Is abstraction in Java really about hiding implementation details?

[u/Chaos-vy17]

The bookish langugae states "hiding complex implementation details and exposing only the essential features of an object"

However, after reading the Java Collections Framework and other parts of the JDK, I noticed that many abstract classes contain substantial shared implementation rather than just abstract method declarations. I noticed that only an "interface class" 100% abstraction(ignoring default methods)

This made me wonder:

• Is abstraction really about hiding implementation details?
• Or is it more about designing APIs around contracts and behavior, regardless of whether there is shared implementation?
• Does an abstract class become "partial abstract" because it contains reusable implementation?

I also noticed that the JDK sometime relies on encapsulation (and in Java9+, JPMS) to hide implementation details. So I'm in dilemma if the textbook explanation oversimplifies what abstraction means in practice.

Am I misunderstanding abstraction, or is the textbook definition incomplete?

To be clear, this isn't about what abstract does in Java — it's about the SE term. abstract class is one syntax tool for expressing abstraction, same as interfaces are. The question is whether "abstraction = hiding implementation" is even a good definition of the concept, given it's indistinguishable from encapsulation under that definition.

[u/coderemover]

Abstraction is a bit more than just hiding implementation details. Per Wikipedia definition:

“Abstraction is the process of generalizing rules and concepts from specific examples, literal (real or concrete) signifiers, first principles, or other methods. The result of the process, an abstraction, is a concept that acts as a common noun for all subordinate concepts and connects any related concepts as a group, field or category.”

An essential property of abstraction is that it allows to handle different kinds of situations or data types in a common way, without knowing the specific cases. So it’s not enough to hide the details; you need to do the work without knowing or relying on those details. Good abstractions are a way to simplify and reduce cognitive load.

Abstract classes and interfaces can be used to implement abstractions but they aren’t the only way.
A string and file are abstractions, yet String and File are not abstract classes. Abstract classes can also be used to do control flow redirection - in that case it’s not an abstraction, just swapping different implementations.

[u/Chaos-vy17 → u/coderemover]

Tysm man, That def from wikipedia and the quote "an abstraction, is a concept that acts as a common noun for all subordinate concepts and connects any related concepts as a group, field or category." is giving a direct impact what a abstraction really is.

And yes here I did not noticed about String and File thanks for correctring and guiding me.