce security and functionality. Constructors, like regular methods, can also be declared as private. You may wonder why we need … Read more
Here is a simple Java example showing Polymorphism: overloading and overriding. Polymorphism means that functions assume different forms at different times. In case of compile time it is called function overloading. Overloading allows related methods to be accessed by use of a common name. It is sometimes called ad hoc polymorphism, as opposed to the … Read more
In this post, I will illustrate what are instance variable initializer, instance initializer, static initializer, and how the instance initializer works in Java. 1. Execution Order Look at the following class, do you know which one gets executed first? public class Foo { //instance variable initializer String s = "abc"; //constructor public Foo() … Read more
Below is a simple example for showing how to use Calendar.getInstance() to get current time. public static String getCurrentTime(){ Calendar c=Calendar.getInstance(); String hour="" + c.get(Calendar.HOUR_OF_DAY); String min="" + c.get(Calendar.MINUTE); String sec="" + c.get(Calendar.SECOND); if (hour.length() == 1) hour="0" + hour; if (min.length() == 1) min="0" + min; if (sec.length() == 1) sec="0" + sec; return … Read more