The following java file can be compiled with command "javac Test.java":

Test.java:
package java.util;
public class Test {
    java.util.List l = new java.util.ArrayList();
}

It's unreasonable. To avoid ambiguity, javac should deny any same package name in different visible modules.

For example, if we have following 2 Test.java, which are in different modules

Test.java:
package my.util;
public class Test {
    //some code
}

Test.java:
package my.util;
public class Test {
    //some code different with former Test.java
}

javac can compile each Test.java individually, and they can run if they can't see each other in runtime, e.g. if the different modules are not readable.

But any module reads java.base, so the package name same as any package in java.base should be denied.