Develop Reference

Compare Lists in ArrayList

I have a text file containing the following strings (which are versions of a software):
1_10_2_0_154
3_10_5_2_10
2_10_4_1
3_10_5_1_37
I'm trying to find the most recent version, in this case 3_10_5_2_10 is the version that I'm trying to display using java.
For the moment, here is my code:
BufferedReader br;
String version;
ArrayList<List<Integer>> array = new ArrayList<List<Integer>>();
List<Integer> liste = new ArrayList<Integer>();
try{
br = new BufferedReader(new FileReader(new File(FILEPATH)));
while((version= br.readLine()) != null)
{
liste = Arrays.asList(version.split("_")).stream().
map(s -> Integer.parseInt(s.trim())).collect(Collectors.toList());
array.add(liste);
}
for(int i = 0; i < array.size(); i++)
{
for (List l: array)
{
Object z = l.get(i);
List<Object> listes = new ArrayList<Object>();
listes.add(z);
System.out.println(listes);
}
}
br.close();
System.out.println(array);
}catch(FileNotFoundException e){
e.printStackTrace();
}catch(IOException e){
e.printStackTrace();
}
I made a loop to save strings to ArrayList> like:
[[1,10,2,0,154] , [3,10,5,2,10], [2,10,4,1], [3,10,5,1,37]]
I want to get the elements of each list and compare them to find the most biggest one (most recent one) but I don't know to do that..

I sugguest you a object approach, define a class named Version with compareTo method, then using method sort on Collections class you can simply sort your versions.
Advantages
Clean and Clear code
Data validation
Main:
public class Main {
public static void main(String[] args){
List<Version> versions = Arrays.asList(
Version.create("1_10_2_0_154"),
Version.create("3_10_5_2_10"),
Version.create("2_10_4_1_49"),
Version.create("3_10_5_1_37"));
versions.sort(Version::compareTo);
System.out.println(versions.get(0).toString());
}
}
Version:
public class Version implements Comparable<Version> {
private final int major;
private final int minor;
private final int bug;
private final int release;
private final int build;
public Version(int major, int minor, int bug, int release, int build) {
this.major = major;
this.minor = minor;
this.bug = bug;
this.release = release;
this.build = build;
}
public int getMajor() {
return major;
}
public int getMinor() {
return minor;
}
public int getBug() {
return bug;
}
public int getRelease() {
return release;
}
public int getBuild() {
return build;
}
#Override
public String toString() {
return "Version{" +
"major=" + major +
", minor=" + minor +
", bug=" + bug +
", release=" + release +
", build=" + build +
'}';
}
public static Version create(String value){
String[] splitRes = value.split("_");
List<Integer> intValues = new ArrayList<>();
for(String v : splitRes){
intValues.add(Integer.parseInt(v));
}
return create(intValues);
}
public static Version create(List<Integer> values){
if(Objects.requireNonNull(values).size() < 5)
throw new IllegalArgumentException();
return new Version(
values.get(0),
values.get(1),
values.get(2),
values.get(3),
values.get(4)
);
}
#Override
public int compareTo(Version that) {
if (this.major > that.major) {
return -1;
} else if (this.major < that.major) {
return 1;
}
if (this.minor > that.minor) {
return -1;
} else if (this.minor < that.minor) {
return 1;
}
if (this.bug > that.bug) {
return -1;
} else if (this.bug < that.bug) {
return 1;
}
if (this.release > that.release) {
return -1;
} else if (this.release < that.release) {
return 1;
}
if (this.build > that.build) {
return -1;
} else if (this.build < that.build) {
return 1;
}
return 0;
}
}
UPDATE 1
As suggested by #Henrik i updated the list sorting with a Java 8 approach.
UPDATE 2
I reversed the compareTo method so now you can simply do plain sort calling sort method on list and passing method reference Version::compareTo
UPDATE 3
A more dynamic solution for Version class:
public class Version implements Comparable<Version> {
private final List<Integer> values;
public Version(List<Integer> values) {
this.values = values;
}
public List<Integer> getValues() {
return values;
}
#Override
public String toString() {
return String.join("_", values
.stream()
.map(Object::toString)
.collect(Collectors.toList()));
}
#Override
public int compareTo(Version that) {
List<Integer> thatValues = that.getValues();
for(int index = 0; index < values.size(); index++){
Integer value = values.get(index);
Integer thatValue = thatValues.get(index);
if (value > thatValue) {
return -1;
} else if (value < thatValue) {
return 1;
}
}
return 0;
}
public static Version create(String value){
String[] splitRes = value.split("_");
List<Integer> intValues = new ArrayList<>();
for(String v : splitRes){
intValues.add(Integer.parseInt(v));
}
return new Version(intValues);
}
}

You can write a Comparator to compare two Lists
Comparator<List<Integer>> comparator = (list1, list2) -> {
Iterator<Integer> iteratorA = list1.iterator();
Iterator<Integer> iteratorB = list2.iterator();
//It iterates through each list looking for an int that is not equal to determine which one precedes the other
while (iteratorA.hasNext() && iteratorB.hasNext()) {
int elementA = iteratorA.next();
int elementB = iteratorB.next();
if (elementA > elementB) {
return 1;
} else if (elementA < elementB) {
return -1;
}
}
//All elements seen so far are equal. Use the list size to decide
return iteratorA.hasNext() ? 1 : iteratorB.hasNext() ? -1 : 0;
};
You can sort it as
Collections.sort(list, comparator);
EDIT: You can refer to David Geirola's answer to convert the version string as a POJO and move the comparator logic inside that. But that is highly tied/coupled to the input string format. My solution works for any List<List<Integer>>.

A simple object oriented approach would be to create object, representing version number, let's call it VersionNumber, which would have a constructor of a factory method that does the parsing of the string. This VersionNumber class should implement interface Comparable and implement method compareTo.
Here is a hint for using Comparable Why should a Java class implement comparable?
Then you can easily write an algorithm to find the max version or google some library that would do it for you.

It is not optimized but should work. You can use both of comparators.
static List<String> versions = Arrays.asList(
"1_10_2_0_154",
"3_10_5_2_10",
"2_10_4_1_49",
"3_10_5_1_37");
static Comparator<List<Integer>> c = (o1,o2) -> {
int length = o1.size()>o2.size()?o2.size():o1.size();
for (int i = 0; i < length; i++) {
int i1 = o1.get(i);
int i2 = o2.get(i);
if (i1 != i2)
return i1 - i2;
}
return 0;
};
static Comparator<List<Integer>> c2 = (o1,o2) -> {
Iterator<Integer> i1=o1.iterator();
Iterator<Integer> i2=o2.iterator();
while (i1.hasNext() && i2.hasNext()){
int i = i1.next()-i2.next();
if (i!=0) return i;
}
return 0;
};
static Optional<List<Integer>> getTheMostRecentVersion(List<String> versions) {
return versions.stream().
map(s -> Arrays.stream(s.split("_")).
map(Integer::parseInt).
collect(Collectors.toList())).max(c2);
}

I think that this text file could be very big and it is better to compare each line on the fly (instead of store all line into collection to sort it after):
public static String getMostRecentVersion(BufferedReader in) throws IOException {
final Comparator<String[]> version = (s1, s2) -> {
int res = 0;
for (int i = 0; i < 5 && res == 0; i++)
res = Integer.compare(Integer.parseInt(s1[i]), Integer.parseInt(s2[i]));
return res;
};
String str;
String resStr = null;
String[] resPparts = null;
while ((str = in.readLine()) != null) {
String[] parts = str.split("_");
if (resStr == null || version.compare(parts, resPparts) > 0) {
resStr = str;
resPparts = parts;
}
}
return resStr;
}

A general ListComparator should help.
static class ListComparator<T extends Comparable<T>> implements Comparator<List<T>> {
#Override
public int compare(List<T> o1, List<T> o2) {
for (int i = 0; i < Math.max(o1.size(), o2.size()); i++) {
int diff =
// Off the end of both - same.
i >= o1.size() && i >= o2.size() ? 0
// Off the end of 1 - the other is greater.
: i >= o1.size() ? -1
: i >= o2.size() ? 1
// Normal diff.
: o1.get(i).compareTo(o2.get(i));
if (diff != 0) {
return diff;
}
}
return 0;
}
}
private static final Comparator<List<Integer>> BY_VERSION = new ListComparator<Integer>().reversed();
public void test(String[] args) {
String[] tests = {
"1_10_2_0_154",
"3_10_5_2_10",
"2_10_4_1_49",
"3_10_5_1_37",
"3_10_5_1_37_0"
};
System.out.println("Before: " + Arrays.toString(tests));
System.out.println("After: " + Arrays.stream(tests)
// Split into parts.
.map(s -> s.split("_"))
// Map String[] to List<Integer>
.map(a -> Arrays.stream(a).map(s -> Integer.valueOf(s)).collect(Collectors.toList()))
// Sort it.
.sorted(BY_VERSION)
// Back to a new list.
.collect(Collectors.toList()));
}

slap your arrays together into a number then just do number comparison.
class Scratch
{
public static void main(String[] args)
{
List<List<Integer>> arr = new ArrayList<>();
arr.add(fromArray(new Integer[]{1,10,2,0,154}));
arr.add(fromArray(new Integer[]{3,10,5,2,10}));
arr.add(fromArray(new Integer[]{2,10,4,1,49}));
arr.add(fromArray(new Integer[]{3,10,5,1,37}));
Integer[] maxLengths = {0,0,0,0,0};
for (List<Integer> v : arr)
{
for(int idx = 0; idx < v.size(); idx++)
{
Integer n = v.get(idx);
int curMaxLen = maxLengths[idx];
maxLengths[idx] = Math.max(n.toString().length(), curMaxLen);
}
}
Long largest = arr.stream().map(v -> {
StringBuilder result = new StringBuilder();
for(int idx = 0; idx < v.size(); idx++)
{
Integer n = v.get(idx);
int maxLen = maxLengths[idx];
result.append(String.format("%-" + maxLen + 's', n).replace(' ', '0'));
}
return result.toString();
}).map(Long::valueOf).max(Comparator.naturalOrder()).get();
System.out.println(largest);
}
public static List<Integer> fromArray(Integer[] array)
{
List<Integer> list = new ArrayList<>();
Collections.addAll(list, array);
return list;
}
}

n-ary Cartesian product inRxJava

Now I hold an Observable<Observable<Integer>, how can I transfer it into Observable<int[]> that contains the n-ary Cartesian product?
For example:
Observable<Observable<Integer> ob = Observable.just(
Observable.just(0,1),
Observable.just(2,3),
Observable.just(4,5)
);
ob...... -> (0,2,4), (0,3,4), (0,2,5), (0,3,5), (1,2,4), (1,3,4), (1,2,5), (1,3,5)

First of all, you need a fixed number of input Observables. Second, there is no need for blocking but there is likely the need for caching because the 2nd, 3rd etc Observables need to be consumed multiple times.
import java.util.*;
import io.reactivex.Observable;
public class Cartesian {
static Observable<int[]> cartesian(Observable<Observable<Integer>> sources) {
return sources.toList().flatMapObservable(list -> cartesian(list));
}
static Observable<int[]> cartesian(List<Observable<Integer>> sources) {
if (sources.size() == 0) {
return Observable.<int[]>empty();
}
Observable<int[]> main = sources.get(0).map(v -> new int[] { v });
for (int i = 1; i < sources.size(); i++) {
int j = i;
Observable<Integer> o = sources.get(i).cache();
main = main.flatMap(v -> {
return o.map(w -> {
int[] arr = Arrays.copyOf(v, j + 1);
arr[j] = w;
return arr;
});
});
}
return main;
}
public static void main(String[] args) {
cartesian(Observable.just(
Observable.just(0, 1),
Observable.just(2, 3),
Observable.just(4, 5)
))
.subscribe(v -> System.out.println(Arrays.toString(v)));
}
}

Creating Cartesian product in an asynchronous way is hard or in some sense is impossible. And if blocking is OK, you can do something like this
public class Main
{
static class ProductIterator<T> implements Iterator<T[]>
{
private final List<List<T>> componentsList;
private final Class<T> componentClass;
private final int[] indices;
private boolean hasNext;
public ProductIterator(List<List<T>> componentsList, Class<T> componentClass)
{
this.componentsList = componentsList;
this.componentClass = componentClass;
this.indices = new int[componentsList.size()];
this.hasNext = this.indices[componentsList.size() - 1] < componentsList.get(componentsList.size() - 1).size();
}
#Override
public boolean hasNext()
{
return hasNext;
}
#Override
public T[] next()
{
T[] res = (T[]) Array.newInstance(componentClass, componentsList.size());
for (int i = 0; i < componentsList.size(); i++)
{
res[i] = componentsList.get(i).get(indices[i]);
}
// move next
indices[0]++;
for (int i = 0; i < componentsList.size() - 1; i++)
{
if (indices[i] == componentsList.get(i).size())
{
indices[i] = 0;
indices[i + 1]++;
}
}
hasNext = indices[componentsList.size() - 1] < componentsList.get(componentsList.size() - 1).size();
return res;
}
}
public static <T> Observable<T[]> product(Observable<Observable<T>> components, Class<T> componentClass)
{
return Observable.fromIterable(new Iterable<T[]>()
{
#Override
public Iterator<T[]> iterator()
{
// postpone blocking up until iterator is requested
// and by this point we can't postpone anymore
Single<List<List<T>>> componentsList = components.map(o -> o.toList().blockingGet()).toList();
return new ProductIterator<T>(componentsList.blockingGet(), componentClass);
}
});
}
public static void main(String[] args) throws Exception
{
Observable<Observable<Integer>> ob = Observable.just(
Observable.just(0, 1),
Observable.just(2, 3),
Observable.just(4, 5)
);
Observable<Integer[]> product = product(ob, Integer.class);
product.forEach(a -> System.out.println(Arrays.toString(a)));
}
}
It is possible to improve this code to avoid blocking but you still will have to cache all results from all Observables and code will be much more complicated. Most probably is blocking is not acceptable for you, trying to get Cartesian product is bad idea anyway.

Well, I can resolve it myself. But is there any more elegant way?
(The toArray method convert an Observable<T> to T[])
Observable<int[]> toObservableArray(Observable<Observable<Integer>> obs) {
List<int[]> list = obs.map(ob -> toArray(ob)).toList().toBlocking().last();
return Observable.create(new SyncOnSubscribe<int[], int[]>() {
#Override
protected int[] generateState() {
int[] array = new int[list.size()];
Arrays.fill(array, 0);
return array;
}
#Override
protected int[] next(int[] state, Observer<? super int[]> observer) {
int[] next = new int[list.size()];
for (int i = 0; i < next.length; i++) {
next[i] = list.get(i)[state[i]];
}
observer.onNext(next);
state[state.length - 1]++;
for (int i = state.length - 1; i >= 0; i--) {
int delta = list.get(i).length - state[i];
if (delta > 0) {
break;
} else if (delta == 0) {
state[i] = 0;
if (i == 0) {
observer.onCompleted();
break;
}
state[i - 1]++;
}
}
return state;
}
});
}

How to design a test interface that times only some part of the code?

I want to time how long my code takes to perform on average over many test runs. In each test run, doWork() performs the work that I want to time. But I also want to checkWork() in each test run without it counting towards the time. I'm going to have many similar Exercise# classes, so I'd like to abstract the testing via a TestInterface. Is my current way a reasonable approach? Or is there a better design pattern / standard approach? Thanks in advance.
#FunctionalInterface
public interface TestInterface {
void test(final int NUM_TESTS);
}
public class TimeTests {
public static void test(TestInterface ti, final int NUM_TESTS, String testName) {
DecimalFormat df = new DecimalFormat("#.####");
long start = System.nanoTime();
ti.test(NUM_TESTS);
System.out.println("DEBUG: " + testName + " took "
+ df.format((System.nanoTime() - start) * 1.0 / NUM_TESTS)
+ " nanoseconds on average for " + NUM_TESTS + " tests");
}
}
public class Exercise1 {
private static final int NUM_TESTS = (int) Math.pow(10, 6);
private static void mainWork(List<Integer> A) {
// do stuff and time it
}
private static void checkWork(List<Integer> A) {
// do stuff but don't count it towards the time
}
public static void main(String[] args) {
TimeTests.test((NUM_TESTS_LOCAL) -> {
for (int i = 0; i < NUM_TESTS_LOCAL; ++i) {
List<Integer> A = new ArrayList<>();
// add random elements to A
mainWork(A);
checkWork(A);
}
}, NUM_TESTS, "Exercise1");
}
}

Okay, I think I managed to put together a decent framework (is this the right word?) for this task. If anybody could chime in to let me know if my approach is any good, I'd really appreciate it.
While my code seems to work fine for my use cases so far, I have a few questions:
In the interface definition of public interface CloneableTestInput<T extends CloneableTestInput<T>>, how is the type template <T extends CloneableTestInput<T> not a circular definition? I'm not sure I fully understand what that type template is saying.
Is there a way to make a generic CloneableList class that implements CloneableTestInput<List>? Currently, I need to make a separate implementation for each Collection type (e.g. ArrayList, LinkedList, ...). Similarly, is it possible to make a generic CloneableSet class that implements CloneableTestInput<Set>?
Thanks in advance :)
Testing Framework
Part I - An interface for test inputs
This allows TimeTests.java to work for generic input types.
public interface CloneableTestInput<T extends CloneableTestInput<T>> extends Cloneable {
T clone();
}
public class CloneableString implements CloneableTestInput<CloneableString> {
public String data;
public CloneableString() {}
public CloneableString(String input) { data = input; }
public CloneableString clone() { return new CloneableString(String.valueOf(data)); }
}
public class CloneableArrayList extends ArrayList implements CloneableTestInput<CloneableArrayList> {
public CloneableArrayList(ArrayList input) {
this.addAll(input);
}
#Override
public CloneableArrayList clone() {
return new CloneableArrayList(this);
}
}
Part II - An interface for timing tests
#FunctionalInterface
public interface TimeTestsInterface<outputType> {
void test(Callable<CloneableTestInput> formInput
, Function<CloneableTestInput, outputType> runAlgorithm
, Function<CloneableTestInput, outputType> getKnownOutput
, BiFunction<outputType, outputType, Boolean> checkResults
, final int NUM_TESTS, String testName);
}
public class TimeTests<outputType> implements TimeTestsInterface<outputType> {
public void test(Callable<CloneableTestInput> formInput
, Function<CloneableTestInput, outputType> runAlgorithm
, Function<CloneableTestInput, outputType> getKnownOutput
, BiFunction<outputType, outputType, Boolean> checkResults
, final int NUM_TESTS, String testName) {
try {
DecimalFormat df = new DecimalFormat("#.####");
long total = 0, start;
for (int i=0; i < NUM_TESTS; ++i) {
CloneableTestInput input = formInput.call();
CloneableTestInput orig_input = input.clone();
start = System.nanoTime();
outputType algorithmResult = runAlgorithm.apply(input);
total += System.nanoTime() - start;
outputType expectedResult = getKnownOutput.apply(orig_input);
assert(checkResults.apply(algorithmResult, expectedResult));
}
System.out.println("DEBUG: " + testName + " took "
+ df.format(total * 1.0 / NUM_TESTS)
+ " nanoseconds on average for " + NUM_TESTS + " tests");
} catch (Exception|AssertionError e) {
System.out.println(e.toString() + " - " + e.getMessage() + " - ");
e.printStackTrace();
}
}
}
Example Usages
Increment a BigInteger (uses CloneableArrayList)
/**
* Problem 6.2 from EPI
* Given an array A of digits encodiing a decimal number D,
* with MSD at A[0]. Update A to hold D + 1.
*/
public class PlusOne {
private static final int NUM_TESTS = (int) Math.pow(10, 5);
private static final int ARR_LENGTH = (int) Math.pow(10, 2);
private static ArrayList<Integer> plusOne(ArrayList<Integer> A) {
int n = A.size() - 1;
A.set(n, A.get(n) + 1);
for (int i = n; i > 0 && A.get(i) == 10; --i) {
A.set(i, 0);
A.set(i-1, A.get(i-1) + 1);
}
if (A.get(0) == 10) {
// Need additional digit up front as MSD
A.set(0,0);
A.add(0,1);
}
return A;
}
private static ArrayList<Integer> randArray(int len) {
ArrayList<Integer> A = new ArrayList<>();
if (len == 0) return A;
Random rgen = new Random();
A.add(rgen.nextInt(9) + 1);
--len;
while (len != 0) {
A.add(rgen.nextInt(10));
--len;
}
return A;
}
public static void main(String[] args) {
Callable<CloneableTestInput> formInput = () -> new CloneableArrayList(randArray(ARR_LENGTH));
Function<CloneableTestInput, ArrayList<Integer>> runAlgorithm =
(input) -> plusOne((ArrayList<Integer>) input);
Function<CloneableTestInput, ArrayList<Integer>> getKnownOutput =
(orig_input) -> {
BigInteger B = new BigInteger(Joiner.on("").join((ArrayList<Integer>) orig_input));
B = B.add(BigInteger.valueOf(1));
ArrayList<Integer> expectedOutput = new ArrayList<>();
while (B.compareTo(BigInteger.valueOf(0)) > 0) {
expectedOutput.add(0, B.mod(BigInteger.valueOf(10)).intValue());
B = B.divide(BigInteger.valueOf(10));
}
return expectedOutput;
};
BiFunction<ArrayList<Integer>, ArrayList<Integer>, Boolean> checkResults = List::equals;
TimeTests<ArrayList<Integer>> algTimer = new TimeTests<>();
algTimer.test(formInput, runAlgorithm, getKnownOutput, checkResults, NUM_TESTS, "PlusOne");
}
}
Can String be rearranged as a palindrome? (uses CloneableString)
public class CanStringBePalindrome {
private static final int INPUT_STRING_LENGTH = (int) Math.pow(10, 2);
private static final int NUM_TESTS = (int) Math.pow(10, 6);
private static boolean canFormPalindromeHash(final String s) {
Map<Character, Integer> charFreqs = new HashMap<>();
for (int i = 0; i < s.length(); ++i) {
char c = s.charAt(i);
if (!charFreqs.containsKey(c))
charFreqs.put(c, 1);
else
charFreqs.put(c, charFreqs.get(c) + 1);
}
int oddFreqCount = 0;
for (Map.Entry<Character, Integer> entry : charFreqs.entrySet()) {
if ((entry.getValue() % 2) != 0 && (++oddFreqCount > 1))
return false;
}
return true;
}
private static boolean canFormPalindromeSorting(final String s) {
// TODO : find faster/simpler way of getting frequency counts
char[] a = s.toCharArray();
Arrays.sort(a);
int oddFreqCount = 0;
int numCurrChar =1;
for (int i = 1; i < a.length && oddFreqCount <= 1; ++i) {
if(a[i] != a[i-1]) {
if ((numCurrChar & 1) != 0)
++oddFreqCount;
numCurrChar = 1;
} else
++numCurrChar;
}
if ((numCurrChar & 1) != 0)
++oddFreqCount;
return oddFreqCount <= 1;
}
private static String randString(int len) {
StringBuilder sb = new StringBuilder();
Random rgen = new Random();
while (len-- > 0)
sb.append((char)(rgen.nextInt(26) + 'A'));
return sb.toString();
}
public static void main(String[] args) {
Callable<CloneableTestInput> formInput = () -> new CloneableString(randString(INPUT_STRING_LENGTH));
Function<CloneableTestInput, Boolean > runAlgorithm =
(input) -> canFormPalindromeHash(((CloneableString)input).data);
Function<CloneableTestInput, Boolean> getKnownOutput =
(orig_input) -> canFormPalindromeSorting(((CloneableString)orig_input).data);
BiFunction<Boolean, Boolean, Boolean> checkResults = Boolean::equals;
TimeTests<Boolean> algTimer = new TimeTests<>();
algTimer.test(formInput, runAlgorithm, getKnownOutput, checkResults
, NUM_TESTS, "CanStringBePalindrome");
}
}

Division of a task to threads - multi threading

I want to generate pairs from a given large pool of numbers. I am using two for loops and threads. My function getAllPairs() in the code generates apairs with a given array of numbers.
I have an array of length 1000. With one thread, output time is nearly 15 sec. Now I want to use 5-6 threads and reduce this output time.I am stuck at dividing this task equally to five threads.If not threads,how to decrease the output time?
Solution with threads is appreciated since I put a lot of time learning multithreading. I would like to implement it.
import java.util.*;
class Pair {
public int x, y;
public Pair(int x, int y) {
this.x = x;
this.y = y;
}
#Override
public String toString(){
return " ( " + x + " ," + y + " ) " ;
}
}
class selectPairs{
private int[] array;
private List<Pair> totalPairs ;
public selectPairs(int[] arr){
array = arr;
}
//set Method
public void settotalPairs(List<Pair> pieces){
totalPairs = pieces;
}
//get Method
public List<Pair> gettotalPairs(){
return totalPairs;
}
// Method to generate pairs
public List<Pair> getAllPairs() {
List<Pair> pairs = new ArrayList<Pair>();
int total = array.length;
for(int i=0; i < total; i++) {
int num1 = array[i];
for(int j=i+1; j < total; j++) {
int num2 = array[j];
pairs.add(new Pair(num1,num2));
}
}
return pairs;
}
}
// Thread class
class ThreadPairs extends Thread {
private Thread t;
selectPairs SP;
ThreadPairs(selectPairs sp){
SP = sp;
}
public void run() {
synchronized(SP) {
List<Pair> PAIRS = SP.getAllPairs();
SP.settotalPairs(PAIRS);
}
}
}
public class TestThread {
public static void main(String args[]) {
int[] a = new int[1000];
for (int i = 0; i < a.length; i++) {
a[i] = i ;
}
selectPairs ob = new selectPairs(a);
ThreadPairs T = new ThreadPairs( ob );
T.start();
while (true) {
try {
T.join();
break;
}
catch(Exception e){
}
}
List<Pair> Total = new ArrayList<Pair>() ;
List<Pair> Temp1 = ob.gettotalPairs();
Total.addAll(Temp1);
System.out.println(Total);
}
}

A solution with a thread-pool, a task split strategy and it collects all results:
public class SelectPairs {
private static final int NUM_THREADS = 8;
private int[] array;
public SelectPairs(int[] arr) {
array = arr;
}
// A splitting task strategy
public List<Pair> getPartialPairs(int threadIndex, int numThreads) {
List<Pair> pairs = new ArrayList<Pair>();
int total = array.length;
for (int i = threadIndex; i < total; i += numThreads) {
int num1 = array[i];
for (int j = i + 1; j < total; j++) {
int num2 = array[j];
pairs.add(new Pair(num1, num2));
}
}
return pairs;
}
// To use Callables or Runnables are better than extends a Thread.
public static class PartialPairsCall implements Callable<List<Pair>> {
private int thread;
private int totalThreads;
private SelectPairs selectPairs;
public PartialPairsCall(int thread, int totalThreads, SelectPairs selectPairs) {
this.thread = thread;
this.totalThreads = totalThreads;
this.selectPairs = selectPairs;
}
#Override
public List<Pair> call() throws Exception {
return selectPairs.getPartialPairs(thread, totalThreads);
}
}
public static void main(String[] args) throws Exception {
int[] a = new int[1000];
for (int i = 0; i < a.length; i++) {
a[i] = i;
}
SelectPairs sp = new SelectPairs(a);
// Create a thread pool
ExecutorService es = Executors.newFixedThreadPool(NUM_THREADS);
List<Future<List<Pair>>> futures = new ArrayList<>(NUM_THREADS);
// Submit task to every thread:
for (int i = 0; i < NUM_THREADS; i++) {
futures.add(es.submit(new PartialPairsCall(i, NUM_THREADS, sp)));
}
// Collect the results:
List<Pair> result = new ArrayList<>(a.length * (a.length - 1));
for (Future<List<Pair>> future : futures) {
result.addAll(future.get());
}
// Shutdown thread pool
es.shutdown();
System.out.println("result: " + result.size());
}
}

regarding the framework of multithreading, you can implement ThreadPoolExecutor as was suggested in a comment.
Regarding splitting the workload, it seems that the key is splitting the iteration on the array which is achievable if you give the Runnable task a start and end index to iterate over.

Can't locate the problems in this simple Genetic Algorithm program

I have written a simple genetic algorithm program in java. What it is does is maximize the decimal value represented by the bits in the chromosome. Somehow mutation is not working as expected, e.g. causing two genes to mutate when just one is to change. The print statements I have included there show which to mutate, but in addition to that some more chromosomes get mutated. I can't figure out what the problem is :-(
Here are my java classes.
Gene.java
public class Gene {
private int value;
public Gene() {
value = Math.random() < 0.5 ? 0 : 1;
}
public Gene(int value) {
if (value != 0 && value != 1) {
throw new IllegalArgumentException("value must be either 0 or 1");
}
else {
this.value = value;
}
}
public void mutate() {
value = 1 - value;
}
public int value() {
return value;
}
#Override
public String toString() {
return String.valueOf(value);
}
}
Chromosome.java
import java.util.ArrayList;
import java.util.List;
public class Chromosome implements Comparable {
private ArrayList<Gene> genes;
private final int chromosomeLength;
public Chromosome(int length) {
this.genes = new ArrayList<>();
this.chromosomeLength = length > 0 ? length : 16;
for (int i = 0; i < chromosomeLength; i++) {
this.genes.add(i, new Gene());
}
}
public List<Gene> getAllele(int fromIndex, int toIndex) {
return new ArrayList<>(genes.subList(fromIndex, toIndex));
}
public void setAllele(int fromIndex, List<Gene> allele) {
int lastIndex = fromIndex + allele.size();
if (lastIndex > chromosomeLength) {
throw new IndexOutOfBoundsException("the allele exceeds beyond the size of the chromosome");
}
for (int i = fromIndex, j = 0; i < lastIndex; i++, j++) {
genes.set(i, allele.get(j));
}
}
public int getChromosomeLength() {
return chromosomeLength;
}
public void setGeneAt(int index, Gene gene) {
genes.set(index, gene);
}
public Gene getGeneAt(int index) {
return genes.get(index);
}
public int value() {
return Integer.parseInt(this.toString(), 2);
}
#Override
public String toString() {
StringBuilder chromosome = new StringBuilder("");
genes.stream().forEach((Gene g) -> chromosome.append(g));
return chromosome.toString();
}
#Override
public int compareTo(Object anotherChromosome) {
Chromosome c = (Chromosome) anotherChromosome;
return this.value() - c.value();
}
}
GenePool.java
import java.util.ArrayList;
import java.util.Arrays;
public class GenePool {
private final ArrayList<Chromosome> genePool;
private final int genePoolSize;
private final int chromosomeLength;
private final double crossOverRate;
private final double mutationRate;
private int[] crossPoints;
public GenePool(int numOfChromosome, int chromosomeLength, double crossOverRate, double mutationRate) {
this.genePoolSize = numOfChromosome;
this.chromosomeLength = chromosomeLength > 0 ? chromosomeLength : 16;
this.crossOverRate = crossOverRate;
this.mutationRate = mutationRate;
crossPoints = new int[1];
crossPoints[0] = this.chromosomeLength / 2;
genePool = new ArrayList<>();
for (int i = 0; i < numOfChromosome; i++) {
genePool.add(new Chromosome(chromosomeLength));
}
}
public int getGenePoolSize() {
return genePoolSize;
}
public Chromosome getChromosomeAt(int index) {
return genePool.get(index);
}
public void setChromosomeAt(int index, Chromosome c) {
genePool.set(index, c);
}
public int getChromosomeLength() {
return chromosomeLength;
}
public Chromosome[] crossOver(Chromosome c1, Chromosome c2) {
Chromosome[] offsprings = new Chromosome[2];
offsprings[0] = new Chromosome(c1.getChromosomeLength());
offsprings[1] = new Chromosome(c1.getChromosomeLength());
Chromosome[] parentChromosomes = {c1, c2};
int selector = 0;
for (int i = 0, start = 0; i <= crossPoints.length; i++) {
int crossPoint = i == crossPoints.length ? c1.getChromosomeLength() : crossPoints[i];
offsprings[0].setAllele(start, parentChromosomes[selector].getAllele(start, crossPoint));
offsprings[1].setAllele(start, parentChromosomes[1 - selector].getAllele(start, crossPoint));
selector = 1 - selector;
start = crossPoint;
}
return offsprings;
}
public void mutateGenePool() {
int totalGeneCount = genePoolSize * chromosomeLength;
System.out.println("Mutating genes:");
for (int i = 0; i < totalGeneCount; i++) {
double prob = Math.random();
if (prob < mutationRate) {
System.out.printf("Chromosome#: %d\tGene#: %d\n", i / chromosomeLength, i % chromosomeLength);
genePool.get(i / chromosomeLength).getGeneAt(i % chromosomeLength).mutate();
}
}
System.out.println("");
}
public int getLeastFitIndex() {
int index = 0;
int min = genePool.get(index).value();
int currentValue;
for (int i = 1; i < genePoolSize; i++) {
currentValue = genePool.get(i).value();
if (currentValue < min) {
index = i;
min = currentValue;
}
}
return index;
}
public void saveFittest(ArrayList<Chromosome> offsprings) {
// sort in ascending order
offsprings.sort(null);
offsprings.stream().forEach((offspring) -> {
int leastFitIndex = getLeastFitIndex();
if (offspring.value() > genePool.get(leastFitIndex).value()) {
genePool.set(leastFitIndex, offspring);
}
});
}
public void evolve(int noOfGeneration) {
for (int generation = 1; generation <= noOfGeneration; generation++) {
System.out.println("Generation :" + generation);
ArrayList<Integer> selection = new ArrayList<>();
for (int i = 0; i < genePoolSize; i++) {
if (Math.random() <= crossOverRate) {
selection.add(i);
}
}
if (selection.size() % 2 == 1) {
selection.remove(selection.size() - 1);
}
ArrayList<Chromosome> offsprings = new ArrayList<>();
for (int i = 0; i < selection.size(); i += 2) {
int index1 = selection.get(i);
int index2 = selection.get(i + 1);
offsprings.addAll(Arrays.asList(crossOver(genePool.get(index1), genePool.get(index2))));
}
System.out.println("Before saving the offsprings");
displayChromosomes(genePool, "GenePool");
displayChromosomes(offsprings, "Offsprings");
saveFittest(offsprings);
System.out.println("Before mutation:");
displayChromosomes(genePool, "GenePool");
mutateGenePool();
System.out.println("After mutation:");
displayChromosomes(genePool, "GenePool");
System.out.println("\n\n");
}
}
public void displayChromosomes(ArrayList<Chromosome> geneList, String name) {
System.out.println(name);
if (geneList.isEmpty()) {
System.out.println("Empty list");
}
geneList.stream().forEach((c) -> {
System.out.println(c + " -> " + c.value());
});
System.out.println("");
}
}
GADemo.java
public class GADemo {
public static void main(String[] args) {
GenePool gp = new GenePool(6, 8, 0.25, 0.01);
gp.evolve(10);
}
}
After evolving for a number of generations, the chromosomes all tend to become exactly the same, or very similar. And the problem is that that value is not the maximum for that many bits, and sometimes even a small value. For example, for 8 bits the values should (tend to) approach 255, but this doesn't do so in my code. Someone please provide a hint where/how to look for and solve the problem.

Focus on these lines and imagine the references. These are from setAllele()
for (int i = fromIndex, j = 0; i < lastIndex; i++, j++) {
genes.set(i, allele.get(j));
}
You are basically copying the reference from one onto the other. They are the same Gene so whatever mutation you do on those genes, will also affect even other Chromosomes.
You must produce a deep copy here.

Initially each chromosome has an own list of genes. But when you do the crossover operation you set gene objects from one chromosome into the gene list of other chromosome.
When you evolve the system, the number of shared genes will rise and therefore ultimately all chromosomes will share the same genes. No matter how you mutate a gene the chromosomes are not affected.
EDIT:
As Incognito also answered the setAllele method seems to be the culprit where gene sharing starts. You may want to introduce a method in the gene class where you can set its value given another gene.