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.

CodePudding user response：

Even 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

CodePudding user response：

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.