Develop Reference

How to improve a cumbersome comparison of 2 objects' fields

We have a program that compares thousands of pairs of Students by checking each field of the Student and counting the diffs:
class Student{
String name;
String address;
String biologyCourse;
.....
// about 100 other fields
}
And the counter POJO class:
class Counters{
long bothStudentsHaveName;
long onlyLeftHasName;
long onlyRightHasName;
......
// number of fields in Student * 3 (both, only left, only right)
}
Our compare function accepts 2 students plus the counters object and needs to scan the fields and update the relevant counters:
public void compareStudents(Student left, Student right, Counters counters){
if (!StringUtils.isEmpty(left.name) && !StringUtils.isEmpty(right.name) ){
counters.bothStudentsHaveName++;
} else if (StringUtils.isEmpty(left.name) && !StringUtils.isEmpty(right.name)){
counters.onlyRightHasName++;
} else if (!StringUtils.isEmpty(left.name) && StringUtils.isEmpty(right.name))){
counters.onlyLeftHasName++;
}
/// and now??
}
At this point, we can add 100s more triplets of if/else like the above - but we believe there should be a much easier way to do that.
Reflection can be an option or maybe X dimensions arrays, but can we somehow write the code so the comparison and counting will be much more generic?

I have solved your problem with one single loop. But here I'm assuming that naming convention for all the fields will be the same as described in your question. Here I am dynamically accessing the Student fields and updating Counter fields accordingly. Here is the complete solution:
Solution Class:
public class Solution {
public void compareStudents(Student left, Student right, Counter counter) throws Exception {
for (Field field : Student.class.getDeclaredFields()) {
Object leftValue = field.get(left);
Object rightValue = field.get(right);
String fieldName = field.getName().substring(0, 1).toUpperCase() + field.getName().substring(1);
if(leftValue != null && rightValue != null) {
Field counterField = Counter.class.getDeclaredField("bothStudentsHave" + fieldName);
counterField.set(counter, (long) counterField.get(counter) + 1);
} else if (leftValue != null) {
Field counterField = Counter.class.getDeclaredField("onlyLeftHas" + fieldName);
counterField.set(counter, (long) counterField.get(counter) + 1);
} else if (rightValue != null) {
Field counterField = Counter.class.getDeclaredField("onlyRightHas" + fieldName);
counterField.set(counter, (long) counterField.get(counter) + 1);
}
}
}
}
Student Class:
class Student {
String name;
String address;
String biologyCourse;
}
Counter Class:
class Counter {
// name
long bothStudentsHaveName;
long onlyLeftHasName;
long onlyRightHasName;
// address
long bothStudentsHaveAddress;
long onlyLeftHasAddress;
long onlyRightHasAddress;
// biologyCourse
long bothStudentsHaveBiologyCourse;
long onlyLeftHasBiologyCourse;
long onlyRightHasBiologyCourse;
// ... and so on
#Override
public String toString() {
return "Counter{" + "\n" +
"\tbothStudentsHaveName = " + bothStudentsHaveName + "\n" +
"\t, onlyLeftHasName = " + onlyLeftHasName + "\n" +
"\t, onlyRightHasName = " + onlyRightHasName + "\n" +
"\t, bothStudentsHaveAddress = " + bothStudentsHaveAddress + "\n" +
"\t, onlyLeftHasAddress = " + onlyLeftHasAddress + "\n" +
"\t, onlyRightHasAddress = " + onlyRightHasAddress + "\n" +
"\t, bothStudentsHaveBiologyCourse = " + bothStudentsHaveBiologyCourse + "\n" +
"\t, onlyLeftHasBiologyCourse = " + onlyLeftHasBiologyCourse + "\n" +
"\t, onlyRightHasBiologyCourse = " + onlyRightHasBiologyCourse + "\n" +
'}';
}
}
Tester Class:
public class Tester {
public static void main(String[] args) throws Exception {
// Creating Dummy Variables
Student student1 = new Student();
student1.name = "Test";
student1.biologyCourse = "Yes";
Student student2 = new Student();
student2.name = "Test1";
student2.address = "abc street";
Counter counter = new Counter();
// Comparing Students
Solution solution = new Solution();
solution.compareStudents(student1, student2, counter);
// Printing Counter
System.out.println(counter);
}
}
Output:
Counter{
bothStudentsHaveName = 1
, onlyLeftHasName = 0
, onlyRightHasName = 0
, bothStudentsHaveAddress = 0
, onlyLeftHasAddress = 0
, onlyRightHasAddress = 1
, bothStudentsHaveBiologyCourse = 0
, onlyLeftHasBiologyCourse = 1
, onlyRightHasBiologyCourse = 0
}

If you keep repreating the same basic pattern of fields, then consider extracting that into a class. For example introduce a FieldComparison class that looks a little like this:
public class FieldComparisonCounter {
public int bothHave;
public int onlyLeftHas;
public int onlyRightHas;
// constructor, getters, setters left as an exercise for the reader
}
Then have a Map<String,FieldComparisonCounter> counters somewhere and a method like this:
public void compareField(String fieldName, String leftValue, String rightValue) {
FieldComparisonCounter counter = counters.get(fieldName);
if (counter == null) {
counter = new FieldComparisonCounter();
counters.put(fieldName, counter);
}
boolean leftHas = !StringUtils.isEmpty(leftValue);
boolean rightHas = !StringUtils.isEmpty(rightValue);
if (leftHas && rightHas) {
counter.bothHave++;
} else if (leftHas) {
counter.onlyLeftHas++;
} else if (rightHas) {
counter.onlyRightHas++;
}
}
Then adding a new field comparison is as simple as calling
compareField("name", left.name, right.name);

Queue not deleting elements (using linkedlist) [java]

I'm trying to get the hang of queues by making a very simple program that simulates a checkout line. For some reason my custom dequeue function (using a linkedlist's removeFirst()) wont actually delete stuff in the queue.
Heres the CustomerQueue Class
public class CustomerQueue<E> {
private LinkedList<E> list = new LinkedList<>();
public void enqueue(E e) { list.addLast(e); }
public E dequeue() { return list.removeFirst(); }
public int size() { return list.size(); }
public E element() { return list.getFirst(); }
#Override
public String toString() { return "Queue: " + list.toString(); }
}
Here is my main:
public static void main(String[] args) {
CustomerQueue<Customer> queue = new CustomerQueue<>();
final int totalTime = 720;
int totalServiced = 0;
int totalQueued = 0;
int totalArrived = 1;
int maxQueuedAtOnce = 0;
Customer next = new Customer();
queue.enqueue(next);
System.out.println(next.getATime() + " <-- ATIME STIME --> " + next.getSTime());
for (int minute = 0; minute < totalTime; minute++) {
System.out.println("-- Minute " + (minute + 1) + " ---------");
if (queue.element().getSTime() == 0) {
totalServiced++;
System.out.println("Current customer (" + queue.dequeue() + ") finished being serviced. (Removed)");
//queue.dequeue();
}
if (next.getATime() == 0) {
System.out.println("Customer ID=" + next.getID() + " has arrived.");
queue.enqueue(next);
System.out.println("Customer ID=" + next.getID() + " in queue.");
next = new Customer();
System.out.println("New Customer generated. ID=" + next.getID() + " ATIME: " + next.getATime() + " STIME: " + next.getSTime());
totalArrived++;
totalQueued++;
}
System.out.println("Customer ID=" + next.getID() + " arrival ticked down.");
next.tickArrivalDown();
queue.element().tickServiceDown();
System.out.println("Current queue customer ID=" + next.getID() + " ticked down.");
if (queue.size() > maxQueuedAtOnce)
maxQueuedAtOnce = queue.size();
}
System.out.println("Total Customers Generated " + next.getNumCustomers());
System.out.println("Total Customers Serviced: " + totalServiced);
System.out.println("Total Customers Arrived: " + totalArrived);
System.out.println("Maximum Customers Queued: " + maxQueuedAtOnce);
System.out.println(queue.element());
}
And of course the Customer Class:
public class Customer {
private int serviceTime;
private int arrivalTime;
private static int numCustomers = 0;
private int ID;
public int getSTime() { return serviceTime; }
public void setSTime(int t) { serviceTime = t; }
public int getATime() { return arrivalTime; }
public void setATime(int t) { arrivalTime = t; }
public int getID() { return ID; }
public static int getNumCustomers() { return numCustomers; }
Customer() {
serviceTime = (int) (Math.random()*3 + 1);
arrivalTime = (int) (Math.random()*3 + 1);
ID = ++numCustomers;
}
public void tickServiceDown() { serviceTime--; }
public void tickServiceUp() { serviceTime++; }
public void tickArrivalDown() { arrivalTime--; }
public void tickArrivalUp() { arrivalTime++; }
#Override
public String toString() {
return "ID: " + ID + " ArrivalTime: " + arrivalTime + " ServiceTime: " + serviceTime;
}
}
The Customer class I have setup generates its own arrival time and completion time when a Customer is instantiated. Basically at the end of the loop, the next customer's arrival time ticks down and the current customer's service time ticks down. When the current customer's service time hits 0, it should dequeue/remove the first element in the LinkedList. For some reason it isnt removing it. Here is the post-loop output:
Total Customers Generated 358
Total Customers Serviced: 1
Total Customers Arrived: 358
Maximum Customers Queued: 357
ID: 1 ArrivalTime: 0 ServiceTime: -717
I'm totally stumped, no amount of googling has helped.

Looking at your output
ID: 1 ArrivalTime: 0 ServiceTime: -717
and ServiceTime being what is being returned in getSTime()
then as 0 != -717 I think the below will be fine
How about if (queue.element().getSTime() <= 0) {

I figured out the answer. The initial if statement:
if (queue.element().getSTime() == 0) needed to become if (queue.size() > 0 && queue.element().getSTime() == 0)
Also needed to make the queue.element().tickServiceDown(); enclosed with the following if-statement if (queue.size() > 0)

Extracting Polynomial Coefficients From input String in JAVA

I made this code for extracting Polynomial coefficients and also evaluating equation in a point,and it is work.
but i want to modify that so the user can enter any shape of polynomial equation.
in my code you have to enter equation like this:
2*x^2+3*x^1+4
but i want :
2*x^5+1*x+6
also if there any term with same power , their coeffs must be added together.
Here is my code in java:
package Priest;
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.List;
public class Equation {
private String Eq;
private final String[] C;
private int Deg;
private final String EqHolder;
public Equation(String Equation) {
this.Eq = Equation;
EqHolder = Equation;
Eq = Eq.replaceAll("[^0-9\\-\\.]+", " ");
Eq = Eq.replaceAll("-", " -");
this.C = Eq.split(" ");
}
public String SourceEquation() {
return EqHolder.toUpperCase().replaceAll("\\*", "").replaceAll("[a-zA-Z]", "\\*(X)").replaceAll("\\+", "\\ + ").replaceAll("\\-", "\\ - ");
}
public List<BigDecimal> CaptureCoeff() {
getDegree();
List<BigDecimal> Temp = new ArrayList<>();
for (String S : C) {
Temp.add(new BigDecimal(S));
}
int Location = Temp.indexOf(BigDecimal.valueOf(Deg));
List<BigDecimal> Coeffs = new ArrayList<>();
for (int Counter = Location - 1; Counter < Temp.size(); Counter += 2) {
Coeffs.add(Temp.get(Counter));
}
return Coeffs;
}
public int getDegree() {
int Degree = 0;
for (int Counter = 0; Counter < C.length; Counter += 2) {
if ((new Double(C[Counter])) != 0) {
Degree = new Integer(C[Counter + 1]);
this.Deg = Degree;
break;
}
}
return Degree;
}
public BigDecimal Evaluate(List<BigDecimal> Coefficients, double EvalPoint) {
BigDecimal Output = BigDecimal.ZERO;
for (int Index = 0; Index < Coefficients.size(); Index++) {
Output = Output.add(Coefficients.get(Index).multiply(BigDecimal.valueOf(EvalPoint).pow(Deg--)));
}
return Output;
}
}
and main class:
package Priest;
import java.math.RoundingMode;
public class MainClass {
public static void main(String[] args) {
long Start = System.nanoTime();
String Str = "3.1415x^5-12.6x^4+6x^3+12*x^2-6*x^1-0";
Equation E = new Equation(Str);
System.out.println("Equation is: " + E.SourceEquation());
System.out.println("Coefficients :" + E.CaptureCoeff());
System.out.println("Polynomial Degree: " + E.getDegree());
double Target = 47.784;
System.out.println("Equation # (X:" + Target + ")= " + E.Evaluate(E.CaptureCoeff(), Target).setScale(15, RoundingMode.HALF_UP));
System.out.println("Elapsed Time: " + String.format("%.20G", (System.nanoTime() - Start) / 1.0e6) + " ms.");
}
}
the output:
run:
Equation is: 3.1415*(X)^5 - 12.6*(X)^4 + 6*(X)^3 + 12*(X)^2 - 6*(X)^1 - 0
Coefficients :[3.1415, -12.6, 6, 12, -6, 0]
Polynomial Degree: 5
Equation # (X:47.784)= 717609084.382589022327914
Elapsed Time: 32.306242000000000000 ms.
BUILD SUCCESSFUL (total time: 0 seconds)

Let's go with the following equation String Str2 = "3.1415x^5+6x^2+12*x-5";
Here is the code that I have added upon your code in order to preprocess this equation and made it compatible to your actual logic so that It will treat it without any major change to your code.
To be totally accurate I had to change the following in your equation class:
public List<BigDecimal> CaptureCoeff() {
getDegree();
List<BigDecimal> Temp = new ArrayList<BigDecimal>();
for (String S : C) {
if (! "".equals(S.trim())) {
Temp.add(new BigDecimal(S));
}
}
So I have added the control to check that none of these S strings is trim - empty.
Here is my preprocessing code.
I have added a method called powerSplitt that allows to splitt the equation on the basis of the '^' char.
Then I created another method called generateNullCoeffPolynomeWithDegree that generate a monome in the form 0*X^k. And a similar one that generate all the similar intermediate monomes between the greater power and the lesser power
Example:
String str3 = generateAllNullCoeffPolynomesWithDegreeExclusiveBetween(5, 2);
System.out.println("all poly = " + str3);
will generate: all poly = 0*x^4+0*x^3
Then I created a buildPreProcessedPolynome that takes the initial equation and pre process it to produce one with the null monomes inside of it. And then I just gave it to your equation program and it could process it fine!!!
Here is the code and a call example all done in the MainClass
import java.math.RoundingMode;
import java.util.ArrayList;
import java.util.List;
public class MainClass {
private static List<String> workList = new ArrayList<String>();
public static void powerSplitt(String equationText) {
char[] charsList = equationText.toCharArray();
boolean foundTargetChar = false;
int index = 0;
for (int i = 0; i < charsList.length; i++) {
index = i;
if (charsList[i] == '^') {
foundTargetChar = true;
break;
}
}
if (foundTargetChar) {
workList.add(equationText.substring(0, index));
if (index +1 < equationText.length()) {
powerSplitt(equationText.substring(index+1));
} else {
workList.add(equationText);
return;
}
} else {
workList.add(equationText);
}
}
public static String generateNullCoeffPolynomeWithDegree(int degree) {
return "0*x^" + degree;
}
public static String generateAllNullCoeffPolynomesWithDegreeExclusiveBetween(int startDegree, int endDegree) {
if (startDegree-endDegree <= 1) {
return "";
}
int index = 0;
StringBuilder builder = new StringBuilder();
for (int i = startDegree -1; i > endDegree; i--) {
if (index > 0) {
builder.append("+");
}
builder.append(generateNullCoeffPolynomeWithDegree(i));
index++;
}
return builder.toString();
}
public static String buildPreProcessedPolynome(String initialEquationText) {
workList.clear();
powerSplitt(initialEquationText);
StringBuilder resultBuilder = new StringBuilder();
assert workList.size() >= 3;
resultBuilder.append(workList.get(0));
for (int i = 1; i <= workList.size()-2; i++) {
int actualPower = Integer.parseInt( workList.get(i).substring(0,1));
int nextFoundPower = Integer.parseInt( workList.get(i+1).substring(0,1));
System.out.print("actual power = " + actualPower + " and next power = " + nextFoundPower);
System.out.println();
String additionalPolyParts = generateAllNullCoeffPolynomesWithDegreeExclusiveBetween(actualPower, nextFoundPower);
resultBuilder.append("^" + actualPower);
resultBuilder.append("+");
resultBuilder.append(additionalPolyParts);
resultBuilder.append(workList.get(i).substring(1));
}
resultBuilder.append("^" + workList.get(workList.size()-1));
return resultBuilder.toString();
}
public static void main(String[] args) {
workList.clear();
String Str2 = "3.1415x^5+6x^2+12*x-5";
powerSplitt(Str2);
for (String part: workList) {
System.out.println("PART:" + part);
}
System.out.println("-----------------");
long Start = System.nanoTime();
String str3 = generateAllNullCoeffPolynomesWithDegreeExclusiveBetween(5, 2);
System.out.println("all poly = " + str3);
String preprocessed = buildPreProcessedPolynome(Str2);
System.out.println("preprocessed = " + preprocessed);
System.out.println();
Equation E = new Equation(preprocessed);
System.out.println("Equation is: " + E.SourceEquation());
System.out.println("Coefficients :" + E.CaptureCoeff());
System.out.println("Polynomial Degree: " + E.getDegree());
double Target = 47.784;
System.out.println("Equation # (X:" + Target + ")= " + E.Evaluate(E.CaptureCoeff(), Target).setScale(15, RoundingMode.HALF_UP));
System.out.println("Elapsed Time: " + String.format("%.20G", (System.nanoTime() - Start) / 1.0e6) + " ms.");
}
}
And here is the produced results (I haved added some System.out.println to check the results of my methods calls. I just noticed I have to take into account the last constant as a monome of type K*X^0, but I will leave that to you):
PART:3.1415x
PART:5+6x
PART:2+12*x-5
all poly = 0*x^4+0*x^3
actual power = 5 and next power = 2
preprocessed = 3.1415x^5+0*x^4+0*x^3+6x^2+12*x-5
Equation is: 3.1415*(X)^5 + 0*(X)^4 + 0*(X)^3 + 6*(X)^2 + 12*(X) - 5
Coefficients :[3.1415, 0, 0, 6, 12]
Polynomial Degree: 5
Equation # (X:47.784)= 782631805.485054892561514
Elapsed Time: 18,441978000000000000 ms.

Topological graph sorting java

I've got some problems with the topological sorting. It can find lops, but it counts some of the tasks (or "nodes" if you want to call it) several times. I think the problem is something with how I read or the Edge class, but I just can't see where it goes wrong. Any help would be really appreciated :)
enter code here
import java.util.*;
import java.io.*;
import java.lang.*;
class Task {
int id, time, staff;
int depA, depB;
String name;
int eStart, lStart;
Edge outEdge;
int cntPredecessors;
boolean visited;
Task(int id, String name, int time, int staff) {
this.id = id;
this.name = name;
this.time = time;
this.staff = staff;
visited = false;
}
public String getName() {
return name;
}
public String toString() {
return name;
}
}
class Edge {
Task id, name, time, staff;
Edge neste;
Task fra, til;
Edge(Task id) {
this.id = id;
}
}
class Input {
public static void main(String[] args) {
if (args.length == 0) {
System.out.println("enter a filename!");
System.exit(1);
} else if (args.length == 1) {
String fil = args[0]+".txt";
LesFraFil(fil);
// skrivUt();
topSort();
} else {
System.out.println("too many parameters, try again...");
}
}
static int antTask;
static Task[] ids;
static int tTid;
static void LesFraFil(String fil) {
int i = 0;
int j;
try {
String lest;
Scanner in = new Scanner(new FileReader(fil));
Edge til;
int counter = 0;
antTask = in.nextInt();
ids = new Task[antTask];
System.out.println(antTask);
while (in.hasNextLine()) {
lest = in.nextLine();
// hvis tom linje, så hopper den over
if(lest.trim().length() == 0) continue;
String split[] = lest.split("\\s+");
int id = Integer.parseInt(split[0]);
String act = split[1];
int tid = Integer.parseInt(split[2]);
int staff = Integer.parseInt(split[3]);
int depA = Integer.parseInt(split[4]);
tTid += tid;
ids[i] = new Task(id, act, tid, staff);
j = 4;
/*
* Lesingen av inputen skal avbrytes når den leser 0.
* j er den som holder på hvor langt vi er i split arrayet
* når den møter på 0
*/
while(split[j].compareTo("0") != 0) {
int tmp = Integer.parseInt(split[j])-1;
// System.out.println(tmp+1 + " Aktivitetens navn : " + act); //+ " tiden aktiviteten tar tid: " + tid + " avhengihet: " + split[j]);
j++;
if (ids[tmp] == null) {
ids[tmp] = new Task(id, act, tid, staff);
ids[tmp].visited = true;
}
ids[i].cntPredecessors++;
if(ids[tmp].outEdge == null) {
ids[tmp].outEdge = new Edge(ids[i]);
} else {
til = ids[tmp].outEdge;
while(til.neste != null) {
til = til.neste;
}
til.neste = new Edge(ids[i]);
}
}
counter++;
i++;
}
if (antTask == counter) {
System.out.println("Lesinga gikk som planlagt av fil: " + fil);
System.out.println("Total arbeidstid: " + tTid);// + antTask + " == " + counter );
} else {
System.out.println("Noe gikk galt avslutter!");
System.out.println(antTask + " || " + counter);
System.exit(2);
}
in.close();
} catch (Exception e) {
System.err.println("ERROR!" + e.getMessage());
}
}
static void skrivUt() {
for (Task sort : ids) {
System.out.print(sort.id + " " + sort.name);
Edge til = sort.outEdge;
while (til != null) {
System.out.print(" " + til.id.id);
til = til.neste;
}
System.out.println();
}
}
static void topSort() {
LinkedList<Task> list = new LinkedList<Task>();
ArrayList<Task> array = new ArrayList<Task>();
Task temp;
int count = 0;
int totalTime = 0;
// Legger taskene i lista
for (Task t : ids) {
if(t.cntPredecessors == 0) {
list.add(t);
totalTime += t.time;
// System.out.println(t);
t.visited = true;
}
}
for (Task t : ids) {
if(t.cntPredecessors == 1) {
list.add(t);
totalTime += t.time;
// System.out.println(t);
t.visited = true;
}
}
// går i evig løkke til lista er tom.
while (!list.isEmpty()) {
temp = list.pop(); // fjerner elementet fra lista
array.add(temp); // legger inn i arraylisten
count++;
// System.out.println(temp);
for(Edge til = temp.outEdge; til!=null;til=til.neste) {
til.id.cntPredecessors--;
if(til.id.cntPredecessors==0) {
list.add(til.id);
}
}
}
if(count < antTask) {
System.out.println("A loop has been found. Terminating...");
System.exit(0);
}
System.out.println("Topological sort: " + Arrays.toString(array.toArray()));// den sorterte "arraylisten"
System.out.println("Total time spend: " + totalTime);
}
} // End class Input
Here is an example of an input file
8
1 Build-walls 4 2 5 0
2 Build-roofs 6 4 1 0
3 Put-on-wallpapers 1 2 1 2 0
4 Put-on-tiles 1 3 2 0
5 Build-foundation 4 2 0
6 Make-floor 2 2 5 0
7 Put-carpet-floor 4 2 6 2 0
8 Move-in 4 4 3 7 0

The problem is with this loop (inside topSort()):
for (Task t : ids) {
if(t.cntPredecessors == 1) {
list.add(t);
totalTime += t.time;
// System.out.println(t);
t.visited = true;
}
}
You just need to remove it.
Reason: this loop adds to list nodes that have 1 incoming edge. Later (in the while loop), it is possible that for these nodes the cntPredecessors field will be decreased to 0 which will make them being pushed back onto list, thus counted twice.
In the future, please try to narrow down your code to something that contains less "noise", that is: the smallset (or nearly smallest) code that illustrates the problem. This will ease the understanding on potential answerers (not to mention that it may help you see the problem yourself).

Is there a Java library that will create a number range from a list of numbers?

I am creating a table of contents, and what I have is a Map of product numbers to pages. So an entry might look like this:
ABC123 => [59, 58, 57, 19, 36, 15, 33, 34, 13, 39, 11, 37, 38, 21, 20, 40, 63, 60, 45, 46, 22, 23, 24, 26, 3, 2, 10, 1, 7, 6, 5, 4, 8]
What I want to get from this is:
1-8,10,11,13,15,19-24,26,33,34,36-38,40,45,46,57-60
I can code this of course, but I figured that someone else has already solved this problem. My Googling has yielded naught.
I appreciate any help you can offer, as always!

You could collect the numbers into a sorted set and then iterate over the numbers.
Quick and dirty example:
SortedSet<Integer> numbers = new TreeSet<Integer>();
numbers.add( 1 );
numbers.add( 2 );
numbers.add( 3 );
numbers.add( 6 );
numbers.add( 7 );
numbers.add( 10 );
Integer start = null;
Integer end = null;
for( Integer num : numbers ) {
//initialize
if( start == null || end == null ) {
start = num;
end = num;
}
//next number in range
else if( end.equals( num - 1 ) ) {
end = num;
}
//there's a gap
else {
//range length 1
if( start.equals( end )) {
System.out.print(start + ",");
}
//range length 2
else if ( start.equals( end - 1 )) {
System.out.print(start + "," + end + ",");
}
//range lenth 2+
else {
System.out.print(start + "-" + end + ",");
}
start = num;
end = num;
}
}
if( start.equals( end )) {
System.out.print(start);
}
else if ( start.equals( end - 1 )) {
System.out.print(start + "," + end );
}
else {
System.out.print(start + "-" + end);
}
Yields: 1-3,6,7,10

Apache Commons has the IntRange type that you can use. Unfortunately I didn't find a good corresponding set of utilities to create them. Here's the basic approach you could use:
//create a list of 1-integer ranges
List<IntRange> ranges = new LinkedList<IntRange>();
for ( int pageNum : pageNums ) {
ranges.add(new IntRange(pageNum));
}
//sort the ranges
Collections.sort(ranges, new Comparator<IntRange>() {
public int compare(IntRange a, IntRange b) {
return Integer.valueOf(a.getMinimumInteger()).compareTo(b.getMinimumInteger());
}
});
List<IntRange> output = new ArrayList<IntRange>();
if ( ranges.isEmpty() ) {
return output;
}
//collapse consecutive ranges
IntRange range = ranges.remove(0);
while ( !ranges.isEmpty() ) {
IntRange nextRange = ranges.remove(0);
if ( range.getMaximumInteger() == nextRange.getMinimumInteger() - 1 ) {
range = new IntRange(range.getMinimumInteger(), nextRange.getMaximumInteger());
} else {
output.add(range);
range = nextRange;
}
}
output.add(range);
Alternatively you could skip the first step and create the ranges directly from the sorted list of page numbers.

Edit: A better description:
I had to deal with something similar to support a sorted collection of finite ranges, I used a mix of Google's Guava Range class and binary search to insert the element at the corresponding range or create a new singleton Range (A range with 1 element), eventually with more inserts the ranges have chances of expanding (Or shrinking/splitting in case of removal), removal is pretty fast because locating the corresponding range where the element is uses a binary search:
import com.google.common.collect.DiscreteDomains;
import com.google.common.collect.Lists;
import com.google.common.collect.Range;
import com.google.common.collect.Ranges;
import java.util.Collection;
import java.util.List;
public class IntRangeCollection
{
private int factor=10;
private List<Range<Integer>> rangeList=null;
public IntRangeCollection()
{
rangeList=Lists.newArrayListWithExpectedSize(1000);
}
public IntRangeCollection(final int size)
{
rangeList=Lists.newArrayListWithExpectedSize(size);
}
public IntRangeCollection(final int size, final int factor)
{
rangeList=Lists.newArrayListWithExpectedSize(size);
this.factor=factor;
}
protected IntRangeCollection(final List<Range<Integer>> rangeList)
{
this.rangeList=rangeList;
}
public static IntRangeCollection buildIntRangesCollectionFromArrays(final List<Integer[]> arrays)
{
final List<Range<Integer>> rangeList=Lists.newArrayListWithCapacity(arrays.size());
for(Integer[] range : arrays){
rangeList.add(range.length == 1 ? Ranges.singleton(range[0]) : Ranges.closed(range[0], range[1]));
}
return new IntRangeCollection(rangeList);
}
public boolean addElements(final Collection<Integer> elements)
{
boolean modified=false;
for(Integer element : elements){
modified=addElement(element) || modified;
}
return modified;
}
public boolean removeElements(final Collection<Integer> elements)
{
boolean modified=false;
for(Integer element : elements){
modified=removeElement(element) || modified;
}
return modified;
}
public boolean addElement(final Integer element)
{
final Range<Integer> elementRange=Ranges.singleton(element);
if(rangeList.isEmpty()){
rangeList.add(elementRange);
} else{
int
start=0, mid=0,
end=rangeList.size() - 1;
Range<Integer> midRange=null;
while(start<=end){
mid=(start + end) / 2;
midRange=rangeList.get(mid);
if(midRange.contains(element)){
return false;
} else if(testLinkable(midRange, element)){
rangeList.set(mid, midRange.span(elementRange));
if(mid>0){
final Range<Integer> a=rangeList.get(mid - 1);
if(testLinkable(a, midRange)){
rangeList.set(mid - 1, a.span(midRange));
rangeList.remove(mid);
mid--;
}
}
if(mid<rangeList.size() - 1){
final Range<Integer> b=rangeList.get(mid + 1);
if(testLinkable(midRange, b)){
rangeList.set(mid, midRange.span(b));
rangeList.remove(mid + 1);
}
}
return true;
} else if(midRange.lowerEndpoint().compareTo(element)<0){
start=mid + 1;
} else{
end=mid - 1;
}
}
//noinspection ConstantConditions
rangeList.add(midRange.lowerEndpoint().compareTo(element)<0 ? mid + 1 : mid, elementRange);
}
return true;
}
public boolean removeElement(final Integer element)
{
final Range<Integer> elementRange=Ranges.singleton(element);
if(rangeList.isEmpty()){
rangeList.add(elementRange);
} else{
int
start=0, mid,
end=rangeList.size() - 1;
while(start<=end){
mid=(start + end) / 2;
final Range<Integer> midRange=rangeList.get(mid);
if(midRange.contains(element)){
final Integer
lower=midRange.lowerEndpoint(),
upper=midRange.upperEndpoint();
if(lower.equals(upper)){
rangeList.remove(mid);
} else if(lower.equals(element)){
rangeList.set(mid, Ranges.closed(element + 1, upper));
} else if(upper.equals(element)){
rangeList.set(mid, Ranges.closed(lower, element - 1));
} else{
rangeList.set(mid, Ranges.closed(element + 1, upper));
rangeList.add(mid, Ranges.closed(lower, element - 1));
}
return true;
} else if(midRange.lowerEndpoint().compareTo(element)<0){
start=mid + 1;
} else{
end=mid - 1;
}
}
}
return false;
}
public List<Integer> getElementsAsList()
{
final List<Integer> result=Lists.newArrayListWithExpectedSize(rangeList.size() * factor);
for(Range<Integer> range : rangeList){
result.addAll(range.asSet(DiscreteDomains.integers()));
}
return result;
}
public List<Integer[]> getRangesAsArray()
{
final List<Integer[]> result=Lists.newArrayListWithCapacity(rangeList.size());
for(Range<Integer> range : rangeList){
final Integer
lower=range.lowerEndpoint(),
upper=range.upperEndpoint();
result.add(lower.equals(upper) ? new Integer[]{lower} : new Integer[]{lower,upper});
}
return result;
}
public int getRangesCount()
{
return rangeList.size();
}
private boolean testLinkable(final Range<Integer> range, final Integer element)
{
return Ranges.closed(range.lowerEndpoint() - 1, range.upperEndpoint() + 1).contains(element);
}
private boolean testLinkable(final Range<Integer> a, final Range<Integer> b)
{
return Ranges.closed(a.lowerEndpoint() - 1, a.upperEndpoint() + 1).isConnected(b);
}
#Override
public String toString()
{
return "IntRangeCollection{" +
"rangeList=" + rangeList +
'}';
}
public static void main(String[] args)
{
final int MAX_NUMBER=1000;
final long startMillis=System.currentTimeMillis();
final IntRangeCollection ranges=new IntRangeCollection();
for(int i=0; i<MAX_NUMBER; i++){
//noinspection UnsecureRandomNumberGeneration
ranges.addElement((int) (Math.random() * MAX_NUMBER));
}
System.out.println(MAX_NUMBER + " contained in " + ranges.rangeList.size() + " ranges done in " + (System.currentTimeMillis() - startMillis) + "ms");
System.out.println(ranges);
for(int i=0; i<MAX_NUMBER / 4; i++){
//noinspection UnsecureRandomNumberGeneration
ranges.removeElement((int) (Math.random() * MAX_NUMBER));
}
System.out.println(MAX_NUMBER + " contained in " + ranges.rangeList.size() + " ranges done in " + (System.currentTimeMillis() - startMillis) + "ms");
System.out.println(ranges);
}
}

You can use Arrays.sort() and find neighbouring duplicates/ranges. However I suspect TreeSet may be simpler to use.

This is a good example, it shows a simple way to accomplish this.