What this topic covers
12 subtopics- Interfaces11
- Python ABCs7
- Pure virtual functions6
- Abstract classes5
- Default method rules3
- Design3
- Functional interfaces3
- Abstract class vs interface2
- Protocols2
- Abstract methods1
- Basics1
- UML1
- Beginner 15
- Intermediate 15
- Advanced 15
About this topic
Abstraction exposes what an object does and hides how. An abstract class can hold state and some implemented methods but cannot be instantiated; an interface is a pure contract that a class promises to fulfil (since Java 8 it may also carry default methods). C++ uses pure virtual functions (= 0), and Python uses abc.ABC with @abstractmethod or structural Protocols. Designs are drawn as UML diagrams, and every program was run.
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Sample questions
Try each one first, then open the answer for the explanation and why the other options are wrong.
Why does this fail to compile?
abstract class Shape {
abstract double area();
}
public class Main {
public static void main(String[] args) {
Shape s = new Shape();
System.out.println(s.area());
}
}Show answer and explanation
Explanation
An abstract class is incomplete – area() has no body – so only concrete subclasses can be created. A variable of type Shape is still allowed.
Why the other options are wrong
- AAbstract methods cannot be static.
- BAbstract methods have no body.
- CAbstract classes are valid.
What does this print?
public class Main {
interface Temperature {
double celsius();
static Temperature ofFahrenheit(double f) { return () -> (f - 32) * 5 / 9; }
default boolean freezing() { return celsius() <= 0; }
}
public static void main(String[] args) {
Temperature t = Temperature.ofFahrenheit(212);
System.out.println(t.celsius() + " " + t.freezing() + " " + Temperature.ofFahrenheit(14).freezing());
}
}Show answer and explanation
Explanation
Static interface methods (Java 8) belong to the interface and are called through its name – handy factories such as List.of. 212°F = 100°C; 14°F = −10°C.
Why the other options are wrong
- BThe factory converts to Celsius.
- CStatic methods are allowed.
- D100°C is not freezing.
A superclass method and an interface default method have the same signature. What does this print?
public class Main {
interface Named { default String name() { return "interface"; } }
static class Base { public String name() { return "class"; } }
static class Thing extends Base implements Named { }
public static void main(String[] args) {
Named n = new Thing();
System.out.println(n.name());
}
}Show answer and explanation
Explanation
Rule 1: "class wins" – a method from the superclass chain beats any interface default. Rule 2: a more specific interface beats a less specific one. Rule 3: otherwise the class must override.
Why the other options are wrong
- AThe class method resolves it.
- BSuperclass methods take priority over defaults.
- COnly one method runs.