How come sometimes 'package' is necessary, sometimes is unnecessary?

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I am new for Java. I followed some tutorial to build a simple Java program using Eclipse as below:

C#

package samplelib;

public class samplelib {
    public static void main (String[] args) {
        System.out.println ("sample lib");
    }
}

I ran it inside Eclipse successfully. But when I compiled it at command line using 'javac' and then ran it using 'java' command, I got message like "Wrong: can't find or unable to load main class samplelib".
I deleted the first line "package samplelib", used 'javac'/'java' commands again. It rans successfully.
Then, I went back to Eclipse and ran it, I got the following error message:

Exception in thread "main" java.lang.Error: Unresolved compilation problem:
at samplelib.samplelib.main(samplelib.java:4)

So, how come sometimes we need 'package statement' but sometimes we don't? Thanks.

Posted 21-Dec-14 21:18pm

Stan Huang

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Richard MacCutchan · Accepted Answer · 2014-12-21T23:08:00

Solution 1

A package is somewhat like a folder, where a set of common files are kept together. All classes in a package are kept together, as described at http://docs.oracle.com/javase/tutorial/java/package/index.html[^]. When you build a Java program that has a package statement, then the compiled class is expected to be stored in a subdirectory with the same name. You then call java to execute it from its parent with a command line of the form:

java packagename/classname

You can also use the -classpath option to tell Java where your classes are stored.

Posted 21-Dec-14 23:08pm

Richard MacCutchan

Comments

CPallini 22-Dec-14 5:16am

5.

Stan Huang 22-Dec-14 5:33am

Thanks Richard MacCutchan. Your command works. But there is still a pending question: how come Eclipse can't accept a Java program without package statement?

Richard MacCutchan 22-Dec-14 6:41am

I think that because using package is the 'normally accepted' method, that eclipse enforces it. It is probably a good idea to stick with it as all your programs will follow a standard model.

Stan Huang 22-Dec-14 19:41pm

Thanks Richard very much. I will keep it in mind.

Richard MacCutchan 22-Dec-14 6:42am

Thanks Carlo, have a good Christmas.

CPallini 22-Dec-14 13:13pm

Merry Christmas to you, Richard!