In this article by Peter Verhas author of the book Java 9 Programming By Example, we will develop a simple sort program. Using this code as an example, we will look at different build tools, which are frequently used for Java projects, and learn the basic features of the Java language.

(For more resources related to this topic, see here.)

The problem we will solve

The sorting problem is one of the oldest programming tasks that an engineer solves. We will have a set of records and we know that we will want to find a specific one sometime later, and we will want to find that one fast. To find it, we will sort the records in a specific order that helps finding the record we want fast.

As an example, we can have the names of the students with some marks on cards. When students will come to the office asking for the result, we can turn all pages one after the other to find the name of the enquiring student. However, it is better if we sort the papers by the name of the students lexicographically. When a student comes, we can search the mark attached to the name much faster. We can look at the middle card; if it shows the name of the student, then we are happy to have found the name and the mark. If the card precedes the name of the student lexicographically, then we will continue searching in the second half, otherwise the first half.

Following that approach, we can find the name of the student in no more steps than as many times the pack of cards can be halved. If we have two cards, then it is two steps at most. If it is four, then we will need three steps at most. If there are eight cards, then we may need four steps, but not more. If there are 1000 cards, then we may need at most 11 steps, while the original, non-sorted set will need 1000 steps, worst case. That is, approximately, it speeds up the search 100 times, so this is worth sorting the cards, unless the sorting itself takes too much time.

In many cases, it is worth sorting the dataset and there are many sorting algorithms to do that. There are simpler and more complex algorithms, and as in many cases, more complex algorithms are the one that run faster.

As we are focusing on the Java programming part and not the algorithm forging, in this article, we will develop a Java code that implements a simple and not-that-fast algorithm.

Bubble sort

The algorithm that we will implement in this article is well known as bubble sort. The approach is very simple. Begin at the start of the cards and compare the first and the second card. If the first card is later in lexicographic order than the second one, then swap the two cards. Then, repeat this for the card that is at the second place now, then the third, and so on. There is a card that is lexicographically the latest, say Wilson, and sometime later, we will get to this card as we go on swapping, the cards going from start to end. When we get this card and start to compare it with the next one, we will always swap them; this way, Wilson's card will travel to the last place where it has to be after the sort. All we have to do is repeat this travelling from the start and the occasional swapping of cards again, but this time only to the last but one element. This time, the second latest element will get to its place—say Wilkinson will be right before Wilson. If we have n cards, and we repeat this n-1 times, all cards will get to their place.

Project structure and build tools

When a project is more complex than a single class, and it usually is, then it is wise to define a project structure. We will have to decide where we store the source files, where the resource files (those that contain some resource for the program, but are not Java source) are, where should the .class files be written by the compiler, and so on. Generally, the structure is mainly the directory setup and configuring the tools that perform the build that use these tools.

The compilation of complex programs cannot be feasibly done using the command line issuing javac commands. If we have a 100 Java source files, the compilation will require that many javac commands to be issued. We can write a simple bash script that does that. First, it will be just 100 lines, each compiling one source Java file to class file. Then, we will realize that this is only time, CPU, and power consuming to compile the files that are not changed since the last compilation. So, we can add some bash programming that checks the time stamp on the source and generated files. Then, we will probably realize that… whatever. At the end, we will end up with a tool that is essentially a build tool. And, this is already done.

Instead of creating one, we will use a build tool that is ready. There are a few of them that can be found at https://en.wikipedia.org/wiki/List_of_build_automation_software

Make

The Make program was originally created in April 1976, so this is not a new tool. It is included in the Unix system so this tool is available without any extra installation on Linux, Mac OS X, or any other Unix-based system. Additionally, there are numerous ports of the tool on Windows and some version is/was included in the Visual C compiler toolset.

The Make is not tied to Java. It was created when the major programming language was C, but it is not tied to C or any other language. Make is a dependency description language that has a very simple syntax.

The Make, just like any other build tool, works controlled by a project description file. In case of make, this file contains a rule set. The description file is usually named Makefile, but in case the name of the description file is different, it can be specified as a command-line option to the make command.

Rules in Makefile follow each other and a it is one or more lines. The first line starts at the first position (there is no tab or space at the start of the line) and the following lines start with a tab character. Thus, Makefile may look something like the following code:

run : hello.jar
    java -cp hello.jar HelloWorld

hello.jar : HelloWorld.class
    jar -cf hello.jar HelloWorld.class

HelloWorld.class : HelloWorld.java
    javac HelloWorld.java

The file defines three so-called targets: run, hello.jar, and HelloWorld.class. To create HelloWorld.class, type the following line at the Command Prompt:

make HelloWorld.class

The make will look at the rule and see that it depends on HelloWorld.java. If the HelloWorld.class file does not exist, or HelloWorld.java is newer than the Java source file, make will execute the command that is written on the next line and it will compile the Java source file. If the class file was created following the last modification of HelloWorld.java, then make knows that there is no need to run the command.

In case of creating HelloWorld.class,the make program has an easy task. The source file was already there. If you issue the make hello.jar command, the procedure is more complex. The make command sees that in order to create hello.jar, it needs HelloWorld.class, which itself is also a target on another rule. Thus, it may need to be created.

First, it starts the problem the same way as before. If HelloWorld.class is there, and is older than hello.jar, there is nothing to do. If it is not there, or is newer than hello.jar, then the jar -cf hello.jar HelloWorld.class command needs to be executed, but not yet. It remembers that this command has to be executed sometime in the future when all the commands that are needed to create HelloWorld.class are already executed successfully. Thus, it continues to create the class file exactly the same way as I already described earlier.

In general, a rule can have the following format:

target : dependencies
    command

The make command can create any target using the make target command by first calculating which commands to execute and then executing them one by one. The commands are shell commands executing in a different process and may pose problems under Windows, which may render the Makefile files operating system dependent.

Note that the run target is not an actual file that make creates. A target can be a file name or just a name for the target. In the latter case, make will never consider the readily available target.

As we do not use make for Java project, there is no room to get into more details. Additionally, I cheated a bit by making the description of a rule simpler than it should be. The make tool has many powerful features out of the scope of this book. There are also several implementations that differ a little from each other. You will most probably meet the one made by the Free Software Foundation—the GNU make. And, of course, just in case of any Unix command-line tool, man is your friend. The man make command will display the documentation of the tool on the screen.

The main points that you should remember about make are as follows:

It defines the dependencies of the individual artifacts (targets) in a declarative way
It defines the actions to create the missing artifacts in an imperative way.

Summary

In this article, we have developed a very basic sort algorithm. It was made purposefully simple so that we could reiterate on the basic and most important Java language elements, classes, packages, variables, methods, and so on.