Coming across NoClassDefFoundError in java

I come across an error while running this in Java:

// Test.java
public class Test {
    public static void main(String [] args) throws IOException {
        String a = "123";
        a = a + "456";
        System.out.println(a);
    }
}

by typing in these commands:

javac Test.java
java Test

And it has reported:

Exception in thread "main" java.lang.NoClassDefFoundError: while resolving class: Test
   at java.lang.VMClassLoader.resolveClass(java.lang.Class) (/usr/lib64/libgcj.so.5.0.0)
   at java.lang.Class.initializeClass() (/usr/lib64/libgcj.so.5.0.0)
   at java.lang.Class.forName(java.lang.String, boolean, java.lang.ClassLoader) (/usr/lib64/libgcj.so.5.0.0)
   at gnu.gcj.runtime.FirstThread.run() (/usr/lib64/libgcj.so.5.0.0)
   at _Jv_ThreadRun(java.lang.Thread) (/usr/lib64/libgcj.so.5.0.0)
   at _Jv_RunMain(java.lang.Class, byte const, int, byte const, boolean) (/usr/lib64/libgcj.so.5.0.0)
   at __gcj_personality_v0 (/home/users/haotianyi/tr/java.version=1.4.2)
   at __libc_start_main (/lib64/tls/libc-2.3.4.so)
   at _Jv_RegisterClasses (/home/users/haotianyi/tr/java.version=1.4.2)
Caused by: java.lang.ClassNotFoundException: java.lang.StringBuilder not found in [file:/usr/share/java/libgcj-3.4.5.jar, file:./, core:/]
   at java.net.URLClassLoader.findClass(java.lang.String) (/usr/lib64/libgcj.so.5.0.0)
   at gnu.gcj.runtime.VMClassLoader.findClass(java.lang.String) (/usr/lib64/libgcj.so.5.0.0)
   at java.lang.ClassLoader.loadClass(java.lang.String, boolean) (/usr/lib64/libgcj.so.5.0.0)
   at _Jv_FindClass(_Jv_Utf8Const, java.lang.ClassLoader) (/usr/lib64/libgcj.so.5.0.0)
   at java.lang.Class.forName(java.lang.String, boolean, java.lang.ClassLoader) (/usr/lib64/libgcj.so.5.0.0)
   at _Jv_BytecodeVerifier.verify_instructions_0() (/usr/lib64/libgcj.so.5.0.0)
   at _Jv_VerifyMethod(_Jv_InterpMethod) (/usr/lib64/libgcj.so.5.0.0)
   at _Jv_PrepareClass(java.lang.Class) (/usr/lib64/libgcj.so.5.0.0)
   at _Jv_WaitForState(java.lang.Class, int) (/usr/lib64/libgcj.so.5.0.0)
   at java.lang.VMClassLoader.linkClass0(java.lang.Class) (/usr/lib64/libgcj.so.5.0.0)
   at java.lang.VMClassLoader.resolveClass(java.lang.Class) (/usr/lib64/libgcj.so.5.0.0)
   ...8 more

But the program runs OK when I remove the line a = a + "456";, i.e.,

// Test.java
public class Test {
    public static void main(String [] args) throws IOException {
        String a = "123";
        // a = a + "456";
        System.out.println(a);
    }
}

I wonder where is the problem and how to solve it?