The Stream API in Java 8 and onwards has transformed how we manipulate collections, making operations more expressive and less verbose. One common task is to transform a list into a map for quick lookups. However, a challenge arises when the list contains elements that result in duplicate keys in the target map. By default, the toMap
collector throws an IllegalStateException
when duplicate keys are encountered. This article explores strategies to handle duplicate keys gracefully when producing maps using Java Stream, ensuring robust and error-free code.
Understanding the Problem
Consider a list of Person
objects where each Person
has a name and age. Our goal is to create a map where each person’s name is a key, and their age is the value.
List<Person> people = Arrays.asList(
new Person("John", 30),
new Person("Jane", 25),
new Person("John", 28) // Duplicate key "John"
);
Attempting to convert this list into a map using the straightforward approach results in an exception:
Map<String, Integer> nameToAgeMap = people.stream()
.collect(Collectors.toMap(Person::getName, Person::getAge));
The code above will throw IllegalStateException
because of the duplicate key “John”.
Strategy 1: Choosing One of the Duplicates
One way to handle duplicates is to decide on a policy for which value to keep. The toMap
method allows you to specify a merge function, which determines how to deal with duplicates.
Keeping the First Encountered Value
You might decide that the first occurrence is the most relevant one.
Map<String, Integer> nameToAgeMap = people.stream()
.collect(Collectors.toMap(
Person::getName,
Person::getAge,
(existingValue, newValue) -> existingValue
));
In this case, the age “30” for “John” will be in the map, and the duplicate “John” with age “28” will be ignored.
Keeping the Last Encountered Value
Alternatively, you might prefer the last encountered value:
Map<String, Integer> nameToAgeMap = people.stream()
.collect(Collectors.toMap(
Person::getName,
Person::getAge,
(existingValue, newValue) -> newValue
));
Now, the map will contain “John” with age “28”, ignoring the first occurrence.
Strategy 2: Combining Duplicate Values
Sometimes, discarding duplicates isn’t ideal, and a better approach is combining them somehow.
Summing Up Values
If the values are numeric, you might sum them up:
Map<String, Integer> nameToTotalAge = people.stream()
.collect(Collectors.toMap(
Person::getName,
Person::getAge,
Integer::sum
));
This results in a map where “John” is associated with the age “58”, the sum of “30” and “28”.
Collecting Duplicates into a List
For non-numeric values or when you want to preserve all duplicate entries, you can collect them into a list:
Map<String, List<Integer>> nameToAges = people.stream()
.collect(Collectors.groupingBy(
Person::getName,
Collectors.mapping(Person::getAge, Collectors.toList())
));
This approach creates a map where each key is associated with a list of ages, handling duplicates gracefully.
Strategy 3: Custom Collection Types
Java’s Collectors
utility provides toMap
, but sometimes you might need a more specialized collection, like a Set
for unique values or even a custom type for complex merge logic.
Using a Set to Avoid Duplicates
If the goal is to ensure each key maps to a unique set of values, consider using a Set
:
Map<String, Set<Integer>> nameToAgeSet = people.stream()
.collect(Collectors.groupingBy(
Person::getName,
Collectors.mapping(Person::getAge, Collectors.toSet())
));
This produces a map where each name is associated with a set of unique ages.
Conclusion
Handling duplicate keys when producing maps with Java Stream is a common challenge that requires thoughtful consideration of how duplicates should be managed. Whether choosing one of the duplicates, combining them, or collecting them into a custom collection, Java Stream provides the flexibility to handle duplicates in a way that best fits the application’s needs. By leveraging the power of collectors and merge functions, developers can ensure their map transformations are robust and their applications are free from unexpected IllegalStateExceptions
.
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