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Abstraction in Java

Use abstract classes and interfaces to define contracts without exposing implementation.

Abstraction means showing what something does and hiding how it does it.

A TV remote hides the circuitry. You press volume up without knowing whether it uses infrared or Bluetooth.

Key Concepts

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When you use a List, you know you can add items and fetch them by position. You do not know or care whether it stores them in an array or a chain of nodes. That is abstraction, and the benefit is that the hidden part can change without breaking your code.
List
2
Java gives you two tools: interfaces and abstract classes.An interface is a list of methods a class promises to provide. A class can implement many of them.
3
An abstract class is a partly-built class. It can hold fields and a constructor, and it can provide some methods while leaving others for subclasses. A class can extend only one.
4
Which to use. Ask what the relationship is.
5
If the types share data and a real "is a kind of" relationship — Manager is an Employee, both have a name and a salary — an abstract class fits, because it can hold those shared fields.
ManagerEmployee
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If the types share only an ability, and are otherwise unrelated — anything can be Comparable, anything can be Runnable — an interface fits, and a class can pick up several.
ComparableRunnable
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What changed in Java 8. Interfaces can now contain working methods, not just signatures:
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    interface Vehicle {
        int wheels();                                    // must implement
        default String describe() {                      // provided
            return "vehicle with " + wheels() + " wheels";
        }
    }
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This was added so that Collection could gain stream() without breaking every class in the world that implemented it.
Collectionstream()
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So "interfaces cannot have any implementation" is no longer true. What interfaces still cannot have is instance fields — they hold no per-object data. That is now the real line between the two.
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When two interfaces provide the same default method, the class must choose:
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    class C implements A, B {
        public String hi() { return A.super.hi(); }   // required
    }
13
Adding an interface is not automatically abstraction. An interface with exactly one implementation, created because a guideline said so, hides nothing and doubles the files you read. The question to ask is whether a second implementation is plausible, or whether this is a boundary you want to keep stable — a database, a payment provider, a clock.
14
What the interviewer is probing.1. "Abstraction or encapsulation?" Probing: whether you can separate two words used interchangeably. Stalls: "They both hide things." Moves up: abstraction hides complexity behind a simpler view; encapsulation protects an object's internal state from outside interference.
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2. "Interface or abstract class?" Probing: judgement. Stalls: "Interfaces are more flexible." Moves up: shared fields and a genuine is-a favour the abstract class; an ability shared by unrelated types favours the interface.
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3. "Can an interface have a constructor?" Probing: the state boundary. Stalls: "No." Moves up: no, because there is no per-object data to set up — the same reason it has no instance fields.
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4. "Two interfaces define the same default method." Probing: the conflict. Stalls: "It will not compile." Moves up: correct, and the class resolves it with A.super.method().
A.super.method()
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5. "When would you not add an interface?" Probing: whether you apply the idea or the ritual. Stalls: "Always add one." Moves up: one implementation and no boundary to protect means indirection with no benefit.
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  Interface vs abstract class, Java 8+
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                            interface          abstract class
  ------------------------------------------------------------------
  Instance fields           no (constants only) yes
  Constructor               no                  yes
  Method bodies             default & static    yes
                            (and private,
                            Java 9+)
  How many per class        many                one
  Access modifiers          public (private
                            for helpers)        any
  Purpose                   capability /        shared state and
                            contract            behaviour
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  Choosing
    "X can do Y"  and X types are unrelated     -> interface
    "X is a kind of Y" with shared fields       -> abstract class
    stable boundary you want to swap (DB, clock,
    payment provider)                           -> interface
    one implementation, no boundary             -> neither; use the
                                                   class
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  The diamond, resolved explicitly
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    interface A { default String hi() { return "A"; } }
    interface B { default String hi() { return "B"; } }
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    class C implements A, B {
        public String hi() { return A.super.hi(); }   // required
    }
Use cases:OOPDesignFundamentals