kukuruku 0 Light Poster

Hi everybody,I have java exam tomorrow and one of the question is going to be tracing code,unfortunately I am not vary good at that.My question is if anybody knows web site on the net with similar questions and answers
Thanks
yesterday the professor give us this example

public class A {

	private int a;
	public A(){
		
		a=1;
	}
	public A(int a){
		
		this.a=(a>10)?10:a;
	}
	public String toString(){
		return "a="+a;
	}
}
public class B {
   public A a;
   public int b;
   public B(){
	   a=new A();
	   b=2;
   }
   public B(int b,A a){
	   this();
	   b++;
   }
   public String toString(){
	   return "b=" + b+ "\" a+\""+a;
   }
}
public class C {

	public static void main(String[] args) {
		B b=new B(4, null);               //Iam not sure about the null here
		System.out.println(b);
		A a=new A(2);
		System.out.println(a+b);       //And I am not sure about the a+b

	}

}

The result of the trace was a=2b=2"a="a=1