I have been working on this problem for several hours now and I just cannot figure out what I am doing wrong here. Could anyone help point me in the right direction?
I was asked to write an Autocomplete program and I've completed everything except for this one method I cannot get working. Each term has: 1. String query and 2. long weight.
Here is the method:
public static Comparator<Term> byReverseWeightOrder() {
return new Comparator<Term>() { // LINE CAUSING PROBLEM
public int compare(Term t1, Term t2) {
if (t1.weight > t2.weight) { // LINE CAUSING PROBLEM
return -1;
} else if (t1.weight == t2.weight) {
return 0;
} else {
return 1;
}
}
};
}
My problem is that no matter how I mess with the method I always result in a NullPointerException(). Which, it points to this method (byReverseWeightOrder) as well as these two statements.
Arrays.sort(matches, Term.byReverseWeightOrder());
Term[] results = autocomplete.allMatches(prefix);
Here is the rest of the code if it can be found helpful:
Term
import java.util.Comparator;
public class Term implements Comparable<Term> {
public String query;
public long weight;
public Term(String query, long weight) {
if (query == null) {
throw new java.lang.NullPointerException("Query cannot be null");
}
if (weight < 0) {
throw new java.lang.IllegalArgumentException("Weight cannot be negative");
}
this.query = query;
this.weight = weight;
}
public static Comparator<Term> byReverseWeightOrder() {
return new Comparator<Term>() {
public int compare(Term t1, Term t2) {
if (t1.weight > t2.weight) {
return -1;
} else if (t1.weight == t2.weight) {
return 0;
} else {
return 1;
}
}
};
}
public static Comparator<Term> byPrefixOrder(int r) {
if (r < 0) {
throw new java.lang.IllegalArgumentException("Cannot order with negative number of characters");
}
final int ref = r;
return
new Comparator<Term>() {
public int compare(Term t1, Term t2) {
String q1 = t1.query;
String q2 = t2.query;
int min;
if (q1.length() < q2.length()) {
min = q1.length();
}
else {
min = q2.length();
}
if (min >= ref) {
return q1.substring(0, ref).compareTo(q2.substring(0, ref));
}
else if (q1.substring(0, min).compareTo(q2.substring(0, min)) == 0) {
if (q1.length() == min) {
return -1;
}
else {
return 1;
}
}
else {
return q1.substring(0, min).compareTo(q2.substring(0, min));
}
}
};
}
public int compareTo(Term that) {
String q1 = this.query;
String q2 = that.query;
return q1.compareTo(q2);
}
public long getWeight() {
return this.weight;
}
public String toString() {
return this.weight + "\t" + this.query;
}
}
BinarySearchDeluxe
import java.lang.*;
import java.util.*;
import java.util.Comparator;
public class BinarySearchDeluxe {
public static <Key> int firstIndexOf(Key[] a, Key key, Comparator<Key> comparator) {
if (a == null || key == null || comparator == null) {
throw new java.lang.NullPointerException();
}
if (a.length == 0) {
return -1;
}
int left = 0;
int right = a.length - 1;
while (left + 1 < right) {
int middle = left + (right - left)/2;
if (comparator.compare(key, a[middle]) <= 0) {
right = middle;
} else {
left = middle;
}
}
if (comparator.compare(key, a[left]) == 0) {
return left;
}
if (comparator.compare(key, a[right]) == 0) {
return right;
}
return -1;
}
public static <Key> int lastIndexOf(Key[] a, Key key, Comparator<Key> comparator) {
if (a == null || key == null || comparator == null) {
throw new java.lang.NullPointerException();
}
if (a == null || a.length == 0) {
return -1;
}
int left = 0;
int right = a.length - 1;
while (left + 1 < right) {
int middle = left + (right - left)/2;
if (comparator.compare(key, a[middle]) < 0) {
right = middle;
} else {
left = middle;
}
}
if (comparator.compare(key, a[right]) == 0) {
return right;
}
if (comparator.compare(key, a[left]) == 0) {
return left;
}
return -1;
}
}
AutoComplete
import java.util.Arrays;
import java.util.Scanner;
import java.io.File;
import java.io.IOException;
import java.util.Comparator;
public class Autocomplete {
public Term[] terms;
public Autocomplete(Term[] terms) {
if (terms == null) {
throw new java.lang.NullPointerException();
}
this.terms = terms.clone();
Arrays.sort(this.terms);
}
public Term[] allMatches(String prefix) {
if (prefix == null) {
throw new java.lang.NullPointerException();
}
Term theTerm = new Term(prefix, 0);
int start = BinarySearchDeluxe.firstIndexOf(terms, theTerm, Term.byPrefixOrder(prefix.length()));
int end = BinarySearchDeluxe.lastIndexOf(terms, theTerm, Term.byPrefixOrder(prefix.length()));
int count = start;
System.out.println("Start: " + start + " End: " + end);
if (start == -1 || end == -1) {
// System.out.println("PREFIX: " + prefix);
throw new java.lang.NullPointerException();
} // Needed?
Term[] matches = new Term[end - start + 1];
//matches = Arrays.copyOfRange(terms, start, end);
for (int i = 0; i < end - start; i++) {
matches[i] = this.terms[count];
count++;
}
Arrays.sort(matches, Term.byReverseWeightOrder());
System.out.println("Finished allmatches");
return matches;
}
public int numberOfMatches(String prefix) {
if (prefix == null) {
throw new java.lang.NullPointerException();
}
Term theTerm = new Term(prefix, 0);
int start = BinarySearchDeluxe.firstIndexOf(terms, theTerm, Term.byPrefixOrder(prefix.length()));
int end = BinarySearchDeluxe.lastIndexOf(terms, theTerm, Term.byPrefixOrder(prefix.length()));
System.out.println("Finished numberMatches");
return end - start + 1; // +1 needed?
}
public static void main(String[] args) throws IOException {
// Read the terms from the file
Scanner in = new Scanner(new File("wiktionary.txt"));
int N = in.nextInt(); // Number of terms in file
Term[] terms = new Term[N];
for (int i = 0; i < N; i++) {
long weight = in.nextLong(); // read the next weight
String query = in.nextLine(); // read the next query
terms[i] = new Term(query.replaceFirst("\t",""), weight); // construct the term
}
Scanner ip = new Scanner(System.in);
// TO DO: Data Validation Here
int k;
do {
System.out.println("Enter how many matching terms do you want to see:");
k = ip.nextInt();
} while (k < 1 || k > N);
Autocomplete autocomplete = new Autocomplete(terms);
// TO DO: Keep asking the user to enter the prefix and show results till user quits
boolean cont = true;
do {
// Read in queries from standard input and print out the top k matching terms
System.out.println("Enter the term you are searching for. Enter * to exit");
String prefix = ip.next();
if (prefix.equals("*")) {
cont = false;
break;
}
Term[] results = autocomplete.allMatches(prefix);
System.out.println(results.length);
for(int i = 0; i < Math.min(k,results.length); i++)
System.out.println(results[i].toString());
} while(cont);
System.out.println("Done!");
}
}
I apologize for the sloppy code, I have been pulling my hair out for awhile now and keep forgetting to clean it up.
Two examples:
Example 1:
int k = 2;
String prefix = "auto";
Enter how many matching terms do you want to see:
2
Enter the term you are searching for. Enter * to exit
auto
619695 automobile
424997 automatic
Example 2:
int k = 5;
String prefix = "the";
Enter how many matching terms do you want to see:
5
Enter the term you are searching for. Enter * to exit
the
5627187200 the
334039800 they
282026500 their
250991700 them
196120000 there