How about you do it the sensible way:

public class A {
 B b;
 public void setB(B b) {
   this.b = b;
 }

 public void foo() {
    System.out.println("Foo!");
    b.bar();
 }
}

public class B {
 A a;
 public void setA(A a) {
   this.a = a;
 }

 public void usesA() {
    a.foo();
 }

 public void bar() {
   System.out.println("Bar!");
 }
}

public class Framework {
 public static void main(String...args) {
      A a = new A();
      B b = new B();
      a.setB(b);
      b.setA(a);

      b.usesA();
 }
}

Its often advantageous to have the instantiation of interrelated
objects done by a coordinator or framework, and have the framework
inform the objects about each other.  That way, if I wanted to have b
talk to an "A" subclass, I don't have to modify B .  Also, that
alleviates the stress of deciding whether A should create B, or visa
versa. The answer is No, they shouldn't create each other, just deal
with each other.

javac will figure out how to compile them, don't worry about that
much. Generally (as others have noted) you'll want to compile all
the .java files at once. (javac *.java works in simple cases,
depending on package structure).

Hope this helps,
Daniel.

First of all,thank you!

Do you mean using "javac *.class", and neither "javac a.class" nor
"javac b.class" will go through while compiled?

In such case,none of the above will work.Right?

Dowson.