Java hashmap get nullpointerexception

Learn Java and WCS

1. Don’t forget to set the hash map before using it.

hm = new HashMapString, String>(); 

2. Do the null check before using HashMap.

3. Do the Null check for values from the hash map

In the above code try to get the value for key “2” which is not there in hm, it will always return “null”

4. Check for the key existence in hasmap using “containsKey”

 
map.containsKey(key)
 
In the above code we can also add a check for key existence in the “HashMap” 
 

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Class HashMap

Type Parameters: K — the type of keys maintained by this map V — the type of mapped values All Implemented Interfaces: Serializable , Cloneable , Map Direct Known Subclasses: LinkedHashMap , PrinterStateReasons

Hash table based implementation of the Map interface. This implementation provides all of the optional map operations, and permits null values and the null key. (The HashMap class is roughly equivalent to Hashtable , except that it is unsynchronized and permits nulls.) This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.

This implementation provides constant-time performance for the basic operations ( get and put ), assuming the hash function disperses the elements properly among the buckets. Iteration over collection views requires time proportional to the «capacity» of the HashMap instance (the number of buckets) plus its size (the number of key-value mappings). Thus, it’s very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important.

An instance of HashMap has two parameters that affect its performance: initial capacity and load factor. The capacity is the number of buckets in the hash table, and the initial capacity is simply the capacity at the time the hash table is created. The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. When the number of entries in the hash table exceeds the product of the load factor and the current capacity, the hash table is rehashed (that is, internal data structures are rebuilt) so that the hash table has approximately twice the number of buckets.

As a general rule, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the lookup cost (reflected in most of the operations of the HashMap class, including get and put ). The expected number of entries in the map and its load factor should be taken into account when setting its initial capacity, so as to minimize the number of rehash operations. If the initial capacity is greater than the maximum number of entries divided by the load factor, no rehash operations will ever occur.

If many mappings are to be stored in a HashMap instance, creating it with a sufficiently large capacity will allow the mappings to be stored more efficiently than letting it perform automatic rehashing as needed to grow the table. Note that using many keys with the same hashCode() is a sure way to slow down performance of any hash table. To ameliorate impact, when keys are Comparable , this class may use comparison order among keys to help break ties.

Note that this implementation is not synchronized. If multiple threads access a hash map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more mappings; merely changing the value associated with a key that an instance already contains is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the map. If no such object exists, the map should be «wrapped» using the Collections.synchronizedMap method. This is best done at creation time, to prevent accidental unsynchronized access to the map:

Map m = Collections.synchronizedMap(new HashMap(. ));

The iterators returned by all of this class’s «collection view methods» are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator’s own remove method, the iterator will throw 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.

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Кофе-брейк #173. В чем разница между HashMap и Hashtable. Как найти и исправить исключение NullPointerException в Java

Источник: Medium Сегодня вы узнаете ответ на один из самых популярных вопросов на собеседованиях по Java: какая разница между HashMap и Hashtable. Hashtable — это структура данных для хранения пар ключей и их значений, основанная на хешировании и реализации интерфейса Map. HashMap также является структурой данных для хранения ключей и значений, основанной на хешировании и реализации интерфейса Map. HashMap позволяет быстро получить значение по ключу. Например, имя пользователя и номер его телефона. Между HashMap и Hashtable в Java есть существенные различия . К ним относятся безопасность потоков, синхронизация и согласованность реализации Map. Важно учитывать эти аспекты при выборе типа структуры данных для использования в вашей Java-программе: HashMap — это несинхронизированная неупорядоченная карта пар ключ-значение (key-value). Она допускает пустые значения и использует хэш-код в качестве проверки на равенство, в то время как Hashtable представляет собой синхронизированную упорядоченную карту пар ключ-значение. Она не допускает пустых значений и использует метод equals() для проверки на равенство. HashMap по умолчанию имеет емкость 16, а начальная емкость Hashtable по умолчанию — 11. Значения объекта HashMap перебираются с помощью итератора, а Hashtable — это единственный класс, кроме вектора, который использует перечислитель (enumerator) для перебора значений объекта Hashtable. Помните, что различия между HashMap и Hashtable — один из самых популярных вопросов на собеседованиях по Java.

Как найти и исправить исключение NullPointerException в Java

Источник: Theflashreads Прочитав эту статью, вы сумеете лучше разбираться в поиске и исправлении распространенной ошибки в Java-коде — исключении NullPointerException.

Что такое исключение NullPointerException?

java.lang.NullPointerException является исключением времени выполнения в Java. Оно возникает, когда происходит попытка доступа к переменной, которая не указывает ни на какой объект и ни на что не ссылается, то есть она равна нулю. Поскольку NullPointerException это исключение времени выполнения, его не нужно перехватывать и обрабатывать явно в коде приложения.

При каких обстоятельствах возникает NullPointerException?

• Доступ к свойствам нулевого объекта.
• Выбрасывание null из метода, который выдает исключение.
• Неправильная конфигурация для инфраструктур внедрения зависимостей, например: (Spring).
• Передача нулевых параметров в метод.
• Вызов методов для нулевого объекта.
• Использование synchronized для нулевого объекта.
• Доступ к элементу index (например, в массиве) нулевого объекта.

Пример исключения NullPointerException

В этом примере мы пытаемся вызвать метод printLength , который принимает строку в качестве параметра и печатает ее длину. Если значение строки, которое передается в качестве параметра, равно нулю, то появится исключение java.lang.NullPointerException .

 public class NullPointerExceptionExample < private static void printLength(String str) < System.out.println(str.length()); >public static void main(String args[]) < String nullString = null; printLength(nullString); >> Exception in thread "main" java.lang.NullPointerException at NullPointerExceptionExample.printLength(NullPointerExceptionExample.java:3) at NullPointerExceptionExample.main(NullPointerExceptionExample.java:8) 

Как избежать возникновения NullPointerException

• Убедитесь, что объект правильно инициализирован, добавив проверку null перед обращением к его методам или свойствам.
• Используйте Apache Commons StringUtils для операций со строками: например, StringUtils.isNotEmpty() для проверки того, пуста ли строка перед ее дальнейшим использованием.

 import org.apache.commons.lang3.StringUtils; public class NullPointerExceptionExample < private static void printLength(String str) < if (StringUtils.isNotEmpty(str)) < System.out.println(str.length()); >else < System.out.println("This time there is no NullPointerException"); >> public static void main(String args[]) < String nullString = null; printLength(nullString); >> 

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