When compiling a class against another class implementing an interface which is defined in some classfile, such classfile must be available in the classpath used by javac during compilation. This is a new requirement as of JDK 8 - a failure to do so will result in a compilation error.

Example:

//Client.java
import p1.A;

class Client {
   void test() {
       new A.m();
   }
}

//p1/A.java
package p1;

public class A implements I {
   public void m() { }
}


//p1/I.java
package p1;

public interface I {
   void m();
}

If neither p1/I.java nor p1/I.class are available when compiling Client.java, the following error will be displayed:

Client.java: error: cannot access I
       new A().m();
            ^
  class file for p1.I not found