[Java] How to use the HashMap class

What is Map

Map is an interface called java.util.Map, which is a data structure used when you want to handle a set of keys and values ​​in Java. By using the Map interface, data structures such as HashMap and` `LinkedHashMap can be handled in a unified manner. In this article, I will write about how to use the HashMap class.

How to use the Map interface

Create your own class to implement the Map interface, or use an implementation class like HashMap or LinkedHashMap. You cannot register the same key more than once in the Map, and each key can only be mapped to one value.

Features of the HashMap class

■ ** Key order is not guaranteed ** ■ ** Key duplication is not allowed ** ■ ** Values ​​can be duplicated **

Declaration method

The declaration of the HashMap class is described as follows. Data type on the right side can be omitted by type inference The value can have List, Set, Map, and the key can be a class.

Map<Key model name,Value type name>Object name= new HashMap<>();

Map<String, List> map = new HashMap<>();  //Specify List as the value
Map<String, Set> map = new HashMap<>();   //Specify Set as the value
Map<String, Map<Integer, Object>> map = new HashMap<>();  //Specify Map as the value
Map<Example>, List> map = new HashMap<>();  //Specify a class called Example as a key

How to use

Specify the key and add the value (put method)

public class Sample {
	public static void main(String[] args) {
		Map<String,String> animal = new HashMap<>();
		animal.put("monkey", "monkey");
		animal.put("dog", "dog");
		animal.put("cat", "Cat");
	}
 }

If the key has already been mapped, it will be overwritten, and if there is no mapping, "null" will be returned.

public class Sample {
	public static void main(String[] args) {
		Map<String,String> animal = new HashMap<>();
		animal.put("monkey", "monkey");
		animal.put("dog", "dog");
		animal.put("dog", "Cat");
		System.out.println(animal.get("dog")); //Cat
		System.out.println(animal.get("pig")); //null
	}
 }

Get the value by specifying the key (get method)

public class Sample {
	public static void main(String[] args) {
		Map<String,String> animal = new HashMap<>();
		animal.put("monkey", "monkey");
		animal.put("dog", "dog");
		animal.put("cat", "Cat");
		System.out.println(animal.get("cat")); //Cat
	}
 }

Get all keys and values ​​(keySet method)

You can get all the Map keys by using the keySet method. Since the keySet method returns the Map key as a Set type, you can get all the keys by repeating the following using the extended for statement.

public class Sample {
	public static void main(String[] args) {
		Map<String,String> animal = new HashMap<>();
		animal.put("monkey", "monkey");
		animal.put("dog", "dog");
		animal.put("cat", "Cat");

		for (String pet : animal.keySet()) {
			System.out.println(pet);// monkey dog cat
		}
	}
 }

Get all values ​​(values ​​method)

You can get all the values ​​of Map by using the values ​​method. Since the values ​​method also returns the Map key as a Set type, you can get all the values ​​like the keySet method by using the extended for statement.

public class Sample {
	public static void main(String[] args) {
		Map<String,String> animal = new HashMap<>();
		animal.put("monkey", "monkey");
		animal.put("dog", "dog");
		animal.put("cat", "Cat");

		for (String pet : animal.values()) {
			System.out.println(pet);//Monkey dog ​​cat
		}
	}
 }

Get the number of elements (size method)

public class Sample {
	public static void main(String[] args) {
		Map<String,String> animal = new HashMap<>();
		animal.put("monkey", "monkey");
		animal.put("dog", "dog");
		animal.put("cat", "Cat");

		System.out.println(animal.size()); // 3
	}
 }

Returns a boolean value for the specified key (containsKey method)

Use the containsKey method to determine if a particular key is included in the map.

public class Sample {
	public static void main(String[] args) {
		Map<String,String> animal = new HashMap<>();
		animal.put("monkey", "monkey");
		animal.put("dog", "dog");
		animal.put("cat", "Cat");
		if (animal.containsKey("dog")) { // true
			System.out.println("Includes dog"); // Includes dog
		}
	}
 }

Returns a boolean value for the specified value (containsValue method)

public class Sample {
	public static void main(String[] args) {
		Map<String,String> animal = new HashMap<>();
		animal.put("monkey", "monkey");
		animal.put("dog", "dog");
		animal.put("cat", "Cat");
		if (animal.containsValue("monkey")) { // true
			System.out.println("Contains monkey"); // Contains monkey
		}
	}
 }

Returns empty or boolean (isEmpty method)

public class Sample {
	public static void main(String[] args) {
		Map<String,String> animal = new HashMap<>();
		animal.put("monkey", "monkey");
		animal.put("dog", "dog");
		animal.put("cat", "Cat");
		animal.clear();
		if (animal.isEmpty()) { // true
			System.out.println("It's empty"); // It's empty
		}
	}
 }

Process the elements in order (forEach method)

public class Sample {
    public static void main(String[] args) {
        Map<String,String> animal = new HashMap<>();
		animal.put("monkey", "monkey");
		animal.put("dog", "dog");
		animal.put("cat", "Cat");
        animal.forEach((key,value) -> System.out.println(key + " " + value)));
        //monkey monkey
        //dog dog
        //cat cat
    }
 }

reference

Java Platform SE 8 Interface Map (https://docs.oracle.com/javase/jp/8/docs/api/java/util/Map.html)