How to Filter a Map Based on Keys in Java - Step by Step Examples

How to Filter a Map Based on Keys in Java ?

Answer

To filter a map based on keys in Java, you can iterate over the map entries and add key-value pairs that meet the filter criteria to a new map. This method allows you to create a subset of the original map based on specific key conditions.

✐ Examples

1 Filtering a Map Based on Key Range

We can filter a map based on a range of keys in Java by iterating over the map entries and adding key-value pairs that fall within the specified range to a new map.

For example,

We start by importing the java.util.HashMap and java.util.Map classes, which provide the necessary functions and data structures for working with maps.
We declare and initialize a `HashMap` named unsortedMap with some key-value pairs. In this example, the map has integer keys and string values.
We declare an empty map named filteredMap to store the filtered key-value pairs.
We define the lower and upper bounds of the key range.
We iterate over the original map entries using a for loop and check if each key falls within the specified range.
We add the key-value pairs that meet the condition to the filteredMap.
We print the filtered map to the console using the System.out.println function to verify the filtering.

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");
        myMap.put(4, "four");
        myMap.put(5, "five");

        // Declare an empty map to store the filtered key-value pairs
        Map<Integer, String> filteredMap = new HashMap<>();

        // Define the key range
        int lowerBound = 2;
        int upperBound = 4;

        // Iterate over the original map and filter based on the key range
        for (Map.Entry<Integer, String> entry : myMap.entrySet()) {
            if (entry.getKey() >= lowerBound && entry.getKey() <= upperBound) {
                filteredMap.put(entry.getKey(), entry.getValue());
            }
        }

        // Print the filtered map
        System.out.println("Filtered Map by Key Range: " + filteredMap);
    }
}

Output

Filtered Map by Key Range: {2=two, 3=three, 4=four}

2 Filtering a Map Based on Specific Keys

We can filter a map based on specific keys in Java by iterating over the map entries and adding key-value pairs that match the specified keys to a new map.

For example,

We start by importing the java.util.HashMap, java.util.HashSet, and java.util.Map classes, which provide the necessary functions and data structures for working with maps and sets.
We declare and initialize a `HashMap` named unsortedMap with some key-value pairs. In this example, the map has integer keys and string values.
We declare an empty map named filteredMap to store the filtered key-value pairs.
We define a set of specific keys that we want to filter the map by.
We iterate over the original map entries using a for loop and check if each key matches one of the specified keys.
We add the key-value pairs that meet the condition to the filteredMap.
We print the filtered map to the console using the System.out.println function to verify the filtering.

Java Program

import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

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");
        myMap.put(4, "four");
        myMap.put(5, "five");

        // Declare an empty map to store the filtered key-value pairs
        Map<Integer, String> filteredMap = new HashMap<>();

        // Define the specific keys to filter by
        Set<Integer> keysToFilter = new HashSet<>();
        keysToFilter.add(1);
        keysToFilter.add(3);
        keysToFilter.add(5);

        // Iterate over the original map and filter based on the specific keys
        for (Map.Entry<Integer, String> entry : myMap.entrySet()) {
            if (keysToFilter.contains(entry.getKey())) {
                filteredMap.put(entry.getKey(), entry.getValue());
            }
        }

        // Print the filtered map
        System.out.println("Filtered Map by Specific Keys: " + filteredMap);
    }
}

Output

Filtered Map by Specific Keys: {1=one, 3=three, 5=five}

3 Filtering a Map with String Keys Based on Prefix

We can filter a map with string keys based on a specific prefix in Java by iterating over the map entries and adding key-value pairs that have keys starting with the given prefix to a new map.

For example,

We start by importing the java.util.HashMap and java.util.Map classes, which provide the necessary functions and data structures for working with maps.
We declare and initialize a `HashMap` named unsortedMap with some key-value pairs. In this example, the map has string keys and string values.
We declare an empty map named filteredMap to store the filtered key-value pairs.
We define a prefix string that we want to filter the map by.
We iterate over the original map entries using a for loop and check if each key starts with the specified prefix.
We add the key-value pairs that meet the condition to the filteredMap.
We print the filtered map to the console using the System.out.println function to verify the filtering.

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<String, String> myMap = new HashMap<>();
        myMap.put("apple", "fruit");
        myMap.put("banana", "fruit");
        myMap.put("carrot", "vegetable");
        myMap.put("apricot", "fruit");
        myMap.put("blueberry", "fruit");

        // Declare an empty map to store the filtered key-value pairs
        Map<String, String> filteredMap = new HashMap<>();

        // Define the prefix to filter by
        String prefix = "ap";

        // Iterate over the original map and filter based on the prefix
        for (Map.Entry<String, String> entry : myMap.entrySet()) {
            if (entry.getKey().startsWith(prefix)) {
                filteredMap.put(entry.getKey(), entry.getValue());
            }
        }

        // Print the filtered map
        System.out.println("Filtered Map by Prefix: " + filteredMap);
    }
}

Output

Filtered Map by Prefix: {apple=fruit, apricot=fruit}

Summary

In this tutorial, we learned How to Filter a Map Based on Keys in Java language with well detailed examples.

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How to Filter a Map Based on Keys in Java - Step by Step Examples