[英]How to implement iterator on nested collection in Java?
我有一個具有此表示形式Collection<Collection<T>>
的嵌套集合。 我已經在類上實現了Iterator,但是next()方法未給出正確的結果。 它僅獲取每個列表的第一個元素。 示例List<List<String>>
,其值為{"1","2"},{"3","4"},{"5","6"}
。 類的完整布局。
import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
import java.util.List;
public class NestedCollectionIterator implements Iterator<Object> {
private Collection<? extends Collection<? extends Object>> _collOfColl = null;
private Iterator<? extends Collection<? extends Object>> itCollection = null;
private Iterator<? extends Object> innerIterator = null;
Object next = null;
public NestedCollectionIterator( Collection<? extends Collection<? extends Object>> collofColl){
_collOfColl = collofColl;
itCollection = _collOfColl.iterator();
}
@Override
public boolean hasNext() {
if(itCollection.hasNext()){
innerIterator = itCollection.next().iterator();
if(innerIterator != null || innerIterator.hasNext()){
next = innerIterator.next();
return true;
}
}
return false;
}
public Object next() {
if(hasNext()){
Object obj = next;
//Need some changes here.
return obj;
}
return null;
}
@Override
public void remove() {}
}
類來測試實現
class Sample{
public static void main(String[] args){
List<List<String>> Nestedlist = new ArrayList<List<String>>();
List<String> l = new ArrayList<String>();
l.add("1");
l.add("2");
Nestedlist.add(l);
l = new ArrayList<String>();
l.add("3");
l.add("4");
Nestedlist.add(l);
l = new ArrayList<String>();
l.add("5");
l.add("6");
Nestedlist.add(l);
NestedCollectionIterator cc = new NestedCollectionIterator(Nestedlist);
while(cc.hasNext()){
System.out.println(cc.next.toString());
}
}
}
結果是1,3,5。 如何使列表首先遍歷列表中的所有元素,然后移至其中的下一個收集項?
謝謝。
這對我有用-它沒有泛化為Collection
但是有實用程序方法可以為您提供多達3個Map
級別的迭代器-迭代器。 我敢肯定,您可以將其適應於一般收藏。
public class NestedIterator<T> implements Iterator<T> {
// Outer iterator. Goes null when exhausted.
Iterator<Iterator<T>> i2 = null;
// Inner iterator. Goes null when exhausted.
Iterator<T> i1 = null;
// Next value.
T next = null;
// Takes a depth-2 iterator.
public NestedIterator(Iterator<Iterator<T>> i2) {
this.i2 = i2;
// Prime the pump.
if (i2 != null && i2.hasNext()) {
i1 = i2.next();
}
}
@Override
public boolean hasNext() {
// Is there one waiting?
if (next == null) {
// No!
// i1 will go null if it is exhausted.
if (i1 == null) {
// i1 is exhausted! Get a new one from i2.
if (i2 != null && i2.hasNext()) {
/// Get next.
i1 = i2.next();
// Set i2 null if exhausted.
if (!i2.hasNext()) {
// Exhausted.
i2 = null;
}
} else {
// Exhausted.
i2 = null;
}
}
// A null i1 now will mean all is over!
if (i1 != null) {
if (i1.hasNext()) {
// get next.
next = i1.next();
// Set i1 null if exhausted.
if (!i1.hasNext()) {
// Exhausted.
i1 = null;
}
} else {
// Exhausted.
i1 = null;
}
}
}
return next != null;
}
@Override
public T next() {
T n = next;
next = null;
return n;
}
@Override
public void remove() {
throw new UnsupportedOperationException("Not supported.");
}
// Iterating across Maps of Maps of Maps.
static <K1, K2, K3, V> Iterator<Iterator<Iterator<V>>> iiiV(Map<K1, Map<K2, Map<K3, V>>> mapMapMap) {
final Iterator<Map<K2, Map<K3, V>>> mmi = iV(mapMapMap);
return new Iterator<Iterator<Iterator<V>>>() {
@Override
public boolean hasNext() {
return mmi.hasNext();
}
@Override
public Iterator<Iterator<V>> next() {
return iiV(mmi.next());
}
@Override
public void remove() {
mmi.remove();
}
};
}
// Iterating across Maps of Maps.
static <K1, K2, V> Iterator<Iterator<V>> iiV(Map<K1, Map<K2, V>> mapMap) {
final Iterator<Map<K2, V>> mi = iV(mapMap);
return new Iterator<Iterator<V>>() {
@Override
public boolean hasNext() {
return mi.hasNext();
}
@Override
public Iterator<V> next() {
return iV(mi.next());
}
@Override
public void remove() {
mi.remove();
}
};
}
// Iterating across Map values.
static <K, V> Iterator<V> iV(final Map<K, V> map) {
return iV(map.entrySet().iterator());
}
// Iterating across Map.Entries.
static <K, V> Iterator<V> iV(final Iterator<Map.Entry<K, V>> mei) {
return new Iterator<V>() {
@Override
public boolean hasNext() {
return mei.hasNext();
}
@Override
public V next() {
return mei.next().getValue();
}
@Override
public void remove() {
mei.remove();
}
};
}
}
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