In Python, the enumerate function allows you to iterate over a sequence of (index, value) pairs. For example:
>>> numbers = ["zero", "one", "two"]
>>> for i, s in enumerate(numbers):
... print i, s
...
0 zero
1 one
2 two
Is there any way of doing this in Java?
For collections that implement the List interface, you can call the listIterator() method to get a ListIterator. The iterator has (amongst others) two methods - nextIndex(), to get the index; and next(), to get the value (like other iterators).
So a Java equivalent of the Python above might be:
import java.util.ListIterator;
import java.util.List;
List<String> numbers = Arrays.asList("zero", "one", "two");
ListIterator<String> it = numbers.listIterator();
while (it.hasNext()) {
System.out.println(it.nextIndex() + " " + it.next());
}
which, like the Python, outputs:
0 zero
1 one
2 two
I find this to be the most similar to the python approach.
Usage
public static void main(String [] args) {
List<String> strings = Arrays.asList("zero", "one", "two");
for(EnumeratedItem<String> stringItem : ListUtils.enumerate(strings)) {
System.out.println(stringItem.index + " " + stringItem.item);
}
System.out.println();
for(EnumeratedItem<String> stringItem : ListUtils.enumerate(strings, 3)) {
System.out.println(stringItem.index + " " + stringItem.item);
}
}
Output
0 zero
1 one
2 two
3 zero
4 one
5 two
Features
Works on any iterable
Does not create an in-memory list copy (suitable for large lists)
Supports native for each syntax
Accepts a start parameter which can be added to the index
Implementation
import java.util.Iterator;
public class ListUtils {
public static class EnumeratedItem<T> {
public T item;
public int index;
private EnumeratedItem(T item, int index) {
this.item = item;
this.index = index;
}
}
private static class ListEnumerator<T> implements Iterable<EnumeratedItem<T>> {
private Iterable<T> target;
private int start;
public ListEnumerator(Iterable<T> target, int start) {
this.target = target;
this.start = start;
}
#Override
public Iterator<EnumeratedItem<T>> iterator() {
final Iterator<T> targetIterator = target.iterator();
return new Iterator<EnumeratedItem<T>>() {
int index = start;
#Override
public boolean hasNext() {
return targetIterator.hasNext();
}
#Override
public EnumeratedItem<T> next() {
EnumeratedItem<T> nextIndexedItem = new EnumeratedItem<T>(targetIterator.next(), index);
index++;
return nextIndexedItem;
}
};
}
}
public static <T> Iterable<EnumeratedItem<T>> enumerate(Iterable<T> iterable, int start) {
return new ListEnumerator<T>(iterable, start);
}
public static <T> Iterable<EnumeratedItem<T>> enumerate(Iterable<T> iterable) {
return enumerate(iterable, 0);
}
}
Strictly speaking, no, as the enumerate() function in Python returns a list of tuples, and tuples do not exist in Java.
If however, all you're interested in is printing out an index and a value, then you can follow the suggestion from Richard Fearn & use nextIndex() and next() on an iterator.
Note as well that enumerate() can be defined using the more general zip() function (using Python syntax):
mylist = list("abcd")
zip(range(len(mylist)), mylist)
gives [(0, 'a'), (1, 'b'), (2, 'c'), (3, 'd')]
If you define your own Tuple class (see Using Pairs or 2-tuples in Java as a starting point), then you could certainly easily write your own zip() function in Java to make use of it (using the Tuple class defined in the link):
public static <X,Y> List<Tuple<X,Y>> zip(List<X> list_a, List<Y> list_b) {
Iterator<X> xiter = list_a.iterator();
Iterator<Y> yiter = list_b.iterator();
List<Tuple<X,Y>> result = new LinkedList<Tuple<X,Y>>();
while (xiter.hasNext() && yiter.hasNext()) {
result.add(new Tuple<X,Y>(xiter.next(), yiter.next()));
}
return result;
}
And once you have zip(), implementing enumerate() is trivial.
Edit: slow day at work, so to finish it off:
public static <X> List<Tuple<Integer,X>> enumerate (List<X> list_in) {
List<Integer> nums = new ArrayList<Integer>(list_in.size());
for (int x = 0; x < list_in.size(); x++) {
nums.add(Integer.valueOf(x));
}
return zip (nums, list_in);
}
Edit 2: as pointed out in the comments to this question, this is not entirely equivalent. While it produces the same values as Python's enumerate, it doesn't do so in the same generative fashion that Python's enumerate does. Thus for large collections this approach could be quite prohibitive.
Simple and straightforward
public static <T> void enumerate(Iterable<T> iterable, java.util.function.ObjIntConsumer<T> consumer) {
int i = 0;
for(T object : iterable) {
consumer.accept(object, i);
i++;
}
}
Sample usage:
void testEnumerate() {
List<String> strings = Arrays.asList("foo", "bar", "baz");
enumerate(strings, (str, i) -> {
System.out.println(String.format("Index:%d String:%s", i, str));
});
}
According to the Python docs (here), this is the closest you can get with Java, and it's no more verbose:
String[] numbers = {"zero", "one", "two"}
for (int i = 0; i < numbers.length; i++) // Note that length is a property of an array, not a function (hence the lack of () )
System.out.println(i + " " + numbers[i]);
}
If you need to use the List class...
List<String> numbers = Arrays.asList("zero", "one", "two");
for (int i = 0; i < numbers.size(); i++) {
System.out.println(i + " " + numbers.get(i));
}
*NOTE: if you need to modify the list as you're traversing it, you'll need to use the Iterator object, as it has the ability to modify the list without raising a ConcurrentModificationException.
Now with Java 8s Stream API together with the small ProtonPack library providing StreamUtils it can be achieved easily.
The first example uses the same for-each notation as in the question:
Stream<String> numbers = Arrays.stream("zero one two".split(" "));
List<Indexed<String>> indexedNumbers = StreamUtils.zipWithIndex(numbers)
.collect(Collectors.toList());
for (Indexed<String> indexed : indexedNumbers) {
System.out.println(indexed.getIndex() + " " + indexed.getValue());
}
Above although does not provide the lazy evaluation as in Python.
For that you must use the forEach() Stream API method:
Stream<String> numbers = Arrays.stream("zero one two".split(" "));
StreamUtils.zipWithIndex(numbers)
.forEach(n -> System.out.println(n.getIndex() + " " + n.getValue()));
The lazy evaluation can be verified with the following infinite stream:
Stream<Integer> infStream = Stream.iterate(0, i -> i++);
StreamUtils.zipWithIndex(infStream)
.limit(196)
.forEach(n -> System.out.println(n.getIndex() + " " + n.getValue()));
No. Maybe there are some libraries for supporting such a functionality. But if you resort to the standard libraries it is your job to count.
List<String> list = { "foo", "bar", "foobar"};
int i = 0;
for (String str : list){
System.out.println(i++ + str );
}
I think this should be the java functionality that resemble the python "enumerate" most, though it is quite complicated and inefficent. Basically, just map the list's indices to its elements, using ListIterator or Collector:
List<String> list = new LinkedList<>(Arrays.asList("one", "two", "three", "four"));
Map<Integer, String> enumeration = new Map<>();
ListIterator iter = list.listIterator();
while(iter.hasNext){
map.put(iter.nextIndex(), iter.next());
}
or using lambda expression:
Set<Integer, String> enumeration = IntStream.range(0, list.size()).boxed.collect(Collectors.toMap(index -> index, index -> list.get(index)));
then you can use it with an enhanced for loop:
for (Map.Entry<Integer, String> entry : enumeration.entrySet){
System.out.println(entry.getKey() + "\t" + entry.getValue());
}
By combining generics with anonymous interfaces, you can essentially create a factory method for handing enumeration. The Enumerator callback hides the messiness of the iterator underneath.
import java.util.Arrays;
import java.util.List;
import java.util.ListIterator;
public class ListUtils2 {
public static interface Enumerator<T> {
void execute(int index, T value);
};
public static final <T> void enumerate(final List<T> list,
final Enumerator<T> enumerator) {
for (ListIterator<T> it = list.listIterator(); it.hasNext();) {
enumerator.execute(it.nextIndex(), it.next());
}
}
public static final void enumerate(final String[] arr,
final Enumerator<String> enumerator) {
enumerate(Arrays.asList(arr), enumerator);
}
public static void main(String[] args) {
String[] names = { "John", "Paul", "George", "Ringo" };
enumerate(names, new Enumerator<String>() {
#Override
public void execute(int index, String value) {
System.out.printf("[%d] %s%n", index, value);
}
});
}
}
Result
[0] John
[1] Paul
[2] George
[3] Ringo
Extended Thoughts
Map, Reduce, Filter
I have taken this a step further and created map, reduce, and filter functions based on this concept.
Both Google's Guava and Apache common-collections dependencies include similar functionality. You can check them out as you wish.
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.ListIterator;
public class ListUtils {
// =========================================================================
// Enumerate
// =========================================================================
public static abstract interface Enumerator<T> {
void execute(int index, T value, List<T> list);
};
public static final <T> void enumerate(final List<T> list,
final Enumerator<T> enumerator) {
for (ListIterator<T> it = list.listIterator(); it.hasNext();) {
enumerator.execute(it.nextIndex(), it.next(), list);
}
}
// =========================================================================
// Map
// =========================================================================
public static interface Transformer<T, U> {
U execute(int index, T value, List<T> list);
};
public static final <T, U> List<U> transform(final List<T> list,
final Transformer<T, U> transformer) {
List<U> result = new ArrayList<U>();
for (ListIterator<T> it = list.listIterator(); it.hasNext();) {
result.add(transformer.execute(it.nextIndex(), it.next(), list));
}
return result;
}
// =========================================================================
// Reduce
// =========================================================================
public static interface Reducer<T, U> {
U execute(int index, T value, U result, List<T> list);
};
public static final <T, U> U reduce(final List<T> list,
final Reducer<T, U> enumerator, U result) {
for (ListIterator<T> it = list.listIterator(); it.hasNext();) {
result = enumerator.execute(it.nextIndex(), it.next(), result, list);
}
return result;
}
// =========================================================================
// Filter
// =========================================================================
public static interface Predicate<T> {
boolean execute(int index, T value, List<T> list);
};
public static final <T> List<T> filter(final List<T> list,
final Predicate<T> predicate) {
List<T> result = new ArrayList<T>();
for (ListIterator<T> it = list.listIterator(); it.hasNext();) {
int index = it.nextIndex();
T value = it.next();
if (predicate.execute(index, value, list)) {
result.add(value);
}
}
return result;
}
// =========================================================================
// Predefined Methods
// =========================================================================
// Enumerate
public static <T> String printTuples(List<T> list) {
StringBuffer buff = new StringBuffer();
enumerate(list, new Enumerator<T>() {
#Override
public void execute(int index, T value, List<T> list) {
buff.append('(').append(index).append(", ")
.append(value).append(')');
if (index < list.size() - 1) {
buff.append(", ");
}
}
});
return buff.toString();
}
// Map
public static List<String> intToHex(List<Integer> list) {
return transform(list, new Transformer<Integer, String>() {
#Override
public String execute(int index, Integer value, List<Integer> list) {
return String.format("0x%02X", value);
}
});
}
// Reduce
public static Integer sum(List<Integer> list) {
return reduce(list, new Reducer<Integer, Integer>() {
#Override
public Integer execute(int index, Integer value, Integer result,
List<Integer> list) {
return result + value;
}
}, 0);
}
// Filter
public static List<Integer> evenNumbers(List<Integer> list) {
return filter(list, new Predicate<Integer>() {
#Override
public boolean execute(int index, Integer value, List<Integer> list) {
return value % 2 == 0;
}
});
}
// =========================================================================
// Driver
// =========================================================================
public static void main(String[] args) {
List<Integer> numbers = Arrays.asList(8, 6, 7, 5, 3, 0, 9);
// Enumerate
System.out.printf("%-10s: %s%n", "Enumerate", printTuples(numbers));
// Map
System.out.printf("%-10s: %s%n", "Map", intToHex(numbers));
// Reduce
System.out.printf("%-10s: %d%n", "Reduce", sum(numbers));
// Filter
System.out.printf("%-10s: %s%n", "Filter", evenNumbers(numbers));
}
}
Pretty much the same syntax using Java8 Streams
ArrayList<String> numbers = new ArrayList<String>();
numbers.add("one");
numbers.add("two");
numbers.add("three");
numbers.stream().forEach(num ->
{
System.out.println(numbers.indexOf(num) + " " + num);
});
Related
What is the most effective way to get a random element from a list with Java8 stream api?
Arrays.asList(new Obj1(), new Obj2(), new Obj3());
Thanks.
Why with streams? You just have to get a random number from 0 to the size of the list and then call get on this index:
Random r = new Random();
ElementType e = list.get(r.nextInt(list.size()));
Stream will give you nothing interesting here, but you can try with:
Random r = new Random();
ElementType e = list.stream().skip(r.nextInt(list.size())).findFirst().get();
Idea is to skip an arbitrary number of elements (but not the last one!), then get the first element if it exists. As a result you will have an Optional<ElementType> which will be non empty and then extract its value with get. You have a lot of options here after having skip.
Using streams here is highly inefficient...
Note: that none of these solutions take in account empty lists, but the problem is defined on non-empty lists.
There are much more efficient ways to do it, but if this has to be Stream the easiest way is to create your own Comparator, which returns random result (-1, 0, 1) and sort your stream:
List<String> strings = Arrays.asList("a", "b", "c", "d", "e", "f");
String randomString = strings
.stream()
.sorted((o1, o2) -> ThreadLocalRandom.current().nextInt(-1, 2))
.findAny()
.get();
ThreadLocalRandom has ready "out of the box" method to get random number in your required range for comparator.
While all the given answers work, there is a simple one-liner that does the trick without having to check if the list is empty first:
List<String> list = List.of("a", "b", "c");
list.stream().skip((int) (list.size() * Math.random())).findAny();
For an empty list this will return an Optional.empty.
In the last time I needed to do something like that I did that:
List<String> list = Arrays.asList("a", "b", "c");
Collections.shuffle(list);
String letter = list.stream().findAny().orElse(null);
System.out.println(letter);
If you HAVE to use streams, I wrote an elegant, albeit very inefficient collector that does the job:
/**
* Returns a random item from the stream (or null in case of an empty stream).
* This operation can't be lazy and is inefficient, and therefore shouldn't
* be used on streams with a large number or items or in performance critical sections.
* #return a random item from the stream or null if the stream is empty.
*/
public static <T> Collector<T, List<T>, T> randomItem() {
final Random RANDOM = new Random();
return Collector.of(() -> (List<T>) new ArrayList<T>(),
(acc, elem) -> acc.add(elem),
(list1, list2) -> ListUtils.union(list1, list2), // Using a 3rd party for list union, could be done "purely"
list -> list.isEmpty() ? null : list.get(RANDOM.nextInt(list.size())));
}
Usage:
#Test
public void standardRandomTest() {
assertThat(Stream.of(1, 2, 3, 4).collect(randomItem())).isBetween(1, 4);
}
Another idea would be to implement your own Spliterator and then use it as a source for Stream:
import java.util.List;
import java.util.Random;
import java.util.Spliterator;
import java.util.function.Consumer;
import java.util.function.Supplier;
public class ImprovedRandomSpliterator<T> implements Spliterator<T> {
private final Random random;
private final T[] source;
private int size;
ImprovedRandomSpliterator(List<T> source, Supplier<? extends Random> random) {
if (source.isEmpty()) {
throw new IllegalArgumentException("RandomSpliterator can't be initialized with an empty collection");
}
this.source = (T[]) source.toArray();
this.random = random.get();
this.size = this.source.length;
}
#Override
public boolean tryAdvance(Consumer<? super T> action) {
if (size > 0) {
int nextIdx = random.nextInt(size);
int lastIdx = size - 1;
action.accept(source[nextIdx]);
source[nextIdx] = source[lastIdx];
source[lastIdx] = null; // let object be GCed
size--;
return true;
} else {
return false;
}
}
#Override
public Spliterator<T> trySplit() {
return null;
}
#Override
public long estimateSize() {
return source.length;
}
#Override
public int characteristics() {
return SIZED;
}
}
public static <T> Collector<T, ?, Stream<T>> toShuffledStream() {
return Collectors.collectingAndThen(
toCollection(ArrayList::new),
list -> !list.isEmpty()
? StreamSupport.stream(new ImprovedRandomSpliterator<>(list, Random::new), false)
: Stream.empty());
}
and then simply:
list.stream()
.collect(toShuffledStream())
.findAny();
Details can be found here.
...but it's definitely an overkill, so if you're looking for a pragmatic approach. Definitely go for Jean's solution.
If you don't know in advance the size of the your list, you could do something like that :
yourStream.collect(new RandomListCollector<>(randomSetSize));
I guess that you will have to write your own Collector implementation like this one to have an homogeneous randomization :
public class RandomListCollector<T> implements Collector<T, RandomListCollector.ListAccumulator<T>, List<T>> {
private final Random rand;
private final int size;
public RandomListCollector(Random random , int size) {
super();
this.rand = random;
this.size = size;
}
public RandomListCollector(int size) {
this(new Random(System.nanoTime()), size);
}
#Override
public Supplier<ListAccumulator<T>> supplier() {
return () -> new ListAccumulator<T>();
}
#Override
public BiConsumer<ListAccumulator<T>, T> accumulator() {
return (l, t) -> {
if (l.size() < size) {
l.add(t);
} else if (rand.nextDouble() <= ((double) size) / (l.gSize() + 1)) {
l.add(t);
l.remove(rand.nextInt(size));
} else {
// in any case gSize needs to be incremented
l.gSizeInc();
}
};
}
#Override
public BinaryOperator<ListAccumulator<T>> combiner() {
return (l1, l2) -> {
int lgSize = l1.gSize() + l2.gSize();
ListAccumulator<T> l = new ListAccumulator<>();
if (l1.size() + l2.size()<size) {
l.addAll(l1);
l.addAll(l2);
} else {
while (l.size() < size) {
if (l1.size()==0 || l2.size()>0 && rand.nextDouble() < (double) l2.gSize() / (l1.gSize() + l2.gSize())) {
l.add(l2.remove(rand.nextInt(l2.size()), true));
} else {
l.add(l1.remove(rand.nextInt(l1.size()), true));
}
}
}
// set the gSize of l :
l.gSize(lgSize);
return l;
};
}
#Override
public Function<ListAccumulator<T>, List<T>> finisher() {
return (la) -> la.list;
}
#Override
public Set<Characteristics> characteristics() {
return Collections.singleton(Characteristics.CONCURRENT);
}
static class ListAccumulator<T> implements Iterable<T> {
List<T> list;
volatile int gSize;
public ListAccumulator() {
list = new ArrayList<>();
gSize = 0;
}
public void addAll(ListAccumulator<T> l) {
list.addAll(l.list);
gSize += l.gSize;
}
public T remove(int index) {
return remove(index, false);
}
public T remove(int index, boolean global) {
T t = list.remove(index);
if (t != null && global)
gSize--;
return t;
}
public void add(T t) {
list.add(t);
gSize++;
}
public int gSize() {
return gSize;
}
public void gSize(int gSize) {
this.gSize = gSize;
}
public void gSizeInc() {
gSize++;
}
public int size() {
return list.size();
}
#Override
public Iterator<T> iterator() {
return list.iterator();
}
}
}
If you want something easier and still don't want to load all your list in memory:
public <T> Stream<T> getRandomStreamSubset(Stream<T> stream, int subsetSize) {
int cnt = 0;
Random r = new Random(System.nanoTime());
Object[] tArr = new Object[subsetSize];
Iterator<T> iter = stream.iterator();
while (iter.hasNext() && cnt <subsetSize) {
tArr[cnt++] = iter.next();
}
while (iter.hasNext()) {
cnt++;
T t = iter.next();
if (r.nextDouble() <= (double) subsetSize / cnt) {
tArr[r.nextInt(subsetSize)] = t;
}
}
return Arrays.stream(tArr).map(o -> (T)o );
}
but you are then away from the stream api and could do the same with a basic iterator
The selected answer has errors in its stream solution...
You cannot use Random#nextInt with a non-positive long, "0" in this case.
The stream solution will also never choose the last in the list
Example:
List<Integer> intList = Arrays.asList(0, 1, 2, 3, 4);
// #nextInt is exclusive, so here it means a returned value of 0-3
// if you have a list of size = 1, #next Int will throw an IllegalArgumentException (bound must be positive)
int skipIndex = new Random().nextInt(intList.size()-1);
// randomInt will only ever be 0, 1, 2, or 3. Never 4
int randomInt = intList.stream()
.skip(skipIndex) // max skip of list#size - 2
.findFirst()
.get();
My recommendation would be to go with the non-stream approach that Jean-Baptiste Yunès put forth, but if you must do a stream approach, you could do something like this (but it's a little ugly):
list.stream()
.skip(list.isEmpty ? 0 : new Random().nextInt(list.size()))
.findFirst();
Sometimes you may want to get a random item somewhere in the stream. If you want to get random items even after filtering your list, this code snippet will work for you:
List<String> items = Arrays.asList("A", "B", "C", "D", "E");
List<String> shuffledAndFilteredItems = items.stream()
.filter(value -> value.equals("A") || value.equals("B"))
//filter, map...
.collect(Collectors.collectingAndThen(
Collectors.toCollection(ArrayList::new),
list -> {
Collections.shuffle(list);
return list;
}));
String randomItem = shuffledAndFilteredItems
.stream()
.findFirst()
.orElse(null);
Of course there may be faster / optimized ways, but it allows you to do it all at once.
I have to write a method which selects the maximum value from a list, and it has to be with Generics. Obviously the List can be Number and String as well. (The return value has to be Opt Object. This is the tasks.)
This is what I have so far, but its not working, I would appreciate your advice:
public static <T> Opt<T> max(List<? extends Object> list) {
T max = (T) list;
for (int i = 0; i < list.size(); i++) {
if (list.get(i) > max) {
max = (T) list.get(i);
}
}
return (Opt<T>) max;
}
And this is the main looks like: (From this one I have to make my method work.)
public static void main(String[] args) {
List<String> stringList = new ArrayList<>();
Utility.addTo(stringList, "aghi");
Utility.addTo(stringList, "fed");
Utility.addTo(stringList, "ghh");
Utility.addTo(stringList, "abc");
Utility.addTo(stringList, "123");
System.out.println("The maximum value: " + Utility.max(stringList).get());
List<Integer> intList = new ArrayList<>();
Utility.addTo(intList, 123);
Utility.addTo(intList, 456);
Utility.addTo(intList, -199);
Utility.addTo(intList, -90);
Utility.addTo(intList, 0);
Utility.addTo(intList, -10);
Utility.addTo(intList, 200);
System.out.println("The maximum value: " + Utility.max(intList).get());
List<Double> doubleList = new ArrayList<>();
Utility.addTo(doubleList, 123.0);
Utility.addTo(doubleList, 456.001);
Utility.addTo(doubleList, -199.0);
Utility.addTo(doubleList, -90.90);
Utility.addTo(doubleList, 0.0);
Utility.addTo(doubleList, -10.20);
Utility.addTo(doubleList, 200.1);
System.out.println("The maximum value: " + Utility.max(doubleList).get());
}
And the Output shoud be:
The maximum value: ghh
The maximum value: 456
The maximum value: 456.001
Using Streams makes it really easy by doing all the work for you:
public static <T extends Comparable<T>> Optional<T> max(List<T> list) {
return list.stream().max(Comparator.naturalOrder());
}
or the Collections class of utility functions:
public static <T extends Comparable<T>> Optional<T> max(List<T> list) {
if (list.isEmpty()) {
return Optional.empty();
} else {
return Optional.of(Collections.max(list, Comparator.naturalOrder()));
}
}
Your code is not working (I mean cannot be compiled) because of this line:
if (list.get(i) > max) {
In Java, there are no overloaded operators as in C++, so you need to find another way...
By Opt you probably meant java.util.Optional class and you can use it like this:
public static <T> Optional<T> max(List<T> list) {
Optional<T> max = Optional.empty();
for (int i = 0; i < list.size(); i++) {
// if (list.get(i) > max) {
max = Optional.of(list.get(i));
// }
}
return max;
}
This is of course not working, it returns last element from list.
When creator of a class expects users might be interested in sorting (comparing), they would implemente java.lang.Comparable, this is the case for String, Long and Double. So instead of T, you can say T extends Comparable like this:
public static <T extends Comparable<T>> Optional<T> max(List<T> list) {
T max = null;
for (int i = 0; i < list.size(); i++) {
final T item = list.get(i);
if (max == null) {
max = item;
} else if (max.compareTo(item) < 0) {
max = item;
}
}
if (max == null) return Optional.empty();
return Optional.of(max);
}
Look at Comparable#compareTo JavaDoc.
Try to understand what this line means exactly (and why you cannot use it with list of java.lang.Objects):
public static <T extends Comparable<T>> Optional<T> max(List<T> list) {
and why we do not need it in addTo:
public static <T> void addTo(List<T> list, T e) {
I would like to get the index of an object in a list by its property in Java.
Example:
List<MyObj> list = new ArrayList<>();
list.add(new MyObj("Ram");
list.add(new MyObj("Girish");
list.add(new MyObj("Ajith");
list.add(new MyObj("Sai");
public class MyObj {
public String name;
public MyObj(String name){
this.name=name;
}
}
Now, I would like to the get the index of an Object which contains the name as "Girish". Please do let me know the code in JAVA.
If you want a solution with stream use this one:
int index = IntStream.range(0, list.size())
.filter(i -> list.get(i).name.equals(searchName))
.findFirst()
.orElse(-1);
In case you have a List, all you can do is to iterate over each element and check required property. This is O(n).
public static int getIndexOf(List<MyObj> list, String name) {
int pos = 0;
for(MyObj myObj : list) {
if(name.equalsIgnoreCase(myObj.name))
return pos;
pos++;
}
return -1;
}
In case you want to increase performance. Then you could implement your own data structure. Note, that key feature is that your key property should be a key of a HashMap and value of HashMap should be index. Then you get O(1) performance.
public static final class IndexList<E> extends AbstractList<E> {
private final Map<Integer, E> indexObj = new HashMap<>();
private final Map<String, Integer> keyIndex = new HashMap<>();
private final Function<E, String> getKey;
public IndexList(Function<E, String> getKey) {
this.getKey = getKey;
}
public int getIndexByKey(String key) {
return keyIndex.get(key);
}
#Override
public int size() {
return keyIndex.size();
}
#Override
public boolean add(E e) {
String key = getKey.apply(e);
if (keyIndex.containsKey(key))
throw new IllegalArgumentException("Key '" + key + "' duplication");
int index = size();
keyIndex.put(key, index);
indexObj.put(index, e);
return true;
}
#Override
public E get(int index) {
return indexObj.get(index);
}
}
Demo:
IndexList<MyObj> list = new IndexList<>(myObj -> myObj.name);
list.add(new MyObj("Ram"));
list.add(new MyObj("Girish"));
list.add(new MyObj("Ajith"));
list.add(new MyObj("Sai"));
System.out.println(list.getIndexByKey("Ajith")); // 2
indexOf() will work if you change the .equals function
I'd suggest just iterating through
int getIndex(String wanted){
for(int i = 0; i<list.size(); i++){
if(list.get(i).name.equals(wanted)){
return i;
}
}
}
indexOf() will return the index of the first occurrence of a value. For example:
int myIndex = list.indexOf("Ram")
(Note though that your arraylist doesn't contain "Ram", it contains an object of type MyObj with a name of "Ram")
Bear in mind ArrayLists start at 0 not one.
What is the most effective way to get a random element from a list with Java8 stream api?
Arrays.asList(new Obj1(), new Obj2(), new Obj3());
Thanks.
Why with streams? You just have to get a random number from 0 to the size of the list and then call get on this index:
Random r = new Random();
ElementType e = list.get(r.nextInt(list.size()));
Stream will give you nothing interesting here, but you can try with:
Random r = new Random();
ElementType e = list.stream().skip(r.nextInt(list.size())).findFirst().get();
Idea is to skip an arbitrary number of elements (but not the last one!), then get the first element if it exists. As a result you will have an Optional<ElementType> which will be non empty and then extract its value with get. You have a lot of options here after having skip.
Using streams here is highly inefficient...
Note: that none of these solutions take in account empty lists, but the problem is defined on non-empty lists.
There are much more efficient ways to do it, but if this has to be Stream the easiest way is to create your own Comparator, which returns random result (-1, 0, 1) and sort your stream:
List<String> strings = Arrays.asList("a", "b", "c", "d", "e", "f");
String randomString = strings
.stream()
.sorted((o1, o2) -> ThreadLocalRandom.current().nextInt(-1, 2))
.findAny()
.get();
ThreadLocalRandom has ready "out of the box" method to get random number in your required range for comparator.
While all the given answers work, there is a simple one-liner that does the trick without having to check if the list is empty first:
List<String> list = List.of("a", "b", "c");
list.stream().skip((int) (list.size() * Math.random())).findAny();
For an empty list this will return an Optional.empty.
In the last time I needed to do something like that I did that:
List<String> list = Arrays.asList("a", "b", "c");
Collections.shuffle(list);
String letter = list.stream().findAny().orElse(null);
System.out.println(letter);
If you HAVE to use streams, I wrote an elegant, albeit very inefficient collector that does the job:
/**
* Returns a random item from the stream (or null in case of an empty stream).
* This operation can't be lazy and is inefficient, and therefore shouldn't
* be used on streams with a large number or items or in performance critical sections.
* #return a random item from the stream or null if the stream is empty.
*/
public static <T> Collector<T, List<T>, T> randomItem() {
final Random RANDOM = new Random();
return Collector.of(() -> (List<T>) new ArrayList<T>(),
(acc, elem) -> acc.add(elem),
(list1, list2) -> ListUtils.union(list1, list2), // Using a 3rd party for list union, could be done "purely"
list -> list.isEmpty() ? null : list.get(RANDOM.nextInt(list.size())));
}
Usage:
#Test
public void standardRandomTest() {
assertThat(Stream.of(1, 2, 3, 4).collect(randomItem())).isBetween(1, 4);
}
Another idea would be to implement your own Spliterator and then use it as a source for Stream:
import java.util.List;
import java.util.Random;
import java.util.Spliterator;
import java.util.function.Consumer;
import java.util.function.Supplier;
public class ImprovedRandomSpliterator<T> implements Spliterator<T> {
private final Random random;
private final T[] source;
private int size;
ImprovedRandomSpliterator(List<T> source, Supplier<? extends Random> random) {
if (source.isEmpty()) {
throw new IllegalArgumentException("RandomSpliterator can't be initialized with an empty collection");
}
this.source = (T[]) source.toArray();
this.random = random.get();
this.size = this.source.length;
}
#Override
public boolean tryAdvance(Consumer<? super T> action) {
if (size > 0) {
int nextIdx = random.nextInt(size);
int lastIdx = size - 1;
action.accept(source[nextIdx]);
source[nextIdx] = source[lastIdx];
source[lastIdx] = null; // let object be GCed
size--;
return true;
} else {
return false;
}
}
#Override
public Spliterator<T> trySplit() {
return null;
}
#Override
public long estimateSize() {
return source.length;
}
#Override
public int characteristics() {
return SIZED;
}
}
public static <T> Collector<T, ?, Stream<T>> toShuffledStream() {
return Collectors.collectingAndThen(
toCollection(ArrayList::new),
list -> !list.isEmpty()
? StreamSupport.stream(new ImprovedRandomSpliterator<>(list, Random::new), false)
: Stream.empty());
}
and then simply:
list.stream()
.collect(toShuffledStream())
.findAny();
Details can be found here.
...but it's definitely an overkill, so if you're looking for a pragmatic approach. Definitely go for Jean's solution.
If you don't know in advance the size of the your list, you could do something like that :
yourStream.collect(new RandomListCollector<>(randomSetSize));
I guess that you will have to write your own Collector implementation like this one to have an homogeneous randomization :
public class RandomListCollector<T> implements Collector<T, RandomListCollector.ListAccumulator<T>, List<T>> {
private final Random rand;
private final int size;
public RandomListCollector(Random random , int size) {
super();
this.rand = random;
this.size = size;
}
public RandomListCollector(int size) {
this(new Random(System.nanoTime()), size);
}
#Override
public Supplier<ListAccumulator<T>> supplier() {
return () -> new ListAccumulator<T>();
}
#Override
public BiConsumer<ListAccumulator<T>, T> accumulator() {
return (l, t) -> {
if (l.size() < size) {
l.add(t);
} else if (rand.nextDouble() <= ((double) size) / (l.gSize() + 1)) {
l.add(t);
l.remove(rand.nextInt(size));
} else {
// in any case gSize needs to be incremented
l.gSizeInc();
}
};
}
#Override
public BinaryOperator<ListAccumulator<T>> combiner() {
return (l1, l2) -> {
int lgSize = l1.gSize() + l2.gSize();
ListAccumulator<T> l = new ListAccumulator<>();
if (l1.size() + l2.size()<size) {
l.addAll(l1);
l.addAll(l2);
} else {
while (l.size() < size) {
if (l1.size()==0 || l2.size()>0 && rand.nextDouble() < (double) l2.gSize() / (l1.gSize() + l2.gSize())) {
l.add(l2.remove(rand.nextInt(l2.size()), true));
} else {
l.add(l1.remove(rand.nextInt(l1.size()), true));
}
}
}
// set the gSize of l :
l.gSize(lgSize);
return l;
};
}
#Override
public Function<ListAccumulator<T>, List<T>> finisher() {
return (la) -> la.list;
}
#Override
public Set<Characteristics> characteristics() {
return Collections.singleton(Characteristics.CONCURRENT);
}
static class ListAccumulator<T> implements Iterable<T> {
List<T> list;
volatile int gSize;
public ListAccumulator() {
list = new ArrayList<>();
gSize = 0;
}
public void addAll(ListAccumulator<T> l) {
list.addAll(l.list);
gSize += l.gSize;
}
public T remove(int index) {
return remove(index, false);
}
public T remove(int index, boolean global) {
T t = list.remove(index);
if (t != null && global)
gSize--;
return t;
}
public void add(T t) {
list.add(t);
gSize++;
}
public int gSize() {
return gSize;
}
public void gSize(int gSize) {
this.gSize = gSize;
}
public void gSizeInc() {
gSize++;
}
public int size() {
return list.size();
}
#Override
public Iterator<T> iterator() {
return list.iterator();
}
}
}
If you want something easier and still don't want to load all your list in memory:
public <T> Stream<T> getRandomStreamSubset(Stream<T> stream, int subsetSize) {
int cnt = 0;
Random r = new Random(System.nanoTime());
Object[] tArr = new Object[subsetSize];
Iterator<T> iter = stream.iterator();
while (iter.hasNext() && cnt <subsetSize) {
tArr[cnt++] = iter.next();
}
while (iter.hasNext()) {
cnt++;
T t = iter.next();
if (r.nextDouble() <= (double) subsetSize / cnt) {
tArr[r.nextInt(subsetSize)] = t;
}
}
return Arrays.stream(tArr).map(o -> (T)o );
}
but you are then away from the stream api and could do the same with a basic iterator
The selected answer has errors in its stream solution...
You cannot use Random#nextInt with a non-positive long, "0" in this case.
The stream solution will also never choose the last in the list
Example:
List<Integer> intList = Arrays.asList(0, 1, 2, 3, 4);
// #nextInt is exclusive, so here it means a returned value of 0-3
// if you have a list of size = 1, #next Int will throw an IllegalArgumentException (bound must be positive)
int skipIndex = new Random().nextInt(intList.size()-1);
// randomInt will only ever be 0, 1, 2, or 3. Never 4
int randomInt = intList.stream()
.skip(skipIndex) // max skip of list#size - 2
.findFirst()
.get();
My recommendation would be to go with the non-stream approach that Jean-Baptiste Yunès put forth, but if you must do a stream approach, you could do something like this (but it's a little ugly):
list.stream()
.skip(list.isEmpty ? 0 : new Random().nextInt(list.size()))
.findFirst();
Sometimes you may want to get a random item somewhere in the stream. If you want to get random items even after filtering your list, this code snippet will work for you:
List<String> items = Arrays.asList("A", "B", "C", "D", "E");
List<String> shuffledAndFilteredItems = items.stream()
.filter(value -> value.equals("A") || value.equals("B"))
//filter, map...
.collect(Collectors.collectingAndThen(
Collectors.toCollection(ArrayList::new),
list -> {
Collections.shuffle(list);
return list;
}));
String randomItem = shuffledAndFilteredItems
.stream()
.findFirst()
.orElse(null);
Of course there may be faster / optimized ways, but it allows you to do it all at once.
I am trying to use a comparator to sort my List 2 based on List 1.
So the two lists are:
ListA = [2,3,4]
ListB = [8,2,4]
I need to sort list 2 based on list 1.
Expected output:
List1: [2,3,4]
List2: [2,8,4]
Here is the code I am trying
Collections.sort(list1);
Collections.sort(list2,new Comparator<Integer>(){
public int compare(Integer left,Integer right){
return Integer.compare(list1.indexOf(left),list1.indexOf(right));
}
})
Here sorting will be based on index of List1 elements. The comparator above is not working for me Please help?
It's not exactly clear to me from your example what you're trying to do, so this answer might not actually address your question. If what you're trying to do is to apply the sort-order of ListA to both ListA and ListB then you can do it as follows.
Create an auxiliary class that allows you to pair up the two lists with respect to the same index:
private class IntPair implements Comparable<IntPair> {
int i1;
int i2;
public IntPair(int i1, int i2) {
this.i1 = i1;
this.i2 = i2;
}
public int compareTo(IntPair other) {
return Integer.compare(i1, other.i1);
}
}
Then create a new list of IntPair instances from your original lists and sort it.
int size = list1.size();
List<IntPair> aux = new ArrayList(size);
for (int i = 0; i < size; i++) {
aux.add(new IntPair(list1.get(i), list2.get(i)))
}
Collections.sort(aux);
Finally, copy back the values of the resulting pair list onto your original lists.
for (int i = 0; i < size; i++) {
IntPair ip = aux.get(i);
list1.set(i, ip.i1);
list2.set(i, ip.i2);
}
Note that in terms of algorithmic complexity, this approach still is O(nlogn).
This might be a little heavy-weight, but it gets the job done and it's generic.
Essentially, you can zip-up two lists, sort/compare them based off an index, unzip them, and return the result.
This does not modify the list in-place. I tried to give the variables meaningful, but discreet names. Please acknowledge me for clarification.
import java.util.*;
public class SortUtil {
public static void main(String[] args) {
List<Integer> indicies = Arrays.asList(3, 2, 4);
List<Integer> values = Arrays.asList(8, 2, 4);
List<Integer> sorted = doSort(indicies, values, new TupleComparator<Integer>(0, 1) {
#Override
public int doCompare(Integer valueA, Integer valueB) {
return Integer.compare(valueA, valueB);
}
});
System.out.println(sorted); // [2, 8, 4]
}
public static List<Integer> doSort(List<Integer> listA, List<Integer> listB, TupleComparator<Integer> comparator) {
List<Tuple<Integer>> tuples = zip(listA, listB);
Collections.sort(tuples, comparator);
return unzip(tuples, comparator.getValuePos());
}
private static <E> List<E> unzip(List<Tuple<E>> tuples, int index) {
List<E> list = new ArrayList<E>();
for (Tuple<E> tuple : tuples) {
list.add(tuple.getData().get(index));
}
return list;
}
private static <E> List<Tuple<E>> zip(List<E> listA, List<E> listB) {
List<Tuple<E>> listC = new ArrayList<Tuple<E>>();
for (int i = 0; i < listA.size(); i++) {
listC.add(new Tuple<E>(Arrays.asList(listA.get(i), listB.get(i))));
}
return listC;
}
private static abstract class TupleComparator<E> implements Comparator<Tuple<E>> {
private int indexPos;
private int valuePos;
public TupleComparator(int indexPos, int valuePos) {
this.indexPos = indexPos;
this.valuePos = valuePos;
}
public int compare(Tuple<E> left, Tuple<E> right) {
E indexA = left.getData().get(this.getIndexPos());
E indexB = right.getData().get(this.getIndexPos());
return doCompare(indexA, indexB);
}
public int getIndexPos() { return indexPos; }
public int getValuePos() { return valuePos; }
public abstract int doCompare(E valueA, E valueB);
}
private static class Tuple<T> {
private List<T> data;
public Tuple(List<T> data) { this.data = data; }
public List<T> getData() {return data; }
}
}