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How to use generics with interfaces

I'm fairly new to programming and we do have an exercise, we have to use the Consumer interface, we have a generic class (genClass) that has a Consumer<T> attribute (conAtt). In another class we have to use the accept method of Consumer, but somehow it doesn't work. I have already read through the Java API of the Consumer interface, but it didn't help.
The error message says:
The method accept(capture#4-of ?) in the type Consumer<capture#4-of ?> is not applicable for the arguments (capture#5-of ?)
I know it says not applicable, but why not?
public abstract class GenClass<T> {
protected Consumer<T> conAtt;
public abstract T getData();
}
class Otherclass{
private List<GenClass<?>> helparray= new ArrayList<>();
private void testmethod() {
Iterator<GenClass<?>> hilfe = helparray.iterator();
while (hilfe.hasNext()) {
GenClass<?> help = hilfe.next();
help.conAtt.accept(help.getData());//here is the problem
}
}
}
public class thirdclass extends GenClass<Character> {
#Override
public Character getData() {
return 't';//t is just an example
}
}

This is not really a question about how the Consumer - or other interfaces - in Java work, but about Generics.
Generics aim to simplify the way of writing code and avoid code repetitions. E.g. you need to do a similar task, but for different types you can write it once by using Generics instead of writing it over and over again, just with concrete types being replaced.
For example one day you have the need to keep track of a list of Strings. As easy as that, your going ahead and implementing a solution for that, whereby the first implementation can look like the following (note: a very simplified example, but it'll show the purpose):
public class CustomListString {
private String[] elements = new String[10];
public void add(String newElement) {
int nextFreeIndex = findNextFreeIndex();
elements[nextFreeIndex] = newElement;
}
public String get(int index) {
return elements[index];
}
}
So you can use the above implementation of the List in your code like the following:
public static void main(String[] args) {
CustomListString listOfStrings = new CustomListString();
listOfStrings.add("A");
listOfStrings.add("B");
}
Simple, specific and sufficient!
But the other day, you also have the requirement to keep track of a list of Integers. What to do now?
A way to solve this is to just repeat your previous approach and to implement another CustomList only for the Integers now. Where the corresponding implementation would look like this (the implementation of CustomListString has been copied and all occurrences of String have been replaced by Integer):
public class CustomListInteger {
private Integer[] elements = new Integer[10];
public void add(Integer newElement) {
int nextFreeIndex = findNextFreeIndex();
elements[nextFreeIndex] = newElement;
}
public Integer get(int index) {
return elements[index];
}
}
As you can imagine now already, this is not flexible and can be very cumbersome in the future. This approach will require a new implementation of each type you want to store in the future. So you might end up to also create implementations like CustomListDouble, CustomListCharacter, ... and so on, in which only the type of the elements within the array change - nothing else which would be of importance!
This will additionally lead to the situation, that you'll duplicate a lot of similar code (like findNextFreeIndex() method would have been) and in case of a bugfix need to adjust it in a lot of places instead of in only one.
To solve this issue and remain the type safety in the CustomList.get method Generics have been introduced to Java!
With the Generics approach you'll be able to create a single implementation of the CustomList to store all of your data types without unnecessarily duplicating any shared, basic code and remain the type safety!
public class CustomList<T> {
private Object[] elements = new Object[10]; // Java doesn't supprort easily support generic arrays, so using Object
// here. But the compiler ensures only elements of the generic type T
// will end up here
public void add(T newElement) {
int nextFreeIndex = findNextFreeIndex();
elements[nextFreeIndex] = newElement;
}
#SuppressWarnings("unchecked")
public T get(int index) {
return (T) elements[index];
}
}
Using the new list following the Generics approach we can use it like this now:
public static void main(String[] args) {
CustomList<String> genericList = new CustomList<>();
genericList.add("Hello World");
genericList.add(5); // Compile error! Integer and String types cannot be mixed in
// a single instance of the list anymore => Nice, prevents errors!
genericList.get(0).substring(6); // No compile error, also the compiler knows Strings
// are contained in the list
}
The generic CustomList can now also be reused for any other type and still provide type safety.
What does it mean for your implementation
You can see how we specified the generic type in the CustomList class as T - this is similar like you specified it with ? (probably you'll also want to replace it with T, since you'll run into other issues later when working with the Consumer). But when we used the implementation in our other classes, it wouldn't have been possible to specify it as CustomList<T> or CustomList<?> anymore. We needed to decide and specifiy which exact type of elements the list should contain. This has been the String class, so we specified it as CustomList<String>.
Note: ? is a generic wildcard and means something like "I don't know the real type of the classes now and I'll also don't know it in the future". That's why it'll be hard for you working with the concrete types later in the Consumer. You'll be not able to call any conrete methods on your objects therein. Therefore ? should be avoided as a generic type argument and something like T should be used instead. T means something like "I don't know the real type of the classes now, but I'll do later, as soon as you tell me". Therfore you'll be able to call concrete methods on the objects later in the Consumer, what will simplify your work there a lot.
For your code this means, wherever you want to use your implementation of GenClass<T> you need to specify with which exact kind of elements the class is going to work with. In case of String it is GenClass<String> in case of Character GenClass<Character>.
So the place you'll need to replace the occurrences of GenClass<?> is wherever you refer to it in Otherclass and Otherclass.testmethod.
The way you used the Consumer is fine

How to return collections' data without returning a collection itself?

I have a class (A.java) that contains two private fields of type ArrayList and HashMap.
I also have another class (B.java) that should have access to their data. I could make two getters, but I don't want to return my collections as is. Class B.java should only have access to data, not to add(), isEmpty(), containsKey() etc.
Can I return my collections in such way, so I could somehow use it with foreach in class B somehow but without giving the possibility to modify them?

Don't return a collection, return a Stream. That way it is easy for the user to know that they are getting a stream of objects, not a collection. And it's easy to change the implementation of the collection without changing the way it's used. It's trivial for the user to filter, map, reduce collect etc.
So:
class A {
private List<C> cs = new ArrayList<>();
public Stream<C> getCs() {
return cs.stream();
}
}
class B {
public void processCs(A a) {
a.getCs().filter(C::hasFooness).forEach(...);
}
}

Create a getter method that returns a "Collections.unmodifiableList()" like this:
List<String> strings = new ArrayList<String>();
List<String> unmodifiable = Collections.unmodifiableList(strings);
unmodifiable.add("New string"); // will fail at runtime
strings.add("Aha!"); // will succeed
System.out.println(unmodifiable);

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.

Automatically merge several collections to one

I have some Guava Functions like Function<String,Set<String>>. Using those with FluentIterable.transform() leads to a FluentIterable<Set<String>>, however I need a FluentIterable<String>. So my idea now would be to subclass FluentIterable<E> and add a new method transform2() which simply merges everything to one collection before returning it.
The original transform method looks like this:
public final <T> FluentIterable<T> transform(Function<? super E, T> function) {
return from(Iterables.transform(iterable, function));
}
I thought of something like this for my subclass and transform2() method:
public abstract class FluentIterable2<E> extends FluentIterable<E>
{
public final <T> FluentIterable<T> transform2(Function<? super E, Collection<T>> function) {
// (PROBLEM 1) Eclipse complains: The field FluentIterable<E>.iterable is not visible
Iterable<Collection<T>> iterables = Iterables.transform(iterable, function);
// (PROBLEM 2) Collection<T> merged = new Collection<T>(); // I need a container / collection - which one?
for(Collection<T> iterable : iterables)
{
// merged.addAll(iterable);
}
// return from(merged);
}
}
Currently I have two problems with my new subclass, marked above with PROBLEM 1 and PROBLEM 2
PROBLEM 1: The iterable field in the original FluentIterable class is private - what can I do about this? Can I create a new private field with the same name in my subclass, will this then be OK? What about methods in my subclass that call super.someMethod() which uses this field? Will they then use the field of the super class, which probably has a different value?
PROBLEM 2: I need some generic collection where I can combine the content of several collections, but collections is an interface, so I can't instantiate it. So, which class can I use there?
It would be acceptable if the solution only works with sets, though I'd prefer a solution that works with sets and lists.
Thanks for any hint on this!

Does FluentIterable.transformAndConcat(stringToSetFunction) not work for your use case?

Why subclass FluentIterable just to do this? You just need a simple loop:
Set<String> union = Sets.newHashSet();
for (Set<String> set : fluentIterableOfSets) {
union.addAll(set);
}

Use FluentIterable.transformAndConcat(f), where f is a Function mapping an element to some kind of iterable over the element type.
In your case, let's say your Function<String, Set<String>> is called TOKENIZE, and your initial Iterable<String> is called LINES.
Then to get a Set<String> holding all the distinct tokens in LINES, do this:
Iterable<String> LINES = ...;
Function<String, Set<String>> TOKENIZE = ...;
Set<String> TOKENS = FluentIterable.from(LINES)
.transformAndConcat(TOKENIZE)
.toSet();
But consider JB Nizet's answer carefully. Try it both ways and see which works better.

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?