Computer Science 210
Data Structures
Fall 2017, Siena College
SpellsArrayGenericPair BlueJ Project
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SpellsArrayGenericPair Source Code
The Java source code for SpellsArrayGenericPair is below. Click on a file name to download it.
/*
* Example SpellsArrayGenericPair: a version of the Spells program that
* uses an array of GenericPair objects
*
* Jim Teresco, Siena College, CSIS 210, Fall 2017
*
*/
import java.io.File;
import java.io.IOException;
import java.util.Scanner;
public class SpellsArrayGenericPair {
public static void main(String args[]) throws IOException {
final int NUM_SPELLS = 10;
// create an array of the magic spells we know.
// Note: due to the way Java handles arrays and generic types,
// we have to leave off the <String,String> in the construction..
GenericPair<String,String> spells[] = new GenericPair[NUM_SPELLS];
Scanner inFile = new Scanner(new File("spells.txt"));
for (int i = 0; i < NUM_SPELLS; i++) {
String spell = inFile.nextLine().trim();
String action = inFile.nextLine().trim();
spells[i] = new GenericPair<String,String>(spell, action);
}
inFile.close();
// we play a little game matching spells to descriptions
// until an invalid spell is specified
Scanner keyboard = new Scanner(System.in);
int spellnum = 0;
while (spellnum >= 0) {
System.out.print("Which spell will you use? ");
String spellName = keyboard.next();
spellnum = -1;
for (int spellIndex = 0; spellIndex < spells.length; spellIndex++) {
if (spellName.equals(spells[spellIndex].getFirst())) {
spellnum = spellIndex;
break;
}
}
if (spellnum >= 0) {
System.out.println(spells[spellnum].getSecond());
}
else {
System.out.println("Your wand doesn't know that one. It explodes. Bye!");
}
}
}
}
/*
* Example GenericPair: our pair structure, but now with type parameters
* to enforce consistency of the usage of a given instance of the class.
*
* Jim Teresco, The College of Saint Rose, CSC 523, Summer 2014
* CSIS 210, Siena College, Fall 2016, Fall 2017
*
*/
// First, notice that we now have "type parameters" after our class name.
// These tell us what kinds of objects we will use for the first (U), and
// second (V) values in our pair.
public class GenericPair<U,V> {
// our instance variables will be the generic types U and V
// which will be filled in with actual types when we create an
// instance of the class, much like an actual parameter gives
// a value to a formal parameter when a method is called.
private U first;
private V second;
// our constructor for a new GenericPair
// note that the formal parameter types are now
// the U and V types
public GenericPair(U first, V second) {
this.first = first;
this.second = second;
}
// accessors, also now returning our generic types
public U getFirst() {
return first;
}
public V getSecond() {
return second;
}
// mutators, including ones to set first, second, or both
public void setFirst(U newFirst) {
first = newFirst;
}
public void setSecond(V newSecond) {
second = newSecond;
}
public void setValues(U newFirst, V newSecond) {
first = newFirst;
second = newSecond;
}
// we should have a toString method - in this case, we'll return
// a String to make this look like a coordinate pair, seems as
// good as anything and likely to be useful in many contexts.
// note that we are implcitly using the toString methods of the
// objects we encapsulate
public String toString() {
return "(" + first + ", " + second + ")";
}
// it's also good to define an equals method for any general purpose
// class -- in this case, we'll say a GenericPair is equal to another
// only if first and second are equal according to their own equals
// methods
public boolean equals(Object o) {
GenericPair<U,V> other = (GenericPair<U,V>)o;
return other.first.equals(first) && other.second.equals(second);
}
// a common way to test a class is to include a main method
// in the class that tests its methods. So let's.
public static void main(String args[]) {
// we'll create a few GenericPair objects and print them out
// note that we need to specify the actual types we
// intend to use for the first and second now, and where
// we want a primitive type, we must specify the "boxed"
// equivalent object type
GenericPair<String,Double> a = new GenericPair<String,Double>("Bob", 9.1);
GenericPair<String,Double> b = new GenericPair<String,Double>("Alice", 0.0);
System.out.println("a: " + a);
System.out.println("b: " + b);
// try out the accessors
System.out.println("a.getFirst(): " + a.getFirst());
System.out.println("a.getSecond(): " + a.getSecond());
System.out.println("b.getFirst(): " + b.getFirst());
System.out.println("b.getSecond(): " + b.getSecond());
// are they equal?
System.out.println("a.equals(b): " + a.equals(b));
// mutators!
a.setFirst("Charlie");
a.setSecond(-23.5);
b.setValues("Charlie", -23.5);
System.out.println("a: " + a);
System.out.println("b: " + b);
// are they equal now? Hope so!
System.out.println("a.equals(b): " + a.equals(b));
// and we can use other types too
GenericPair<Integer,String> c = new GenericPair<Integer,String>(2, "Jeter");
// uncomment the following to see what happens if we send
// the wrong type of object to the equals method
//System.out.println("a.equals(\"hi there\"): " + a.equals("high there"));
}
}