ogp-notes

Interfaces

Suppose we are implementing a programming language. Our language supports the datatypes boolean, int, and string, amongst others:

public abstract class Type {}

public class BooleanType extends Type {
    private BooleanType() {}
    public static final BooleanType INSTANCE = new BooleanType();
}

public class IntType extends Type {
    private IntType() {}
    public static final IntType INSTANCE = new IntType();
}

public class StringType extends Type {
    private StringType() {}
    public static final StringType INSTANCE = new StringType();
}

public abstract class Value {
    public abstract Type getType(); 
}

public class BooleanValue extends Value {
    public final boolean value;
    public Type getType() { return BooleanType.INSTANCE; }
    private BooleanValue(boolean value) { this.value = value; }
    public final static BooleanValue TRUE = new BooleanValue(true);
    public final static BooleanValue FALSE = new BooleanValue(false);
    public static BooleanValue of(boolean value) { return value ? TRUE : FALSE; }
}

public class IntValue extends Value {
    public final int value;
    public Type getType() { return IntType.INSTANCE; }
    private IntValue(int value) { this.value = value; }
    public final static IntValue ZERO = new IntValue(0);
    public static IntValue of(int value) {
        return value == 0 ? ZERO : new IntValue(value);
    }
}

public class StringValue extends Value {
    public final String value;
    public Type getType() { return StringType.INSTANCE; }
    private StringValue(String value) { this.value = value; }
    public final static StringValue EMPTY = new StringValue("");
    public static StringValue of(String value) {
        return value.equals("") ? EMPTY : new StringValue(value);
    }
}

First of all, notice the following:

Classes BooleanType, IntType, and StringType are examples of the Singleton Pattern: it does not make sense to create more than one instance of class BooleanType, so, instead of offering to clients a way to create new instances of class BooleanType, the class offers only a static field INSTANCE that refers to the only instance of class BooleanType that will ever exist. A static field differs from a regular field in that it is a property of the class itself, rather than a property of each instance of the class.
Classes BooleanValue, IntValue, and StringValue are examples of immutable value classes. Instead of exposing a constructor to clients directly, class BooleanValue offers a static method of that clients can use to obtain a BooleanValue instance corresponding to a particular boolean value. Instead of creating a new instance of BooleanValue at each call, method of reuses a BooleanValue instance stored in a static field of the class. Similarly, method of of class IntValue reuses an IntValue instance stored in static field ZERO if an IntValue instance corresponding to value 0 is requested; if some other value is requested, a new IntValue instance is created. Class StringValue implements this pattern as well.
Names of static final fields are often written in all-uppercase.

Now suppose that in our programming language, like in Java, we can use the + operator to add int values and to concatenate string values, but we cannot use it on boolean values. Furthermore, suppose that, again like in Java, we can use the logical AND operator & to compute the bitwise AND of two int values and to compute the logical AND of two boolean values, but we cannot use it on string values. To implement this, it would make sense to make classes IntType and StringType extend an abstract class AddableType with an abstract method add, and to make classes BooleanType and IntType extend an abstract class AndableType with an abstract method and. We could then implement a method evaluate as follows:

public abstract class AddableType {
    public abstract Value add(Value leftOperand, Value rightOperand);
}
public abstract class AndableType {
    public abstract Value and(Value leftOperand, Value rightOperand);
}
public class BooleanType extends Type, AndableType { // ERROR
    // ...
    public Value and(Value leftOperand, Value rightOperand) {
        return BooleanValue.of(((BooleanValue)leftOperand).value
            & ((BooleanValue)rightOperand).value);
    }
}
public class IntType extends Type, AddableType, AndableType { // ERROR
    // ...
    public Value add(Value leftOperand, Value rightOperand) {
        return IntValue.of(((IntValue)leftOperand).value
            + ((IntValue)rightOperand).value);
    }
    public Value and(Value leftOperand, Value rightOperand) {
        return IntValue.of((IntValue)leftOperand).value
            & ((IntValue)rightOperand).value);
    }
}
public class StringType extends Type, AddableType { // ERROR
    // ...
    public Value add(Value leftOperand, Value rightOperand) {
        return StringValue.of(((StringValue)leftOperand).value
            + ((StringValue)rightOperand).value);
    }
}
public class Interpreter {
    public static Value evaluate(Value value1, char operator, Value value2) {
        Type type = value1.getType();
        if (type != value2.getType())
            throw new UnsupportedOperationException(
                "The operand types do not match");
        switch (operator) {
            case '+':
                if (!(type instanceof AddableType))
                    throw new UnsupportedOperationException(
                        "Type " + type + " does not support the + operator");
                return ((AddableType)type).add(value, value2);
            case '&':
                if (!(type instanceof AndableType))
                    throw new UnsupportedOperationException(
                        "Type " + type + " does not support the & operator");
                return ((AndableType)type).and(value1, value2);
            // ...
         }
    }
}

Unfortunately, this is not valid Java code: Java does not allow a class to extend multiple superclasses. (Some other programming languages, such as C++, do allow such multiple inheritance.) However, Java does allow a restricted form of multiple inheritance: it allows a class to extend from one superclass and zero or more interfaces. An interface, declared using the interface keyword, is in most ways just like a class, except that it is not allowed to declare any instance (i.e. non-static) fields and it is not allowed to declare any constructors. We can therefore turn the incorrect program above into the correct program below:

public interface AddableType {
    Value add(Value leftOperand, Value rightOperand);
}
public interface AndableType {
    Value and(Value leftOperand, Value rightOperand);
}
public class BooleanType extends Type implements AndableType {
    // ...
    public Value and(Value leftOperand, Value rightOperand) {
        return BooleanValue.of(((BooleanValue)leftOperand).value
            & ((BooleanValue)rightOperand).value);
    }
}
public class IntType extends Type implements AddableType, AndableType {
    // ...
    public Value add(Value leftOperand, Value rightOperand) {
        return IntValue.of(((IntValue)leftOperand).value
            + ((IntValue)rightOperand).value);
    }
    public Value and(Value leftOperand, Value rightOperand) {
        return IntValue.of((IntValue)leftOperand).value
            & ((IntValue)rightOperand).value);
    }
}
public class StringType extends Type implements AddableType {
    // ...
    public Value add(Value leftOperand, Value rightOperand) {
        return StringValue.of(((StringValue)leftOperand).value
            + ((StringValue)rightOperand).value);
    }
}
public class Interpreter {
    public static Value evaluate(Value value1, char operator, Value value2) {
        Type type = leftOperand.getType();
        if (type != rightOperand.getType())
            throw new UnsupportedOperationException(
                "The operand types do not match");
        switch (operator) {
            case '+':
                if (!(type instanceof AddableType))
                    throw new UnsupportedOperationException(
                        "Type " + type + " does not support the + operator");
                return ((AddableType)type).add(value1, value2);
            case '&':
                if (!(type instanceof AndableType))
                    throw new UnsupportedOperationException(
                        "Type " + type + " does not support the & operator");
                return ((AndableType)type).and(value1, value2);
            // ...
         }
    }
}

Notice the following:

Interfaces are like abstract classes in that you cannot directly instantiate an interface (although you can instantiate a class that implements the interface). Interfaces are always abstract; it is not necessary to specify the abstract keyword.
Methods declared by interfaces are public and abstract by default; these keywords need not be specified explicitly.
A class can extend at most one superclass, but it can additionally implement any number of interfaces. The interfaces are specified after the implements keyword. A class that implements an interface must implement each of the interface’s methods (i.e. declare, for each of the interface’s methods, a non-abstract method that overrides it), unless the class is declared abstract itself.
You can use the instanceof operator to test if an object implements an interface, in exactly the same way that you can test if it is an instance of some class. Furthermore, you can use a typecast to cast an object to an interface type, and then call the interface’s methods on it. Just like when casting to a class type, a run-time check will be performed to check that the object’s class indeed implements the interface; if not, a ClassCastException is thrown.