Java hashset and null

Java собеседование. Коллекции vs null

В топике Java собеседование. Коллекции подробно изложен вопрос работы с Set & Map в Java. Но у меня ещё есть парочка любимых вопросов из этой области:

1. Может ли null использоваться в качестве ключа в Map?
2. Может ли Set содержать null?

 public V get(Object key) < if (key == null) return getForNullKey(); int hash = hash(key.hashCode()); for (Entrye = table[indexFor(hash, table.length)]; e != null; e = e.next) < Object k; if (e.hash == hash && ((k = e.key) == key || key.equals(k))) return e.value; >return null; > /** * Offloaded version of get() to look up null keys. Null keys map * to index 0. This null case is split out into separate methods * for the sake of performance in the two most commonly used * operations (get and put), but incorporated with conditionals in * others. */ private V getForNullKey() < for (Entrye = table[0]; e != null; e = e.next) < if (e.key == null) return e.value; >return null; > 

Предполагается, что пытливый читатель самостоятельно поразмыслит над ответами и затем сравнит их с моими. Самые нетерпеливые могут сразу проследовать под кат.

1. Может ли null использоваться в качестве ключа в Map?

HashMap

Map map = new HashMap(); map.put(null, "test"); // момент истины . ошибки нет! 

System.out.println(map.size()); // вывод: 1 System.out.println(map.get(null)); // вывод: test 

При добавлении новой пары ключ-значение, вычисляет хеш-код ключа, на основании которого вычисляется номер корзины (номер ячейки массива), в которую попадет новый элемент.

То есть получается, что вычисляется hash-code от null… хммм. Но как же он вычисляется без объекта, без hashCode()? Мой ответ — не знаю, но дебаггер показывает, что для null hash=0. Видимо где-то есть отдельная проверка.

Дальше все без сюрпризов, другой объект с hash=0 попадает в ту же «корзину».

map.put(0, "0"); System.out.println(map.size()); // вывод: 2 

Map map = new HashMap(); map.put(null, null); map.get(null); // результат null говорит нам, что значения по этому ключу нет, значит и самого ключа тоже нет map.containsKey(null); // результат true: а ключ на самом деле есть, и значение тоже есть 

TreeMap

Map map = new TreeMap(); map.put(null, "null"); // ошибки нет! 

System.out.println(map.size()); // вывод: 1 System.out.println(map.get(null)); // БАБАХ!! Exception in thread "main" java.lang.NullPointerException 

А ведь так все хорошо начиналось! Положить положили, оно там лежит (size=1), но достать обратно не можем. Ну ок, допустим нам вовсе и не надо ничего доставать. А мы хотим только класть, вот такие мы странные. Пробуем.

Map map = new TreeMap(); map.put(null, "null"); // ошибки нет System.out.println(map.size()); // вывод: 1 map.put(0, "0"); // БАБАХ!! Exception in thread "main" java.lang.NullPointerException 

Map map = new TreeMap(); map.put(0, "0"); // ошибки нет map.put(1, 1); // ошибки нет System.out.println(map.size()); // вывод: 2 map.put(null, "null"); // БАБАХ!! Exception in thread "main" java.lang.NullPointerException 

Это что же за ромашка получается, null то работает, то не работает?! А получается вот что. Когда мы добавляем ключ null в пустое дерево — он попадает в root. А root, товарищи, он и в Африке root, он равнее всех прочих, для него не происходит вызова compareTo() и null спокойно занимает своё место в корне дерева. А если дерево не пустое — начинаются попытки сравнения с имеющимся содержимым, и мы получаем «законный» NPE. Никаких особых условий для null здесь не предусмотрено.

Ответ №2: В пустой TreeMap можно положить единственный ключ-null, все остальные операции (кроме size() и clear(), кстати) после этого не работают. В непустой TreeMap положить null-ключ нельзя из-за обязательного вызова compareTo().

2. Может ли Set содержать null?

Ответ на этот вопрос повторяет предыдущие, с учетом того, что Set, собственно, реализован на основе Map. HashSet работает «на ура», TreeSet — только для первого элемента.

Источник

Java hashset and null

This class implements the Set interface, backed by a hash table (actually a HashMap instance). It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the null element. This class offers constant time performance for the basic operations (add, remove, contains and size), assuming the hash function disperses the elements properly among the buckets. Iterating over this set requires time proportional to the sum of the HashSet instance’s size (the number of elements) plus the «capacity» of the backing HashMap instance (the number of buckets). Thus, it’s very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important. Note that this implementation is not synchronized. If multiple threads access a hash set concurrently, and at least one of the threads modifies the set, it must be synchronized externally. This is typically accomplished by synchronizing on some object that naturally encapsulates the set. If no such object exists, the set should be «wrapped» using the Collections.synchronizedSet method. This is best done at creation time, to prevent accidental unsynchronized access to the set:

Set s = Collections.synchronizedSet(new HashSet(. ));

The iterators returned by this class’s iterator method are fail-fast: if the set is modified at any time after the iterator is created, in any way except through the iterator’s own remove method, the Iterator throws a ConcurrentModificationException . Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs. This class is a member of the Java Collections Framework.

Constructor Summary

Constructs a new, empty set; the backing HashMap instance has default initial capacity (16) and load factor (0.75).

Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and default load factor (0.75).

Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and the specified load factor.

Method Summary

Methods inherited from class java.util.AbstractSet

Methods inherited from class java.util.AbstractCollection

Methods inherited from class java.lang.Object

Methods inherited from interface java.util.Set

Methods inherited from interface java.util.Collection

Methods inherited from interface java.lang.Iterable

Constructor Detail

HashSet

Constructs a new, empty set; the backing HashMap instance has default initial capacity (16) and load factor (0.75).

HashSet

Constructs a new set containing the elements in the specified collection. The HashMap is created with default load factor (0.75) and an initial capacity sufficient to contain the elements in the specified collection.

HashSet

public HashSet(int initialCapacity, float loadFactor)

Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and the specified load factor.

HashSet

public HashSet(int initialCapacity)

Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and default load factor (0.75).

Method Detail

iterator

size

isEmpty

contains

Returns true if this set contains the specified element. More formally, returns true if and only if this set contains an element e such that (o==null ? e==null : o.equals(e)).

add

Adds the specified element to this set if it is not already present. More formally, adds the specified element e to this set if this set contains no element e2 such that (e==null ? e2==null : e.equals(e2)). If this set already contains the element, the call leaves the set unchanged and returns false.

remove

Removes the specified element from this set if it is present. More formally, removes an element e such that (o==null ? e==null : o.equals(e)), if this set contains such an element. Returns true if this set contained the element (or equivalently, if this set changed as a result of the call). (This set will not contain the element once the call returns.)

clear

clone

spliterator

Creates a late-binding and fail-fast Spliterator over the elements in this set. The Spliterator reports Spliterator.SIZED and Spliterator.DISTINCT . Overriding implementations should document the reporting of additional characteristic values.

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Источник

Class HashSet

Type Parameters: E — the type of elements maintained by this set All Implemented Interfaces: Serializable , Cloneable , Iterable , Collection , Set Direct Known Subclasses: JobStateReasons , LinkedHashSet

This class implements the Set interface, backed by a hash table (actually a HashMap instance). It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the null element.

This class offers constant time performance for the basic operations ( add , remove , contains and size ), assuming the hash function disperses the elements properly among the buckets. Iterating over this set requires time proportional to the sum of the HashSet instance’s size (the number of elements) plus the «capacity» of the backing HashMap instance (the number of buckets). Thus, it’s very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important.

Note that this implementation is not synchronized. If multiple threads access a hash set concurrently, and at least one of the threads modifies the set, it must be synchronized externally. This is typically accomplished by synchronizing on some object that naturally encapsulates the set. If no such object exists, the set should be «wrapped» using the Collections.synchronizedSet method. This is best done at creation time, to prevent accidental unsynchronized access to the set:

Set s = Collections.synchronizedSet(new HashSet(. ));

The iterators returned by this class’s iterator method are fail-fast: if the set is modified at any time after the iterator is created, in any way except through the iterator’s own remove method, the Iterator throws a ConcurrentModificationException . Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

This class is a member of the Java Collections Framework.

Источник