visual basic barcode program 2: Declarations and Access Control in Java

Making PDF 417 in Java 2: Declarations and Access Control

2: Declarations and Access Control
PDF 417 Decoder In Java
Using Barcode Control SDK for Java Control to generate, create, read, scan barcode image in Java applications.
PDF 417 Creation In Java
Using Barcode printer for Java Control to generate, create PDF-417 2d barcode image in Java applications.
Look for syntax errors on import statements. Can you spot what s wrong with the following code
Scanning PDF-417 2d Barcode In Java
Using Barcode scanner for Java Control to read, scan read, scan image in Java applications.
Paint Bar Code In Java
Using Barcode generation for Java Control to generate, create barcode image in Java applications.
import java.util.Arraylist.*; // Wildcard import import java.util; // Explicit class import
Barcode Decoder In Java
Using Barcode recognizer for Java Control to read, scan read, scan image in Java applications.
Generating PDF417 In C#
Using Barcode generator for .NET Control to generate, create PDF417 image in Visual Studio .NET applications.
The first import looks like it should be a valid wildcard import, but ArrayList is a class, not a package, so it makes no sense (not to mention making the compiler cranky) to use the wildcard import on a single class. Pay attention to the syntax detail of the import statement, by looking at how the statement ends. If it ends with .*; (dot, asterisk, semicolon), then it must be a wildcard statement; therefore, the thing immediately preceding the .*; must be a package name, not a class name. Conversely, the second import looks like an explicit class import, but util is a package, not a class, so you can t end that statement with a semicolon.
Make PDF 417 In VS .NET
Using Barcode printer for ASP.NET Control to generate, create PDF 417 image in ASP.NET applications.
PDF417 Printer In .NET Framework
Using Barcode creator for Visual Studio .NET Control to generate, create PDF417 image in VS .NET applications.
Think about another dilemma for a moment: what happens if you have two classes with the same name, from two different packages, and you want to use both in the same source code In that case, you have to use the fully qualified names in code. Even in the core class libraries you ll find more than one class using the same name. You ll find a List class, for example, in both java.awt and java.util. If you want to use both, you ll have to make it clear to the compiler. Wildcard imports alone won t work properly since importing both packages still doesn t help the compiler figure out which version of the List class you want. The following code shows the problem of trying to use two classes of the same name (although different packages):
Encoding PDF 417 In VB.NET
Using Barcode generation for Visual Studio .NET Control to generate, create PDF-417 2d barcode image in VS .NET applications.
Code128 Printer In Java
Using Barcode drawer for Java Control to generate, create USS Code 128 image in Java applications.
import java.awt.*; import java.util.*; class TestImport { void doStuff() { List fromAWT = new List(); // How will the compiler know which to use List fromUtil = new List(); // How will the compiler know which to use } }
Painting UPC-A Supplement 2 In Java
Using Barcode printer for Java Control to generate, create UPC-A image in Java applications.
Draw Bar Code In Java
Using Barcode encoder for Java Control to generate, create bar code image in Java applications.
The preceding code confuses the compiler (never a pretty thing), and you ll get a message similar to this:
Print 2 Of 5 Standard In Java
Using Barcode generator for Java Control to generate, create 2 of 5 Standard image in Java applications.
Paint Bar Code In VS .NET
Using Barcode creation for ASP.NET Control to generate, create barcode image in ASP.NET applications.
TestImport.java:6: reference to List is ambiguous, both class java.util.List in java.util and class java.awt.List in java.awt match
Matrix 2D Barcode Drawer In Visual Studio .NET
Using Barcode printer for VS .NET Control to generate, create Matrix 2D Barcode image in .NET applications.
GTIN - 12 Encoder In Java
Using Barcode maker for Eclipse BIRT Control to generate, create UPC-A Supplement 2 image in BIRT reports applications.
Declaration Rules (Exam Objective 4.1)
Encode Barcode In Java
Using Barcode maker for BIRT Control to generate, create barcode image in BIRT applications.
EAN / UCC - 13 Drawer In Java
Using Barcode generation for BIRT reports Control to generate, create UPC - 13 image in Eclipse BIRT applications.
List w = new List(); ^
Barcode Drawer In None
Using Barcode drawer for Microsoft Excel Control to generate, create bar code image in Excel applications.
Make UPC - 13 In VB.NET
Using Barcode encoder for .NET framework Control to generate, create EAN13 image in Visual Studio .NET applications.
Formatting the Main() Method
When you want your code to actually run, you have to get the ball rolling with a main() method. The following rules apply to the main() method:
It must be marked static. It must have a void return type. It must have a single String array argument. You can name the argument anything you want. It should be declared public (for the purposes of the exam, assume it
must be public). There s nothing special about the main() method; it s just another static method in your class. The only thing that makes it different from other methods is that it has the signature the JVM is looking for when you invoke Java as follows:
java MyClass
Typing that at the command line starts the JVM looking for the class file named MyClass, and when it finds it, it looks for the main() method the one with a signature matching what the JVM is searching for. If it finds the matching method, you re good to go. If it doesn t, you get a runtime error like this:
Exception in thread "main" java.lang.NoSuchMethodError: main
The tricky thing about this error is that you can get it even when there is a main() method. The following code compiles fine, but still produces the previous NoSuchMethodError when you try to invoke this class from the command line:
class MyClass { public void main (String [] args) { } }
Did you spot the problem There is a main() method, but it isn t static. So when we say the main() method, you need to know whether we mean a method that
2: Declarations and Access Control
happens to be named main() (which you re allowed to have) or the Main() Method the one the JVM looks for.
Look for lots of subtle variations surrounding the main() method. You might see classes with a main() method similar to the preceding example, where the signature doesn t match what the JVM wants. You must know that not having a proper main() method is a runtime error, not a compiler error! So while you re completely free to have as many methods named main() as you like (or none at all), if no methods match the main() method the JVM looks for, then you won t be able to run the class by invoking Java using that class name. You can still instantiate the class from other code (or invoke its static methods once the JVM is already running), it just can t be used to crank up a virtual machine and bootstrap your program. If the main() method doesn t look like this:
public static void main (String [] args) { }
you won t be able to run the class. You actually do have a few slight variations you can make to the main() method. For example, the following is a perfectly legal, executable main() method:
static public void main (String whatever []) { }
In other words, you re allowed to name the String array argument whatever you like, and the static and public modifiers can be used in a different order. The most important point for the exam is to know that not having the able-to-run main() method is a runtime, rather than compiler, error. A class with a legal, nonstatic main() method, for example, will compile just fine, and other code is free to call that method. But when it comes time to use that class to invoke the JVM, that nonstatic main() method just won t cut it, and you ll get the runtime error.
We ve covered everything we need for this objective except for interface declarations, which we ll look at next, and inner class declarations, which we ll look at in 8. The key points for this objective are the structure of a source code file (where to place the package, import, and class declarations) and the signature of the main() method (public static void main (String [] args)). Next, we re going to dive into the rules for declaring and implementing interfaces.
Interface Implementation (Exam Objective 4.2)
Copyright © OnBarcode.com . All rights reserved.