why `java.lang.SecurityException: Prohibited package name: java` is required?

Question

I created a class \"String\" and placed that in package \"java\" [ actually i wanted to create java.lang to see which class is loaded by classLoader as

Answer 1

java is a reserved package name. Only classes inside the JVM can reside in this package.

If anyone could write in the Java package, that could result in libraries arbitrarily replacing core Java classes by their own implementations. That could result in a lot of thinks, from breaking core Java features to execution of malicious code.