Develop Reference

ArrayList and Passing values

I am new to programming and we just learned ArrayLists in my class today and I have an easy question for you guys, I just can't seem to find it in the notes on what to set the passing value equal to. The point of this practice program is to take in a Number Object (that class has already been created) and those Numbers in the ArrayList are supposed to be counted as odds, evens, and perfect numbers. Here is the first couple of lines of the program which is all you should need.
import java.util.ArrayList;
import static java.lang.System.*;
public class NumberAnalyzer {
private ArrayList<Number> list;
public NumberAnalyzer() {
list = new ArrayList<Number>();
}
public NumberAnalyzer(String numbers) {
}
public void setList(String numbers) {
}
What am I supposed to set (String numbers) to in both NumberAnalyzer() and setList()? Thanks in advance for answering a noob question!
NumberAnalyzer test = new NumberAnalyzer("5 12 9 6 1 4 8 6");
out.println(test);
out.println("odd count = "+test.countOdds());
out.println("even count = "+test.countEvens());
out.println("perfect count = "+test.countPerfects()+"\n\n\n");
This is the Lab16b Class that will run the program. ^^
public class Number
{
private Integer number;
public Number()
{
number = 0;
}
public Number(int num)
{
number = num;
}
public void setNumber(int num)
{
number = num;
}
public int getNumber()
{
return 0;
}
public boolean isOdd()
{
return number % 2 != 0;
}
public boolean isPerfect()
{
int total=0;
for(int i = 1; i < number; i++)
{
if(number % i == 0)
{
total = total + i;
}
}
if(total == number)
{
return true;
}
else
{
return false;
}
}
public String toString( )
{
return "";
}
}
Here is the Number class. ^^

Based on the information you provided, this is what I feel NumberAnalyzer should look like. The setList function is presently being used to take a String and add the numbers in it to a new list.
public class NumberAnalyzer {
private List<Number> list;
public NumberAnalyzer() {
this.list = new ArrayList<Number>();
}
public NumberAnalyzer(String numbers) {
setList(numbers);
}
public void setList(String numbers) {
String[] nums = numbers.split(" ");
this.list = new ArrayList<Number>();
for(String num: nums)
list.add(new Number(Integer.parseInt(num)));
}
}

Analyze to learn something.
public static void main(String[] args) {
String temp = "5 12 9 6 1 4 8 6";
NumberAnalyzer analyzer = new NumberAnalyzer(temp);
//foreach without lambda expressions
System.out.println("without Lambda");
for (NeverNumber i : analyzer.getList()) {
i.print();
}
//with the use of lambda expressions, which was introduced in Java 8
System.out.println("\nwith Lambda");
analyzer.getList().stream().forEach((noNumber) -> noNumber.print());
NeverNumber number = new NeverNumber(31);
number.print();
number.setNumber(1234);
number.print();
}
public class NumberAnalyzer {
private List<NeverNumber> list; //List is interface for ArrayList
public NumberAnalyzer(String numbers) {
String[] numb=numbers.split(" ");
this.list=new ArrayList<>();
for (String i : numb) {
list.add(new NeverNumber(Integer.parseInt(i)));
}
}
public void setList(List<NeverNumber> numbers) {
List<NeverNumber> copy=new ArrayList<>();
numbers.stream().forEach((i) -> {
copy.add(i.copy());
});
this.list=copy;
}
public List<NeverNumber> getList() {
List<NeverNumber> copy=new ArrayList<>();
this.list.stream().forEach((i) -> {
copy.add(i.copy());
});
return copy;
}
public NeverNumber getNumber(int index) {
return list.get(index).copy();
}
}
public class NeverNumber { //We do not use the names used in the standard library.
//In the library there is a class Number.
private int number; // If you can use simple types int instead of Integer.
public NeverNumber() {
number = 0;
}
public NeverNumber(int num) {
number = num;
}
private NeverNumber(NeverNumber nn) {
this.number=nn.number;
}
public void setNumber(int num) {
number = num;
}
public int getNumber() {
return this.number;
}
public boolean isOdd() {
return number % 2 != 0;
}
public boolean isPerfect() {
long end = Math.round(Math.sqrt(number)); //Method Math.sqrt(Number) returns a double, a method Math.round(double) returns long.
for (int i = 2; i < end + 1; i++) {
if (number % i == 0) {
return false;
}
}
return true;
}
public NeverNumber copy(){
return new NeverNumber(this);
}
public void print() {
System.out.println("for: " + this.toString() + " isPer: " + this.isPerfect() + " isOdd: " + this.isOdd() + "\n");
}
#Override //Every class in Java inherits from the Object class in which it is toString(),
//so we have to override our implementation.
public String toString() {
return this.number + ""; //The object of any class + "" creates a new object of the String class,
//that is for complex types, calls the toString () method implemented in this class,
//override the toString () from the Object class. If the runs, we miss class toString()
//calls from the Object class.
}
}

How to pass a comparator to the constructor of a class

A class has the following definition:
public class MinPQ<Key> implements Iterable<Key> {
private Key[] pq; // store items at indices 1 to N
private int N; // number of items on priority queue
private Comparator<Key> comparator; // optional comparator
public MinPQ(int initCapacity) {
pq = (Key[]) new Object[initCapacity + 1];
N = 0;
}
public MinPQ() { this(1); }
public MinPQ(int initCapacity, Comparator<Key> comparator) {
this.comparator = comparator;
pq = (Key[]) new Object[initCapacity + 1];
N = 0;
}
public MinPQ(Comparator<Key> comparator) { this(1, comparator); }
........
}
I want to define a variable by the following statements in another class, but there is error in the last line. I wonder why and how it should be defined. Thanks!
class Searchnode {
Board prebd;
Board curbd;
int moves;
}
class BoardComparator implements Comparator<Board> {
public int compare(Board bd1, Board bd2) {
if (bd1.manhattan() > bd2.manhattan())
return 1;
else if (bd1.manhattan() < bd2.manhattan())
return -1;
else
return 0;
}
}
private MinPQ<Searchnode> pq = new MinPQ<Searchnode>(new BoardComparator());

Just from a casual glance it looks like your MinPQ is MinPQ<Searchnode> and your comparator is Comparator<Board> and your class is defined as
public class MinPQ<Key> implements Iterable<Key>
{
public MinPQ(Comparator<Key> comparator) { this(1, comparator); }
}
So i would expect you to have to pass a comparator<SearchNode> to that constructor.

How to make my custom Iterator work with every List?

I have implemented a SubListIterator which is a small utility Iterator for iterating over sublists of a given list.
Suppose I have a List containing 13500 elements and I want to split it into 7 sublists and use them.
#Test
public void shouldSplitTheGivenListIntoSmallerLists() {
List<Long> given = new ArrayList<Long>();
for (int count = 0; count < 13500; count++) {
given.add(Long.valueOf(count));
}
List<List<Long>> actualSubLists = new ArrayList<List<Long>>();
for (List<Long> subList : SubListIterator.subList(given, 2000)) { // Line got compilation error
actualSubLists.add(subList);
}
assertEquals(7, actualSubLists.size());
}
Everything works well if I implement the SubListIterator directly with List<Long>.
Then I wanted to extend my SubListIterator to work with every List regardless their generic type, so I went changing List<Long> to List<?> and get the compilation error:
Type mismatch: cannot convert from element type List<?> to List<Long>
I tried with List<T> and it doesn't work either.
My question is: Is there anyway to achieve my goal which is making the SubListIterator to work with every List, not just List<Long>?
Below is the SubListIterator:
public class SubListIterator implements Iterator<List<?>>, Iterable<List<?>> {
public static SubListIterator subList(List<?> given, int itemsEachSubList) {
return new SubListIterator(given, itemsEachSubList);
}
private final List<?> whole;
private final int elementsEachPart;
private int fromIndex;
private int toIndex;
public SubListIterator(List<?> whole, int itemsEach) {
this.whole = whole;
this.elementsEachPart = itemsEach;
this.fromIndex = 0;
this.toIndex = elementsEachPart;
}
#Override
public boolean hasNext() {
return fromIndex < toIndex;
}
#Override
public List<?> next() {
List<?> nextSubList = whole.subList(fromIndex, toIndex);
fromIndex = toIndex;
toIndex = Math.min(toIndex + elementsEachPart, whole.size());
return nextSubList;
}
#Override
public void remove() {
throw new UnsupportedOperationException("This method is not supported");
}
#Override
public Iterator<List<?>> iterator() {
return this;
}
}
Thanks for your support

You need to generalize your SubListIterator.
public class SubListIterator<T> implements Iterator<List<T>>, Iterable<List<T>> {
public static <C> SubListIterator<C> subList(List<C> given, int itemsEachSubList) {
return new SubListIterator<C>(given, itemsEachSubList);
}
private final List<T> whole;
private final int elementsEachPart;
private int fromIndex;
private int toIndex;
public SubListIterator(List<T> whole, int itemsEach) {
this.whole = whole;
this.elementsEachPart = itemsEach;
this.fromIndex = 0;
this.toIndex = elementsEachPart;
}
#Override
public boolean hasNext() {
return fromIndex < toIndex;
}
#Override
public List<T> next() {
List<T> nextSubList = whole.subList(fromIndex, toIndex);
fromIndex = toIndex;
toIndex = Math.min(toIndex + elementsEachPart, whole.size());
return nextSubList;
}
#Override
public void remove() {
throw new UnsupportedOperationException("This method is not supported");
}
#Override
public Iterator<List<T>> iterator() {
return this;
}
}

Real time sorted by value, auto-discarding, bounded collection ?

I spent some time to try to make a collection that:
1) is sorted by value (not by key)
2) is sorted each time an element is added or modified
3) is fixed size and discard automatically smallest/biggest element depending of the sort way
4) is safe thread
So 3) and 4) I think it is quite ok. For 1) and 2) it was a bit more tricky. I spent quite a long time on this thread, experimenting the different sample, but one big issue is that the collection are sorted only once when object are inserted.
Anyway, I try to implement my own collection, which is working (shouldn't be used for huge data as it is sorted quite often) but I'm not so happy with the design. Especially in the fact that my value objects are constrained to be Observable (which is good) but not comparable so I had to use a dirty instanceof + exception for this.
Any sugestion to improve this ?
Here is the code:
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Observable;
import java.util.Observer;
public class SortedDiscardingSyncArray<K, V extends Observable> implements Observer {
// Comparison way (ascendent or descendant)
public static enum ComparisonWay
{
DESC,
ASC;
}
// this is backed by a List (and ArrayList impl)
private List<ArrayElement> array;
// Capacity, configurable, over this limit, an item will be discarded
private int MAX_CAPACITY = 200;
// default is descending comparison
private ComparisonWay compareWay = ComparisonWay.DESC;
public SortedDiscardingSyncArray(ComparisonWay compareWay, int mAX_CAPACITY) {
super();
this.compareWay = compareWay;
MAX_CAPACITY = mAX_CAPACITY;
array = new ArrayList <ArrayElement>(MAX_CAPACITY);
}
public SortedDiscardingSyncArray(int mAX_CAPACITY) {
super();
MAX_CAPACITY = mAX_CAPACITY;
array = new ArrayList<ArrayElement>(MAX_CAPACITY);
}
public SortedDiscardingSyncArray() {
super();
array = new ArrayList <ArrayElement>(MAX_CAPACITY);
}
public boolean put(K key, V value)
{
try {
return put (new ArrayElement(key, value, this));
} catch (Exception e) {
e.printStackTrace();
return false;
}
finally
{
sortArray();
}
}
private synchronized boolean put(ArrayElement ae)
{
if (array.size() < MAX_CAPACITY)
{
return array.add(ae);
}
// check if last one is greater/smaller than current value to insert
else if (ae.compareTo(array.get(MAX_CAPACITY-1)) < 0)
{
array.remove(MAX_CAPACITY - 1);
return array.add(ae);
}
// else we don't insert
return false;
}
public V getValue (int index)
{
return array.get(index).getValue();
}
public V getValue (K key)
{
for (ArrayElement ae : array)
{
if (ae.getKey().equals(key)) return ae.getValue();
}
return null;
}
public K getKey (int index)
{
return array.get(index).getKey();
}
private void sortArray()
{
Collections.sort(array);
}
public synchronized void setValue(K key, V newValue) {
for (ArrayElement ae : array)
{
if (ae.getKey().equals(key))
{
ae.setValue(newValue);
return;
}
}
}
public int size() {
return array.size();
}
#Override
public void update(java.util.Observable arg0, Object arg1) {
sortArray();
}
public static void main(String[] args) {
// some test on the class
SortedDiscardingSyncArray<String, ObservableSample> myData = new SortedDiscardingSyncArray<String, ObservableSample>(ComparisonWay.DESC, 20);
String Ka = "Ka";
String Kb = "Kb";
String Kc = "Kc";
String Kd = "Kd";
myData.put(Ka, new ObservableSample(0));
myData.put(Kb, new ObservableSample(3));
myData.put(Kc, new ObservableSample(1));
myData.put(Kd, new ObservableSample(2));
for (int i=0; i < myData.size(); i++)
{
System.out.println(myData.getKey(i).toString() + " - " + myData.getValue(i).toString());
}
System.out.println("Modifying data...");
myData.getValue(Kb).setValue(12);
myData.getValue(Ka).setValue(34);
myData.getValue(Kd).setValue(9);
myData.getValue(Kc).setValue(19);
for (int i=0; i < myData.size(); i++)
{
System.out.println(myData.getKey(i).toString() + " - " + myData.getValue(i).toString());
}
}
private class ArrayElement implements Comparable <ArrayElement> {
public ArrayElement(K key, V value, Observer obs) throws Exception {
super();
// don't know how to handle that case
// maybe multiple inheritance would have helped here ?
if (! (value instanceof Comparable)) throw new Exception("Object must be 'Comparable'");
this.key = key;
this.value = value;
value.addObserver(obs);
}
public String toString()
{
StringBuffer sb = new StringBuffer();
sb.append(key);
sb.append(" - ");
sb.append(value);
return sb.toString();
}
private K key;
private V value;
public K getKey() {
return key;
}
public V getValue() {
return value;
}
public synchronized void setValue(V value) {
this.value = value;
}
#SuppressWarnings("unchecked")
#Override
public int compareTo(ArrayElement o) {
int c;
if (compareWay == ComparisonWay.DESC) c = ((Comparable<V>) o.getValue()).compareTo(this.getValue());
else c = ((Comparable<V>) this.getValue()).compareTo(o.getValue());
if (c != 0) {
return c;
}
Integer hashCode1 = o.getValue().hashCode();
Integer hashCode2 = this.getValue().hashCode();
// we don't check the compare way for hash code (useless ?)
return hashCode1.compareTo(hashCode2);
}
}
}
And the other class for testing purpose:
import java.util.Observable;
public class ObservableSample extends Observable implements Comparable <ObservableSample>
{
private Integer value = 0;
public ObservableSample(int value) {
this.value = value;
setChanged();
notifyObservers();
}
public String toString()
{
return String.valueOf(this.value);
}
public void setValue(Integer value) {
this.value = value;
setChanged();
notifyObservers();
}
public Integer getValue() {
return value;
}
#Override
public int compareTo(ObservableSample o) {
int c;
c = (this.getValue()).compareTo(o.getValue());
if (c != 0) {
return c;
}
Integer hashCode1 = o.getValue().hashCode();
Integer hashCode2 = this.getValue().hashCode();
// we don't check the compare way for hash code (useless ?)
return hashCode1.compareTo(hashCode2);
}
}

Collections are difficult to write, maybe you should look for an existing implementation.
Try checking out ImmutableSortedSet from Guava.

You can have a marker interface
public interface ComparableObservable extends Observable, Comparable {
}
and then change
SortedDiscardingSyncArray<K, V extends Observable>
to
SortedDiscardingSyncArray<K, V extends ComparableObservable>
to avoid the explicit cast.
Other than that the code is quite verbose and I didn't follow it completely. I would also suggest having a look at guava or (apache) commons-collections library to explore if you can find something reusable.

You can write generic wildcards with multiple bounds. So change your declaration of <K, V extends Observable> to <K, V extends Observable & Comparable<V>> and then you can treat V as if it implements both interfaces, without an otherwise empty and useless interface.
Another few things: Pick a naming convention, and stick with it. The one I use is that a name such as MAX_CAPACITY would be used for a static final field (i.e. a constant, such as a default) and that the equivalent instance field would be maxCapacity Names such as mAX_CAPACITY would be right out of the question.
See: Oracle's naming conventions for Java
Instead of using a ComparisonWay enum, I would take a custom Comparator. Much more flexible, and doesn't replicate something that already exists.
See: the Comparator API docs
Your code, as written, is not thread safe. In particular an observed element calling the unsynchronized update method may thus invoke sortArray without obtaining the proper lock. FindBugs is a great tool that catches a lot of problems like this.
Your ObservableSample does not really follow good practices with regards to how it implements Comparable, in that it does not really compare data values but instead the hashCode. The hashCode is essentially arbitrary and collisions are quite possible. Additionally, the Comparable interface requests that usually you should be "consistent with Equals", for which you also might want to take a look at the documentation for the Object class's equals method
Yes, it sounds like a lot of work, but if you go through it and do it right you will save yourself astounding amounts of debugging effort down the road. If you do not do these properly and to the spec, you will find that when you place it in Sets or Maps your keys or values strangely disappear, reappear, or get clobbered. And it will depend on which version of Java you run, potentially!

Here is a version updated. Still not completly sure it is safe thread but findbugs tool didn't give so usefull tips. Also for the comparisonWay, I don't want to constraint the user to develop its own comparator, I want to keep the things simple.
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Observable;
import java.util.Observer;
public class SortedDiscardingSyncArray<K, V extends Observable & Comparable<V>> implements Observer {
// Comparison way (ascendent or descendant)
public static enum ComparisonWay { DESC, ASC; }
// this is backed by a List (and ArrayList)
private List<ArrayElement> array;
// Capacity, configurable, over this limit, an item will be discarded
private int maxCapacity = 200;
// default is descending comparison
private ComparisonWay compareWay = ComparisonWay.DESC;
public SortedDiscardingSyncArray(ComparisonWay compareWay, int maxCapacity) {
super();
this.compareWay = compareWay;
this.maxCapacity = maxCapacity;
array = new ArrayList <ArrayElement>(maxCapacity);
}
public SortedDiscardingSyncArray(int maxCapacity) {
super();
this.maxCapacity = maxCapacity;
array = new ArrayList<ArrayElement>(maxCapacity);
}
public SortedDiscardingSyncArray() {
super();
array = new ArrayList <ArrayElement>(maxCapacity);
}
// not synchronized, but calling internal sync put command
public boolean put(K key, V value)
{
try {
return put (new ArrayElement(key, value, this));
} catch (Exception e) {
e.printStackTrace();
return false;
}
finally
{
sortArray();
}
}
private synchronized boolean put(ArrayElement ae)
{
if (array.size() < maxCapacity) return array.add(ae);
// check if last one is greater/smaller than current value to insert
else if (ae.compareTo(array.get(maxCapacity-1)) < 0)
{
array.remove(maxCapacity - 1);
return array.add(ae);
}
// else we don't insert and return false
return false;
}
public V getValue (int index)
{
return array.get(index).getValue();
}
public V getValue (K key)
{
for (ArrayElement ae : array)
{
if (ae.getKey().equals(key)) return ae.getValue();
}
return null;
}
public K getKey (int index)
{
return array.get(index).getKey();
}
private synchronized void sortArray()
{
Collections.sort(array);
}
public synchronized void setValue(K key, V newValue) {
for (ArrayElement ae : array)
{
if (ae.getKey().equals(key))
{
ae.setValue(newValue);
return;
}
}
}
public int size() {
return array.size();
}
#Override
public void update(java.util.Observable arg0, Object arg1) {
sortArray();
}
public static void main(String[] args) {
// some test on the class
SortedDiscardingSyncArray<String, ObservableSample> myData = new SortedDiscardingSyncArray<String, ObservableSample>(ComparisonWay.DESC, 20);
String Ka = "Ka";
String Kb = "Kb";
String Kc = "Kc";
String Kd = "Kd";
myData.put(Ka, new ObservableSample(0));
myData.put(Kb, new ObservableSample(3));
myData.put(Kc, new ObservableSample(1));
myData.put(Kd, new ObservableSample(2));
for (int i=0; i < myData.size(); i++)
{
System.out.println(myData.getKey(i).toString() + " - " + myData.getValue(i).toString());
}
System.out.println("Modifying data...");
myData.getValue(Kb).setValue(12);
myData.getValue(Ka).setValue(34);
myData.getValue(Kd).setValue(9);
myData.getValue(Kc).setValue(19);
for (int i=0; i < myData.size(); i++)
{
System.out.println(myData.getKey(i).toString() + " - " + myData.getValue(i).toString());
}
}
private class ArrayElement implements Comparable <ArrayElement> {
public ArrayElement(K key, V value, Observer obs) throws Exception {
super();
this.key = key;
this.value = value;
value.addObserver(obs);
}
public String toString()
{
StringBuffer sb = new StringBuffer();
sb.append(key);
sb.append(" - ");
sb.append(value);
return sb.toString();
}
private K key;
private V value;
public K getKey() {
return key;
}
public V getValue() {
return value;
}
public synchronized void setValue(V value) {
this.value = value;
}
#Override
public int compareTo(ArrayElement o) {
int c;
if (compareWay == ComparisonWay.DESC) c = o.getValue().compareTo(this.getValue());
else c = this.getValue().compareTo(o.getValue());
if (c != 0) {
return c;
}
Integer hashCode1 = o.getValue().hashCode();
Integer hashCode2 = this.getValue().hashCode();
// we don't check the compare way for hash code (useless ?)
return hashCode1.compareTo(hashCode2);
}
}
}

writing a Comparator for a compound object for binary searching

I have a class, and list of instances, that looks something like this (field names changed to protect the innocent/proprietary):
public class Bloat
{
public long timeInMilliseconds;
public long spaceInBytes;
public long costInPennies;
}
public class BloatProducer
{
final private List<Bloat> bloatList = new ArrayList<Bloat>();
final private Random random = new Random();
public void produceMoreBloat()
{
int n = bloatList.size();
Bloat previousBloat = (n == 0) ? new Bloat() : bloatList.get(n-1);
Bloat newBloat = new Bloat();
newBloat.timeInMilliseconds =
previousBloat.timeInMilliseconds + random.nextInt(10) + 1;
newBloat.spaceInBytes =
previousBloat.spaceInBytes + random.nextInt(10) + 1;
newBloat.costInPennies =
previousBloat.costInPennies + random.nextInt(10) + 1;
bloatList.add(newBloat);
}
/* other fields/methods */
public boolean testMonotonicity()
{
Bloat previousBloat = null;
for (Bloat thisBloat : bloatList)
{
if (previousBloat != null)
{
if ((previousBloat.timeInMilliseconds
>= thisBloat.timeInMilliseconds)
|| (previousBloat.spaceInBytes
>= thisBloat.spaceInBytes)
|| (previousBloat.costInPennies
>= thisBloat.costInPennies))
return false;
}
previousBloat = thisBloat;
}
return true;
}
BloatProducer bloatProducer;
The list bloatList is kept internally by BloatProducer and is maintained in such a way that it only appends new Bloat records, does not modify any of the old ones, and each of the fields is monotonically increasing, e.g. bloatProducer.testMonotonicity() would always return true.
I would like to use Collections.binarySearch(list,key,comparator) to search for the Bloat record by either the timeInMilliseconds, spaceInBytes, or costInPennies fields. (and if the number is between two records, I want to find the previous record)
What's the easiest way to write a series of 3 Comparator classes to get this to work? Do I have to use a key that is a Bloat object with dummy fields for the ones I'm not searching for?

You'll need to write a separate comparator for each field you want to compare on:
public class BloatTimeComparator implements Comparator<Bloat> {
public int compare(Bloat bloat1, Bloat bloat2) {
if (bloat1.timeInMilliseconds > bloat2.timeInMilliseconds) {
return 1;
} else if (bloat1.timeInMilliseconds < bloat2.timeInMilliseconds) {
return -1;
} else {
return 0;
}
}
}
And so on for each property in Bloat you want to compare on (you'll need to create a comparator class for each). Then use the Collections helper method:
Collections.binarySearch(bloatList, bloatObjectToFind,
new BloatTimeComparator());
From the Java documentation for the binarySearch method, the return value will be:
the index of the search key, if it is contained in the list; otherwise, (-(insertion point) - 1). The insertion point is defined as the point at which the key would be inserted into the list: the index of the first element greater than the key, or list.size() if all elements in the list are less than the specified key. Note that this guarantees that the return value will be >= 0 if and only if the key is found.
Which is the index you specified that you wanted.

You will need to have 3 separate Comparators if you want to search by each of the 3 properties.
A cleaner option would be to have a generic Comparator which receives a parameter which tells it by which field to compare.
A basic generic comparator should look something like this:
public class BloatComparator implements Comparator<Bloat>
{
CompareByEnum field;
public BloatComparator(CompareByEnum field) {
this.field = field;
}
#Override
public int compare(Bloat arg0, Bloat arg1) {
if (this.field == CompareByEnum.TIME){
// compare by field time
}
else if (this.field == CompareByEnum.SPACE) {
// compare by field space
}
else {
// compare by field cost
}
}
}

Here's a test-driven approach to writing the first comparator:
public class BloatTest extends TestCase{
public class Bloat {
public long timeInMilliseconds;
public long spaceInBytes;
public long costInPennies;
public Bloat(long timeInMilliseconds, long spaceInBytes, long costInPennies) {
this.timeInMilliseconds = timeInMilliseconds;
this.spaceInBytes = spaceInBytes;
this.costInPennies = costInPennies;
}
}
public void testMillisecondComparator() throws Exception {
Bloat a = new Bloat(5, 10, 10);
Bloat b = new Bloat(3, 12, 12);
Bloat c = new Bloat(5, 12, 12);
Comparator<Bloat> comparator = new MillisecondComparator();
assertTrue(comparator.compare(a, b) > 0);
assertTrue(comparator.compare(b, a) < 0);
assertEquals(0, comparator.compare(a, c));
}
private static class MillisecondComparator implements Comparator<Bloat> {
public int compare(Bloat a, Bloat b) {
Long aTime = a.timeInMilliseconds;
return aTime.compareTo(b.timeInMilliseconds);
}
}
}

If you want to leverage the binary search for all three properties, you have to create comparators for them and have additional Lists or TreeSets sorted by the comparators.

test program (MultiBinarySearch.java) to see if these ideas work properly (they appear to):
package com.example.test;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.Random;
class Bloat
{
final public long timeInMilliseconds;
final public long spaceInBytes;
final public long costInPennies;
static final private int N = 100;
public Bloat(long l1, long l2, long l3) {
timeInMilliseconds = l1;
spaceInBytes = l2;
costInPennies = l3;
}
public Bloat() { this(0,0,0); }
public Bloat moreBloat(Random r)
{
return new Bloat(
timeInMilliseconds + r.nextInt(N) + 1,
spaceInBytes + r.nextInt(N) + 1,
costInPennies + r.nextInt(N) + 1
);
}
public String toString() {
return "[bloat: time="+timeInMilliseconds
+", space="+spaceInBytes
+", cost="+costInPennies
+"]";
}
static int compareLong(long l1, long l2)
{
if (l2 > l1)
return -1;
else if (l1 > l2)
return 1;
else
return 0;
}
public static class TimeComparator implements Comparator<Bloat> {
public int compare(Bloat bloat1, Bloat bloat2) {
return compareLong(bloat1.timeInMilliseconds, bloat2.timeInMilliseconds);
}
}
public static class SpaceComparator implements Comparator<Bloat> {
public int compare(Bloat bloat1, Bloat bloat2) {
return compareLong(bloat1.spaceInBytes, bloat2.spaceInBytes);
}
}
public static class CostComparator implements Comparator<Bloat> {
public int compare(Bloat bloat1, Bloat bloat2) {
return compareLong(bloat1.costInPennies, bloat2.costInPennies);
}
}
enum Type {
TIME(new TimeComparator()),
SPACE(new SpaceComparator()),
COST(new CostComparator());
public Comparator<Bloat> comparator;
Type(Comparator<Bloat> c) { this.comparator = c; }
}
}
class BloatProducer
{
final private List<Bloat> bloatList = new ArrayList<Bloat>();
final private Random random = new Random();
public void produceMoreBloat()
{
int n = bloatList.size();
Bloat newBloat =
(n == 0) ? new Bloat() : bloatList.get(n-1).moreBloat(random);
bloatList.add(newBloat);
}
/* other fields/methods */
public boolean testMonotonicity()
{
Bloat previousBloat = null;
for (Bloat thisBloat : bloatList)
{
if (previousBloat != null)
{
if ((previousBloat.timeInMilliseconds
>= thisBloat.timeInMilliseconds)
|| (previousBloat.spaceInBytes
>= thisBloat.spaceInBytes)
|| (previousBloat.costInPennies
>= thisBloat.costInPennies))
return false;
}
previousBloat = thisBloat;
}
return true;
}
public int searchBy(Bloat.Type t, Bloat key)
{
return Collections.binarySearch(bloatList, key, t.comparator);
}
public void showSearch(Bloat.Type t, Bloat key)
{
System.out.println("Search by "+t+": ");
System.out.println(key);
int i = searchBy(t,key);
if (i >= 0)
{
System.out.println("matches");
System.out.println(bloatList.get(i));
}
else
{
System.out.println("is between");
i = -i-1;
Bloat b1 = (i == 0) ? null : bloatList.get(i-1);
System.out.println(b1);
Bloat b2 = (i >= bloatList.size()) ? null : bloatList.get(i);
System.out.println("and");
System.out.println(b2);
}
}
}
public class MultiBinarySearch {
private static int N = 1000;
public static void main(String[] args)
{
BloatProducer bloatProducer = new BloatProducer();
for (int i = 0; i < N; ++i)
{
bloatProducer.produceMoreBloat();
}
System.out.println("testMonotonicity() returns "+
bloatProducer.testMonotonicity());
Bloat key;
key = new Bloat(10*N, 20*N, 30*N);
bloatProducer.showSearch(Bloat.Type.COST, key);
bloatProducer.showSearch(Bloat.Type.SPACE, key);
bloatProducer.showSearch(Bloat.Type.TIME, key);
key = new Bloat(-10000, 0, 1000*N);
bloatProducer.showSearch(Bloat.Type.COST, key);
bloatProducer.showSearch(Bloat.Type.SPACE, key);
bloatProducer.showSearch(Bloat.Type.TIME, key);
}
}