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1. Overview

In this quick tutorial, we'll explore various ways to find the maximum value in a Java Map. We'll also see how new features in Java 8 have simplified this operation.

Before we begin let's briefly recap how objects are compared in Java.

Typically objects can express a natural ordering by implementing the method compareTo() from the Comparable interface. However, an ordering other than natural ordering can be employed through a Comparator object. We'll see these in more details as we go on.

2. Before Java 8

Let's start first exploring how can we find the highest value without Java 8 features.

2.1. Using Simple Iteration

Using iteration we could simply go through all the entries of a Map to pick the highest value, storing the current highest in a variable:

public <K, V extends Comparable<V>> V maxUsingIteration(Map<K, V> map) {
    Map.Entry<K, V> maxEntry = null;
    for (Map.Entry<K, V> entry : map.entrySet()) {
        if (maxEntry == null || entry.getValue()
            .compareTo(maxEntry.getValue()) > 0) {
            maxEntry = entry;
        }
    }
    return maxEntry.getValue();
}

Here, we're also making use of Java generics to build a method that can be applied to different types.

2.2. Using Collections.max()

Now let's see how the utility method max() in the Collections class can save us from writing a lot of this ourselves:

public <K, V extends Comparable<V>> V maxUsingCollectionsMax(Map<K, V> map) {
    Entry<K, V> maxEntry = Collections.max(map.entrySet(), new Comparator<Entry<K, V>>() {
        public int compare(Entry<K, V> e1, Entry<K, V> e2) {
            return e1.getValue()
                .compareTo(e2.getValue());
        }
    });
    return maxEntry.getValue();
}

In this example, we're passing a Comparator object to max() which can leverage the natural ordering of the Entry values through compareTo() or implement a different ordering altogether.

3. After Java 8

Java 8 features can simplify our attempt above to get the max value from a Map in more ways than one.

3.1. Using Collections.max()  with a Lambda Expression

Let's begin by exploring how lambda expressions can simplify the call to Collections.max():

public <K, V extends Comparable<V>> V maxUsingCollectionsMaxAndLambda(Map<K, V> map) {
    Entry<K, V> maxEntry = Collections.max(map.entrySet(), (Entry<K, V> e1, Entry<K, V> e2) -> e1.getValue()
        .compareTo(e2.getValue()));
    return maxEntry.getValue();
}

As we can see here, lambda expressions save us from defining the full-fledged functional interface and provide a concise way of defining the logic. To read more about lambda expressions, also check out our previous article.

3.2. Using Stream

The Stream API is another addition to Java 8 which has largely simplified working with collections:

public <K, V extends Comparable<V>> V maxUsingStreamAndLambda(Map<K, V> map) {
    Optional<Entry<K, V>> maxEntry = map.entrySet()
        .stream()
        .max((Entry<K, V> e1, Entry<K, V> e2) -> e1.getValue()
            .compareTo(e2.getValue())
        );
    
    return maxEntry.get().getValue();
}

This API offers a lot of data processing queries like map-reduce transformations on collections. Here, we've used max() over a stream of Map Entry which is a special case of a reduction operation. More details about the Stream API are available here.

We're also making use of the Optional API here which is a container object added in Java 8 that may or may not contain a non-null value. More details about Optional can be obtained here.

3.3. Using Stream with Method Reference

Lastly, let's see how method references can further simplify our use of lambda expressions:

public <K, V extends Comparable<V>> V maxUsingStreamAndMethodReference(Map<K, V> map) {
    Optional<Entry<K, V>> maxEntry = map.entrySet()
        .stream()
        .max(Comparator.comparing(Map.Entry::getValue));
    return maxEntry.get()
        .getValue();
}

In cases where lambda expressions are merely calling an existing method, a method reference allows us to do this using the method name directly. For more details about method references have a look at this previous article.

4. Conclusion

In this article, we've seen multiple ways of finding the highest value in a Java Map, some of which were using features added as part of Java 8.

As always, the code for the examples is available over on GitHub.

LS Price Increase Launch

The Price of all “Learn Spring” course packages will increase by $40 at the end of this week:

>> GET ACCESS NOW

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