How to Iterate Over Entries (Key-Value Pairs) in a Map in Java - Step by Step Examples

How to Iterate Over Entries (Key-Value Pairs) in a Map in Java ?

Answer

To iterate over entries (key-value pairs) in a map in Java, you can use a for-each loop with the entrySet method or an iterator. This allows you to access each key-value pair in the map.

✐ Examples

1 Iterating Over Entries Using for-each Loop

We can iterate over entries in a map in Java using a for-each loop with the entrySet method, which provides a convenient syntax for iterating over all key-value pairs in the map.

For example,

We start by importing the java.util.HashMap package, which provides the HashMap class needed to create and manipulate the map.
We declare and initialize a map named myMap with some key-value pairs. In this example, the map has integer keys and string values.
We use the entrySet method to get a set of the key-value pairs in the map.
We iterate over the entries using a for-each loop, accessing each key and value from the entry set.
We print each key and value to the console using the System.out.println function to verify that we are iterating over all entries.

Java Program

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

public class Main {
    public static void main(String[] args) {
        // Declare and initialize a map
        Map<Integer, String> myMap = new HashMap<>();
        myMap.put(1, "one");
        myMap.put(2, "two");
        myMap.put(3, "three");

        // Iterate over entries using for-each loop
        for (Map.Entry<Integer, String> entry : myMap.entrySet()) {
            System.out.println("Key: " + entry.getKey() + ", Value: " + entry.getValue());
        }
    }
}

Output

Key: 1, Value: one
Key: 2, Value: two
Key: 3, Value: three

2 Iterating Over Entries Using Iterator

We can iterate over entries in a map in Java using an iterator, which provides a flexible way to access each key-value pair in the map.

For example,

We start by importing the java.util.HashMap and java.util.Iterator packages, which provide the HashMap and Iterator classes needed to create and manipulate the map and iterate over its entries.
We declare and initialize a map named myMap with some key-value pairs. In this example, the map has integer keys and string values.
We declare an iterator for the set of entries in the map using the entrySet and iterator methods.
We use a while loop to iterate over the entries of the map by incrementing the iterator. We access each key and value from the iterator's entry set.
We print each key and value to the console using the System.out.println function to verify that we are iterating over all entries.

Java Program

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

public class Main {
    public static void main(String[] args) {
        // Declare and initialize a map
        Map<Integer, String> myMap = new HashMap<>();
        myMap.put(1, "one");
        myMap.put(2, "two");
        myMap.put(3, "three");

        // Declare an iterator for the set of entries in the map
        Iterator<Map.Entry<Integer, String>> iterator = myMap.entrySet().iterator();

        // Iterate over entries using iterator
        while (iterator.hasNext()) {
            Map.Entry<Integer, String> entry = iterator.next();
            System.out.println("Key: " + entry.getKey() + ", Value: " + entry.getValue());
        }
    }
}

Output

Key: 1, Value: one
Key: 2, Value: two
Key: 3, Value: three

3 Storing and Iterating Over Entries Separately

We can store entries in a separate list and then iterate over the list in Java, which provides a way to work with key-value pairs independently of the map.

For example,

We start by importing the java.util.HashMap and java.util.ArrayList packages, which provide the HashMap and ArrayList classes needed to create and manipulate the map and store the entries.
We declare and initialize a map named myMap with some key-value pairs. In this example, the map has integer keys and string values.
We declare a list named entries to store the key-value pairs from the map.
We use the entrySet method to get a set of the key-value pairs in the map and add each entry to the entries list using the addAll method.
We use a for-each loop to iterate over the entries list and print each key and value to the console using the System.out.println function to verify that we have stored all entries separately.

Java Program

import java.util.HashMap;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

public class Main {
    public static void main(String[] args) {
        // Declare and initialize a map
        Map<Integer, String> myMap = new HashMap<>();
        myMap.put(1, "one");
        myMap.put(2, "two");
        myMap.put(3, "three");

        // Declare a list to store the entries
        List<Map.Entry<Integer, String>> entries = new ArrayList<>(myMap.entrySet());

        // Iterate over the entries list
        for (Map.Entry<Integer, String> entry : entries) {
            System.out.println("Key: " + entry.getKey() + ", Value: " + entry.getValue());
        }
    }
}

Output

Key: 1, Value: one
Key: 2, Value: two
Key: 3, Value: three

Summary

In this tutorial, we learned How to Iterate Over Entries (Key-Value Pairs) in a Map in Java language with well detailed examples.

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How to Iterate Over Entries (Key-Value Pairs) in a Map in Java - Step by Step Examples