IterableAndIterator
Class should not implement both `Iterable` and `Iterator`

Category
Severity
WARNING
Maturity

The problem

An Iterator is a state-ful instance that enables you to check whether it has more elements (via hasNext()) and moves to the next one if any (via next()), while an Iterable is a representation of literally iterable elements. An Iterable can generate multiple valid Iterators, though.

Suppression

Suppress false positives by adding the suppression annotation @SuppressWarnings("IterableAndIterator") to the enclosing element.


Positive examples

IterableAndIteratorPositiveCases.java

/*
* Copyright 2016 Google Inc. All Rights Reserved.
*
* Licensed 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 com.google.errorprone.bugpatterns.testdata;

import java.util.Iterator;

/** @author jsjeon@google.com (Jinseong Jeon) */
public class IterableAndIteratorPositiveCases {

/** Test Node */
public static class MyNode {
String tag;
MyNode next;
}

/** Test List that implements both Iterator and Iterable */
// BUG: Diagnostic contains: both
public static class MyBadList implements Iterator<MyNode>, Iterable<MyNode> {
private MyNode head;

public MyBadList() {
head = null;
}

@Override
public boolean hasNext() {
return head != null;
}

@Override
public MyNode next() {
if (hasNext()) {
MyNode ret = head;
head = head.next;
return ret;
}
return null;
}

@Override
public void remove() {
throw new UnsupportedOperationException("remove is not supported");
}

public void add(MyNode node) {
if (!hasNext()) {
head.next = node;
}
head = node;
}

@Override
public Iterator<MyNode> iterator() {
return this;
}
}

/** Test List that extends the above bad implementation Diagnostic should bypass this */
public static class MyBadListInherited extends MyBadList {
public MyBadListInherited() {}
}

/** Test List that implements only Iterator */
public static class MyGoodList implements Iterator<MyNode> {
private MyNode head;

public MyGoodList() {
head = null;
}

@Override
public boolean hasNext() {
return head != null;
}

@Override
public MyNode next() {
if (hasNext()) {
MyNode ret = head;
head = head.next;
return ret;
}
return null;
}

@Override
public void remove() {
throw new UnsupportedOperationException("remove is not supported");
}

public void add(MyNode node) {
if (!hasNext()) {
head.next = node;
}
head = node;
}
}

/** Test List that implicitly implements both interfaces */
// BUG: Diagnostic contains: both
public static class MyImplicitlyBadList extends MyGoodList implements Iterable<MyNode> {

public MyImplicitlyBadList() {}

@Override
public Iterator<MyNode> iterator() {
return this;
}
}
}

Negative examples

IterableAndIteratorNegativeCases.java

/*
* Copyright 2016 Google Inc. All Rights Reserved.
*
* Licensed 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 com.google.errorprone.bugpatterns.testdata;

import java.util.Iterator;

/** @author jsjeon@google.com (Jinseong Jeon) */
public class IterableAndIteratorNegativeCases {

/** Test Node */
public static class MyNode {
String tag;
MyNode next;
}

/** Test List that implements only Iterator */
public static class MyList1 implements Iterator<MyNode> {
private MyNode head;

public MyList1() {
head = null;
}

@Override
public boolean hasNext() {
return head != null;
}

@Override
public MyNode next() {
if (hasNext()) {
MyNode ret = head;
head = head.next;
return ret;
}
return null;
}

@Override
public void remove() {
throw new UnsupportedOperationException("remove is not supported");
}

public void add(MyNode node) {
if (!hasNext()) {
head.next = node;
}
head = node;
}
}

/** Test List that implements only Iterable */
public static class MyList2 implements Iterable<MyNode> {

@Override
public Iterator<MyNode> iterator() {
MyList1 l = new MyList1();
// code to populate the list goes here
return l;
}
}
}