What this topic covers
8 subtopics- static members14
- final and const11
- Static initialisation6
- this5
- classmethod and staticmethod4
- super3
- Class vs instance attributes1
- Class-level hooks1
- Beginner 15
- Intermediate 15
- Advanced 15
About this topic
Four small keywords cause a large share of OOPs output puzzles. static members belong to the class (one copy shared by all objects); instance members belong to each object. final (Java) and const (C++) stop reassignment – of a variable, an override or a subclass – but not mutation of the object a final reference points to. this is the current object; super reaches the parent's version of a member. Python spells these ideas as class attributes, @classmethod, @staticmethod, self and super().
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Sample questions
Try each one first, then open the answer for the explanation and why the other options are wrong.
What does this print?
public class Main {
static class Visitor {
static int total = 0;
final int id;
Visitor() { total++; id = total; }
}
public static void main(String[] args) {
Visitor a = new Visitor();
Visitor b = new Visitor();
Visitor c = new Visitor();
System.out.println(Visitor.total + " " + b.id + " " + a.id);
}
}Show answer and explanation
Explanation
total is static, so all Visitors share one counter; id is an instance field, so each object keeps the value it had when it was created.
Why the other options are wrong
- Aid is copied at construction time.
- BEach visitor gets the next number.
- CThe static counter is shared, not per object.
Static fields are initialised in the order they appear. What does this print?
public class Main {
static int a = Main.b + 1;
static int b = 5;
static int c = b + 1;
public static void main(String[] args) {
System.out.println(a + " " + b + " " + c);
}
}Show answer and explanation
Explanation
When a is initialised, b still holds its default 0, so a = 1. (Writing just b there is an "illegal forward reference" compile error; qualifying it as Main.b sidesteps the check and exposes the default value.)
Why the other options are wrong
- AThe qualified name compiles.
- Bb is still 0 when a is computed.
- Ca adds 1.
What goes wrong when this runs?
public class Main {
static class Rates {
static final int DIVISOR = load();
static final int RATE = 100 / DIVISOR;
static int load() { return 0; }
}
public static void main(String[] args) {
System.out.println(Rates.RATE);
}
}Show answer and explanation
Explanation
An exception thrown while initialising a class is wrapped in ExceptionInInitializerError; later uses of the class throw NoClassDefFoundError. Keep static initialisers simple.
Why the other options are wrong
- AInteger division by zero throws.
- COnly floating-point division gives Infinity.
- DIt compiles.