org.apache.kafka.common.utils.CircularIterator Maven / Gradle / Ivy
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* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.kafka.common.utils;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.Objects;
/**
* An iterator that cycles through the {@code Iterator} of a {@code Collection}
* indefinitely. Useful for tasks such as round-robin load balancing. This class
* does not provide thread-safe access. This {@code Iterator} supports
* {@code null} elements in the underlying {@code Collection}. This
* {@code Iterator} does not support any modification to the underlying
* {@code Collection} after it has been wrapped by this class. Changing the
* underlying {@code Collection} may cause a
* {@link ConcurrentModificationException} or some other undefined behavior.
*/
public class CircularIterator implements Iterator {
private final Iterable iterable;
private Iterator iterator;
private T nextValue;
/**
* Create a new instance of a CircularIterator. The ordering of this
* Iterator will be dictated by the Iterator returned by Collection itself.
*
* @param col The collection to iterate indefinitely
*
* @throws NullPointerException if col is {@code null}
* @throws IllegalArgumentException if col is empty.
*/
public CircularIterator(final Collection col) {
this.iterable = Objects.requireNonNull(col);
this.iterator = col.iterator();
if (col.isEmpty()) {
throw new IllegalArgumentException("CircularIterator can only be used on non-empty lists");
}
this.nextValue = advance();
}
/**
* Returns true since the iteration will forever cycle through the provided
* {@code Collection}.
*
* @return Always true
*/
@Override
public boolean hasNext() {
return true;
}
@Override
public T next() {
final T next = nextValue;
nextValue = advance();
return next;
}
/**
* Return the next value in the {@code Iterator}, restarting the
* {@code Iterator} if necessary.
*
* @return The next value in the iterator
*/
private T advance() {
if (!iterator.hasNext()) {
iterator = iterable.iterator();
}
return iterator.next();
}
/**
* Peek at the next value in the Iterator. Calling this method multiple
* times will return the same element without advancing this Iterator. The
* value returned by this method will be the next item returned by
* {@code next()}.
*
* @return The next value in this {@code Iterator}
*/
public T peek() {
return nextValue;
}
@Override
public void remove() {
throw new UnsupportedOperationException();
}
}