Map вывод всех элементов java

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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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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Print HashMap in Java

This is the most basic and easiest method to print out HashMap in java. Pass the HashMap reference to the System.out.println, and the HashMap will output the key-value of the elements enclosed in curly brackets.

We will use a HashMap constructor and a pass Map of elements to the constructor, which provides an easier way to initialize a HashMap with values using the Map.of() method.

A Map is an interface that forms the basis for all map implementations, including a HashMap. The Map.of() method returns an unmodifiable map containing the number of elements you create.

If there are any duplicate keys, the method throws IllegalArgumentException and a NullPointerException if any key or value is null .

Print HashMap using foreach method with keyset()

The HashMap get(Object key) is a method that returns the value that belongs to a particular key.

To get the key for every value in the HashMap, we use a for-loop with the keySet() method.

The key set method returns a set of unique keys, and we pass the keys to the get method to retrieve each value.

Print HashMap using Consumer with entrySet()

To use Consumer functional interface, you have to use the entrySet() method, which returns a Set containing the same type of key-value elements.

Since the Set is a Collection class that implements Iterable , use the forEach() method to print out the elements of the HashMap.

The forEach() is a method from the Iterable class, and it accepts only a Consumer as the parameter.

The for-each method iterates through the elements in the HashMap until they are exhausted as it prints them out using System.out.println.

Print HashMap using Arrays’s asList() method

To print out the elements of a HashMap using this class, pass the HashMap reference to the asList() method of the Arrays class.

This method will print out a list of elements in the HashMap as an Array.

When the reference of the array is is null , it throws a NullpointerException except where noted.

Print HashMap using Collections’s singletonList()

The singletonList() is a static method from the Collections class hence no instantiation is required to use the method.

singletonList() returns an immutable list which is simply a list that can not be modified by either adding or removing elements from it once it has been created.

When you try to add or remove elements from the singleton list it throws an UnsupportedOperationException indicating that it is not supported in the list.

The singletonList method is generic meaning that it can handle any data type and, in our case just pass the HashMap reference and it will automatically infer the type.

Print HashMap using getkey() and getValue with entrySet()

To use the getKey() and getValue() methods, we use the entrySet() method which returns a Set of Map entries Map.Entry .

The map entry contains the key-value elements in the HashMap, and the elements are only available during the iteration period.

Since Set is a Collection the only way to reference the Map entry is by using an iterator inherited from the Iterable class.

A for loop will iterate through the elements in the Set of Map entries and print out the key for each value using the get key method and the value using the get value method.

Print HashMap using BiConsumer

BiConsumer is a Functional Interface that represents an operation that accepts two arguments and returns no value.

The BiConsumer functional interface is a parameter of the forEach() method. The for-each method is inherited by the HashMap from the Map interface.

The type declared in the HashMap is the only type of value that will be accepted by the BiConsumer .

The functional interface will use the accept() method behind the scenes to receive the key and value parameters from the HashMap.

The action of our consumer will be printing out the elements in the HashMap. Note that if the action is null or an entry is removed during iteration, the for-each method will throw a NullPointerException and CurrentModificationException , respectively.

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How To Print HashMap In Java Map вывод всех элементов java

Do you want to know how to print HashMap values and keys in JAVA? In this post, I’ll show you how to print the hash map’s values and keys in JAVA.

HashMap is an implementation of the Map interface that is used to group elements into key-value pairs. We can use a variety of methods to print its elements. Examples include keySet(), values(), entrySet(), and asList(). Let us look at some examples.

Ways to Print Values and Keys in JAVA for HashMap

As you may be aware, the keySet() method returns a set view of the map’s keys, while the values() method returns a set view of the map’s values.

As a result, we can use keySet() to print all keys in the map and values() to print all values. There are numerous ways to accomplish this.

To get a set of keys, we can use the keySet() function, and then in the for loop, we can use the get() method to get the value. The get() function returns the value associated with the provided key. Examine the example below.

Using Println function to Print HashMap in JAVA

Using this method is the simplest way to print HashMap in Java. Simply pass the HashMap reference to the println() method, and the key-value pairs will be printed inside the curly braces. Take a look at the example below.

import java.util.HashMap; import java.util.Map; class Main < // Program to print all keys present in the map using `keySet()` in Java public static void main(String[] args) < MaphashMap = new HashMap<>(); hashMap.put(1, "First Element"); hashMap.put(2, "Second Element"); System.out.println(hashMap); > >

Using for-each loop To Print HashMap In Java

The for-each loop can be used by any object that implements the Iterable interface. Because Set extends Iterable, we can use a for-each loop to loop through the keySet and values.

import java.util.HashMap; import java.util.Map; class Solution < // Program to print all keys present in the map using `keySet()` in Java public static void main(String[] args) < MaphashMap = new HashMap<>(); hashMap.put(1, "First Element"); hashMap.put(2, "Second Element"); // Using For Each loop to print HashMap for (Integer key: hashMap.keySet()) < System.out.println(key); >> >

Using Iterator in Java 8 forEachRemaining()

The Iterator interface’s forEachRemaining() method can print each element until all of them have been processed.

import java.util.HashMap; import java.util.Map; class Solution < // Program to print all keys present in the map using `keySet()` in Java public static void main(String[] args) < MaphashMap = new HashMap<>(); hashMap.put(1, "First Element"); hashMap.put(2, "Second Element"); // Using ForEachRemaining for printing HashMap hashMap.keySet().iterator() .forEachRemaining(System.out::println); > >

Using Stream.forEach in Java 8

To print each stream element, we can use the Stream.forEach() method to loop through the keySet and values.

import java.util.HashMap; import java.util.Map; class Solution < // Program to print all keys present in the map using `keySet()` in Java public static void main(String[] args) < MaphashMap = new HashMap<>(); hashMap.put(1, "First Element"); hashMap.put(2, "Second Element"); // Using Java 8 – Collection.stream() + Stream.forEach() to print the HashMap hashMap.keySet().stream() .forEach(System.out::println); > >

Using toString method ()

To display all keys and values on the map, we can simply print the string representations of keySet() and values().

import java.util.HashMap; import java.util.Map; import java.util.stream.Stream; class Solution < // Program to print all keys present in the map using `keySet()` in Java public static void main(String[] args) < MaphashMap = new HashMap<>(); hashMap.put(1, "First Element"); hashMap.put(2, "Second Element"); // Using Convert to a string method to print HashMap System.out.println(hashMap.keySet().toString()); // Java 8 Stream.of(hashMap.keySet().toString()) .forEach(System.out::println); > >

Using Simple Iterator

Map does not have its own iterator because it does not extend the Collection interface. Because the set extends the Collection interface, we can get an iterator from both keySet() and values().

import java.util.HashMap; import java.util.Iterator; import java.util.Map; class Main < // Program to print all keys present in the map using `keySet()` in Java public static void main(String[] args) < MaphashMap = new HashMap<>(); hashMap.put(1, "First Element"); hashMap.put(2, "Second Element"); // Using an iterator for Printing the HashMap Iterator mapIterator = hashMap.keySet().iterator(); while (mapIterator.hasNext()) < System.out.println(mapIterator.next()); >> >

Wrap Up

I hope you now have a better understanding of how to print a hashmap in Java. I’ve provided more than five methods that you can use to print the keys and values of a HashMap in Java, which you can find in the code below.

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