class A {    
        B b;    
        setB(B b) {
            B oldb = this.b;
            this.b = b;
            
            if (this.b.equals(oldb)) {
                return;
            }
            
            if (oldb.getA().equals(this)) {
                oldb.setA(null);
            }

            if (b != null && !b.getA().equals(this)) {
                b.setA(this);    
            }    
        }    
    }    
        
    class B {    
        A a;    
        setA(A a) { 
            A olda = this.a;    
            this.a = a; 
            
            if (this.a.equals(olda)) {
                return;
            }
            
            if (olda.getB().equals(this)) {
                olda.setB(null);
            }
  
            it (a != null && !a.getB().equals(this)) {    
                a.setB(this);    
            }    
        }    
    }