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Do arraylist needs to be specified as return type in method after modification?

I would like to know what is the best practice to return 'updated' ArrayList?
For example, if I am adding in a new element, it seems that whether if I did or did not specify the return type (see the addNewA() and addNewB()), the ArrayList in my main() method will still be 'updated' nonetheless.
Should I or should I not specify the return type?
Currently in my client program, most of the methods, I have specified it as void (no return type) and while the overall program still does works as intended, thought I would like to get this clarified and make the necessary changes if necessary.
public class MyClient
{
public static ArrayList<Person> addNewA(ArrayList<Person> myArray)
{
Person jack = new Person("jack", 24);
myArray.add(jack);
return myArray;
}
public static void addNewB(ArrayList<Person> myArray)
{
Person ben= new Person("ben", 19);
myArray.add(ben);
}
public static void main(String[] args)
{
ArrayList<Person> personArray= new ArrayList();
addNewA(personArray); // will return me an array size of 1
addNewB(personArray); // will return me an array size of 2
}
}

In a case like this, you should not return the list and should make your method void.
Different languages have different conventions, but one of Java's is that methods that operate by modifying their arguments (or the object they're called on) should not return values. Returning a value implies to someone using your code that a different object is being returned, since otherwise there is no use in returning an object the caller already has1. A method that is void, on the other hand, couldn't possibly be returning a copied-and-extended list, so it's very clear that it's intended to operate by modifying the list that you give it in the first place.
(By the way, you should also just use List<Person>, and you should pay attention to the warning you get about using new ArrayList() instead of new ArrayList<>().)
1 There is a specific exception to this, called the fluent builder pattern, but it's not easily confused with general code like this.

In java (and most high level strict type languages) Objects are passed by reference and primitives passed by value.
When using the new keyword you create an object.
While primitives (like int, char, double ect) are passed by value (meaning that a copy of the value of the variable will be sent to the invoked function), Object types are passed by reference, meaning that the actual original object is passed to the function.
To sum up - since you are using object here (ArrayList), you don't need a return type since the original object is changing.

Which is good practice - Modifying a List in the method, or returning a new List in the method?

Example code:
modifyMyList(myList);
public void modifyMyList(List someList){
someList.add(someObject);
}
or:
List myList = modifyMyList(myList);
public List modifyMyList(List someList){
someList.add(someObject)
return someList;
}
There is also a 3rd option I believe: You can create a new List in modifyMyList method and return this new List...
( 3rd option is here, I was too lazy but someone already added it in the answers: )
List myList = modifyMyList(myList);
public List modifyMyList(List someList){
List returnList = new ArrayList();
returnList.addAll(someList);
returnList.add(someObject);
return Collections.unmodifiableList(returnList);
}
Is there any reason why I should choose one over another? What should be considered in such case?

I have a (self imposed) rule which is "Never mutate a method parameter in a public method". So, in a private method, it's ok to mutate a parameter (I even try to avoid this case too). But when calling a public method, the parameters should never be mutated and should be considered immutable.
I think that mutating method arguments is a bit hacky and can lead to bugs that are harder to see.
I have been known to make exceptions to this rule but I need a really good reason.

Actually there is no functional difference.
You'll come to know the difference when you want the returned list
List someNewList = someInstnace.modifyMyList(list);

The second is probably confusing as it implies a new value is being created and returned - and it isn't.
An exception would be if the method was part of a 'fluent' API, where the method was an instance method and was modifying its instance, and then returning the instance to allow method chaining: the Java StringBuilder class is an example of this.
In general, however, I wouldn't use either.
I'd go for your third option: I write a method that creates and returns a new list with the appropriate change. This is a bit artificial in the case of your example, as the example is really just reproducing List.add(), but...
/** Creates a copy of the list, with val appended. */
public static <T> List<T> modifyMyList(List<T> list, T val) {
List<T> xs = new ArrayList<T>(list);
xs.add(val);
return xs;
}
Aside: I wouldn't, as suggested by Saket return an immutable list. His argument for immutability and parallelism is valid. But most of the time Java programmers expect to be able to modify a collection, except in special circumstances. By making you method return an immutable collection, you limit it's reusability to such circumstances. (The caller can always make the list immutable if they want to: they know the returned value is a copy and won't be touched by anything else.) Put another way: Java is not Clojure. Also, if parallelism is a concern, look at Java 8 and streams (the new kind - not I/O streams).
Here's a different example:
/** Returns a copy of a list sans-nulls. */
public static <T> List<T> compact(Iterable<T> it) {
List<T> xs = new ArrayList<T>();
for(T x : it)
if(x!=null) xs.add(x);
return xs;
}
Note that I've genercized the method and made it more widely applicable to taking an Iterable instead of a list. In real code, I'd have two overloaded versions, one taking an Iterable and one an Iterator. (The first would be implemented by calling the second, with the iterable's iterator.) Also, I've made it static as there was no reason for your method to be an instance method (it does not depend on state from the instance).
Sometimes, though, if I'm writing library code, and if it is not clear whether a mutating or non-mutating implementation is more generally useful, I create both. Here's a fuller example:
/** Returns a copy of the elements from an Iterable, as a List, sans-nulls. */
public static <T> List<T> compact(Iterable<T> it) {
return compact(it.iterator());
}
public static <T> List<T> compact(Iterator<T> iter) {
List<T> xs = new ArrayList<T>();
while(iter.hasNext()) {
T x = iter.next();
if(x!=null) xs.add(x);
}
return xs;
}
/** In-place, mutating version of compact(). */
public static <T> void compactIn(Iterable<T> it) {
// Note: for a 'fluent' version of this API, have this return 'it'.
compactIn(it.iterator());
}
public static <T> void compactIn(Iterator<T> iter) {
while(iter.hasNext()) {
T x = iter.next();
if(x==null) iter.remove();
}
}
If this was in a real API I'd check the arguments for null and throw IllegalArgumentException. (NOT NullPointerException - though it is often used for this purpose. NullPointerException happens for other reasons as well, e.g. buggy code. IllegalArgumentException is better for invalid parameters.)
(There'd also be more Javadoc than actual code too!)

The first and second solution are very similar, The advantage of the second is to permit chaining. The question of "is it a good practise" is subjected to debate as we can see here:
Method Chaining in Java
So the real question is between the first solution with mutable list and the third with a unmutable list, and again, there is not a unique response, it is the same debate between returning String, which are immutable and using Stringbuffer, which are mutable but permits better performance.
If you need reliablility of your API , and if you don't have performance issues use immutable (the third solution). Use it if your lists are always small.
If you need only performance use a mutable list (the first solution)

I will recommend creating a new list in the method and returning an immutable list. That way your code will work even when you are passed in an Immutable list. It is generally a good practice to create immutable objects as we generally move towards functional programming and try to scale across multiple processor architectures.
List myList = modifyMyList(myList);
public List modifyMyList(List someList){
List returnList = new ArrayList();
returnList.addAll(someList);
returnList.add(someObject);
return Collections.unmodifiableList(returnList);
}

As I said in my other answer, I don't think you should mutate the list parameter. But there are times where you also don't want to take a copy of the original list and mutate the copy.
The original list might be large so the copy is expensive
You want the copy to be kept up-to-date with any updates to the original list.
In these scenarios, you could create a MergedList which is a view over two (or perhaps more) lists
import java.util.*;
public class MergedList<T> extends AbstractList<T> {
private final List<T> list1;
private final List<T> list2;
public MergedList(List<T> list1, List<T> list2) {
this.list1 = list1;
this.list2 = list2;
}
#Override
public Iterator<T> iterator() {
return new Iterator<T>() {
Iterator<T> it1 = list1.iterator();
Iterator<T> it2 = list1.iterator();
#Override
public boolean hasNext() {
return it1.hasNext() || it2.hasNext();
}
#Override
public T next() {
return it1.hasNext() ? it1.next() : it2.next();
}
};
}
#Override
public T get(int index) {
int size1 = list1.size();
return index < size1 ? list1.get(index) : list2.get(index - size1);
}
#Override
public int size() {
return list1.size() + list2.size();
}
}
The you could do
public List<String> modifyMyList(List<String> someList){
return new MergedList(someList, List.of("foo", "bar", "baz"));
}

Both ways will work because in this case java works with the reference of the List but i prefer the secound way because this solution works for pass by value too, not only for pass by reference.

Functionally both are same.
However when you expose your method as an API, second method may give an impression that it returns a new modified list other than the original passed list.
While the first method would make it clear (of-course based on method naming convention) that it will modify the original list (Same object).
Also, the second method returns a list, so ideally the caller should check for a null return value even if the passed list is non null (The method can potentially return a null instead of modified list).
Considering this I generally prefer to use method one over second.

How to move contents of one ArrayList to another?

Is there a way to move the entire contents of an ArrayList to another instance of ArrayList in O(1)?
I.e.: only the reference to the backing array is passed from one instance to the other (elements are not copied one by one).
For example:
ArrayList<String> a = new ArrayList<>(Arrays.asList("A", "B", "C"));
ArrayList<String> b = new ArrayList<>();
a.moveContentsTo(b);
// 'a' is now empty, while 'b' contains everything that 'a' did before and 'a != b'
// It is desired that the 'moveContentsTo' method is O(1)
Even better, is there an ArrayList#swapContents(ArrayList) method?
Further explanation and use-case:
Further explanation 1: the references of 'a' and 'b' must not be exchanged. I am not looking for tmp = a; a = b; b = tmp; type of solutions.
Further explanation 2: The operation must be ~O(1) in time.
The use-case: This is useful when an object wants to encapsulate a list constructed outside:
public class A {
private ArrayList<String> items = new ArrayList<>();
/**
* This method takes the sole ownership of the contents. Whoever
* passed the list from the outside will not be able to modify
* contents of 'this.items' from outside the class.
*/
public AnImmutableObject(ArrayList<String> items) {
if (items != null) {
items.moveContentsTo(this.items);
}
}
/**
* Other collections that do not provide the 'move' functionality
* must be copied. If we just stored the reference to 'items' we
* would break encapsulation as whoever called the constructor
* still have write capabilities to the collection.
*/
public A(Collection<String> items) {
if (items != null) {
this.items.addAll(items);
}
}
public List<String> getItems() {
return Collections.unmodifiableList(items);
}
}
Notice that we want to avoid making a copy (to increase speed and decrease memory usage). The crucial bit is that the callee must lose the ability to modify the (now encapsulated) ArrayList.

#Lirik answer is greate +1. However, if you are looking for a real ArrayList#swapContents(ArrayList), here is how you can do it:
public static void swapContents(ArrayList<String> listA, ArrayList<String> listB)
{
List<String> temp = new ArrayList<String>(listA);
listA.clear();
listA.addAll(listB);
listB.clear();
listB.addAll(temp);
}

AFAIK, it's very not Java-like to keep track of "ownership" of references (at least on the programmer's side) which is why I doubt that you'll find the std::move()-like functionality that you want. It just isn't very commonly needed in Java.
I guess C++ needs to keep track of object ownership explicitly because there is no garbage collection.
I think that your best bet is to create a defensive copy in your constructor and save space by relying on copy constructors of immutable objects:
public class AnImmutableObject {
private final List<String> items;
/**
* Creates a new immutable object with the given list of items.
* Always deep copy from an outside source, because we can't know if the
* calling code will modify that collection later.
*/
public AnImmutableObject(Collection<String> items) {
// It's a constructor. We know items needs to be set.
// We are creating a new array list, using its constructor which deep
// copies the collection, and immediately wrapping it to make it truly
// immutable. We are guaranteed that no one will hold a reference to the
// mutable view of the new ArrayList.
this.items = Collections.unmodifiableList(new ArrayList<String>(items));
}
/**
* Creates a new immutable object with the same items as another.
* Copying the reference here is completely safe because we
* enforce the immutability of the items array.
*/
public AnImmutableObject(AnImmutableObject source) {
items = source.items;
}
public List<String> getItems() {
return items;
}
}
At this point, you can "pass the arrays around" (really share them) in O(1) between your own immutable objects:
ImmutableObject a = new ImmutableObject(Arrays.asList("A", "B", "C")); // O(n)
ImmutableObject b = new ImmutableObject(a); // O(1)
Hopefully, something like this can suit your purposes.
Another route you could go is use Guava's ImmutableList. Since these are immutable, you can safely copy the reference to the ImmutableList in a constructor.
The main approach is about making it safe for you to copy references to the lists rather than taking ownership over them.

This should do it:
ArrayList<String> a = new ArrayList<>(Arrays.asList("A", "B", "C"));
ArrayList<String> b = a;
a = new ArrayList<>();
Conceptually speaking, a is now empty and b contains what a contained before. There was a single assignment and no copying of data, which is about the fastest you can do it. Does that satisfy your requirement, or do you actually want a to still reference the same array except that the given array should now be empty?
Update
I don't believe that in C++ the time complexity for the move operation is O(1) either. It's also prudent to point out that "because classes in Java use reference semantics, there are never any implicit copies of objects in those languages. The problem move semantics solve does not and has never existed in Java." (see the answer by FredOverflow: C++ Rvalue references and move semantics)
Is there a way to move the entire contents of an ArrayList to another ArrayList so that only the reference to the backing array is passed from one to the other (i.e., so that elements are not copied one by one).
Given the above statement, then if you copy something from array a to array b in Java, both arrays will reference the same data. All that you do with move semantics in C++ is that you save the temporary object which needs to be created in order to make such a copy:
X foo();
X x;
// perhaps use x in various ways
x = foo();
The last one does:
destructs the resource held by x,
clones the resource from the temporary returned by foo,
destructs the temporary and thereby releases its resource.
Move semantics does:
swap resource pointers (handles) between x and the temporary,
temporary's destructor destruct x's original resource.
You save one destruct, but only in C++... the above problem does not exist in Java! See this article for more details on move semantics: http://thbecker.net/articles/rvalue_references/section_02.html

How do I perform an action on each element of a List and return the result (without affecting the original of course)?

How do I write a static method in Java that will take a List, perform an action on each element, and return the result (without affecting the original of course)?
For example, if I want to add 2 to each element what goes in the ... here? The concrete return type must be the same, e.g. if my List is a LinkedList with values 1,2,3 I should get back a LinkedList with values 3,4,5. Similarly for ArrayList, Vector, Stack etc, which are all Lists.
I can see how to do this using multiple if (lst instanceof LinkedList) ... etc... any better way?
import java.util.List;
public class ListAdd {
static List<Integer> add2 (List<Integer> lst) {
...
return result;
}
}

There are already many answers, but I'd like to show you a different way to think of this problem.
The operation you want to perform is known as map in the world of functional programming. It is something we do really all the time in functional languages.
Let M<A> be some kind of container (in your case, M would be List, and A would be Integer; however, the container can be lots of other things). Suppose you have a function that transforms As into Bs, that is, f: A -> B. Let's write this function as of type F<A, B>, to use a notation closer to Java. Note that you can have A = B, as in the example you give (in which A = B = Integer).
Then, the operation map is defined as follows:
M<B> map(M<A>, F<A, B>)
That is, the operation will return a M<B>, presumably by applying F<A, B> to each A in M<A>.
In practice...
There's a brilliant library developed by Google, called Guava, which brings lot's of functional idioms to Java.
In Guava, the map operation is called transform, and it can operate on any Iterable. It has also more specific implementations that work directly on lists, sets, etc.
Using Guava, the code you want to write would look like this:
static List<Integer> add2(List<Integer> ns) {
return Lists.transform(ns, new Function<Integer, Integer>() {
#Override Integer apply(Integer n) { return n + 2; }
}
}
Simple as that.
This code won't touch the original list, it will simply provide a new list that calculates its values as needed (that is, the values of the newly created list won't be calculated unless needed -- it's called a lazy operation).
As a final consideration, it is not possible for you to be absolutely sure that you will be able to return exactly the same implementation of List. And as many others pointed out, unless there's a very specific reason for this, you shouldn't really care. That's why List is an interface, you don't care about the implementation.

Fundamentally, the List interface doesn't make any guarantees that you'll have a way to duplicate it.
You may have some luck with various techniques:
Using clone() on the passed in List, although it may throw, or (since it is protected in Object) simply not be accessible
Use reflection to look for a public no-argument constructor on the passed-in List
Try to serialize and deserialize it in order to perform a "deep clone"
Create some sort of factory and build in knowledge of how to duplicate each different kind of List your code may encounter (What if it's a wrapper created by unmodifiableList(), or some oddball custom implementation backed by a RandomAccessFile?)
If all else fails, either throw, or return an ArrayList or a Vector for lack of better options

You could use reflection to look for a public zero-arg constructor on the result of lst.getClass() and then invoke() it to obtain the List into which you'll place your results. The Java Collections Framework recommends that any derivative of Collection offer a zero-arg constructor. That way, your results we be of the same runtime class as the argument.

Here is a variant which does neither copies nor modifies the original list. Instead, it wraps the original list by another object.
public List<Integer> add2(final List<Integer> lst) {
return new AbstractList<Integer>() {
public int size() {
return lst.size();
}
public Integer get(int index) {
return 2 + lst.get(index);
}
};
}
The returned list is not modifiable, but will change whenever the original list changes.
(This implements the iterator based on index access, thus it will be slow for a linked list. Then better implement it based on AbstractSequentialList.)
Of course, the resulting list will obviously not be of the same class as the original list.
Use this solution only if you really only need a read-only two added view of your original list, not if you want a modified copy with similar properties.

The whole point of using an interface, in this case List, is to abstract the fact that the implementation is hidden behind the interface.
Your intention is clear to me, however: the Clonable interface supports creating a new instance with the same state. This interface might not be defined on your List.
Often it's a good idea to rethink this situation: why do you need to clone the List in this place, this class? Shouldn't your list-creator be responsible for cloning the list? Or shouldn't the caller, who knows the type, make sure he passes in a clone of his list?
Probably, if you look for the semantics as you defined it, you can implement all your supported Lists:
static Vector<Integer> addTwo(Vector<Integer> vector) {
Vector<Integer> copy = null; // TODO: copy the vector
return addTwo_mutable(copy);
}
static ArrayList<Integer> addTwo(ArrayList<Integer> aList) {
ArrayList<Integer> copy = null; // TODO: copy the array list
return addTwo_mutable(copy);
}
static LinkedList<Integer> addTwo(LinkedList<Integer> lList) {
LinkedList<Integer> copy = null; // TODO: copy the linked list
return addTwo_mutable(copy);
}
private <T extends List<Integer>> static T addTwo_mutable(T list) {
return list; // TODO: implement
}
Even, when you don't support a data-type, you'll get a nice compiler error that the specified method does not exists.
(code not tested)

Just to show you that what you want to do is not possible in the general case, consider the following class:
final class MyList extends ArrayList<Integer> {
private MyList() {
super.add(1);
super.add(2);
super.add(3);
}
private static class SingletonHolder {
private static final MyList instance = new MyList();
}
public static MyList getInstance() {
return SingletonHolder.instance;
}
}
It is a singleton (also, a lazy, thread-safe singleton by the way), it's only instance can be obtained from MyList.getInstance(). You cannot use reflection reliably (because the constructor is private; for you to use reflection, you'd have to rely on proprietary, non-standard, non-portable APIs, or on code that could break due to a SecurityManager). So, there's no way for you to return a new instance of this list, with different values.
It's final as well, so that you cannot return a child of it.
Also, it would be possible to override every method of ArrayList that would modify the list, so that it would be really an immutable singleton.
Now, why would you want to return the exact same implementation of List?

OK well someone mentioned reflection. It seems to be an elegant solution:
import java.util.*;
public class ListAdd {
static List<Integer> add2 (List<Integer> lst) throws Exception {
List<Integer> result = lst.getClass().newInstance();
for (Integer i : lst) result.add(i + 2);
return result;
}
}
Concise, but it thows an checked exception, which is not nice.
Also, wouldn't it be nicer if we could use the method on concrete types as well, e.g. if a is an ArrayList with values 1, 2, 3, we could call add2(a) and get an ArrayList back? So in an improved version, we could make the signature generic:
static <T extends List<Integer>> T add2 (T lst) {
T res;
try {
res = (T) lst.getClass().newInstance();
} catch (InstantiationException e) {
throw new IllegalArgumentException(e);
} catch (IllegalAccessException e) {
throw new RuntimeException(e);
}
for (Integer i : lst) res.add(i + 2);
return res;
}
I think throwing a runtime exception is the least worst option if a list without a nullary construcor is passed in. I don't see a way to ensure that it does. (Java 8 type annotations to the rescue maybe?) Returning null would be kind of useless.
The downside of using this signature is that we can't return an ArrayList etc as the default, as we could have done as an alternative to throwing an exception, since the return type is guaranteed to be the same type as that passed in. However, if the user actually wants an ArrayList (or some other default type) back, he can make an ArrayList copy and use the method on that.
If anyone with API design experience reads this, I would be interested to know your thoughts on which is the preferable option: 1) returning a List that needs to be explicity cast back into the original type, but enabling a return of a different concrete type, or 2) ensuring the return type is the same (using generics), but risking exceptions if (for example) a singleton object without a nullary constructor is passed in?

Java Object Oriented Design: Returning multiple objects in java

The below code in Java throws Null pointer exception.
public class New{
int i;
New(int i)
{
this.i = i;
}
public void func(New temp)
{
temp.i = 10;
temp = new New(20);
}
public static void main(String[] args)
{
New n = null;
n.func(n);
System.out.println("value "+ n.i);
}
}
The reason being, java passes objects references by value. If I wanted to return one object, then I can return it from the function.
But, If I have multiple objects, the only way I could return the object references is, by keeping them into another object, like having some container which has references to all the objects.
Is there a better way to do it?
In C++, I normally just pass the address of pointer to handle this scenario. If I wanted to just return two objects of a single type, creating a container and passing it is a over kill.
What is the problem with returning multiple objects from a function? Why cannot the semantics of the function in all these languages be changed?

Most often you create an object to hold the combination of objects you want to return.
For a more general-purpose solution, you can either return a collection, and array or some sort of tuple, such as Pair, Triple, etc (the latter you will need to create).
Note, you don't generally pass a mutable object as a parameter, but return an immutable one:
public Pair<Integer,Integer> getLowHighTemp() {
int low,hgh;
// do stuff...
return new Pair(low,hgh);
}

This is more of 2 questions than one.
Firstly the problem with your code is that you are not declaring n before you use it. That is throwing the exception.
Secondly if you would like to return 2 objects, you need to have a container object that will hold 2 objects.

You can return some kind of Collection. Returning a Map or List is pretty common.