Conditions: do not modifiy the original lists; JDK only, no external libraries. Bonus points for a one-liner or a JDK 1.3 version.
Is there a simpler way than:
List<String> newList = new ArrayList<String>();
newList.addAll(listOne);
newList.addAll(listTwo);
Off the top of my head, I can shorten it by one line:
List<String> newList = new ArrayList<String>(listOne);
newList.addAll(listTwo);
Another Java 8 one-liner:
List<String> newList = Stream.of(listOne, listTwo)
.flatMap(Collection::stream)
.collect(Collectors.toList());
As a bonus, since Stream.of() is variadic, you may concatenate as many lists as you like.
List<String> newList = Stream.of(listOne, listTwo, listThree)
.flatMap(Collection::stream)
.collect(Collectors.toList());
You could do it with a static import and a helper class
nb the generification of this class could probably be improved
public class Lists {
private Lists() { } // can't be instantiated
public static List<T> join(List<T>... lists) {
List<T> result = new ArrayList<T>();
for(List<T> list : lists) {
result.addAll(list);
}
return results;
}
}
Then you can do things like
import static Lists.join;
List<T> result = join(list1, list2, list3, list4);
You can do a oneliner if the target list is predeclared.
(newList = new ArrayList<String>(list1)).addAll(list2);
Almost of answers suggest to use an ArrayList.
List<String> newList = new LinkedList<>(listOne);
newList.addAll(listTwo);
Prefer to use a LinkedList for efficient add operations.
ArrayList add is O(1) amortized, but O(n) worst-case since the array must be resized and copied. While LinkedList add is always constant O(1).
more infos https://stackoverflow.com/a/322742/311420
I'm not claiming that it's simple, but you mentioned bonus for one-liners ;-)
Collection mergedList = Collections.list(new sun.misc.CompoundEnumeration(new Enumeration[] {
new Vector(list1).elements(),
new Vector(list2).elements(),
...
}))
Use a Helper class.
I suggest:
public static <E> Collection<E> addAll(Collection<E> dest, Collection<? extends E>... src) {
for(Collection<? extends E> c : src) {
dest.addAll(c);
}
return dest;
}
public static void main(String[] args) {
System.out.println(addAll(new ArrayList<Object>(), Arrays.asList(1,2,3), Arrays.asList("a", "b", "c")));
// does not compile
// System.out.println(addAll(new ArrayList<Integer>(), Arrays.asList(1,2,3), Arrays.asList("a", "b", "c")));
System.out.println(addAll(new ArrayList<Integer>(), Arrays.asList(1,2,3), Arrays.asList(4, 5, 6)));
}
This is simple and just one line, but will add the contents of listTwo to listOne. Do you really need to put the contents in a third list?
Collections.addAll(listOne, listTwo.toArray());
Here is a java 8 solution using two lines:
List<Object> newList = new ArrayList<>();
Stream.of(list1, list2).forEach(newList::addAll);
Be aware that this method should not be used if
newList is not known and it may already be shared with other threadsnewList is a parallel stream and access to newList is not synchronized or threadsafedue to side effect considerations.
Both of the above conditions do not apply for the above case of joining two lists, so this is safe.
Based on this answer to another question.
import java.util.AbstractList;
import java.util.List;
/**
* The {@code ConcatList} is a lightweight view of two {@code List}s.
* <p>
* This implementation is <em>not</em> thread-safe even though the underlying lists can be.
*
* @param <E>
* the type of elements in this list
*/
public class ConcatList<E> extends AbstractList<E> {
/** The first underlying list. */
private final List<E> list1;
/** The second underlying list. */
private final List<E> list2;
/**
* Constructs a new {@code ConcatList} from the given two lists.
*
* @param list1
* the first list
* @param list2
* the second list
*/
public ConcatList(final List<E> list1, final List<E> list2) {
this.list1 = list1;
this.list2 = list2;
}
@Override
public E get(final int index) {
return getList(index).get(getListIndex(index));
}
@Override
public E set(final int index, final E element) {
return getList(index).set(getListIndex(index), element);
}
@Override
public void add(final int index, final E element) {
getList(index).add(getListIndex(index), element);
}
@Override
public E remove(final int index) {
return getList(index).remove(getListIndex(index));
}
@Override
public int size() {
return list1.size() + list2.size();
}
@Override
public boolean contains(final Object o) {
return list1.contains(o) || list2.contains(o);
}
@Override
public void clear() {
list1.clear();
list2.clear();
}
/**
* Returns the index within the corresponding list related to the given index.
*
* @param index
* the index in this list
*
* @return the index of the underlying list
*/
private int getListIndex(final int index) {
final int size1 = list1.size();
return index >= size1 ? index - size1 : index;
}
/**
* Returns the list that corresponds to the given index.
*
* @param index
* the index in this list
*
* @return the underlying list that corresponds to that index
*/
private List<E> getList(final int index) {
return index >= list1.size() ? list2 : list1;
}
}
You could do it with a static import and a helper class
nb the generification of this class could probably be improved
public class Lists {
private Lists() { } // can't be instantiated
public static List<T> join(List<T>... lists) {
List<T> result = new ArrayList<T>();
for(List<T> list : lists) {
result.addAll(list);
}
return results;
}
}
Then you can do things like
import static Lists.join;
List<T> result = join(list1, list2, list3, list4);
Not simpler, but without resizing overhead:
List<String> newList = new ArrayList<>(listOne.size() + listTwo.size());
newList.addAll(listOne);
newList.addAll(listTwo);
public static <T> List<T> merge(List<T>... args) {
final List<T> result = new ArrayList<>();
for (List<T> list : args) {
result.addAll(list);
}
return result;
}
Almost of answers suggest to use an ArrayList.
List<String> newList = new LinkedList<>(listOne);
newList.addAll(listTwo);
Prefer to use a LinkedList for efficient add operations.
ArrayList add is O(1) amortized, but O(n) worst-case since the array must be resized and copied. While LinkedList add is always constant O(1).
more infos https://stackoverflow.com/a/322742/311420
I'm not claiming that it's simple, but you mentioned bonus for one-liners ;-)
Collection mergedList = Collections.list(new sun.misc.CompoundEnumeration(new Enumeration[] {
new Vector(list1).elements(),
new Vector(list2).elements(),
...
}))
Here is a java 8 solution using two lines:
List<Object> newList = new ArrayList<>();
Stream.of(list1, list2).forEach(newList::addAll);
Be aware that this method should not be used if
newList is not known and it may already be shared with other threadsnewList is a parallel stream and access to newList is not synchronized or threadsafedue to side effect considerations.
Both of the above conditions do not apply for the above case of joining two lists, so this is safe.
Based on this answer to another question.
A little shorter would be:
List<String> newList = new ArrayList<String>(listOne);
newList.addAll(listTwo);
Slightly simpler:
List<String> newList = new ArrayList<String>(listOne);
newList.addAll(listTwo);
I can't improve on the two-liner in the general case without introducing your own utility method, but if you do have lists of Strings and you're willing to assume those Strings don't contain commas, you can pull this long one-liner:
List<String> newList = new ArrayList<String>(Arrays.asList((listOne.toString().subString(1, listOne.length() - 1) + ", " + listTwo.toString().subString(1, listTwo.length() - 1)).split(", ")));
If you drop the generics, this should be JDK 1.4 compliant (though I haven't tested that). Also not recommended for production code ;-)
public class TestApp {
/**
* @param args
*/
public static void main(String[] args) {
System.out.println("Hi");
Set<List<String>> bcOwnersList = new HashSet<List<String>>();
List<String> bclist = new ArrayList<String>();
List<String> bclist1 = new ArrayList<String>();
List<String> object = new ArrayList<String>();
object.add("BC11");
object.add("C2");
bclist.add("BC1");
bclist.add("BC2");
bclist.add("BC3");
bclist.add("BC4");
bclist.add("BC5");
bcOwnersList.add(bclist);
bcOwnersList.add(object);
bclist1.add("BC11");
bclist1.add("BC21");
bclist1.add("BC31");
bclist1.add("BC4");
bclist1.add("BC5");
List<String> listList= new ArrayList<String>();
for(List<String> ll : bcOwnersList){
listList = (List<String>) CollectionUtils.union(listList,CollectionUtils.intersection(ll, bclist1));
}
/*for(List<String> lists : listList){
test = (List<String>) CollectionUtils.union(test, listList);
}*/
for(Object l : listList){
System.out.println(l.toString());
}
System.out.println(bclist.contains("BC"));
}
}
You can create your generic Java 8 utility method to concat any number of lists.
@SafeVarargs
public static <T> List<T> concat(List<T>... lists) {
return Stream.of(lists).flatMap(List::stream).collect(Collectors.toList());
}
I'm not claiming that it's simple, but you mentioned bonus for one-liners ;-)
Collection mergedList = Collections.list(new sun.misc.CompoundEnumeration(new Enumeration[] {
new Vector(list1).elements(),
new Vector(list2).elements(),
...
}))
Java 8 (
Stream.ofandStream.concat)
The proposed solution is for three lists though it can be applied for two lists as well. In Java 8 we can make use of Stream.of or Stream.concat as:
List<String> result1 = Stream.concat(Stream.concat(list1.stream(),list2.stream()),list3.stream()).collect(Collectors.toList());
List<String> result2 = Stream.of(list1,list2,list3).flatMap(Collection::stream).collect(Collectors.toList());
Stream.concat takes two streams as input and creates a lazily concatenated stream whose elements are all the elements of the first stream followed by all the elements of the second stream. As we have three lists we have used this method (Stream.concat) two times.
We can also write a utility class with a method that takes any number of lists (using varargs) and returns a concatenated list as:
public static <T> List<T> concatenateLists(List<T>... collections) {
return Arrays.stream(collections).flatMap(Collection::stream).collect(Collectors.toList());
}
Then we can make use of this method as:
List<String> result3 = Utils.concatenateLists(list1,list2,list3);
Slightly simpler:
List<String> newList = new ArrayList<String>(listOne);
newList.addAll(listTwo);
You can do a oneliner if the target list is predeclared.
(newList = new ArrayList<String>(list1)).addAll(list2);
One of your requirements is to preserve the original lists. If you create a new list and use addAll(), you are effectively doubling the number of references to the objects in your lists. This could lead to memory problems if your lists are very large.
If you don't need to modify the concatenated result, you can avoid this using a custom list implementation. The custom implementation class is more than one line, obviously...but using it is short and sweet.
CompositeUnmodifiableList.java:
public class CompositeUnmodifiableList<E> extends AbstractList<E> {
private final List<E> list1;
private final List<E> list2;
public CompositeUnmodifiableList(List<E> list1, List<E> list2) {
this.list1 = list1;
this.list2 = list2;
}
@Override
public E get(int index) {
if (index < list1.size()) {
return list1.get(index);
}
return list2.get(index-list1.size());
}
@Override
public int size() {
return list1.size() + list2.size();
}
}
Usage:
List<String> newList = new CompositeUnmodifiableList<String>(listOne,listTwo);
If you want to do this statically you can the following.
The examples uses 2 EnumSets in natural-order (==Enum-order) A, B and joins then in an ALL list.
public static final EnumSet<MyType> CATEGORY_A = EnumSet.of(A_1, A_2);
public static final EnumSet<MyType> CATEGORY_B = EnumSet.of(B_1, B_2, B_3);
public static final List<MyType> ALL =
Collections.unmodifiableList(
new ArrayList<MyType>(CATEGORY_A.size() + CATEGORY_B.size())
{{
addAll(CATEGORY_A);
addAll(CATEGORY_B);
}}
);
You could do it with a static import and a helper class
nb the generification of this class could probably be improved
public class Lists {
private Lists() { } // can't be instantiated
public static List<T> join(List<T>... lists) {
List<T> result = new ArrayList<T>();
for(List<T> list : lists) {
result.addAll(list);
}
return results;
}
}
Then you can do things like
import static Lists.join;
List<T> result = join(list1, list2, list3, list4);
I can't improve on the two-liner in the general case without introducing your own utility method, but if you do have lists of Strings and you're willing to assume those Strings don't contain commas, you can pull this long one-liner:
List<String> newList = new ArrayList<String>(Arrays.asList((listOne.toString().subString(1, listOne.length() - 1) + ", " + listTwo.toString().subString(1, listTwo.length() - 1)).split(", ")));
If you drop the generics, this should be JDK 1.4 compliant (though I haven't tested that). Also not recommended for production code ;-)
Probably not simpler, but intriguing and ugly:
List<String> newList = new ArrayList<String>() { { addAll(listOne); addAll(listTwo); } };
Don't use it in production code... ;)
In Java 8:
List<String> newList = Stream.concat(listOne.stream(), listTwo.stream())
.collect(Collectors.toList());
The smartest in my opinion:
/**
* @param smallLists
* @return one big list containing all elements of the small ones, in the same order.
*/
public static <E> List<E> concatenate (final List<E> ... smallLists)
{
final ArrayList<E> bigList = new ArrayList<E>();
for (final List<E> list: smallLists)
{
bigList.addAll(list);
}
return bigList;
}
public static <T> List<T> merge(@Nonnull final List<T>... list) {
// calculate length first
int mergedLength = 0;
for (List<T> ts : list) {
mergedLength += ts.size();
}
final List<T> mergedList = new ArrayList<>(mergedLength);
for (List<T> ts : list) {
mergedList.addAll(ts);
}
return mergedList;
}
Use a Helper class.
I suggest:
public static <E> Collection<E> addAll(Collection<E> dest, Collection<? extends E>... src) {
for(Collection<? extends E> c : src) {
dest.addAll(c);
}
return dest;
}
public static void main(String[] args) {
System.out.println(addAll(new ArrayList<Object>(), Arrays.asList(1,2,3), Arrays.asList("a", "b", "c")));
// does not compile
// System.out.println(addAll(new ArrayList<Integer>(), Arrays.asList(1,2,3), Arrays.asList("a", "b", "c")));
System.out.println(addAll(new ArrayList<Integer>(), Arrays.asList(1,2,3), Arrays.asList(4, 5, 6)));
}
Found this question looking to concatenate arbitrary amount of lists, not minding external libraries. So, perhaps it will help someone else:
com.google.common.collect.Iterables#concat()
Useful if you want to apply the same logic to a number of different collections in one for().
You could use the Apache commons-collections library:
List<String> newList = ListUtils.union(list1, list2);
public class TestApp {
/**
* @param args
*/
public static void main(String[] args) {
System.out.println("Hi");
Set<List<String>> bcOwnersList = new HashSet<List<String>>();
List<String> bclist = new ArrayList<String>();
List<String> bclist1 = new ArrayList<String>();
List<String> object = new ArrayList<String>();
object.add("BC11");
object.add("C2");
bclist.add("BC1");
bclist.add("BC2");
bclist.add("BC3");
bclist.add("BC4");
bclist.add("BC5");
bcOwnersList.add(bclist);
bcOwnersList.add(object);
bclist1.add("BC11");
bclist1.add("BC21");
bclist1.add("BC31");
bclist1.add("BC4");
bclist1.add("BC5");
List<String> listList= new ArrayList<String>();
for(List<String> ll : bcOwnersList){
listList = (List<String>) CollectionUtils.union(listList,CollectionUtils.intersection(ll, bclist1));
}
/*for(List<String> lists : listList){
test = (List<String>) CollectionUtils.union(test, listList);
}*/
for(Object l : listList){
System.out.println(l.toString());
}
System.out.println(bclist.contains("BC"));
}
}
public static <T> List<T> merge(@Nonnull final List<T>... list) {
// calculate length first
int mergedLength = 0;
for (List<T> ts : list) {
mergedLength += ts.size();
}
final List<T> mergedList = new ArrayList<>(mergedLength);
for (List<T> ts : list) {
mergedList.addAll(ts);
}
return mergedList;
}
I can't improve on the two-liner in the general case without introducing your own utility method, but if you do have lists of Strings and you're willing to assume those Strings don't contain commas, you can pull this long one-liner:
List<String> newList = new ArrayList<String>(Arrays.asList((listOne.toString().subString(1, listOne.length() - 1) + ", " + listTwo.toString().subString(1, listTwo.length() - 1)).split(", ")));
If you drop the generics, this should be JDK 1.4 compliant (though I haven't tested that). Also not recommended for production code ;-)
Not simpler, but without resizing overhead:
List<String> newList = new ArrayList<>(listOne.size() + listTwo.size());
newList.addAll(listOne);
newList.addAll(listTwo);
another one liner solution using Java8 stream, since flatMap solution is already posted, here is a solution without flatMap
List<E> li = lol.stream().collect(ArrayList::new, List::addAll, List::addAll);
or
List<E> ints = Stream.of(list1, list2).collect(ArrayList::new, List::addAll, List::addAll);
code
List<List<Integer>> lol = Arrays.asList(Arrays.asList(1, 2, 3), Arrays.asList(4, 5, 6));
List<Integer> li = lol.stream().collect(ArrayList::new, List::addAll, List::addAll);
System.out.println(lol);
System.out.println(li);
output
[[1, 2, 3], [4, 5, 6]]
[1, 2, 3, 4, 5, 6]
Probably not simpler, but intriguing and ugly:
List<String> newList = new ArrayList<String>() { { addAll(listOne); addAll(listTwo); } };
Don't use it in production code... ;)
You can do a oneliner if the target list is predeclared.
(newList = new ArrayList<String>(list1)).addAll(list2);
I'm not claiming that it's simple, but you mentioned bonus for one-liners ;-)
Collection mergedList = Collections.list(new sun.misc.CompoundEnumeration(new Enumeration[] {
new Vector(list1).elements(),
new Vector(list2).elements(),
...
}))
Slightly simpler:
List<String> newList = new ArrayList<String>(listOne);
newList.addAll(listTwo);
If you want to do this statically you can the following.
The examples uses 2 EnumSets in natural-order (==Enum-order) A, B and joins then in an ALL list.
public static final EnumSet<MyType> CATEGORY_A = EnumSet.of(A_1, A_2);
public static final EnumSet<MyType> CATEGORY_B = EnumSet.of(B_1, B_2, B_3);
public static final List<MyType> ALL =
Collections.unmodifiableList(
new ArrayList<MyType>(CATEGORY_A.size() + CATEGORY_B.size())
{{
addAll(CATEGORY_A);
addAll(CATEGORY_B);
}}
);
Use a Helper class.
I suggest:
public static <E> Collection<E> addAll(Collection<E> dest, Collection<? extends E>... src) {
for(Collection<? extends E> c : src) {
dest.addAll(c);
}
return dest;
}
public static void main(String[] args) {
System.out.println(addAll(new ArrayList<Object>(), Arrays.asList(1,2,3), Arrays.asList("a", "b", "c")));
// does not compile
// System.out.println(addAll(new ArrayList<Integer>(), Arrays.asList(1,2,3), Arrays.asList("a", "b", "c")));
System.out.println(addAll(new ArrayList<Integer>(), Arrays.asList(1,2,3), Arrays.asList(4, 5, 6)));
}
Here's an approach using streams and java 8 if your lists have different types and you want to combine them to a list of another type.
public static void main(String[] args) {
List<String> list2 = new ArrayList<>();
List<Pair<Integer, String>> list1 = new ArrayList<>();
list2.add("asd");
list2.add("asdaf");
list1.add(new Pair<>(1, "werwe"));
list1.add(new Pair<>(2, "tyutyu"));
Stream stream = Stream.concat(list1.stream(), list2.stream());
List<Pair<Integer, String>> res = (List<Pair<Integer, String>>) stream
.map(item -> {
if (item instanceof String) {
return new Pair<>(0, item);
}
else {
return new Pair<>(((Pair<Integer, String>)item).getKey(), ((Pair<Integer, String>)item).getValue());
}
})
.collect(Collectors.toList());
}
You could do it with a static import and a helper class
nb the generification of this class could probably be improved
public class Lists {
private Lists() { } // can't be instantiated
public static List<T> join(List<T>... lists) {
List<T> result = new ArrayList<T>();
for(List<T> list : lists) {
result.addAll(list);
}
return results;
}
}
Then you can do things like
import static Lists.join;
List<T> result = join(list1, list2, list3, list4);
Probably not simpler, but intriguing and ugly:
List<String> newList = new ArrayList<String>() { { addAll(listOne); addAll(listTwo); } };
Don't use it in production code... ;)
A little shorter would be:
List<String> newList = new ArrayList<String>(listOne);
newList.addAll(listTwo);
Java 8 (
Stream.ofandStream.concat)
The proposed solution is for three lists though it can be applied for two lists as well. In Java 8 we can make use of Stream.of or Stream.concat as:
List<String> result1 = Stream.concat(Stream.concat(list1.stream(),list2.stream()),list3.stream()).collect(Collectors.toList());
List<String> result2 = Stream.of(list1,list2,list3).flatMap(Collection::stream).collect(Collectors.toList());
Stream.concat takes two streams as input and creates a lazily concatenated stream whose elements are all the elements of the first stream followed by all the elements of the second stream. As we have three lists we have used this method (Stream.concat) two times.
We can also write a utility class with a method that takes any number of lists (using varargs) and returns a concatenated list as:
public static <T> List<T> concatenateLists(List<T>... collections) {
return Arrays.stream(collections).flatMap(Collection::stream).collect(Collectors.toList());
}
Then we can make use of this method as:
List<String> result3 = Utils.concatenateLists(list1,list2,list3);
Off the top of my head, I can shorten it by one line:
List<String> newList = new ArrayList<String>(listOne);
newList.addAll(listTwo);
In Java 8 (the other way):
List<?> newList =
Stream.of(list1, list2).flatMap(List::stream).collect(Collectors.toList());
The smartest in my opinion:
/**
* @param smallLists
* @return one big list containing all elements of the small ones, in the same order.
*/
public static <E> List<E> concatenate (final List<E> ... smallLists)
{
final ArrayList<E> bigList = new ArrayList<E>();
for (final List<E> list: smallLists)
{
bigList.addAll(list);
}
return bigList;
}
You can create your generic Java 8 utility method to concat any number of lists.
@SafeVarargs
public static <T> List<T> concat(List<T>... lists) {
return Stream.of(lists).flatMap(List::stream).collect(Collectors.toList());
}
A little shorter would be:
List<String> newList = new ArrayList<String>(listOne);
newList.addAll(listTwo);
I can't improve on the two-liner in the general case without introducing your own utility method, but if you do have lists of Strings and you're willing to assume those Strings don't contain commas, you can pull this long one-liner:
List<String> newList = new ArrayList<String>(Arrays.asList((listOne.toString().subString(1, listOne.length() - 1) + ", " + listTwo.toString().subString(1, listTwo.length() - 1)).split(", ")));
If you drop the generics, this should be JDK 1.4 compliant (though I haven't tested that). Also not recommended for production code ;-)
another one liner solution using Java8 stream, since flatMap solution is already posted, here is a solution without flatMap
List<E> li = lol.stream().collect(ArrayList::new, List::addAll, List::addAll);
or
List<E> ints = Stream.of(list1, list2).collect(ArrayList::new, List::addAll, List::addAll);
code
List<List<Integer>> lol = Arrays.asList(Arrays.asList(1, 2, 3), Arrays.asList(4, 5, 6));
List<Integer> li = lol.stream().collect(ArrayList::new, List::addAll, List::addAll);
System.out.println(lol);
System.out.println(li);
output
[[1, 2, 3], [4, 5, 6]]
[1, 2, 3, 4, 5, 6]
public static <T> List<T> merge(List<T>... args) {
final List<T> result = new ArrayList<>();
for (List<T> list : args) {
result.addAll(list);
}
return result;
}
We can join 2 lists using java8 with 2 approaches.
List<String> list1 = Arrays.asList("S", "T");
List<String> list2 = Arrays.asList("U", "V");
1) Using concat :
List<String> collect2 = Stream.concat(list1.stream(), list2.stream()).collect(toList());
System.out.println("collect2 = " + collect2); // collect2 = [S, T, U, V]
2) Using flatMap :
List<String> collect3 = Stream.of(list1, list2).flatMap(Collection::stream).collect(toList());
System.out.println("collect3 = " + collect3); // collect3 = [S, T, U, V]
Slightly simpler:
List<String> newList = new ArrayList<String>(listOne);
newList.addAll(listTwo);
No way near one-liner, but I think this is the simplest:
List<String> newList = new ArrayList<String>(l1);
newList.addAll(l2);
for(String w:newList)
System.out.printf("%s ", w);
import java.util.AbstractList;
import java.util.List;
/**
* The {@code ConcatList} is a lightweight view of two {@code List}s.
* <p>
* This implementation is <em>not</em> thread-safe even though the underlying lists can be.
*
* @param <E>
* the type of elements in this list
*/
public class ConcatList<E> extends AbstractList<E> {
/** The first underlying list. */
private final List<E> list1;
/** The second underlying list. */
private final List<E> list2;
/**
* Constructs a new {@code ConcatList} from the given two lists.
*
* @param list1
* the first list
* @param list2
* the second list
*/
public ConcatList(final List<E> list1, final List<E> list2) {
this.list1 = list1;
this.list2 = list2;
}
@Override
public E get(final int index) {
return getList(index).get(getListIndex(index));
}
@Override
public E set(final int index, final E element) {
return getList(index).set(getListIndex(index), element);
}
@Override
public void add(final int index, final E element) {
getList(index).add(getListIndex(index), element);
}
@Override
public E remove(final int index) {
return getList(index).remove(getListIndex(index));
}
@Override
public int size() {
return list1.size() + list2.size();
}
@Override
public boolean contains(final Object o) {
return list1.contains(o) || list2.contains(o);
}
@Override
public void clear() {
list1.clear();
list2.clear();
}
/**
* Returns the index within the corresponding list related to the given index.
*
* @param index
* the index in this list
*
* @return the index of the underlying list
*/
private int getListIndex(final int index) {
final int size1 = list1.size();
return index >= size1 ? index - size1 : index;
}
/**
* Returns the list that corresponds to the given index.
*
* @param index
* the index in this list
*
* @return the underlying list that corresponds to that index
*/
private List<E> getList(final int index) {
return index >= list1.size() ? list2 : list1;
}
}
No way near one-liner, but I think this is the simplest:
List<String> newList = new ArrayList<String>(l1);
newList.addAll(l2);
for(String w:newList)
System.out.printf("%s ", w);
Java 8 version with support for joining by object key:
public List<SomeClass> mergeLists(final List<SomeClass> left, final List<SomeClass> right, String primaryKey) {
final Map<Object, SomeClass> mergedList = new LinkedHashMap<>();
Stream.concat(left.stream(), right.stream())
.map(someObject -> new Pair<Object, SomeClass>(someObject.getSomeKey(), someObject))
.forEach(pair-> mergedList.put(pair.getKey(), pair.getValue()));
return new ArrayList<>(mergedList.values());
}
Found this question looking to concatenate arbitrary amount of lists, not minding external libraries. So, perhaps it will help someone else:
com.google.common.collect.Iterables#concat()
Useful if you want to apply the same logic to a number of different collections in one for().
Here's an approach using streams and java 8 if your lists have different types and you want to combine them to a list of another type.
public static void main(String[] args) {
List<String> list2 = new ArrayList<>();
List<Pair<Integer, String>> list1 = new ArrayList<>();
list2.add("asd");
list2.add("asdaf");
list1.add(new Pair<>(1, "werwe"));
list1.add(new Pair<>(2, "tyutyu"));
Stream stream = Stream.concat(list1.stream(), list2.stream());
List<Pair<Integer, String>> res = (List<Pair<Integer, String>>) stream
.map(item -> {
if (item instanceof String) {
return new Pair<>(0, item);
}
else {
return new Pair<>(((Pair<Integer, String>)item).getKey(), ((Pair<Integer, String>)item).getValue());
}
})
.collect(Collectors.toList());
}
In Java 8:
List<String> newList = Stream.concat(listOne.stream(), listTwo.stream())
.collect(Collectors.toList());
You could use the Apache commons-collections library:
List<String> newList = ListUtils.union(list1, list2);
A little shorter would be:
List<String> newList = new ArrayList<String>(listOne);
newList.addAll(listTwo);
This is simple and just one line, but will add the contents of listTwo to listOne. Do you really need to put the contents in a third list?
Collections.addAll(listOne, listTwo.toArray());
We can join 2 lists using java8 with 2 approaches.
List<String> list1 = Arrays.asList("S", "T");
List<String> list2 = Arrays.asList("U", "V");
1) Using concat :
List<String> collect2 = Stream.concat(list1.stream(), list2.stream()).collect(toList());
System.out.println("collect2 = " + collect2); // collect2 = [S, T, U, V]
2) Using flatMap :
List<String> collect3 = Stream.of(list1, list2).flatMap(Collection::stream).collect(toList());
System.out.println("collect3 = " + collect3); // collect3 = [S, T, U, V]
Off the top of my head, I can shorten it by one line:
List<String> newList = new ArrayList<String>(listOne);
newList.addAll(listTwo);
Use a Helper class.
I suggest:
public static <E> Collection<E> addAll(Collection<E> dest, Collection<? extends E>... src) {
for(Collection<? extends E> c : src) {
dest.addAll(c);
}
return dest;
}
public static void main(String[] args) {
System.out.println(addAll(new ArrayList<Object>(), Arrays.asList(1,2,3), Arrays.asList("a", "b", "c")));
// does not compile
// System.out.println(addAll(new ArrayList<Integer>(), Arrays.asList(1,2,3), Arrays.asList("a", "b", "c")));
System.out.println(addAll(new ArrayList<Integer>(), Arrays.asList(1,2,3), Arrays.asList(4, 5, 6)));
}
Off the top of my head, I can shorten it by one line:
List<String> newList = new ArrayList<String>(listOne);
newList.addAll(listTwo);
One of your requirements is to preserve the original lists. If you create a new list and use addAll(), you are effectively doubling the number of references to the objects in your lists. This could lead to memory problems if your lists are very large.
If you don't need to modify the concatenated result, you can avoid this using a custom list implementation. The custom implementation class is more than one line, obviously...but using it is short and sweet.
CompositeUnmodifiableList.java:
public class CompositeUnmodifiableList<E> extends AbstractList<E> {
private final List<E> list1;
private final List<E> list2;
public CompositeUnmodifiableList(List<E> list1, List<E> list2) {
this.list1 = list1;
this.list2 = list2;
}
@Override
public E get(int index) {
if (index < list1.size()) {
return list1.get(index);
}
return list2.get(index-list1.size());
}
@Override
public int size() {
return list1.size() + list2.size();
}
}
Usage:
List<String> newList = new CompositeUnmodifiableList<String>(listOne,listTwo);
Another Java 8 one-liner:
List<String> newList = Stream.of(listOne, listTwo)
.flatMap(Collection::stream)
.collect(Collectors.toList());
As a bonus, since Stream.of() is variadic, you may concatenate as many lists as you like.
List<String> newList = Stream.of(listOne, listTwo, listThree)
.flatMap(Collection::stream)
.collect(Collectors.toList());
You can do a oneliner if the target list is predeclared.
(newList = new ArrayList<String>(list1)).addAll(list2);
Java 8 version with support for joining by object key:
public List<SomeClass> mergeLists(final List<SomeClass> left, final List<SomeClass> right, String primaryKey) {
final Map<Object, SomeClass> mergedList = new LinkedHashMap<>();
Stream.concat(left.stream(), right.stream())
.map(someObject -> new Pair<Object, SomeClass>(someObject.getSomeKey(), someObject))
.forEach(pair-> mergedList.put(pair.getKey(), pair.getValue()));
return new ArrayList<>(mergedList.values());
}
In Java 8 (the other way):
List<?> newList =
Stream.of(list1, list2).flatMap(List::stream).collect(Collectors.toList());
Source: Stackoverflow.com