org.opentripplanner.util.lang.CollectionsView Maven / Gradle / Ivy
package org.opentripplanner.util.lang;
import java.io.Serializable;
import java.util.AbstractCollection;
import java.util.Collection;
import java.util.Iterator;
/**
* This class provides a read-only view for a fixed set of collections. It does not copy the
* elements, instead it keeps a reference to the underlying collections. There is a very small
* overhead when iterating over the collection and when calculating the size.
*
* Use this when you want to concatenate several big {@link Collection}s for read-only access.
*
* This list will reflect any changes in the underlying {@link Collection}s, so keeping a reference
* to it is safe.
*
* Any attempts to modify the view will throw an exception - it is READ-ONLY, even when the
* underlying collections are modifiable.
*
* This class is serializable, but be aware that the underlying collections also must be
* serializable for it to work. The view is very light-weight, so in general it is better to
* avoid serialization. In most cases you can create a new instance every time you need a view.
*/
public class CollectionsView
extends AbstractCollection
implements Collection, Serializable {
private final Collection[] collections;
@SafeVarargs
public CollectionsView(Collection... collections) {
this.collections = collections;
}
@Override
public Iterator iterator() {
return new Iterator() {
int i = 0;
Iterator it = collections[0].iterator();
@Override
public boolean hasNext() {
if (i == collections.length) {
return false;
}
if (it.hasNext()) {
return true;
}
++i;
while (i != collections.length) {
it = collections[i].iterator();
if (it.hasNext()) {
return true;
}
++i;
}
it = null;
return false;
}
@Override
public T next() {
return it.next();
}
};
}
@Override
public int size() {
int size = 0;
for (Collection it : collections) {
size += it.size();
}
return size;
}
}