Interfaces and User-Defined Behaviour in Java (ISC Class 12 Computer Science)

Understand Interfaces and User-Defined Behaviour in Java

Interfaces and User-Defined Behaviour in Java: key facts

Interface rules

• •Methods are public abstract by default
• •Variables are public static final
• •No objects of an interface type
• •No constructors

Implementing methods must be written public.

Using an interface

• •class C implements I
• •class C extends B implements I, J
• •I ref = new C();
• •Only methods of I are callable through ref

implements may list many interfaces.

Interface versus abstract class

• •Interface: behaviour only, many allowed
• •Abstract class: may hold variables, constructors and code, only one extended
• •Both cannot be instantiated

A class can do both: extend one and implement several.

Compile-time rules

• •Every interface method must be defined or the class is abstract
• •Reduced access (not public) is an error
• •Interface constants cannot be changed

Common error spots in questions.

How to answer Interfaces and User-Defined Behaviour in Java questions

1. 1Write the interface with method headers only (semicolon, no body).
2. 2Write each class with implements and define every method as public.
3. 3In main, create objects and store them in interface-type variables or an array.
4. 4Trace each call to the implementation of the actual object.
5. 5For a user-defined behaviour (such as an ordering), pass the interface object to the method that uses it.
6. 6Check that a call through the interface uses only methods named in the interface.

Interfaces and User-Defined Behaviour in Java in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Tricks to remember Interfaces and User-Defined Behaviour in Java

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Interface = promise, class = keep

  The interface promises methods; each implementing class keeps the promise.

  Use it when: Understanding implements.

• implements for interfaces, extends for classes

  Class to class: extends. Class to interface: implements.

  Use it when: Keyword choice.

• Always public

  Interface methods are public, so the implementation must be public too.

  Use it when: Avoiding compile errors.

• Two meanings

  The construct interface versus the word interface (the public methods).

  Use it when: Definition questions.

Interfaces and User-Defined Behaviour in Java: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

interface Shape { double area(); }
class Rect implements Shape {
    double l, b;
    Rect(double l, double b) { this.l = l; this.b = b; }
    public double area() { return l * b; }
}
class Sq implements Shape {
    double s;
    Sq(double s) { this.s = s; }
    public double area() { return s * s; }
}
// in main
Shape[] a = { new Rect(3, 4), new Sq(5) };
double t = 0;
for (int i = 0; i < a.length; i++)
    t += a[i].area();
System.out.println(t);

interface Order { boolean before(int a, int b); }
class Asc  implements Order { public boolean before(int a, int b) { return a < b; } }
class Desc implements Order { public boolean before(int a, int b) { return a > b; } }

static void sort(int[] v, Order o) {
    for (int i = 0; i < v.length - 1; i++)
        for (int j = 0; j < v.length - 1 - i; j++)
            if (o.before(v[j + 1], v[j])) {
                int t = v[j]; v[j] = v[j + 1]; v[j + 1] = t;
            }
}

interface Tax {
    double RATE = 0.5;
    double calc(double x);
}
class Half implements Tax {
    public double calc(double x) { return x * RATE; }
}
// in main
System.out.println(new Half().calc(300) + " " + Tax.RATE);

interface Printable { void print(); }
interface Savable   { void save(); }
class Doc implements Printable, Savable {
    public void print() { System.out.print("P "); }
    public void save()  { System.out.print("S "); }
}

Where students lose marks in Interfaces and User-Defined Behaviour in Java

The slips examiners see most often, and what to write instead.

• ✗Mistake: Weaker access in the implementing method

  Interface methods are public, so the implementation cannot be package-private.

  Wrong working

  double area() { ... } in a class implementing Shape

  Right working

  public double area() { ... }

  ✓Do this: Write public on each implemented method.

  Usually costs: 1 mark

• ✗Mistake: Instantiating an interface

  An interface has no implementation.

  Wrong working

  Shape s = new Shape();

  Right working

  Shape s = new Rect(3, 4);

  ✓Do this: Create an object of an implementing class.

  Usually costs: 1 mark

• ✗Mistake: Using extends for an interface

  A class implements an interface.

  Wrong working

  class Rect extends Shape

  Right working

  class Rect implements Shape

  ✓Do this: implements for interfaces.

  Usually costs: 1 mark

Practice questions on Interfaces and User-Defined Behaviour in Java

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   By default, the methods declared in an interface are
   (a) private and static (b) public and abstract (c) protected and final (d) public and static

   Hint

   They have no body.

   Answer

   (b) public and abstract

2. Q2 · 2 marks · Short answerBoard level

   Distinguish between the Java construct interface and the word interface used for the set of method headers of a class.

   Hint

   One is a keyword, the other is a description.

   Answer

   The construct interface is a Java language element declared with the keyword interface that a class implements. The word interface in the general sense means the public methods through which any class is used; every class has one, whether or not a Java interface is declared.

3. Q3 · 4 marks · Short answerHarder board level

   Write an interface Discount with a method double apply(double price), and two classes Flat (subtracts 50) and Quarter (subtracts one quarter of the price). Show apply for price 400 with each class.

   Hint

   Implementing methods are public.

   Answer

   interface Discount { double apply(double price); }
   class Flat implements Discount {
       public double apply(double price) { return price - 50; }
   }
   class Quarter implements Discount {
       public double apply(double price) { return price - price * 0.25; }
   }

   For 400: Flat gives 400 - 50 = 350.0; Quarter gives 400 - 100 = 300.0.

4. Q4 · 3 marks · Short answerHarder board level

   State two differences between an interface and an abstract class, and write the header of a class C that extends a class B and implements interfaces I and J.

   Hint

   Think of variables, constructors and how many can be used.

   Answer

   An abstract class can have instance variables, constructors and ordinary methods; an interface has only method headers and constants. A class can extend only one abstract class but can implement many interfaces. Header: class C extends B implements I, J

5. Q5 · 4 marks · Case-basedHOTS / competency

   Use the Order, Asc, Desc and sort code from the worked example. (i) Show the array after sort({2, 9, 5}, new Desc()). (ii) Explain why the sort method needs no change to sort in the other order. (iii) Name the idea shown.

   Hint

   Trace each pass with before(v[j + 1], v[j]).

   Answer

   (i) i = 0: j = 0: before(9, 2) is true, swap, giving {9, 2, 5}; j = 1: before(5, 2) is true, swap, giving {9, 5, 2}. i = 1: j = 0: before(5, 9) is false. Result {9, 5, 2}. (ii) The decision of which element comes first is made by the Order object, so passing new Asc() reverses the order. (iii) User-defined behaviour through an interface (polymorphism).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Which keyword does a class use for an interface?Show answer

   implements.

2. 2.Are interface variables changeable?Show answer

   No, they are public static final constants.

3. 3.Can an interface be instantiated?Show answer

   No.

4. 4.Can a class implement more than one interface?Show answer

   Yes, separated by commas.

5. 5.What must an implementing class do for each interface method?Show answer

   Define it as a public method, or be declared abstract.

6. 6.What problem does an interface solve in a sorting class?Show answer

   It lets the user supply the comparison and the order.

Interfaces and User-Defined Behaviour in Java: frequently asked questions