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You can run a Java program from within an environment or by executing the interpreter from the command line:
java Test
where
Test is the name of a class containing a
main method.
Suggestion: It is convenient to write
main methods in most classes and to use then
for testing classes individually. After the classes are integrated into a singleprogram, you only need ensure that the interpreter is invoked on the class that
contains the “real”
main method.
Two runtime errors can occur if the interpreter is not successful in finding and running the program.
The interpreter must be able to find the file containing a
class with the
main method, for example,
Test.class .
If packages are not used, this must be in the directory where theinterpreter is executed. Check that the name of the class is the same
as the name of the file without the extension. Case is significant!
Warning! If you are compiling and running Java from a command window or shell with a history facility, you are likely to encounter this error. First,you compile the program:
javac MyClass.java
and then you recall that line and erase the
c from
javac to
obtain the command for the interpreter:
java MyClass.java
Unfortunately,
MyClass.java is
not the name of the class so you
will get this exception. You must erase the
.java extension as well to
run the interpreter:
java MyClass
If you are using packages, the main class must be in a subdirectory of that name. For example, given:
package project1.userinterface;
class Test { public static void main(String[] args) { ...
}}
the file
Test.class must be located in the directory
userinterface that is a subdirectory of
project1 that is a subdirectory of the
directory where the interpreter is executed:
c:\projects> dir
<DIR> project1
c:\projects> dir project1
<DIR> userinterface
c:\projects> dir project1\userinterface
Test.class
The program is invoked by giving the fully qualified name made up of the package names and the class name:
c:\projects> java project1.userinterface.Test
This error will occur if there is no method
main in the class, or if
the declaration is not precisely correct: the
static modifier,
the
void return type, the method name
main written in lower case, and one
parameter of type
String[] . If you forget the
public modifier,
the error message is
Main method not public .
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