How do I run a Java class in a package?

Question

I have two java classes as follows `App1` without a package: ``` class App1 { public static void main(String[] args) { System.out.println("App1 hello world..."); } } ``` `App2` in a package: ``` package java.java.package1; class App2 { public static void main(String[] args) { System.out.println("App2 hello world..."); } } ``` I can compile them both: ``` D:\javaTest>javac App1.java D:\javaTest>javac App2.java ``` However, I can only run the first one: ``` D:\javaTest>java App1 App1 hello world... D:\javaTest>java java.java.package1.App2 Exception in thread "main" java.lang.NoClassDefFoundError: java/java/package1/App2 Caused by: java.lang.ClassNotFoundException: java.java.package1.App2 at java.net.URLClassLoader$1.run(Unknown Source) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(Unknown Source) at java.lang.ClassLoader.loadClass(Unknown Source) at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source) at java.lang.ClassLoader.lo…

Accepted Answer

If you put the source in an appropriate directory hierarchy matching the package name (`D:\javaTest\java\java\package1\App1.java`), and compile/run from the root of the hierarchy (`D:\javaTest`), you wouldn't have this problem:

```
D:\javaTest>javac java\java\package1\App1.java

D:\javaTest>java java.java.package1.App1
App2 hello world...
```

You can also compile using the `-d` option so that the classes are moved into such a directory hierarchy:

```
javac -d . App2.java
java java.java.package1.App2
```

Note you shouldn't use a package name starting with `java`, and later versions of the JDK will throw a SecurityException. See this question for more information.

How do I run a Java class in a package?

Solution

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