java迭代器模式实现正反向遍历

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import java.util.*;
abstract class Iterator
{
public abstract Object First();
public abstract Object Next();
public abstract boolean IsDone();
public abstract Object CurrentItem();

}
abstract class Aggregate
{
//public abstract Iterator CreatIterator();
}
class ConcreteIterator1 extends Iterator
{
private ConcreteAggregate aggregate;
private int current=0;
public ConcreteIterator1(ConcreteAggregate aggregate)
{
this.aggregate=aggregate;
}
public Object First()
{
return aggregate.get(0);
}
public Object Next()
{
Object ret=null;
current++;