[Java] How to use Map

Programming study diary

January 5, 2021 A brief summary of how to handle Maps in Java.

What is Map?

Map can store multiple data consisting of two elements, key and value. A key is like giving a name to a value, and each value has a key. The feature of Map is that this key and value are paired. Therefore, when searching for a value from Map, you can search for the key as a marker.

How to initialize with HashMap

Map is an interface, so you have to use a class that implements it to instantiate it. The HashMap class is often used to instantiate a Map.

How to declare using the HashMap class

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

You must specify the key and value data types when declaring.

Sample code

import java.util.HashMap;
import java.util.Map;

public class Sample {
    public static void main(String[] args) throws Exception {
        //When the key is Integer and the value is String
        Map<Integer, String> map = new HashMap<>();
    }
}

Frequently used methods in Map

put method

A method that can add data to Map, specify the key in the first argument and the value in the second argument.

Sample code

import java.util.HashMap;
import java.util.Map;

public class Sample {
           public static void main(String[] args) throws Exception {
                      Map<Integer, String> map = new HashMap<>();
                      map.put(1, "Tanaka");
                      map.put(3, "Suzuki");
                      map.put(5, "Yamada");
           }
}

get method

It is a method that can get the data stored in Map, and you can get the value of the key by specifying the key in the argument.

Sample code

import java.util.HashMap;
import java.util.Map;

public class Sample {
           public static void main(String[] args) throws Exception {
                      Map<Integer, String> map = new HashMap<>();
                      map.put(1, "Tanaka");
                      map.put(3, "Suzuki");
                      map.put(5, "Yamada");

                      System.out.println(map.get(1));
                      System.out.println(map.get(3));
                      System.out.println(map.get(5));
           }
}

Execution result

Tanaka
Suzuki
Yamada

keySet method

A method that returns the value of the key contained in the Map. This time, I will introduce how to output all the keys of Map using the keySet method.

Sample code

import java.util.HashMap;
import java.util.Map;

public class Sample {
           public static void main(String[] args) throws Exception {
                      Map<Integer, String> map = new HashMap<>();
                      map.put(1, "Tanaka");
                      map.put(3, "Suzuki");
                      map.put(5, "Yamada");

                      for(Integer key:map.keySet()) {
                                 System.out.println(key);
                      }
           }
}

Execution result

1
3
5

values ​​method

The values ​​method can receive the defined Map and enumerator collectively. If you want to get all the values ​​of Map, use values ​​method.

Sample code

#### **`Sample code`**
```java

import java.util.HashMap;
import java.util.Map;

public class Sample {
           public static void main(String[] args) throws Exception {
                      Map<Integer, String> map = new HashMap<>();
 map.put (1, "Tanaka");
 map.put (3, "Suzuki");
 map.put (5, "Yamada");

                      for(String values: map.values()) {
                                 System.out.println(values);
                      }
           }
}

Execution result

 Tanaka
 Suzuki
 Yamada

#References [For beginners] Explain how to use Java map! Introducing sample code! [Introduction to Java] Summary of how to use Map(Initialize with HashMap, sort values)