What is import statement for ?

Question

Hi all,
Sorry for asking a silly question. Since it is a new to java forum I am asking this.
What is happening when an import statement is triggered at compile time and at runtime.
What is the difference/advantages/disadvantages between importing an entire package and importing required classes only.
Is there any size limit on the generated class file.
rgds
Antony Paul

Answer 1

looks like we are both beguinners
i just received a newsletter from sun and in it has this
MONITORING CLASS LOADING AND GARBAGE COLLECTION
Have you ever wondered what classes are loaded when you launch an application or from where the classes are loaded? Have you ever wondered when garbage collection runs or how long it takes? The java command line tool offers several different command line options that you can use to get answers to those questions.
You might already be familiar with a number of command line options available with the java command line tool, such as -cp, -Xms, and -Xmx. The -cp option is used for specifying the classpath. The -Xms and -Xmx options are used to specify the heap size. For example, instead of setting the CLASSPATH environment variable, you can use the -cp option to tell the system to look in a specific directory for necessary class files:
java -cp ExampleDir MyExample
Here, the system will look in the ExampleDir subdirectory for the MyExample.class file and anything else needed besides the system classes. The ExampleDir in the command line tells the system to look only in the ExampleDir directory (assume that it's the parent directory). If MyExample.class is located in the current working directory, the system would not find it.
Two less frequently used command line features report on class loading and garbage collection. The -verbose:class option reports when a class is loaded into the Java virtual machine and from where it came. For instance, if you use the -verbose:class option when loading the SwingSet2 demo that comes with the J2SE 1.4.2 SDK, you get a report on the many different classes that are loaded as part of the demo, such the following two:
java -verbose:class -jar
C:\j2sdk1.4.2\demo\jfc\SwingSet2\SwingSet2.jar
[Loaded FilePreviewer]
[Loaded javax.swing.plaf.TableUI from
     C:\j2sdk1.4.2\jre\lib\rt.jar]
The first line indicates that the class came from the main JAR for the demo (assuming it was started with java -jar SwingSet2.jar). The second line indicates that the TableUI class was loaded from the rt.jar file that comes with the runtime located in the c:\j2sdk1.4.2\jre directory. (From there, the rt.jar file is located in the lib subdirectory.) Different implementations of the Java platform can have different formats here. The only requirement is that -verbose:class displays messages as classes get loaded and unloaded.
Let's see when classes are loaded, and how many classes are needed for the following simple program:
public class Sample {
public static void main(String args[]) {
System.out.println("Hello, World");
Compile the Sample class. Then run it with the -verbose:class option enabled:
java -verbose:class Sample
When you run the command, you'll see that this simple program requires the opening of five jar files (such as rt.jar) and the loading of almost 250 classes.
To see an example of a class unloading message, try the -verbose:class command line option with the RunItReload class shown in the August 19, 2003 Tech Tip titled Unloading and Reloading Classes.
The -verbose:gc option reports on each garbage collection event. This includes the time for garbage collection to run, and the before and after heap sizes. This is demonstrated in the following lines:
[GC 27872K->26296K(42216K), 0.0069590 secs]
[GC 28973K->26455K(42216K), 0.0036812 secs]
[GC 29134K->26474K(42216K), 0.0016388 secs]
[GC 29117K->26487K(42216K), 0.0008859 secs]
[GC 29134K->26498K(42216K), 0.0009197 secs]
[GC 29180K->26479K(42216K), 0.0008711 secs]
[GC 29149K->26484K(42216K), 0.0008716 secs]
Like the output for -verbose:class, there is no requirement for output format, and it is subject to change without notice. The "GC" at the beginning indicates what kind of collection occurred. The number before the "->" is the heap occupancy before the collection. The number after the "->" is the heap occupancy after the collection. The number in parentheses is the currently allocated size of the heap. The seconds are the duration of the collection.
This information can be useful in debugging. For example, it could help you determine if garbage collection happened at a critical point in time, and might have caused a program to crash. This sometimes happens when mixing Java and C/C++ code with JNI, especially when there is an underlying bug on the C/C++ code side.
If you're ever curious about why it takes so long for an application to start, or if garbage collection in the middle of an operation appears to cause a problem, be sure to try out these command line options.
hope it helps