Input: map = {1=1, 2=2, 3=null, 4=4, 5=null, 6=null}, defaultValue = 0
Output: {1=1, 2=2, 3=0, 4=4, 5=0, 6=0}

Input: map = {1=A, 2=B, 3=null, 4=D, 5=null, 6=null}, defaultValue = ‘Z’
Output: {1=A, 2=B, 3=Z, 4=D, 5=Z, 6=Z}

Approach:

1. Get the map with null values and therefore the default value to get replaced with.
2. Get the set view of the Map using Map.entrySet() method.
3. Convert the obtained set view into stream using stream() method.
4. Now map the null values to default value with the assistance of map() method.
5. Collect the modified stream into Map using collect() method.
6. The null values has been successfully replaced with default value.

Below is the implementation of the above approach:

Example 1: With integers.

// Java program to replace null values
// of a map with a default value

import java.util.*;
import java.util.stream.*;

class GFG {

	// Function to replace the null values
	public static <T, K> Map<K, T>
	replaceNullValues(Map<K, T> map, T defaultValue)
	{

		// Replace the null value
		map = map.entrySet()
				.stream()
				.map(entry -> {
					if (entry.getValue() == null)
						entry.setValue(defaultValue);
					return entry;
				})
				.collect(Collectors.toMap(Map.Entry::getKey,
											Map.Entry::getValue));

		return map;
	}

	public static void main(String[] args)
	{

		// Get the map
		Map<Integer, Integer> map = new HashMap<>();
		map.put(1, 1);
		map.put(2, 2);
		map.put(3, null);
		map.put(4, 4);
		map.put(5, null);
		map.put(6, null);

		// Get the default value
		int defaultValue = 0;

		// Print the original map
		System.out.println("Map with null values: "
						+ map);

		// Replace the null values with the defaultValue
		map = replaceNullValues(map, defaultValue);

		// Print the modified map
		System.out.println("Map with null value replaced: "
						+ map);
	}
}

Output:

Map with null values: {1=1, 2=2, 3=null, 4=4, 5=null, 6=null}
Map with null value replaced: {1=1, 2=2, 3=0, 4=4, 5=0, 6=0}

Example 2: With characters.

// Java program to replace null values
// of a map with a default value

import java.util.*;
import java.util.stream.*;

class GFG {

	// Function to replace the null values
	public static <T, K> Map<K, T>
	replaceNullValues(Map<K, T> map, T defaultValue)
	{

		// Replace the null value
		map = map.entrySet()
				.stream()
				.map(entry -> {
					if (entry.getValue() == null)
						entry.setValue(defaultValue);
					return entry;
				})
				.collect(Collectors.toMap(Map.Entry::getKey,
											Map.Entry::getValue));

		return map;
	}

	public static void main(String[] args)
	{

		// Get the map
		Map<Integer, Character> map = new HashMap<>();
		map.put(1, 'A');
		map.put(2, 'B');
		map.put(3, null);
		map.put(4, 'D');
		map.put(5, null);
		map.put(6, null);

		// Get the default value
		char defaultValue = 'Z';

		// Print the original map
		System.out.println("Map with null values: "
						+ map);

		// Replace the null values with the defaultValue
		map = replaceNullValues(map, defaultValue);

		// Print the modified map
		System.out.println("Map with null value replaced: "
						+ map);
	}
}

Output:

Map with null values: {1=A, 2=B, 3=null, 4=D, 5=null, 6=null}
Map with null value replaced: {1=A, 2=B, 3=Z, 4=D, 5=Z, 6=Z}