Great question! As a Java beginner, this is such a logical thing to question—after all, if you can't create an instance of an abstract class, why bother giving it a constructor? Let's break this down step by step.

Abstract Classes & Constructors: The "Hidden" Purpose #

Even though you can't directly new an abstract class, its constructor plays a critical role when subclasses are instantiated. Here's why:

• Abstract classes are designed to be inherited. They often hold shared state (like instance variables) that subclasses need. The constructor initializes this shared state so subclasses don't have to repeat the code.
• When you create an instance of a subclass, Java requires that the parent class (abstract or not) is initialized first. That means the subclass's constructor will automatically call the abstract class's constructor (via super()—you might have seen this in code!).

Let's look at a concrete example to make this real:

abstract class Vehicle {
    protected String model;
    protected int year;

    // Abstract class constructor
    public Vehicle(String model, int year) {
        this.model = model;
        this.year = year;
        System.out.println("Vehicle initialized: " + model + " (" + year + ")");
    }

    public abstract void startEngine();
}

class Car extends Vehicle {
    private int doors;

    public Car(String model, int year, int doors) {
        super(model, year); // Must call abstract parent's constructor first
        this.doors = doors;
        System.out.println("Car initialized with " + doors + " doors");
    }

    @Override
    public void startEngine() {
        System.out.println(model + " engine roars to life!");
    }
}

public class Main {
    public static void main(String[] args) {
        Car myCar = new Car("Tesla Model 3", 2024, 4);
        // Output:
        // Vehicle initialized: Tesla Model 3 (2024)
        // Car initialized with 4 doors
        myCar.startEngine(); // Tesla Model 3 engine roars to life!
    }
}

Without the abstract class's constructor, we'd have to manually set model and year in every subclass—duplicating code and risking inconsistencies. The constructor ensures all subclasses start with a valid, initialized state from the abstract parent.

Why Interfaces Can't Have Constructors #

Interfaces are fundamentally different from abstract classes—they're meant to define behavior contracts, not hold state. Here's why constructors don't make sense for them:

• Interfaces can't have instance variables (all variables are implicitly public static final—constants). There's no state to initialize, so a constructor would have nothing to do.
• A class can implement multiple interfaces. If each interface had a constructor, Java wouldn't know which one to call first when instantiating the class. This creates an ambiguous scenario that the language avoids.
• Interfaces aren't meant to be inherited in the same way as abstract classes. Classes "implement" interfaces to promise they'll provide certain methods—there's no parent-child hierarchy that requires initialization of an interface's state.

Quick Recap #

• Abstract classes have constructors to initialize shared state for their subclasses, which rely on this initialization when they're instantiated.
• Interfaces don't need constructors because they don't hold state, and their multi-implementation model makes constructor calls impossible to resolve.

内容的提问来源于stack exchange，提问作者Faisal