How do I remove a map entry for the specified key-value?

Beginning from Java 8, the Map interface includes the remove(Object key, Object value) method, which removes the entry for the specified key only if it is currently mapped to the specified value.

Here is a Java 8 way of accomplishing this:

package org.kodejava.util;

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

public class MapRemoveKeyValueExample {
    public static void main(String[] args) {
        Map<String, String> myMap = new HashMap<>();

        myMap.put("key1", "value1");
        myMap.put("key2", "value2");

        System.out.println("Map before: " + myMap);

        myMap.remove("key1", "value1");

        System.out.println("Map after: " + myMap);
    }
}

Output:

Map before: {key1=value1, key2=value2}
Map after: {key2=value2}

It’s important to note, however, that this method will do nothing if the initially passed value does not match the value currently mapped by the key in the map. The method also returns a boolean indicating whether the removal was successful (i.e., the key/value pair was in the map).

The remove(Object key, Object value) method is indeed a more concise way to accomplish this task in Java 8 or above, as it does not require an explicit condition check as in the previous approach.

How do I use the compute operations of the map object in Java?

The compute(), computeIfAbsent(), and computeIfPresent() methods introduced in Java 8 provide powerful functionality to modify an existing map in a thread-safe manner.

Here’s an example of how you might use each:

  • compute(): Performs the given mapping function to the entry for the specified key. The function is applied even if key is not present or is null.
Map<Integer, String> map = new HashMap<>();
map.put(1, "one");
map.put(2, "two");
map.put(3, "three");

map.compute(1, (key, value) -> value + " hundred");

System.out.println(map.get(1)); // prints "one hundred"
  • Word Frequency Count (using compute())

A common use case is when counting the frequency of words in a text. This is where compute() can come in handy:

package org.kodejava.util;

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

public class MapComputeExample {
    public static void main(String[] args) {
        Map<String, Integer> wordCounts = new HashMap<>();
        String sentence = "This is a sample sentence with repeated sample words sample sample";

        for (String word : sentence.split(" ")) {
            wordCounts.compute(word, (key, value) -> value == null ? 1 : value + 1);
        }
        System.out.println("wordCounts = " + wordCounts);
    }
}

In the snippet above, for each word, we increment its count in the wordCounts map, initializing with 1 if the word doesn’t exist yet.

  • computeIfAbsent(): If the specified key is not already associated with a value (or is mapped to null), computes its value using the given mapping function and enters it into this map unless null.
Map<Integer, String> map = new HashMap<>();
map.put(1, "one");
map.put(2, "two");
map.put(3, "three");

map.computeIfAbsent(4, key -> "four");

System.out.println(map.get(4)); // prints "four"
  • Caching objects (using computeIfAbsent())

Using computeIfAbsent(), you can create a cache that computes values the first time they are requested:

package org.kodejava.util;

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

public class MapComputeIfAbsentExample {
    Map<String, String> cache = new HashMap<>();

    public static void main(String[] args) {
        MapComputeIfAbsentExample demo = new MapComputeIfAbsentExample();
        String imageDir = demo.fetchFromCache("image_dir");
        System.out.println("imageDir = " + imageDir);
    }

    public String fetchFromCache(String key) {
        return cache.computeIfAbsent(key, this::fetchFromDataBase);
    }

    public String fetchFromDataBase(String key) {
        // Simulating actual fetching from a DB
        return "Data for " + key;
    }
}

In this case, whenever data is fetched from the cache, if the key doesn’t exist, computeIfAbsent() will automatically fetch it from the database and store it in the map for future access.

  • computeIfPresent(): If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.
Map<Integer, String> map = new HashMap<>();
map.put(1, "one");
map.put(2, "two");
map.put(3, "three");

map.computeIfPresent(1, (key, value) -> value + " hundred");

System.out.println(map.get(1)); // prints "one hundred"
  • Modifying map entries upon certain conditions (using computeIfPresent())

Suppose we have a map of users and their loyalty points. You want to double the points of a user only if the user exists in the map.

package org.kodejava.util;

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

public class MapComputeIfPresentExample {
    public static void main(String[] args) {
        Map<String, Integer> loyaltyPoints = new HashMap<>();
        loyaltyPoints.put("User1", 10);
        loyaltyPoints.put("User2", 20);

        loyaltyPoints.computeIfPresent("User1", (key, value) -> value * 2);

        System.out.println(loyaltyPoints.get("User1")); // prints 20
    }
}

computeIfPresent() will only modify the entries if the keys exactly exist in the map. This can be useful for making conditional updates to a map.

These methods are interesting when you want to modify the map in one atomic operation, which can be useful in multithreaded environments. Moreover, they allow cleaner and more concise code by combining the operations of testing, inserting, removing, and modifying into a single method call.

How do I use Map.getOrDefault() default method in Java?

The Map.getOrDefault(Object key, V defaultValue) method in Java 8 is a convenience default method to return the value for a given key. If the map does not contain a mapping for the key, then it returns the default value.

This method can be particularly useful in situations where you’re working with a map and need to fetch a value for a key, but aren’t sure if the key exists in the map. It helps you handle these scenarios without a need to write extra conditional code to check if the key is present (i.e., using containsKey(Object key)) before trying to get the value.

Here’s a common use case without getOrDefault():

Map<String, Integer> map = new HashMap<>();
// fill map...

Integer value;
if (map.containsKey("key")) {
    value = map.get("key");
} else {
    value = -1;
}

Here’s a basic example of how to use getOrDefault():

package org.kodejava.util;

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

public class MapGetOrDefaultExample {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();
        map.put("A", 1);
        map.put("B", 2);
        map.put("C", 3);

        // Get a value of the key "A". It will return value 1 as
        // "A" is present in the map.
        Integer value = map.getOrDefault("A", -1);
        System.out.println("Value: " + value);

        // Try to get a value of the key "Z". As "Z" is not present
        // in the map, it will return the default value -1.
        value = map.getOrDefault("Z", -1);
        System.out.println("Value: " + value);
    }
}

In this example, “Value: 1” and then “Value: -1” will be printed in the console. In the first case, the key “A” is in the map, so the associated value 1 is returned. In the second case, the key “Z” does not exist in the map, so the default value of -1 is returned.

How do I sort entries of a map by its keys or values?

To sort the entries of a map by keys or values in Java, you can convert your Map to a Stream, sort it, and then collect it back into a Map.

Here’s an example of sorting by keys:

package org.kodejava.util;

import java.util.HashMap;
import java.util.Map;
import java.util.LinkedHashMap;
import java.util.stream.Collectors;

public class MapSortComparingByKey {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();
        map.put("Apple", 10);
        map.put("Orange", 20);
        map.put("Banana", 30);

        Map<String, Integer> sortedByKey = map.entrySet().stream()
                .sorted(Map.Entry.comparingByKey())
                .collect(Collectors.toMap(
                        Map.Entry::getKey,
                        Map.Entry::getValue,
                        (oldValue, newValue) -> oldValue,
                        LinkedHashMap::new
                ));

        sortedByKey.forEach((key, value) -> System.out.println("Key: " + key + ", Value: " + value));
    }
}

Output:

Key: Apple, Value: 10
Key: Banana, Value: 30
Key: Orange, Value: 20

In the example map.entrySet().stream() creates a Stream consisting of the entries in the map. The sorted(Map.Entry.comparingByKey()) method sorts the entries based on keys. The sorted entries are collected back into a new LinkedHashMap (which maintains the order of its elements).

You can sort by values in a similar way:

package org.kodejava.util;

import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.stream.Collectors;

public class MapSortComparingByValue {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();
        map.put("Apple", 10);
        map.put("Orange", 20);
        map.put("Banana", 30);

        Map<String, Integer> sortedByValue = map.entrySet().stream()
                .sorted(Map.Entry.<String, Integer>comparingByValue().reversed())
                .collect(Collectors.toMap(
                        Map.Entry::getKey,
                        Map.Entry::getValue,
                        (oldValue, newValue) -> oldValue,
                        LinkedHashMap::new
                ));

        sortedByValue.forEach((key, value) -> System.out.println("Key: " + key + ", Value: " + value));
    }
}

Output:

Key: Banana, Value: 30
Key: Orange, Value: 20
Key: Apple, Value: 10

In this example, sorted(Map.Entry.<String, Integer>comparingByValue().reversed()) sorts the entries based on values in descending order. The reversed() method is used to reverse the natural ordering. If you want to sort in ascending order, omit the reversed() call.

How do I use the Map.forEach() default method?

The forEach() method in the Map interface in Java 8, allows you to iterate over each entry in the map, allowing you to use each key-value pair in some way.

Here’s a basic usage of the forEach() method:

package org.kodejava.util;

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

public class MapForEachExample {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();
        map.put("Apple", 10);
        map.put("Orange", 20);
        map.put("Banana", 30);

        // Use the forEach method. Here, each key-value pair is printed.
        map.forEach((key, value) -> System.out.println("Key: " + key + ", Value: " + value));
    }
}

Output:

Key: Apple, Value: 10
Key: Orange, Value: 20
Key: Banana, Value: 30

In this example, a HashMap is created and populated with some data. The forEach method is then called on this map, with a lambda expression that accepts a key and a value, then prints them. The key and value parameters represent the current key-value pair the forEach method is handling. In this lambda expression, they are printed to the console.

This operation is applied to each entry in the map, hence the name forEach.

Using the forEach method with lambda expressions has several benefits:

  1. Improved Readability: Traditional iteration requires creating an iterator, a while or for loop, and handling each element. With forEach and lambdas, you can express what you want to do with each element clearly and concisely, making the code easier to read and understand
  2. Concurrency Safety: The forEach method is inherently safer to use in concurrent environments. You don’t need to worry about ConcurrentModificationException errors which you might get while using an Iterator and modifying the collection concurrently.
  3. Less Boilerplate Code: The forEach function in combination with a lambda function provides a way to iterate over a collection with fewer lines of code compared to using iterators
  4. Functional Programming: Lambda expressions and functional interfaces pave the way towards functional programming in Java, which allows for more expressive ways to manipulate collections.

Remember, although forEach can make your code more concise, it does not necessarily make it faster.