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Java programming FAQ

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Programming techniques

Q: How can I copy process information from Windows task manager?

A: Getting information from Windows system applications relies on the programs having command line interfaces that a Java program can read from. Windows Task Manager program does not seem to support command line arguments to interrogate running processes, nor standard output, so you may need to interface with a different program to get the information you seek.

The example below uses the Java ProcessBuilder class to create a Process that opens the Task Manager graphical user interface application. The same approach can be used to run other system tools or installed applications, provided you know the path to the executable program and can supply any appropriate arguments.

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Q: How do I call another class in the same folder?

Assuming the Java classes are in the same package, one class should instantiate the other to call an instance method, or use a class name reference to call a static method, as below.

public class Example {

  public static void main(String[] args) {

    Other otherInstance = new Other();

    otherInstance.instanceMethod();

    Other.staticMethod();
  }
}
      

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Q: How do I get the name of the operating system?

A: To get the operating system name in Java use the static System.getProperty("os.name") method, which returns a string. Other operating system property keys are "os.arch" for the hardware architecture and "os.version" for the version number.

You can also get the current user name using the method System.getProperty("user.name").

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Q: What is a good beginner's book about Java?

A: Thinking In Java by Bruce Eckel is a good beginner's book on Java, which is available as a free download too.

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Q: What is a recursive method?

A: A recursive method is one whose method body includes a call to itself, so that it is called repeatedly until an expected condition is met or it cannot continue the recursion any longer. These methods often take an object or numeric argument that is subject to progressive interrogation or mathematical processing at each pass. Recursive methods must be designed carefully to ensure that they do not result in a very deep or endless recursion, which is likely to cause an OutOfMemoryError.

A simple example of a recursive method is the getNodeByName(String) method below, which iterates through all child nodes in an object structure until it finds one that matches, or returns null.

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Java arithmetic

Q: How do I add two numbers in Java?

A: To add two numbers in Java, use the simple mathematical plus operator, which may be applied to any numeric variable type, as below:

int variable1 = 26;
int variable2 = 4;

int result = variable1 + variable2;
      

The result of the addition does not have to be assigned to a variable, it can also be used anonymously in control statements, as below.

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Q: How can I add two numbers using a method?

A: For simple mathematical functions a method would usually declare the input variables as method arguments and a return type that matches the required numeric type for the result. In the simplest case an add(int, int) method would return an int value, as below:

public final int add(final int firstInt,
                     final int secondInt) {

  return firstInt + secondInt;
}
      

To add two numbers you would call the method and assign the return value to a variable so it can be used later.

int sum = add(5, 2);
      

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Q: How should I round a double up to an int?

A: To round a double value up and convert it to an int value takes two operations. The static Math.ceil(double) method returns a double value that is equal to the "next highest" integer. You must then give an explicit down cast to an int, as below.

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Handling exceptions

Q: Can you give an example of catching an exception?

A: One common example of catching an exception is when you try to read from a file that may not exist. You may enter a file name as the first argument on the command line, for example:

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Q: How can I throw a NumberFormatException back to the main() method?

A: NumberFormatException is an un-checked exception. That means that the compiler will not enforce that your application catches the exception and handle the case. For example, users may enter an invalid number at runtime and the application would throw an exception and crash.

For runtime exceptions it is important that your application handles the exception in the method in which they may occur. The callers of such methods cannot be expected to know that they would throw a runtime exception and so catch the exception. Your methods should include validation and handling for specific, anticipated runtime exceptions.

If you want to signal a problem that cannot be handled locally by your method, you should catch the runtime exception and throw a checked exception, as below. This is a typical case for creating your own checked exception type, though rather heavyweight for this simple example.

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Java applications

Q: How do Java front- and back-end skill sets differ?

A: This is a somewhat arbitrary distinction to make about Java development, since applications often have front- and back-end components and it is important to understand how both aspects are integrated. In general terms, back-end development is concerned with database storage and retrieval, servlets, Web application frameworks and Enterprise Java Beans. This requires a good understanding of SQL and database applications, JDBC, network principles, servlet containers, the HTTP protocol and an appreciation of concurrent programming issues.

Front-end development for servlets is concerned with the delivery of HTML content to Web browsers, especially forms, and touches on most aspects of markup design, Cascading Style Sheets and Javascript. Usually, front-end developers will work to visual designs provided by others, and will implement the work using a combination of formats including JSP, JSP tag libraries and other template frameworks, such as Spring.

Front-end developers may also be involved in creating pure Java user interfaces for stand-alone Swing applications or (less commonly these days) the Abstract Windowing Toolkit (AWT) for applets. In this case, it is important to have a good knowledge of the Swing API components, their intended use, and the data structures they operate on.

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Q: How can I make spelling checker with a LinkedList dictionary?

A: A basic spelling checker that processes an input stream of some kind might use a StreamTokenizer to identify the words from the input and use the LinkedList's contains(Object) method to check whether the given word is recognised. How you deal with un-matched words is up to the application interface, this example just lists the line number and word on the console output.

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Q: How can I use SAX to parse a Web page via JTidy?

A: The JTidy project help page is not all that helpful. The original HTML Tidy configuration options are a better reference to the equivalent methods in the Java implementation and the Code Style JTidy development notes article should help you get started.

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Q: How can I create a search engine in Java?

A: This is a big question and there are many aspects to consider. You may find it helpful to take a look at the MKSearch system. This is an open source project, so you can review the code yourself. The project Web site includes Java documentation, configuration notes and how to guides to get you started.

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Q: How can I design my own fonts using Java?

A: It is puzzling that you would want to design fonts using Java, the language is not particularly suited for this purpose. There are several very good applications for designing and converting fonts such as Fontographer for example.

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