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what is the difference between user defined package and system defined java package with respect to accessing static content

Q) With respect to accessing static content, what is the difference
between user defined package and java system package (say java.lang
etc)
I'm preparing for ocjp6. using 1.6.26 java version
my java program has a package named "pack" in PackageTest.java
package pack;
public class PackageTest {
public static final int i=20;
}
}
javac -d . PackageTest.java
created PackageTest.class file in pack folder
now accessing static contents of PackageTest class from another java
program (TestStaticContents.java) as below
import pack.PackageTest.*;
// here importing all contents of PackageTest class
class TestStaticContents {
public static void main(String[] args){
System.out.println("normal import, accessing i value with class name: "+PackageTest.i);
}
}
javac TestStaticContent.java
displaying Compilation Error:
TestStaticContents.java: cannot access PackageTest bad class file: .\PackageTest.java
If i try accessing static contents of Math class from my java program
its not displaying any compilation error i.e
import java.lang.Math.*;
// here importing all contents of Math class
class TestMathStaticContents {
public static void main(String[] args){
System.out.println("normal import, accessing pi value with class name : "+ Math.PI);
}
}
javac TestMathStaticContents.java
No Compilation Error, and PI value is printed as expected.
Why this behavior is different compared to User defined package?

I believe I understand your problem now. You've placed both PackageTest.java and TestStaticComponents.java in the same package, pack. Two classes that share the same package cannot explicitly import one another.
You've hinted at this yourself by providing TestStaticComponents.java with a package-private class identifier.
Your import statement works perfectly if both classes reside in different packages.

import java.lang.Math.* attempts to import the nested types from Math, not the static members. You should be using import static:
import static java.lang.Math.PI; //recommended to avoid wildcards
When using imported static members, you do not need to reference the class when using the member: You can use PI instead of Math.PI.
No Compilation Error, and PI value is printed as expected.
This is because java.lang.Math, like all types in the java.lang package, is automatically imported: there's no need to import it. Because of this, Math.PI isn't causing an error like you'd expect.
If you used PI instead of Math.PI, you would have gotten an error, informing you the static member wasn't imported.

How to write an import statement to refer to a java class in a jar?

How can I refer to a Java class in stdlib1.jar when the directory structure is like this? How to write the import statement?
I want to call a method under stdlib1.jar, I have configured it.

The classes are in the default package. According to this answer, it is not possible to import classes from the default package. So, they have to be moved to another package or you have to use reflection.

You call a method from a class and not from a package.
You don't need to specify the jar when you call a method from a class belonging to it. Which matters is your jar is in the classpath
In your screenshot if the lib makes part of the classpath folders, you can import and use classes from it in your code.
Here the classes of your jar use the default package (no package name) which seems weird for a third-party library. Default package is not recommended since it doesn't allow to naturally reference and use the classes of the archive from the client code.
I am not sure you are using the correct version of the jar. Look at that :
http://grepcode.com/snapshot/repo1.maven.org/maven2/com.googlecode.princeton-java-introduction/stdlib/1.0.1
This contains classes in the edu.princeton.cs package :
With package, you could declare this :
For example :
You could create a class like that and use BinaryIn like that:
package main;
import edu.princeton.cs.BinaryIn;
public class MyClass(){
public static void main(String args[]){
BinaryIn in = new BinaryIn();
}
}

How to import a custom package into another? Getting error package "packagename" does not exist

So right now in my code I have this:
package pokemonbattle;
import pokemon.Pokemon.*;
public class PokemonBattle { //Do stuff }
and then I also have:
package pokemon;
public class Pokemon {
public static void main(String[] args) {}
public String getName(int pokemon) {
//stuff
}
Except that I'm getting the error "Package pokemon.Pokemon does not exist." When it does. The file name it's under is called Pokemon, but I've tried using different capitalization but nothing is working. Any suggestions? Thanks!

Packages are like directories and you should put your files into the place specified by their package. In each directory can either be files (classes, interfaces, ..) or other sub directories.
When you do
package pokemon;
public class Pokemon {
You express that this is the Pokemon.java file in the \pokemon directory. In other words \pokemon\Pokemon.java.
To use that class in another place you do either
import pokemon.Pokemon;
to import just that one class specifically or you do
import pokemon.*;
to import all the classes in that package (but not sub-packages thereof) When you say pokemon.Pokemon.* it's looking for a directory named \pokemon\Pokemon\ which doesn't exist.

Java classes and packages

Hello everybody I am a beginner of Java. I am blocked at this point with the following
program:
import prog.io.Orario;
import prog.io.ConsoleOutputManager;
class primoprogramma{
public static void main(String[] args){
ConsoleOutputManager video=new ConsoleOutputManager();
video.println("ciao");
}
}
That gives me the error:
bad class file: ./prog/io/Orario.class
class file contains wrong class: prog.utili.Orario
Please remove or make sure it appears in the correct subdirectory of the classpath.
I did everything I could tried in those days but nothing works. Here there the class
Orario:
package prog.utili;
public class Orario {
private static char separaratore=';';
}
Thank you for any advice

Your class Orario has the wrong package declaration (package prog.utili; instead of package prog.io;)
The compiler scans your import of prog.io.Orario.
It searches for class Orario in a file Orario.class in directory prog/io.
The class found has the package prog.utili declared which is not the desired one - Error

In java, directorys are the same as package names.
So, a class Orario in the package prog.utili
have to be in a directory prog/utili instead of prog/io

Should I import a math library, and if so how?

So I want to print out the math functions, "PI" and "E."
Here is how I'm trying to do it:
System.out.println(Math.PI);
System.out.println(Math.E);
I think I have to import the math library.

E and PI are not functions, they're static fields (data members). That code will output their values correctly. You don't have to import anything, the Math class is in the java.lang package, which is imported by default. (It's the only package imported by default, I believe.)

You don't have to import anything here. The java.lang.Math class should already be available as java.lang package is imported by default

This works just fine:
/**
<P>{#code java MathXmpl}</P>
**/
public class MathXmpl {
public static final void main(String[] igno_red) {
System.out.println(Math.PI);
System.out.println(Math.E);
}
}
Output:
[C:\java_code\]java MathXmpl
3.141592653589793
2.718281828459045
Since Math is in the java.lang package, it does not need to be imported. java.lang is the "default package" and everything in it is already implicitly imported for you.