Reading 16, Part 2: Writing a Program with Abstract Data Types

Bringing together what we’ve learned about abstract data types, this reading demonstrates the development of an ADT to solve a simple problem.

Recipes for programming

Recall the test-first programming approach for
Writing a procedure (a static method):

Spec. Write the spec, including the method signature (name, argument types, return types, exceptions), and the precondition and the postcondition as a Javadoc comment.
Test. Create systematic test cases and put them in a JUnit class so you can run them automatically.
1. You may have to go back and change your spec when you start to write test cases. Just the process of writing test cases puts pressure on your spec, because you’re thinking about how a client would call the method. So steps 1 and 2 iterate until you’ve got a better spec and some good test cases.
2. Make sure at least some of your tests are failing at first. A test suite that passes all tests even when you haven’t implemented the method is not a good test suite for finding bugs.
Implement. Write the body of the method. You’re done when the tests are all green in JUnit.
1. Implementing the method puts pressure on both the tests and the specs, and you may find bugs in them that you have to go back and fix. So finishing the job may require changing the implementation, the tests, and the specs, and bouncing back and forth among them.

Let’s broaden this to a recipe for
Writing an abstract data type:

Spec. Write specs for the operations of the datatype, including method signatures, preconditions, and postconditions.
Test. Write test cases for the ADT’s operations.
1. Again, this puts pressure on the spec. You may discover that you need operations you hadn’t anticipated, so you’ll have to add them to the spec.
Implement. For an ADT, this part expands to:
1. Choose rep. Write down the private fields of a class, or the variants of a recursive datatype. Write down the rep invariant as a comment.
2. Assert rep invariant. Implement a checkRep() method that enforces the rep invariant. This is critically important if the rep invariant is nontrivial, because it will catch bugs much earlier.
3. Implement operations. Write the method bodies of the operations, making sure to call checkRep() in them. You’re done when the tests are all green in JUnit.

And let’s broaden it further to a recipe for
Writing a program (consisting of ADTs and procedures):

Choose datatypes. Decide which ones will be mutable and which immutable.
Choose procedures. Write your top-level procedure and break it down into smaller steps.
Spec. Spec out the ADTs and procedures. Keep the ADT operations simple and few at first. Only add complex operations as you need them.
Test. Write test cases for each unit (ADT or procedure).
Implement simply first. Choose simple, brute-force representations. The point here is to put pressure on the specs and the tests, and try to pull your whole program together as soon as possible. Make the whole program work correctly first. Skip the advanced features for now. Skip performance optimization. Skip corner cases. Keep a to-do list of what you have to revisit.
Reimplement and iterate and optimize. Now that it’s all working, make it work better.

Problem: matrix multiplication

Suppose we want to compute matrix multiplications and we want to do them faster.

For example, if a, b are scalar constants, and X is a big matrix, then

(aX)b

is slow to compute because it loops over the matrix X twice: once to multiply it by a, and then again to multiply it by b. It’d be equivalent and cheaper to do:

(ab)X

That’s just one example of an optimization we could make by rearranging the products in a matrix multiplication. (Remember however that matrix multiplication is associative but not commutative; only scalars commute.)

Choose datatypes

Let’s call this a MatrixExpression. To make the definitions easier to read, we’ll abbreviate that to MatExpr.

Let’s define some operations:

make: double → MatExpr
// effects: returns an expression consisting of just the given scalar

make: double[][] → MatExpr
// requires: array.length > 0, and array[i].lengths are equal and > 0, for all i
// effects: returns an expression consisting of just the given matrix

times: MatExpr × MatExpr → MatExpr
// requires: m1 and m2 are compatible for multiplication
// effects: returns m1×m2

isIdentity: MatExpr → boolean
// effects: returns true if the expression is the multiplicative identity

And the one we really want:

optimize: MatExpr → MatExpr
// effects: returns an expression with the same value, but which may be faster to compute

Test

Let’s test optimize(). Partitions:

Number of scalars in expression: 0, 1, 2, >2
Position of scalar in expression tree: immediate left, immediate right, left-of-left, left-of-right, right-of-left, right-of-right

Test case		Partitions covered
X ⇒ X	covers	0 scalars
aX ⇒ aX		1 scalar, immediate left
a(Xb) ⇒ (ab)X		2 scalars, immediate left, right-of-right
(aX)b ⇒ (ab)X		2 scalars, immediate right, left-of-left
(Xa)(bY) ⇒ (((ab)X)Y)		2 scalars, left-of-right, right-of-left

Choose a rep

This problem is a natural one for a recursive datatype.

MatExpr = Identity + Scalar(double) + Matrix(double[][]) + Product(MatExpr, MatExpr)

/** Represents an immutable expression of matrix and scalar products. */
public interface MatrixExpression {
    // ...
}

class Identity implements MatrixExpression {
    public Identity() {
    }
}

class Scalar implements MatrixExpression {
    private final double value;
    public Scalar(double value) {
        this.value = value;
    }
}

class Matrix implements MatrixExpression {
    private final double[][] array;
    // RI: array.length > 0, and all array[i] are equal nonzero length
    public Matrix(double[][] array) {
        this.array = array; // note: danger!
    }
}

class Product implements MatrixExpression {
    private final MatrixExpression m1;
    private final MatrixExpression m2;
    // RI: m1's column count == m2's row count, or m1 or m2 is scalar
    public Product(MatrixExpression m1, MatrixExpression m2) {
        this.m1 = m1;
        this.m2 = m2;
    }
}

reading exercises

Danger

In the code above, what is note: danger! concerned with?

(missing explanation)

Choose an identity

It’s always good to have a value in the datatype that represents nothing, so that we can avoid using null. For a matrix product, the natural choice is the identity matrix — an empty product expression is just the identity anyway. So let’s define that:

/** Identity for all matrix computations. */
public static final MatrixExpression I = new Identity();

Unfortunately, we’ll see that this is not a perfect situation: other MatExprs might also be the identity.

Implementing `make`: use factory methods

Let’s start implementing, starting with the make() creators.

We don’t want to expose our concrete rep classes Scalar, Matrix, and Product, so that clients won’t depend on them and we’ll be able to change them later (being ready for change).

So we’ll create static methods in MatrixExpression to implement make():

/** @return a matrix expression consisting of just the scalar value */
public static MatrixExpression make(double value) {
    return new Scalar(value);
}

/** @return a matrix expression consisting of just the matrix given */
public static MatrixExpression make(double[][] array) {
    return new Matrix(array);
}

These are called factory methods — static methods that play the role of constructors. The factory-method pattern is a common design pattern that you’ll see throughout object-oriented programming, in many languages.

Implementing `isIdentity`: don’t use `instanceof`

Now let’s implement the isIdentity observer. Here’s a bad way to do it:

// don't do this
public static boolean isIdentity(MatrixExpression m) {
    if (m instanceof Scalar) {
        return ((Scalar)m).value == 1;
    } else if (m instanceof Matrix) {
        // ... check for 1s on the diagonal and 0s everywhere else
    } else ... // do the right thing for other variant classes
}

In general, using instanceof in object-oriented programming is a bad smell. Break the operation down into pieces that are appropriate for each class, and write instance methods instead:

reading exercises

Identity

class Identity implements MatrixExpression {
    // no fields
    ...
    public boolean isIdentity() {
(missing answer)
    }
}

(missing explanation)

Scalar

class Scalar implements MatrixExpression {
    private final double value;
    ...
    public boolean isIdentity() {
(missing answer)
    }
}

(missing explanation)

Matrix

class Matrix implements MatrixExpression {
    private final double[][] array;
    ...
    public boolean isIdentity() { 
        for (int row = 0; row < array.length; row++) {
            for (int col = 0; col < array[row].length; col++) {
                double expected = (row == col) ? 1 : 0;
                if (array[row][col] != expected) {
(missing answer)
                }
            }
        }(missing answer)
    }
}

(missing explanation)

Product

class Product implements MatrixExpression {
    private final MatrixExpression m1, m2;
    ...
    public boolean isIdentity() { 
        return m1.isIdentity() ❓ m2.isIdentity();
    }
}

Pick the best operator to replace ❓:

(missing explanation)

Implementing isIdentity exposes an issue that we should have first discovered by writing test cases: we will not always return true for a Product whose value is the identity matrix (e.g. A × A^-1). We probably want to resolve this by weakening the spec of isIdentity.

Implementing `optimize` without `instanceof`

Let’s implement optimize(). Again, here’s a bad way to do it, which will quickly get us mired in weeds:

// don't do this
class Product implements MatrixExpression {
    public MatrixExpression optimize() {
        if (m1 instanceof Scalar) {
            ...
        } else if (m2 instanceof Scalar) {
            ...
        }
        ...
    }
}

If you find yourself writing code with instanceof checks all over the place, you need to take a step back and rethink the problem.

In particular, to optimize the scalars, we’re going to need two recursive helper operations, so we’ll add them to our abstract datatype:

scalars: MatExpr → MatExpr
// effects: returns a MatExpr with no matrices in it, only the scalars

matrices: MatExpr → MatExpr
// effects: returns a MatExpr with no scalars in it, only matrices in the same order they appear in the input expression

These expressions will allow us to pull the scalars out of an expression and move them together in a single multiplication expression.

/** Represents an immutable expression of matrix and scalar products. */
public interface MatrixExpression {

    // ...

    /** @return the product of all the scalars in this expression */
    public MatrixExpression scalars();

    /** @return the product of all the matrices in this expression.
     * times(scalars(), matrices()) is equivalent to this expression. */
    public MatrixExpression matrices();
}

Now we’ll implement them as expected:

class Identity implements MatrixExpression {
    // no fields
    ...
    public MatrixExpression scalars() { return this; }
    public MatrixExpression matrices() { return this; }
}

class Scalar implements MatrixExpression {
    private final double value;
    ...
    public MatrixExpression scalars() { return this; }
    public MatrixExpression matrices() { return I; }
}

class Matrix implements MatrixExpression {
    private final double[][] array;
    ...
    public MatrixExpression scalars() { return I; }
    public MatrixExpression matrices() { return this; }
}

class Product implements MatrixExpression {
    private final MatrixExpression m1, m2;
    ...
    public MatrixExpression scalars() {
        return times(m1.▶▶A◀◀, m2.▶▶B◀◀);
    }
    public MatrixExpression matrices() {
        return times(m1.▶▶C◀◀, m2.▶▶D◀◀);
    }
}

reading exercises

Product.scalars and matrices

Fill in the blanks in the implementation above…

(missing answer)
(missing answer)
(missing answer)
(missing answer)

(missing explanation)

With these helper functions, optimize() can just separate the scalars and the matrices:

class Identity implements MatrixExpression {
    ...
    public MatrixExpression optimize() { return this; }
}

class Scalar implements MatrixExpression {
    ...
    public MatrixExpression optimize() { return this; }
}

class Matrix implements MatrixExpression {
    ...
    public MatrixExpression optimize() { return ▶▶A◀◀; }
}

class Product implements MatrixExpression {
    ...
    public MatrixExpression optimize() {
        return times(▶▶B◀◀);
    }
}

reading exercises

Matrix and Product.optimize

Fill in the blanks in the implementations above…

(missing answer)
(missing answer)

Hint: times takes two arguments, so answer B should have a comma.

(missing explanation)

Reading 16, Part 2: Writing a Program with Abstract Data Types

Recipes for programming

Problem: matrix multiplication

Choose datatypes

Test

Choose a rep

reading exercises

Choose an identity

Implementing make: use factory methods

Implementing isIdentity: don’t use instanceof

reading exercises

Implementing optimize without instanceof

reading exercises

reading exercises

Next: Summary

Implementing `make`: use factory methods

Implementing `isIdentity`: don’t use `instanceof`

Implementing `optimize` without `instanceof`