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Java - Use of nonpublic class from the current package makes compiling error

I've the following two source files
File World.java
package planets;
public class World {
public static void main(String[] args) {
Mars.land();
}
}
File Moon.java
package planets;
public class Moon {
public static void land() {
System.out.println("Hello Moon");
}
}
class Mars {
public static void land() {
System.out.println("Hello Mars");
}
}
As we can see, the Moon.java contains two classes: the public Moon class and the nonpublic Mars class.
The files are located inside planets directory, below is showed the directory tree
+current-dir:
+----+planets:
+----+World.java
+----+Moon.java
Now, if I try to compile from Windows command prompt (I'm inside current-dir folder) typing
javac planets\World.java
I receive this error message:
planets\World.java:5: error: cannot find symbol
Mars.land();
^
symbol: variable Mars
location: class World
1 error
It's very strange, because I know that the compiler searches for nonpublic classes inside all the source files of the current package.
Also Cay Horstmann's Core Java Vol 1, 10th ed. at pp. 192-193 says that:
[...]you can import nonpublic classes from the current package. These
classes may be defined in source files with different names. If you
import a class from the current package, the compiler searches all
source files of the current package to see which one defines the
class.
In addition I tried to write these files using Eclipse Oxygen and it compile without problems. But I know that Eclipse use a different compiler.
Why does javac compiler fail?
EDIT: I have not set CLASSPATH variable. So by default compiler looks inside current directory.

you need to type the following commands in order (inside your 'current-dir')
javac planets\Moon.java
javac -cp . planets\World.java
java -cp . planets.World

Not able to compile an abstract class having an interface implemented

This is first of my question .I have got two classes and an interface .i am not able to compile the following code.Can anyone please help we out why the code is not compiling and running.
public interface Animals {
public void run();
public void eat();
public void excreate();
public void mate();
}
public abstract class Mammals implements Animals {
public void run() {
System.out.println("Run with Four Legs");
}
public void eat() {
System.out.println("Eat Grass");
}
public void excreate() {
System.out.println("Excreate Excreata");
}
public void mate() {
System.out.println("Mammals Mate with a Mammal");
}
public abstract void giveBirth();
}
public class Horse extends Mammals {
public void mate() {
System.out.println("Horse Mates With A Mare");
}
public void giveBirth() {
System.out.println("Giving Birth To Foal");
}
public static void main(String[] args) {
Horse h1 = new Horse();
h1.eat();
h1.run();
h1.mate();
h1.giveBirth();
}
}
The Animal code Compiles successfully while the mammals and Horse class is not compiling .
The error is attached in the screen shot below
scrren1

The issue is that you're compiling files one by one, and javac can't find Animals when compiling Mammals. From the javac docs,
When compiling a source file, the compiler often needs information about a type whose definition did not appear in the source files given on the command line.
When the compiler needs type information, it looks for a source file or class file which defines the type. The compiler searches for class files first in the bootstrap and extension classes, then in the user class path (which by default is the current directory). The user class path is defined by setting the CLASSPATH environment variable or by using the -classpath command line option. (For details, see Setting the Class Path).
If you set the -sourcepath option, the compiler searches the indicated path for source files; otherwise the compiler searches the user class path for both class files and source files.
As you can see from your screenshot, the directory you're compiling in isn't being searched for source or class files. javac is effectively unaware of what Animals is.
You can either set the source path/class path to contain your source folder or directly compile multiple source files, as the javac docs show:
Compiling Multiple Source Files
This example compiles all the source files in the package greetings.
C:\>dir /B
greetings
C:\>dir greetings /B
Aloha.java
GutenTag.java
Hello.java
Hi.java
C:\>javac greetings\*.java
C:\>dir greetings /B
Aloha.class
Aloha.java
GutenTag.class
GutenTag.java
Hello.class
Hello.java
Hi.class
Hi.java

Compile java file with a package using Oracle JDK in ubuntu

I have successfully compiled this file and saved.
class A
{
public static void main(String[] args)
{
System.out.println("Hey!");
}
}
using
javac a.java
java A
but when I compile this file:
package B;
class A
{
public static void main(String[] args)
{
System.out.println("Hey!");
}
}
now, again using the same commands it do compile but never run
javac a.java
java A
// could not find or load main class
Please guide me the exact command for the terminal to run the file.
Note: The file is named "a.java".

You need to specify the fully-qualified name, i.e. packageName.ClassName:
java B.A

you have to change the directory to the directory wich contains the path 'B' (your package name) and than call java B.A

As mentioned above, You need to specify the fully-qualified name, i.e. packageName.ClassName:
>> javac a.java
>> java B.A
But you need to create the directory named "packageName" yourself as jdk does not create one for you implicitly.

Java SPI JARs not loaded from directory without setting -Djava.ext.dirs

I can't get the Service Provider Interface to load an implementation from another JAR in the same directory. It only works when I use -Djava.ext.dirs=. on the command line. Should it not work without?
I have the following interface:
package playground;
public interface TestIface {
public String test();
}
which is implemented here:
package playground;
public class TestImpl implements TestIface {
public String test() {
return "TEST";
}
}
Here I try to load the implementation:
package playground;
import java.util.Iterator;
import java.util.ServiceLoader;
public class Lalala {
public static void main(String[] args) {
ServiceLoader<TestIface> loader = ServiceLoader.load(TestIface.class);
Iterator<TestIface> it = loader.iterator();
while (it.hasNext()) {
TestIface a = it.next();
System.out.println(a.test());
}
System.out.println("DONE");
}
}
The interface and the last class are packaged in main.jar, the implementation in impl.jar.
main.jar has the Main class set and impl.jar has the META-INF/services/playground.TestIface file which contains "playground.TestImpl". Both JARs are in the same directory.
Running
java -jar main.jar
only prints "DONE", the implementation apparently is not found.
If I instead run
java -Djava.ext.dirs=. -jar main.jar
it also prints "TEST" as it should.
What am I doing wrong? Why is the implementation from the other JAR not loaded unless I change the java.ext.dirs setting?

The java.ext.dirs setting automatically adds all jar-files found in the specified directory to the main classloader, which is why the ServiceLoader can find and load TestIface.class (from the Apidocs: "Creates a new service loader for the given service type, using the current thread's context class loader.").
But you should not use java.ext.dirs for this (see here for one of the dangers). And when you use java -jar you cannot use java -cp to set a classpath (you can only use one of them). This leaves you the option to use the URLClassLoader to load additional jars and then call ServiceLoader.load(class from another jar).
Tip: to load other jar-files, use the location of the main.jar file as explained in the answers of this question. Other variables (like startup directory) depend on how and from where Java was started and can easily result in not finding the other jar-files.

Confusion regarding Java Packages and Windows Directory?

When preparing for the SCJP exam, we were going through the following code:
package certificaton;
public class OtherClass
{
public void testIt()
{
System.out.println("otherclass");
}
}
And this:
package somethingElse;
import certification.OtherClass;
public class AccessClass
{
public static void main( String args[])
{
OtherClass o= new OtherClass();
o.testIt();
}
}
I placed both the above files in the following directory: C:\scjp\temp8 ; and the strange thing is that, the .java files are compiling and results in two .class files being created in the same directory. The thing I want to ask, is that, the difference between packages and directory. Isn't it true that the class files could be created in a directory other than the one stated in the package declaration? And the package declaration is something 'virtual', and disregards the windows directory structure. In addition, isn't it also true that, by executing the following command:
javac -d . OtherClass.java
The directories are created conforming to the package declaration, which isn't always mandatory?

The directories are created conforming
to the package declaration, which
isn't always mandatory?
No, the package and directory structures must match. It's mandatory, not optional.