Reading 12: Defining ADTs with Interfaces, Generics, Enums, and Functions

Java’s interface is a useful language mechanism for expressing an abstract data type. An interface in Java is a list of method signatures without method bodies. A class implements an interface if it declares the interface in its implements clause, and provides method bodies for all of the interface’s methods. So one way to define an abstract data type in Java is as an interface. The type’s implementation is then a class implementing that interface.

One advantage of this approach is that the interface specifies the contract for the client and nothing more. The interface is all a client programmer needs to read to understand the ADT. The client can’t create inadvertent dependencies on the ADT’s rep, because fields can’t be put in an interface at all. The implementation is kept well and truly separated, in a different class altogether.

Another advantage is that multiple different representations of the abstract data type can coexist in the same program, as different classes implementing the interface. When an abstract data type is represented just as a single class, without an interface, it’s harder to have multiple representations. In the MyString example from Abstract Data Types, MyString was a single class. We explored two different representations for MyString, but we couldn’t have both representations for the ADT in the same program.

Java’s static type checking allows the compiler to catch many mistakes in implementing an ADT’s contract. For instance, it is a compile-time error to omit one of the required methods, or to give a method the wrong return type. Unfortunately, the compiler doesn’t check for us that the code adheres to the specs of those methods that are written in documentation comments.

About interfaces in Java

      /** Represents an immutable curve in the plane. */
      public interface Curve {

          /** make a one-point curve */
/* A */   public Curve(double x, double y);

          /** @return true if the point (x,y) lies on this curve */
          public boolean contains(double x,y);

          /** @return a curve formed by connecting this with that */
/* B */   public ArrayCurve join(Curve that);

      }

      /** Implementation of Curve. */
      public class ArrayCurve implements Curve {

          /** make a one-point curve */
          public ArrayCurve(double x, double y) { ... }

          /** @return a curve formed by connecting this with that */
          public ArrayCurve join(Curve that) { ... }

          /** extend this curve with a segment to the point (x,y) */
          public void add(double x, double y) { ... }

      }

Recall that a type is a set of values. The Java List type is defined by an interface. If we think about all possible List values, none of them are actually objects of type List: we cannot create instances of an interface. Instead, those values are all ArrayList objects, or LinkedList objects, or objects of another class that implements List. A subtype is simply a subset of the supertype: ArrayList and LinkedList are subtypes of List.

“B is a subtype of A” means “every B is an A.” In terms of specifications: “every B satisfies the specification for A.”

That means B is only a subtype of A if B’s specification is at least as strong as A’s specification. When we declare a class that implements an interface, the Java compiler enforces part of this requirement automatically: for example, it ensures that every method in A appears in B, with a compatible type signature. Class B cannot implement interface A without implementing all of the methods declared in A.

But the compiler cannot check that we haven’t weakened the specification in other ways: strengthening the precondition on some inputs to a method, weakening a postcondition, weakening a guarantee that the interface abstract type advertises to clients. If you declare a subtype in Java — implementing an interface is our current focus — then you must ensure that the subtype’s spec is at least as strong as the supertype’s.

How to declare subtypes in Java

To declare that a class B is a subtype of an interface A, use implements:

class ArrayList<E> implements List<E> {
    ...
}

This declaration requires ArrayList to implement (provide method bodies for) all the method signatures found in List, with specs at least as strong as the specs in List. Java statically checks the method signatures, but a human programmer must check the rest of the spec.

You can also declare that an interface is a subtype of another interface, using extends:

interface SortedSet<E> extends Set<E> {
    ...
}

Because SortedSet is also an interface, it doesn’t provide any implementations of Set methods. But it may strengthen the spec of the type, by strengthening existing Set methods or adding new methods.

/**  ... spec says nothing at all about mutation or mutability ... */
List<String> toUpperCase(List<String> names)

/*1*/ List<String> list = new ArrayList<String>();
/*2*/ list = Collections.unmodifiableList(list);
/*3*/ list = List.of("glorp");
/*4*/ list.add("fleeb");
/*5*/ list = new List<String>();
/*6*/ Set<String> set = list; //list has no repeats!
/*7*/ Collection<String> collection = list;

/*1*/ List<String> list = new ArrayList<String>();

/*2*/ list = Collections.unmodifiableList(list);

/*3*/ list = List.of("glorp");

/*4*/ list.add("fleeb");

/*5*/ list = new List<String>();

/*6*/ Set<String> set = list; //list has no repeats!

/*7*/ Collection<String> collection = list;

/** An immutable rectangle. */
public interface ImmutableRectangle {
    /** @return the width of this rectangle */
    public int getWidth();
    /** @return the height of this rectangle */
    public int getHeight();
}

/** An immutable square. */
public class ImmutableSquare implements ImmutableRectangle {
    private final int side;
    /** Make a new side x side square. */
    public ImmutableSquare(int side) { this.side = side; }
    /** @return the width of this square */
    public int getWidth() { return side; }
    /** @return the height of this square */
    public int getHeight() { return side; }
}

/** A mutable rectangle. */
public interface MutableRectangle {
    // ... same methods as ImmutableRectangle above ...
    /** Set this rectangle's dimensions to width x height. */
    public void setSize(int width, int height);
}

/** A mutable square. */
public class MutableSquare implements MutableRectangle /* hopefully? */ {
    private int side;
    // ... same constructor and methods as ImmutableSquare above ...
    // TODO implement setSize(..)
}

/** 
 * Set this square's dimensions to width x height.
 * Requires width = height.
 */
public void setSize(int width, int height) { ... }

/** 
 * Set this square's dimensions to width x height.
 * @throws BadSizeException if width != height
 */
public void setSize(int width, int height) throws BadSizeException { ... }

/**
 * If width = height, set this square's dimensions to width x height.
 * Otherwise, new dimensions are unspecified.
 */
public void setSize(int width, int height) { ... }

/** 
 * Set this square's dimensions to side x side.
 */
public void setSize(int side) { ... }

Let’s revisit MyString. Using an interface instead of a class for the ADT, we can support multiple implementations:

/**
 * MyString represents an immutable sequence of characters.
 */
public interface MyString { 

    // We'll skip this creator operation for now
    // /**
    //  * @param b a boolean value
    //  * @return string representation of b, either "true" or "false"
    //  */
    // public static MyString valueOf(boolean b) { ... }

    /**
     * @return number of characters in this string
     */
    public int length();

    /**
     * @param i character position (requires 0 <= i < string length)
     * @return character at position i
     */
    public char charAt(int i);

    /**
     * Get the substring between start (inclusive) and end (exclusive).
     * @param start starting index
     * @param end ending index.  Requires 0 <= start <= end <= string length.
     * @return string consisting of charAt(start)...charAt(end-1)
     */
    public MyString substring(int start, int end);
}

We’ll skip the static valueOf method and come back to it in a minute. Instead, let’s go ahead using a different technique from our toolbox of ADT concepts in Java: constructors.

Here’s our first implementation:

public class SimpleMyString implements MyString {

    private char[] a;

    /**
     * Create a string representation of b, either "true" or "false".
     * @param b a boolean value
     */
    public SimpleMyString(boolean b) {
        a = b ? new char[] { 't', 'r', 'u', 'e' } 
              : new char[] { 'f', 'a', 'l', 's', 'e' };
    }

    // private constructor, used internally by producer operations
    private SimpleMyString(char[] a) {
        this.a = a;
    }

    @Override public int length() { return a.length; }

    @Override public char charAt(int i) { return a[i]; }

    @Override public MyString substring(int start, int end) {
        char[] subArray = new char[end - start];
        System.arraycopy(this.a, start, subArray, 0, end - start);
        return new SimpleMyString(subArray);
    }
}

And here’s the optimized implementation:

public class FastMyString implements MyString {

    private char[] a;
    private int start;
    private int end;

    /**
     * Create a string representation of b, either "true" or "false".
     * @param b a boolean value
     */
    public FastMyString(boolean b) {
        a = b ? new char[] { 't', 'r', 'u', 'e' } 
              : new char[] { 'f', 'a', 'l', 's', 'e' };
        start = 0;
        end = a.length;
    }

    // private constructor, used internally by producer operations.
    private FastMyString(char[] a, int start, int end) {
        this.a = a;
        this.start = start;
        this.end = end;
    }

    @Override public int length() { return end - start; }

    @Override public char charAt(int i) { return a[start + i]; }

    @Override public MyString substring(int start, int end) {
        return new FastMyString(this.a, this.start + start, this.start + end);
    }
}

• Compare these classes to the implementations of MyString in Abstract Data Types. Notice how the code that previously appeared in static valueOf methods now appears in the constructors, slightly changed to refer to the rep of this.

• Also notice the use of @Override. This annotation informs the compiler that the method must have the same signature as one of the methods in the interface we’re implementing. @Override also tells readers of the code to look for the spec of that method in the interface. Repeating the spec wouldn’t be DRY, but saying nothing at all makes the code harder to understand.

• And notice that we have added a private constructor for producers like substring(..) to use to make new instances of the class. Its arguments are the rep fields. We didn’t have to write this special constructor before, because Java provides an empty constructor by default when we don’t declare any other constructors. Adding the constructors that take boolean b means we have to declare another constructor explicitly for producer operations to use.

How will clients use this ADT? Here’s an example:

MyString s = new FastMyString(true);
System.out.println("The first character is: " + s.charAt(0));

This code looks very similar to the code we write to use the Java collections classes:

List<String> s = new ArrayList<String>();
...

Fortunately, (as of Java 8) interfaces are allowed to contain static methods, so we can implement the creator operation valueOf as a static factory method in the interface MyString:

public interface MyString { 

    /**
     * @param b a boolean value
     * @return string representation of b, either "true" or "false"
     */
    public static MyString valueOf(boolean b) {
        return new FastMyString(b);
    }

    // ...

Now a client can use the ADT without breaking the abstraction barrier:

MyString s = MyString.valueOf(true);
System.out.println("The first character is: " + s.charAt(0));

Hiding the implementation completely is a tradeoff, because sometimes the client wants a choice of implementations with different characteristics. That’s why ArrayList and LinkedList are exposed by the Java library, because they vary in performance of operations like get() and insert().

Interfaces are used pervasively in real Java code. Not every class is associated with an interface, but there are a few good reasons to bring an interface into the picture.

• Documentation for both the compiler and for humans. Not only does an interface help the compiler catch ADT implementation bugs, but it is also much more useful for a human to read than the code for a concrete implementation. Such an implementation intersperses ADT-level types and specs with implementation details.
• Allowing performance trade-offs. Different implementations of the ADT can provide methods with very different performance characteristics. Different applications may work better with different choices, but we would like to code these applications in a way that is representation-independent. From a correctness standpoint, it should be possible to drop in any new implementation of a key ADT with simple, localized code changes.
• Methods with intentionally underdetermined specifications. An ADT for finite sets could leave unspecified the element order one gets when converting to a list. Some implementations might use slower method implementations that manage to keep the set representation in some sorted order, allowing quick conversion to a sorted list. Other implementations might make many methods faster by not bothering to support conversion to sorted lists.
• Multiple views of one class. A Java class may implement multiple interfaces. For instance, a user interface widget displaying a drop-down list is natural to view as both a widget and a list. The class for this widget could implement both interfaces. In other words, we don’t always implement an ADT multiple times just because we are choosing different data structures. Those implementations may be various different sorts of objects that can be seen as special cases of the ADT.
• More and less trustworthy implementations. Another reason to implement an interface more than once might be that it is easy to build a simple implementation that you believe is correct, while you can work harder to build a fancier version that is more likely to contain bugs. You can choose implementations for applications based on how bad it would be to get bitten by a bug.

public class Rational {
    ...
}

public interface Rational {
    ...
}

public class IntFraction implements Rational {
    ...
}

private final int numerator;
private final int denominator;

//   denominator > 0
//   numerator and denominator are relatively prime

//   AF(numerator, denominator) = numerator / denominator

    /**
     * @param that another Rational
     * @return a Rational equal to (this / that)
     */

    public boolean isZero()

    return numer == 0;

Implementing an interface is one way to make a subtype of another type. Another technique, which you may have seen before, is subclassing. Subclassing means defining a class as an extension, or subclass, of another class, so that:

• the subclass automatically inherits the instance methods of its superclass, including their method bodies
  • the subclass can override any of those inherited method bodies by substituting its own method body
• the subclass also inherits the fields of its superclass
• the subclass can also add new instance methods and fields for its own purposes

The boldfaced parts in the list above show how subclassing differs from implementing an interface. An interface has neither method bodies nor fields, so it’s not possible to inherit them when you implement an interface.

Subclassing is a common feature of object-oriented programming languages. You may have already encountered it in Python, where you can write class SpottedTurtle(Turtle): ... to define a new class SpottedTurtle as a subclass of the class Turtle. Java provides the same effect with the syntax class SpottedTurtle extends Turtle { ... }.

Just like implementing an interface, subclassing should imply subtyping. If SpottedTurtle is a subclass of Turtle, then SpottedTurtle needs to have at least as strong a spec as Turtle, because Java will allow SpottedTurtle objects to be used anywhere that a Turtle might be expected.

But unlike implementing an interface, a subclass inherits not only the spec of its superclass but also its rep – the fields that implement it. At first this seems like a great idea. Reusing implementation ought to make our code more DRY, and more ready for change.

But years of experience by API designers have revealed subtle challenges to making subclassing safe from bugs and ready for change. You can read more if you’re interested, but essentially, inheriting the rep means:

• rep exposure between the superclass and all its subclasses
• rep dependence between superclass and subclasses
• superclass and subclass can inadvertently break each other’s rep invariants

Designing for safe subclassing means that the superclass must now offer two contracts: one for interaction with clients, and one for subclasses. These issues simply do not arise with interfaces.

Overriding and dynamic dispatch

FastMyString fms = new FastMyString(true); // recall that this represents the 4-character string "true"
fms.toString() → "FastMyString@504bae78" // not useful! just the class name followed by the object's address in memory

public class FastMyString {
    ...
    @Override
    public String toString() {
        String s = "";
        for (int i = 0; i < this.length(); ++i) {
            s += this.charAt(i);
        }
        return s;
    }
}

FastMyString fms = new FastMyString(true);
fms.toString() → "true"

Object obj = new FastMyString(true);
obj.toString() → ?

Object obj = new FastMyString(true);
obj.toString() → "true"

List<String> list = new ArrayList<>(List.of("abc"));
Object obj = list;

List<String> list = new ArrayList<>(List.of("abc"));
Object obj = list;
/* next expression here */

List<String> list = new ArrayList<>(List.of("abc"));
Object obj = list;
/* next expression here */

A useful feature of the Java type system, and other modern statically-typed languages too, is a generic type: a type whose specification is in terms of a placeholder type to be filled in later.

Java’s collection classes are good examples of this. Set<E> is the ADT of finite sets of elements of some other type E. Instead of writing separate specifications and implementations for Set<String>, Set<Integer>, and so on, we can design one interface Set<E>, which represents an entire family of set ADTs.

Here is a simplified version of the Set interface:

/**
 * A mutable set.
 * @param <E> type of elements in the set
 */
public interface Set<E> {

Let’s start with a creator operation:

    // example creator operation
    /**
     * Make an empty set.
     * @param <F> type of elements in the set
     * @return a new set instance, initially empty
     */
    public static <F> Set<F> make() { ... }

The make operation is implemented as a static factory method. Clients will write code like:
Set<String> strings = Set.make();
and the compiler will understand that the new Set is a set of String objects.

(An obscure Java syntax requirement arises in the method signature above: a static method like make needs to independently declare its type parameter at the start of the signature, here shown as public static <F>. We called it F instead of E just to highlight that it’s a different type parameter. The instance methods below don’t have this extra <...> declaration because they always use the type parameter declared by the enclosing class or interface, public interface Set<E>.)

    // example observer operations

    /**
     * Get size of the set.
     * @return the number of elements in this set
     */
    public int size();

    /**
     * Test for membership.
     * @param e an element
     * @return true iff this set contains e
     */
    public boolean contains(E e);

Next we have two observer methods. Notice how the specs are in terms of our abstract notion of a set; it would be malformed to mention the details of any particular implementation of sets with particular private fields. These specs should apply to any valid implementation of the Set ADT.

    // example mutator operations

    /**
     * Modifies this set by adding e to the set.
     * @param e element to add
     */
    public void add(E e);

    /**
     * Modifies this set by removing e, if found.
     * If e is not found in the set, has no effect.
     * @param e element to remove
     */
    public void remove(E e);

The story for these mutators is basically the same as for the observers. We still write specs at the level of our abstract model of sets.

Implementing generic interfaces

public class CharSet implements Set<Character>

public interface Set<E> {

    // ...

    /**
     * Test for membership.
     * @param e an element
     * @return true iff this set contains e
     */
    public boolean contains(E e);

    /**
     * Modifies this set by adding e to the set.
     * @param e element to add
     */
    public void add(E e);

    // ...
}

public class CharSet implements Set<Character> {

    private String s = "";


    // ...


    @Override
    public boolean contains(Character e) {
        checkRep();
        return s.indexOf(e) != -1;
    }

    @Override
    public void add(Character e) {
        if (!contains(e)) s += e;
        checkRep();
    }
    // ...
}

public interface Set<E> {

    // ...
                                               

public class HashSet<E> implements Set<E> {

    // ...
     

public interface Set<E> {
    public boolean contains(E e);
    // ...
}

      /** Represents a set of integers. */
/*1*/ public class IntSet implements Set<Integer> {
/*2*/     public boolean contains(E e) { ... }
          // ...
      }

      /** Represents a set that can grow but never shrink. */
/*1*/ public interface GrowingSet<Z> extends Set<E> {
/*2*/     public boolean contains(Z z);
          // ...
      }

      /** Represents a set whose for-iteration returns the elements in the order they were added. */
/*1*/ public class OrderedSet<E> implements Set<E> {
/*2*/     public boolean contains(E e) { ... }
          // ...
      }

public interface Set<E> {
    /**
     * Picks an element from the set.
     * @return an element in the set
     * @throws NoSuchElementException if set is empty
     */
    public E pick() throws NoSuchElementException;

    // ...
}

public class SimpleSet<E> implements Set<E> {

    private final List<E> elementList = new ArrayList<>();
    // ...

    /**
     * <new spec here>
     */
    @Override
    public E pick() throws NoSuchElementException {
        ...
    }
}

Set<String> set = new HashSet<String>();

Set<String> set = Set.empty();

Sometimes an ADT has a small, finite set of immutable values, such as:

• months of the year: January, February, …
• days of the week: Monday, Tuesday, …
• compass points: north, south, east, west
• line caps in a line drawing: butt, round, square

A type like this may be used as part of a more complex type (like DateTime or Latitude), or as a parameter that changes the behavior of a method (like drawLine).

When the set of values is small and finite, it makes sense to define all the values as named constants, called an enumeration. Java has the enum construct to make this convenient:

public enum Month { JANUARY, FEBRUARY, MARCH, ..., DECEMBER };

This enum defines a new type name, Month, in the same way that class and interface define new type names. It also defines a set of named values, which we write in all-caps because they are effectively public static final constants. So you can now write:

Month thisMonth = MARCH;

This idea is called an enumeration because you are explicitly listing the elements of the set, and Java is assigning numbers to them as their default rep values.

In the simplest use case for enumerations, the only operation you need is testing equality between values:

if (day.equals(SATURDAY) || day.equals(SUNDAY)) {
    System.out.println("It's the weekend");
}

You may see code like this as well, which uses == instead of equals():

if (day == SATURDAY || day == SUNDAY) {
    System.out.println("It's the weekend");
}

If the day were represented as a String, this code would be very unsafe, because == tests whether two expressions refer to the same object at the same memory location, which may or may not be true for two arbitrary strings "Saturday". That’s why we always use equals() for comparing objects. But one of the advantages of using an enumeration type is that there is only ever one object in memory representing each value of the enumeration, and there is no way for a client to create more (no constructors!) So == is just as good as equals() for an enumeration. In fact == is more fail-fast, because it does a static check that both sides have the same enum type, whereas equals() delays that check until runtime.

In that sense, using an enumeration can feel like you’re using primitive int constants. Java even supports using them in switch statements (which otherwise only allow primitive integer types and their wrappers, and String, but not other objects):

switch (direction) {
    case NORTH: return "polar bears";
    case SOUTH: return "penguins";
    case EAST:  return "elephants";
    case WEST:  return "llamas";
}

But unlike int values, enumerations have more static checking:

Month firstMonth = MONDAY; // static error: MONDAY has type DayOfWeek, not type Month 

Also unlike primitive values, a variable of enum type can be null, which we must avoid the same way we do with other object types.

An enum declaration can contain all the usual fields and methods that a class can. So you can define additional operations for the ADT, and define your own rep as well. Here is an example that has a rep, an observer, and a producer:

public enum Month {
    // the values of the enumeration, written as calls to the private constructor below
    JANUARY(31),
    FEBRUARY(28),
    MARCH(31),
    APRIL(30),
    MAY(31),
    JUNE(30),
    JULY(31),
    AUGUST(31),
    SEPTEMBER(30),
    OCTOBER(31),
    NOVEMBER(30),
    DECEMBER(31);

    // rep
    private final int daysInMonth;

    // enums also have an automatic, invisible rep field:
    //   private final int ordinal;
    // which takes on values 0, 1, ... for each value in the enumeration.

    // rep invariant:
    //   daysInMonth is the number of days in this month in a non-leap year
    // abstraction function:
    //   AF(ordinal,daysInMonth) = the (ordinal+1)th month of the Gregorian calendar
    // safety from rep exposure:
    //   all fields are private, final, and have immutable types

    // Make a Month value. Not visible to clients, only used to initialize the
    // constants above.
    private Month(int daysInMonth) {
        this.daysInMonth = daysInMonth;
    }

    /**
     * @param isLeapYear true iff the year under consideration is a leap year
     * @return number of days in this month in a normal year (if !isLeapYear) 
     *                                           or leap year (if isLeapYear)
     */
    public int getDaysInMonth(boolean isLeapYear) {
        if (this == FEBRUARY && isLeapYear) {
            return daysInMonth+1;
        } else {
            return daysInMonth;
        }
    }

    /**
     * @return first month of the semester after this month
     */
    public Month startOfNextSemester() {
        switch (this) {
            case JANUARY:
                return FEBRUARY;
            case FEBRUARY:   // cases with no break or return
            case MARCH:      // fall through to the next case
            case APRIL:
            case MAY:
                return JUNE;
            case JUNE:
            case JULY:
            case AUGUST:
                return SEPTEMBER;
            case SEPTEMBER:
            case OCTOBER:
            case NOVEMBER:
            case DECEMBER:
                return JANUARY;
            default:
                throw new RuntimeException("can't get here");
        }
    }
}

All enum types also have some automatically-provided operations, defined by Enum:

• ordinal() is the index of the value in the enumeration, so JANUARY.ordinal() returns 0.
• compareTo() compares two values based on their ordinal numbers.
• name() returns the name of the value’s constant as a string, e.g. JANUARY.name() returns "JANUARY".
• toString() has the same behavior as name().

Finally, a word about where an enumeration like Month might be declared in Java. It can be in a file by itself, Month.java, just like a class or interface. But often, an enumeration type is a subsidiary to another module, such as a Date type. Then it would make sense to define the enumeration as a public declaration inside Date, and external clients would be able to refer to it using Date.Month.

startRegistrationFor("Fall", 2023);

public static final String FALL = "Fall";
...
startRegistrationFor(FALL, 2023);

public enum Semester { IAP, SPRING, SUMMER, FALL };
...
startRegistrationFor(FALL, 2023);

One more way to define an abstract data type is simply as a group of globally-accessible functions that operate on an opaque data type. This pattern is rarely seen in object-oriented languages like Java and Python, but often appears in older programming languages like C. Here is an example of some file I/O in C:

FILE* f = fopen("out.txt", "w"); // open a file for writing
fputs("hello", f); // write to the file
fclose(f);  // close the file

In this code, the abstract data type is FILE, representing an open file. The functions fopen, fputs, and fclose are operations of the type, taking a FILE as an argument or returning it as a return value. fputs expects the file as its second argument, oddly enough.

Because C is not object-oriented, there is no notion of classes, methods, fields, or even private, but representation independence is still achieved. The client has no way to look inside a FILE value. The only way to use a FILE is by passing it to operations provided by the type.

One key takeaway from this is that the notion of an abstract data type does not depend on language features like classes, or interfaces, or public/private access control. Data abstraction is a powerful design pattern that is ubiquitous in software engineering.

We’ve now completed our Java toolbox of ADT concepts from the first ADTs reading:

Java interfaces help us formalize the idea of an abstract data type as a set of operations that must be supported by a type.

This helps make our code…

• Safe from bugs. An ADT is defined by its operations, and interfaces do just that. When clients use an interface type, static checking ensures that they only use methods defined by the interface. If the implementation class exposes other methods — or worse, has visible representation — the client can’t accidentally see or depend on them. When we have multiple implementations of a data type, interfaces provide static checking of the method signatures.

• Easy to understand. Clients and maintainers know exactly where to look for the specification of the ADT. Since the interface doesn’t contain fields or implementations of instance methods, it’s easier to keep details of the implementation out of the specifications.

• Ready for change. We can easily add new implementations of a type by adding classes that implement an interface. If we avoid constructors in favor of static factory methods, clients will only see the interface. That means we can switch which implementation class clients are using without changing their code at all.

Java enumerations allow defining an ADT with a small finite set of immutable values. Compared to the old-fashioned alternatives of special integer values or special strings, enumerations help make our code:

• Safe from bugs. Static checking ensures that clients can’t use values outside the finite set, or confuse two different enumerated types.

• Easy to understand. Named constants are not as magical as integer literals, and named types explain themselves better than int or String.

• Ready for change. The code that uses a particular enumeration is easy to find by searching for the enumeration’s type name, so that it can be reviewed and changed when the enumeration changes. (IDE refactoring tools can even do some of these changes automatically.) Code that depends on a particular set of string or integer constants is much harder to distinguish from code that uses strings and integers for other purposes.

If you would like to get more practice with the concepts covered in this reading, you can visit the question bank. The questions in this bank were written in previous semesters by students and staff, and are provided for review purposes only – doing them will not affect your classwork grades.