Transform and filter a Java Map with streams

Question

I have a Java Map that I\'d like to transform and filter. As a trivial example, suppose I want to convert all values to Integers then remove the odd entries.

Map         


        

Answer 1

Another way to do this is to remove the values you don't want from the transformed Map:

Map output = input.entrySet().stream()
        .collect(Collectors.toMap(
                Map.Entry::getKey,
                e -> Integer.parseInt(e.getValue()),
                (a, b) -> { throw new AssertionError(); },
                HashMap::new
         ));
output.values().removeIf(v -> v % 2 != 0);

This assumes you want a mutable Map as the result, if not you can probably create an immutable one from output.

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If you are transforming the values into the same type and want to modify the Map in place this could be alot shorter with replaceAll:

input.replaceAll((k, v) -> v + " example");
input.values().removeIf(v -> v.length() > 10);

This also assumes input is mutable.

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I don't recommend doing this because It will not work for all valid Map implementations and may stop working for HashMap in the future, but you can currently use replaceAll and cast a HashMap to change the type of the values:

((Map)input).replaceAll((k, v) -> Integer.parseInt((String)v));
Map output = (Map)input;
output.values().removeIf(v -> v % 2 != 0);

This will also give you type safety warnings and if you try to retrieve a value from the Map through a reference of the old type like this:

String ex = input.get("a");

It will throw a ClassCastException.

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You could move the first transform part into a method to avoid the boilerplate if you expect to use it alot:

public static > M transformValues(
        Map old, 
        Function f, 
        Supplier mapFactory){
    return old.entrySet().stream().collect(Collectors.toMap(
            Entry::getKey, 
            e -> f.apply(e.getValue()), 
            (a, b) -> { throw new IllegalStateException("Duplicate keys for values " + a + " " + b); },
            mapFactory));
}

And use it like this:

    Map output = transformValues(input, Integer::parseInt, HashMap::new);
    output.values().removeIf(v -> v % 2 != 0);

Note that the duplicate key exception can be thrown if, for example, the old Map is an IdentityHashMap and the mapFactory creates a HashMap.