Collectors toMap() method in Java with Examples

Collectors.toMap() method is used with Java Streams to convert stream elements into a Map. It creates map keys and values by applying the given functions to each element of the stream.

• Keys must be unique.
• If duplicate keys occur and no merge function is provided, an IllegalStateException is thrown.

1. toMap(keyMapper, valueMapper)

Convert a stream to a Map when all keys are unique.

Syntax

Collectors.toMap(Function keyMapper, Function valueMapper)

Parameters:

• keyMapper: maps stream elements to keys
• valueMapper: maps stream elements to values

Return Type: Collector<T, ?, Map<K, V>>

Example: This example demonstrates how to use Collectors.toMap() to convert a stream of key–value pairs into a Map when all keys are unique.

{g=geeks, GFG=GeeksForGeeks}

Explanation:

• The stream contains key–value pairs stored as string arrays.
• Collectors.toMap(p -> p[0], p -> p[1]) maps the first element as the key and the second element as the value.

2. toMap(keyMapper, valueMapper, mergeFunction)

Handle duplicate keys in the stream.

Syntax

Collectors.toMap(Function keyMapper, Function valueMapper, BinaryOperator<U> mergeFunction)

Parameters:

• keyMapper: produces keys
• valueMapper: produces values
• mergeFunction: resolves duplicate key conflicts

Return Type: Collector<T, ?, Map<K, V>>

Example: This code demonstrates using Collectors.toMap() to handle duplicate keys.

{GFG=GeeksForGeeks, Geeks, g=geeks}

Explanation:

• The stream contains key–value pairs with the same key ("GFG"), which would normally cause an error.
• p -> p[0] extracts the key and p -> p[1] extracts the value.
• The merge function (v1, v2) -> v1 + ", " + v2 combines values of duplicate keys.
• The result is a Map where duplicate values are merged instead of throwing an exception.

3. toMap(keyMapper, valueMapper, mergeFunction, mapSupplier)

Handle duplicates and use a custom Map implementation (e.g., LinkedHashMap).

Syntax

Collectors.toMap(Function keyMapper, Function valueMapper,BinaryOperator<U> mergeFunction, Supplier<M> mapSupplier)

Parameters:

• keyMapper: maps keys
• valueMapper: maps values
• mergeFunction: handles duplicate keys
• mapSupplier: supplies the Map type

Return Type: Collector<T, ?, M extends Map<K, V>>

Example: This example demonstrates using Collectors.toMap() to handle duplicate keys and maintain insertion order with a LinkedHashMap.

{GFG=GeeksForGeeks, Geeks, g=geeks}

Explanation:

• The stream contains key–value pairs with duplicate keys ("GFG").
• The merge function (v1, v2) -> v1 + ", " + v2 combines values for duplicate keys.
• LinkedHashMap::new ensures the order of insertion is preserved in the final map.

Related Topics:

• Java Stream API
• Collectors class
• Collectors.groupingBy()
• Map vs HashMap vs LinkedHashMap