ImmutableMap构建器如何创建抽象类的实例？

Question

I am learning to use guava library and referred to this I see use of builder to construct an instance of immutable map. How is the builder constructor able to create an abstract class instance?

   static final ImmutableMap<String, Integer> WORD_TO_INT =
       new ImmutableMap.Builder<String, Integer>()
           .put("one", 1)
           .put("two", 2)
           .put("three", 3)
           .build();

Answer 1

I does not, the Builder creates an implementation of an ImmutableMap (a class that extends the ImmutableMap ).

To understand clearly, start here before working with Guava.

Update: see comment by @Louis Wasserman. Indeed an essential comment.

Answer 2

It is not the constructor of the builder class that returns an instance of an immutable map.

You are first creating the builder by calling new ImmutableMap.Builder<String, Integer>() and then you are calling methods in a chain on this ImmutableMap.Builder instance - three times the put method and then the build method.

The build method is what is called last, and that is what creates and returns the instance of ImmutableMap .

The "trick" here is that the put method of ImmutableMap.Builder returns the builder itself (it has a statement return this; at the end) so that you can chain method calls like this.

And, indeed, the build method returns an instance of a subclass of ImmutableMap because class ImmutableMap is abstract, so it cannot be directly instantiated.

Answer 3

ImmutableMap (Guava: Google Core Libraries for Java 21.0-SNAPSHOT API) states that it is "a Map whose contents will never change, with many other important properties detailed at ImmutableCollection ".

The "other important properties detailed at ImmutableCollection " include the following guarantees :

Each makes the following guarantees:

• Shallow immutability. Elements can never be added, removed or replaced in this collection. This is a stronger guarantee than that of Collections.unmodifiableCollection(java.util.Collection<? extends T>) , whose contents change whenever the wrapped collection is modified.
• Null-hostility. This collection will never contain a null element.
• Deterministic iteration. The iteration order is always well-defined, depending on how the collection was created (see the appropriate factory method for details). View collections such as Multiset.elementSet() iterate in the same order as the parent, except as noted.
• Thread safety. It is safe to access this collection concurrently from multiple threads.
• Integrity. This type cannot be subclassed outside this package (which would allow these guarantees to be violated).

The last guarantee, integrety , alludes to the fact that internally Guava has concrete implementations (non-abstract) of ImmutableMap and other immutable objects which is what is actually returned by these builders.

Furthermore, the source is open; you can go find out for yourself how the builder is able to do it (eg you might start here ).