ConcurrentHashMap size() Method in Java

The size() method in Java’s ConcurrentHashMap class is used to retrieve the number of key-value mappings in the map. It has the following signature: 

int size()

The size() method works in a concurrent environment, which means that it can be called from multiple threads without causing any data race or synchronization issues.

When a size() operation is performed, the method first acquires the lock on all the segments of the map and then calculates the total number of key-value mappings in the map.

Here is an example of using the size() method:

Java

import java.util.concurrent.ConcurrentHashMap;

public class ConcurrentHashMapExample {
    public static void main(String[] args) {
        ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
        map.put("Alice", 25);
        map.put("Bob", 30);
        map.put("Charlie", 35);

        int size = map.size();
        System.out.println("Size of the map: " + size); // 3
    }
}

Output

Size of the map: 3

In this example, we first create a ConcurrentHashMap and add three key-value pairs to it using the put() method.

Next, we perform a size() operation to retrieve the number of key-value mappings in the map. The method returns 3, and we print out the result.

The java.util.concurrent.ConcurrentHashMap.size() method is an in-built function in Java that counts the number of key-value mappings in this map and returns the integer value. 

Syntax:

public int size()

Return Value: The function returns an integer value that denotes the number of key-value mappings in this map. Below program illustrates the use of the size() method: 

Program 1:

Java

// Java Program Demonstrate size()
// method of ConcurrentHashMap

import java.util.concurrent.*;

class GFG {
    public static void main(String[] args)
    {

        ConcurrentHashMap<Integer, String> chm
            = new ConcurrentHashMap<Integer, String>();

        chm.put(100, "Geeks");
        chm.put(101, "for");
        chm.put(102, "Geeks");

        // Display the number of
        // key-value mappings
        System.out.println("The number of "
                           + "key-value mappings is "
                           + chm.size());
    }
}

The number of key-value mappings is 3

Program 2:

Java

// Java Program Demonstrate size()
// method of ConcurrentHashMap

import java.util.concurrent.*;

class GFG {
    public static void main(String[] args)
    {

        ConcurrentHashMap<Integer, Integer> chm
            = new ConcurrentHashMap<Integer, Integer>();

        chm.put(1, 100);
        chm.put(2, 200);
        chm.put(3, 300);
        chm.put(4, 400);
        chm.put(5, 500);
        chm.put(6, 600);

        // Display the number of
        // key-value mappings
        System.out.println("The number of "
                           + "key-value mappings is "
                           + chm.size());
    }
}

The number of key-value mappings is 6