Hashmap java получить все значения

Hashmap java получить все значения

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.

Nested Class Summary

Nested classes/interfaces inherited from class java.util.AbstractMap

Nested classes/interfaces inherited from interface java.util.Map

Constructor Summary

Constructs an empty HashMap with the default initial capacity (16) and the default load factor (0.75).

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Hashmap java получить все значения

“Анна Ивановна Решетникова, 4211 717171” И как они собирались хранить такой номер паспорта в типе Integer? Тем самым показав, что номер паспорта хранить в базе данных как число — это не лучшая идея. Так как номера паспорта могут содержать дефисы, пробелы, буквы и т.д. Поправьте меня, если я не прав. Иначе у меня вопросы к господину Милану.

Скажите, почему хронология вывода на экран поменялась?

Получение списка всех ключей и значений Еще одна удобная особенность HashMap — можно по-отдельности получить список всех ключей и всех значений. Для этого используются методы keySet() и values() Для чего в примере создаются Set и ArrayList? Если в sout можно прямо вызвать методы keySet() и values()?

Возможно ли в HashMap переопределить метод toString() и если да, то каким образом? P/S: Спросил нынче модный ChatGPT, так он какую-то чушь городит: то пишет можно и даёт нерабочие коды, каждый раз разные, то пишет что нельзя потому что HashMap происходит от абстрактного класса. Хотя я уже понял что верить ему нельзя во всем, на вопрос можно ли реализовать в Java множественное наследование (не через интерфейсы) он бодро рапортовал, что конечно можно и вывалил код типа public сlass A extends B, C, который естественно не работает. Извините за возможный оффтоп. Так что роботы пока не завоюют мир, поэтому, вопрос по toString() все еще актуален. )

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Распечатайте все ключи и значения с карты в Java

В этом посте будут обсуждаться различные методы вывода всех ключей и значений из карты в Java.

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Мы знаем, что keySet() метод возвращает установленное представление ключей, содержащихся в карте, и values() метод возвращает установленное представление значений, содержащихся в карте. Итак, мы можем использовать keySet() распечатать все ключи, присутствующие на карте, и values() для печати всех значений. Есть несколько способов сделать это:

1. Использование Iterator

Map не имеет собственного итератора, поскольку он не расширяет Collection интерфейс. Оба keySet() а также values() возвращает набор, а набор расширяет Collection интерфейс, мы можем получить итератор.

2. Для каждого цикла

Цикл For-each доступен для любого объекта, реализующего Iterable интерфейс. В качестве Set расширяет Iterable интерфейс, мы можем использовать цикл for-each для перебора набора ключей и значений.

3. Java 8 — Iterator.forEachRemaining()

The Iterator интерфейс обеспечивает forEachRemaining() метод, который может печатать каждый элемент, пока все элементы не будут обработаны.

4. Java 8 – Stream.forEach()

Мы можем использовать цикл по набору ключей и значениям, используя Stream.forEach() метод для печати каждого элемента потока.

5. Использование toString()

Для отображения всех ключей или значений, присутствующих на карте, мы можем просто напечатать строковое представление keySet() а также values() , соответственно.

Ниже приведена простая программа на Java, которая печатает все ключи карты, используя keySet() в Java:

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Java Print HashMap Example

This example shows how to print HashMap in Java. The example also shows how to print all keys, all values, and all key-value pairs of HashMap using different ways.

How to print HashMap in Java?

The AbstractMap class, the parent class of the HashMap class, has overridden the toString method which returns a string representation of the map. All key-value pairs are enclosed in < and >and separated by a comma (,). The iterator of the entry set returns the order of the key-value pairs.

How to print all keys and values of HashMap using entrySet?

If you do not want the default formatting done by the toString method, you can get the entry set from the HashMap and print all key-value pairs one by one using the for loop as given below.

Java 8 and above

If you are using Java version 8 and above, you can use the below given code to print all keys and values of HashMap.

How to print all the keys of HashMap?

The keySet method of the HashMap class returns a Set view containing all the keys of the HashMap.

You can also use the System.out.println statement instead of using the for loop if you do not want to change the output format.

How to print all the values of the HashMap?

The values method of the HashMap returns a Collection view containing all the values contained in the HashMap.

You can also use System.out.println statement instead of using the for loop if you do not want to change the output format.

How to print HashMap containing custom class object as keys or values?

In all of the above examples, we printed Integer and String objects using the System.out.println statement. They were printed fine because both of these classes have overridden the toString method. Let’s try to print HashMap containing custom class objects.

We have put objects of Emp class as values in the HashMap in below given example.

As you can see from the output, the keys were printed fine but the values were not. It is because our Emp class has not overridden the toString method so it inherited the method from the Object class.

The toString method of the Object class returns the class name of the object, followed by @, followed by the hexadecimal hash code of the object. The format is not readable and hence it is suggested for all the subclasses to override the toString method to produce informative text.

Let’s override the toString method in the Emp class and try again.

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Java HashMap values()

The collection view only shows all values of the hashmap as one of the collection. The view does not contain actual values. To learn more about the view in Java, visit the view of a collection.

Note: The values() method returns the collection view. It is because unlike keys and entries, there can be duplicate values in hashmap.

Example 1: Java HashMap values()

import java.util.HashMap; class Main < public static void main(String[] args) < // create an HashMap HashMapprices = new HashMap<>(); // insert entries to the HashMap prices.put("Shoes", 200); prices.put("Bag", 300); prices.put("Pant", 150); System.out.println("HashMap: " + prices); // return view of all values System.out.println("Values: " + prices.values()); > >

HashMap: Values: [150, 300, 200]

In the above example, we have created a hashmap named prices . Notice the expression,

Here, the values() method returns a view of all the values present in the hashmap.

The values() method can also be used with the for-each loop to iterate through each value of the hashmap.

Example 2: values() Method in for-each Loop

import java.util.HashMap; class Main < public static void main(String[] args) < // Creating a HashMap HashMapnumbers = new HashMap<>(); numbers.put("One", 1); numbers.put("Two", 2); numbers.put("Three", 3); System.out.println("HashMap: " + numbers); // access all values of the HashMap System.out.print("Values: "); // values() returns a view of all values // for-each loop access each value from the view for(int value: numbers.values()) < // print each value System.out.print(value + ", "); >> >

In the above example, we have created a hashmap named numbers . Notice the line,

Integer value: numbers.values()

Here, the values() method returns a view of all values. The variable value access each value from the view.

Note: The Value of HashMap is of Integer type. Hence, we have used the int variable to access the values.

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