Below are three files: TreeInfo.java, TreeScanner.java and Tree.java.
If they are compiled with
	javac p/Tree.java p/TreeScanner.java p/TreeInfo.java
the compilation succeeds.

If they are compiled with 
	javac p/TreeInfo.java p/TreeScanner.java p/Tree.java
you get the following error:

p/TreeScanner.java:9: unreported exception java.lang.Throwable; must be caught or declared to be thrown
	if(tree!=null) tree.accept(this);
                                  ^
1 error



------Tree.java--------------
package p;

public abstract class Tree {

    /** Visit this tree with a given visitor.
     */
    public abstract <E extends Throwable> void accept(Visitor<E> v) throws E;


    /** A generic visitor class for trees.
     */
    public static abstract class Visitor<E extends Throwable> {
	public void visitTree(Tree that)                   throws E { assert false; }
    }
}


------TreeInfo.java----------
package p;
import p.Tree.*;

public class TreeInfo {

    public static void declarationFor(final Tree tree) {
	class DeclScanner extends TreeScanner<Error> {
            public void scan(Tree tree) {
            }
	}
	DeclScanner s = new DeclScanner();
	tree.accept(s);
    }
}


------TreeScanner.java-------
package p;
import p.Tree.*;

public class TreeScanner<E extends Throwable> extends Visitor<E> {

    /** Visitor method: Scan a single node.
     */
    public void scan(Tree tree) throws E {
	if(tree!=null) tree.accept(this);
    }
}