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/*******************************************************************************
* Copyright (c) 2005, 2008 Oracle. All rights reserved.
* This program and the accompanying materials are made available under the
* terms of the Eclipse Public License v1.0, which accompanies this distribution
* and is available at http://www.eclipse.org/legal/epl-v10.html.
*
* Contributors:
* Oracle - initial API and implementation
******************************************************************************/
package org.eclipse.jpt.utility.internal.iterators;
import java.util.Iterator;
import java.util.NoSuchElementException;
import org.eclipse.jpt.utility.Filter;
import org.eclipse.jpt.utility.internal.StringTools;
/**
* A <code>FilteringIterator</code> wraps another <code>Iterator</code>
* and uses a <code>Filter</code> to determine which elements in the
* nested iterator are to be returned by calls to <code>next()</code>.
* <p>
* As an alternative to building a <code>Filter</code>, a subclass
* of <code>FilteringIterator</code> can override the
* <code>accept(Object)</code> method.
* <p>
* One, possibly undesirable, side-effect of using this iterator is that
* the nested iterator's <code>next()</code> method will be invoked
* <em>before</em> the filtered iterator's <code>next()</code>
* method is invoked. This is because the "next" element must be
* checked for whether it is to be accepted before the filtered iterator
* can determine whether it has a "next" element (i.e. that the
* <code>hasNext()</code> method should return <code>true</code>).
* This also prevents a filtered iterator from supporting the optional
* <code>remove()</code> method.
*/
public class FilteringIterator<E1, E2>
implements Iterator<E2>
{
private final Iterator<? extends E1> nestedIterator;
private final Filter<E1> filter;
private E2 next;
private boolean done;
/**
* Construct an iterator with the specified nested
* iterator and a disabled filter.
* Use this constructor if you want to override the
* <code>accept(Object)</code> method instead of building
* a <code>Filter</code>.
*/
public FilteringIterator(Iterator<? extends E1> nestedIterator) {
this(nestedIterator, Filter.Disabled.<E1>instance());
}
/**
* Construct an iterator with the specified nested
* iterator and filter.
*/
public FilteringIterator(Iterator<? extends E1> nestedIterator, Filter<E1> filter) {
super();
this.nestedIterator = nestedIterator;
this.filter = filter;
this.loadNext();
}
public boolean hasNext() {
return ! this.done;
}
public E2 next() {
if (this.done) {
throw new NoSuchElementException();
}
E2 result = this.next;
this.loadNext();
return result;
}
/**
* Because we need to pre-load the next element
* to be returned, we cannot support the <code>remove()</code>
* method.
*/
public void remove() {
throw new UnsupportedOperationException();
}
/**
* Load next with the next valid entry from the nested
* iterator. If there are none, next is set to <code>END</code>.
*/
private void loadNext() {
this.done = true;
while (this.nestedIterator.hasNext() && (this.done)) {
E1 temp = this.nestedIterator.next();
if (this.accept(temp)) {
// assume that if the object was accepted it is of type E
this.next = this.cast(temp);
this.done = false;
} else {
this.next = null;
this.done = true;
}
}
}
/**
* We have to assume the filter will only "accept" objects that can
* be cast to E2.
*/
@SuppressWarnings("unchecked")
private E2 cast(E1 o) {
return (E2) o;
}
/**
* Return whether the <code>FilteringIterator</code>
* should return the specified next element from a call to the
* <code>next()</code> method.
* <p>
* This method can be overridden by a subclass as an
* alternative to building a <code>Filter</code>.
*/
protected boolean accept(E1 o) {
return this.filter.accept(o);
}
@Override
public String toString() {
return StringTools.buildToStringFor(this, this.nestedIterator);
}
}
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