New to Java

Hello everybody,
I have just installed jdk 6u1 on my linux RedHat as 4 system. I just executed the file and then added a PATH to my environment. I run apps like HelloWorld with no problem, but when it comes to a bit more complex ones I get these errors:
Exception in thread "main" java.lang.NoClassDefFoundError: while resolving class : Leaf
at java.lang.VMClassLoader.resolveClass(java.lang.Class) (/usr/lib/libgcj.so. 5.0.0)
at java.lang.Class.initializeClass() (/usr/lib/libgcj.so.5.0.0)
at java.lang.Class.forName(java.lang.String, boolean, java.lang.ClassLoader) (/usr/lib/libgcj.so.5.0.0)
at java.lang.Class.forName(java.lang.String) (/usr/lib/libgcj.so.5.0.0)
at gnu.gcj.runtime.FirstThread.run() (/usr/lib/libgcj.so.5.0.0)
at JvThreadRun(java.lang.Thread) (/usr/lib/libgcj.so.5.0.0)
at JvRunMain(java.lang.Class, byte const, int, byte const, boolean) (/usr/l ib/libgcj.so.5.0.0)
at __gcj_personality_v0 (/home/oracle/orako/java/java.version=1.4.2)
at __libc_start_main (/lib/tls/libc-2.3.4.so)
at JvRegisterClasses (/home/oracle/orako/java/java.version=1.4.2)
Caused by: java.lang.ClassNotFoundException: java.lang.StringBuilder not found i n [file:/usr/share/java/libgcj-3.4.6.jar, file:./, core:/]
at java.net.URLClassLoader.findClass(java.lang.String) (/usr/lib/libgcj.so.5. 0.0)
at gnu.gcj.runtime.VMClassLoader.findClass(java.lang.String) (/usr/lib/libgcj .so.5.0.0)
at java.lang.ClassLoader.loadClass(java.lang.String, boolean) (/usr/lib/libgc j.so.5.0.0)
at JvFindClass(_Jv_Utf8Const, java.lang.ClassLoader) (/usr/lib/libgcj.so.5. 0.0)
at java.lang.Class.forName(java.lang.String, boolean, java.lang.ClassLoader) (/usr/lib/libgcj.so.5.0.0)
at JvBytecodeVerifier.verify_instructions_0() (/usr/lib/libgcj.so.5.0.0)
at JvVerifyMethod(_Jv_InterpMethod) (/usr/lib/libgcj.so.5.0.0)
at JvPrepareClass(java.lang.Class) (/usr/lib/libgcj.so.5.0.0)
at JvWaitForState(java.lang.Class, int) (/usr/lib/libgcj.so.5.0.0)
at java.lang.VMClassLoader.linkClass0(java.lang.Class) (/usr/lib/libgcj.so.5. 0.0)
at java.lang.VMClassLoader.resolveClass(java.lang.Class) (/usr/lib/libgcj.so. 5.0.0)
...9 more
Note that my app runs on Windows well. I my installation OK? What is up??