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The abstract concept of a unary function can be modeled by the following interface.
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interface UnaryFunLambda { Object apply(Object arg); } |
An anonymous UnaryFun Lambda class has the following form
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new UnaryFunLambda() { Object apply(Object arg) { // concrete code to compute something and return the result // …... } } |
Since Object is the superclass of all concrete classes in Java, in practice, we often have to perform type-casting on the arg in order to perform the desired operation. More than often, we need to type-cast the returned result in order to make proper use of it.
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Use DrJava to compile the above UnaryFun Lambda interface.
Now create and compile a new file containing the following class.
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Add the following method to FingerExercisesLab8Exercises.
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Object doSomething() {
return new UnaryFunLambda() {
Object apply(Object arg) {
return (Integer)arg * (Integer)arg;}
}.apply(5);
}
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int x_minus_y(int x, int y) {
return (Integer)new UnaryFunLambda() {
Object apply(Object arg) {
return (Integer)arg - y;}
}.apply(x);
}
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What does this anonymous inner class do?
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UnaryFunLambda whatIsIt() { return new UnaryFunLambda() { Object apply(Object s1) { return new UnaryFunLambda() { Object apply(Object s2) { return new UnaryFunLambda() { Object apply(Object s3) { return s1 + " " + s2 + " " + s3; } }; } }; } }; } |
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public class Currying {
UnaryFunLambda whatIsIt() {
return new UnaryFunLambda() {
public Object apply(final Object s1) {
return new UnaryFunLambda() {
public Object apply(final Object s2) {
return new UnaryFunLambda() {
public Object apply(final Object s3) {
return s1 + " " + s2 + " " + s3;
}
};
}
};
}
};
}
}
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Notice in exercise #5 how the anonymous inner class new UnaryFunLambda inside of x_minus_y is allowed to reference y in its computation? y is said to be in the closure of this anonymous inner class.
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abstract class ObjectList {
ObjectList cons(Object n) { return new ConsObjectList(n, this); }
/** Applies f to each element in this list and returns the list containing
* the results of this application. */
abstract ObjectList map(UnaryFunLambda f);
/** Returns a String representation of this list as specified in each concrete subclass. */
abstract String listString();
/** Returns s String containing the elements of this, where each element is preceded by ' ' */
abstract String listStringHelp();
}
class EmptyObjectList extends ObjectList {
static EmptyObjectList ONLY = new EmptyObjectList();
private EmptyObjectList() { }
ObjectList map(UnaryFunLambda f) { return null; /*to do */ }
String listString() { return "()"; }
String listStringHelp() { return ""; }
}
class ConsObjectList extends ObjectList {
Object first;
ObjectList rest;
ObjectList map(UnaryFunLambda f) { return null; /* ... rest.map(f) ...to do*/ }
String listString() { return "(" + first + rest.listStringHelp() + ")"; }
String listStringHelp() { return " " + first + rest.listStringHelp(); }
}
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