What does K k; and V v; mean in this Java Method in the Map.Class?

Question

I'm new to Java and am looking at the forEach method in the Map.class in Java... here is an excerpt of that method:

default void forEach(BiConsumer<? super K, ? super V> action) {
    Objects.requireNonNull(action);
    for (Map.Entry<K, V> entry : entrySet()) {
        K k;
        V v;
        //more code follows...

My question is... what is going on with K k; and V v;? The method goes on the use k and v as local variables so I'm guessing the K k; and V v; lines are assigning the arguments to local variables... but if that's true why doesn't this syntax need a type? Thank you in advance for the help.

I didn't try anything or expect anything... just want to better understand what this code is doing.

Does this answer your question? [What are Generics in Java?](https://stackoverflow.com/questions/7815528/what-are-generics-in-java) — Turing85, Jan 18 '23 at 19:50
The types are there. They are `K` and `V`. The term you should be looking for is "generics" — Federico klez Culloca, Jan 18 '23 at 19:50
Wouldn't it be nice if the "generics" (ie, type parameter) naming convention were NOT from the 1970's? `KeyType` and `ValueType` would be so much more intuitive than`K` and `V`. — E-Riz, Jan 18 '23 at 19:57

m.antkowicz · Answer 1 · 2023-02-03T10:20:57.343

0

Even when understanding what are generics this code alone is not really clear.

Therefore you need to take a look at the top of the Map interface declaration

public interface Map<K,V>

Read also:

Lesson: Generics

edited Feb 03 '23 at 10:20

answered Jan 18 '23 at 19:55

m.antkowicz

13,268
18
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score 0 · Accepted Answer · answered Jan 19 '23 at 04:36

Map<K, V> is a parameterised type, which means that it represents a whole family of different types, such as

Map<String, Integer> - a Map where the keys are Strings and the values are Integers
Map<Integer, LocalDate> - a Map where the keys are Integers and the values are LocalDates

and so on. But there's a kind of sameness about all these types - they're all a Map where the keys have some type and the values have some other type.

Now the processing inside the Map is going to be much the same, regardless of the type of its keys or the type of its values. So none of the code inside Map.class should specify what those types are. Instead, it uses K to represent whatever type the keys will be, and V to represent whatever type the values will be. These letters are called type parameters, and they are used in Map.class as a way to refer to the types that will be used in the Map.

For example, if you have a Map<String, Integer>, then K means String and V means Integer. If you have a Map with different key type and value type, then K and V will mean something else.

In the method that you've shown in your question, k is a local variable of the type represented by K and v is a local variable of the type represented by V. Again, if you have a Map<String, Integer>, k is effectively a String variable, and v is an Integer variable. But these could actually be any types at all.

Where you've written //more code follows... the code in question will just be pulling a key and value out of the Map, pointing the variables k and v at them, and then applying the BiConsumer called action to them.

What does K k; and V v; mean in this Java Method in the Map.Class?

2 Answers2