Due to the implementation of Java generics, you can't have code like this:
public class GenSet<E> {
private E a[];
public GenSet() {
a = new E[INITIAL_ARRAY_LENGTH]; // error: generic array creation
}
}
How can I implement this while maintaining type safety?
I saw a solution on the Java forums that goes like this:
import java.lang.reflect.Array;
class Stack<T> {
public Stack(Class<T> clazz, int capacity) {
array = (T[])Array.newInstance(clazz, capacity);
}
private final T[] array;
}
But I really don't get what's going on.
I have to ask a question in return: is your GenSet "checked" or "unchecked"?
What does that mean?
Checked: strong typing. GenSet knows explicitly what type of objects it contains (i.e. its constructor was explicitly called with a Class<E> argument, and methods will throw an exception when they are passed arguments that are not of type E. See Collections.checkedCollection.
-> in that case, you should write:
public class GenSet<E> {
private E[] a;
public GenSet(Class<E> c, int s) {
// Use Array native method to create array
// of a type only known at run time
#SuppressWarnings("unchecked")
final E[] a = (E[]) Array.newInstance(c, s);
this.a = a;
}
E get(int i) {
return a[i];
}
}
Unchecked: weak typing. No type checking is actually done on any of the objects passed as argument.
-> in that case, you should write
public class GenSet<E> {
private Object[] a;
public GenSet(int s) {
a = new Object[s];
}
E get(int i) {
#SuppressWarnings("unchecked")
final E e = (E) a[i];
return e;
}
}
Note that the component type of the array should be the erasure of the type parameter:
public class GenSet<E extends Foo> { // E has an upper bound of Foo
private Foo[] a; // E erases to Foo, so use Foo[]
public GenSet(int s) {
a = new Foo[s];
}
...
}
All of this results from a known, and deliberate, weakness of generics in Java: it was implemented using erasure, so "generic" classes don't know what type argument they were created with at run time, and therefore can not provide type-safety unless some explicit mechanism (type-checking) is implemented.
You can do this:
E[] arr = (E[])new Object[INITIAL_ARRAY_LENGTH];
This is one of the suggested ways of implementing a generic collection in Effective Java; Item 26. No type errors, no need to cast the array repeatedly. However this triggers a warning because it is potentially dangerous, and should be used with caution. As detailed in the comments, this Object[] is now masquerading as our E[] type, and can cause unexpected errors or ClassCastExceptions if used unsafely.
As a rule of thumb, this behavior is safe as long as the cast array is used internally (e.g. to back a data structure), and not returned or exposed to client code. Should you need to return an array of a generic type to other code, the reflection Array class you mention is the right way to go.
Worth mentioning that wherever possible, you'll have a much happier time working with Lists rather than arrays if you're using generics. Certainly sometimes you don't have a choice, but using the collections framework is far more robust.
Here's how to use generics to get an array of precisely the type you’re looking for while preserving type safety (as opposed to the other answers, which will either give you back an Object array or result in warnings at compile time):
import java.lang.reflect.Array;
public class GenSet<E> {
private E[] a;
public GenSet(Class<E[]> clazz, int length) {
a = clazz.cast(Array.newInstance(clazz.getComponentType(), length));
}
public static void main(String[] args) {
GenSet<String> foo = new GenSet<String>(String[].class, 1);
String[] bar = foo.a;
foo.a[0] = "xyzzy";
String baz = foo.a[0];
}
}
That compiles without warnings, and as you can see in main, for whatever type you declare an instance of GenSet as, you can assign a to an array of that type, and you can assign an element from a to a variable of that type, meaning that the array and the values in the array are of the correct type.
It works by using class literals as runtime type tokens, as discussed in the Java Tutorials. Class literals are treated by the compiler as instances of java.lang.Class. To use one, simply follow the name of a class with .class. So, String.class acts as a Class object representing the class String. This also works for interfaces, enums, any-dimensional arrays (e.g. String[].class), primitives (e.g. int.class), and the keyword void (i.e. void.class).
Class itself is generic (declared as Class<T>, where T stands for the type that the Class object is representing), meaning that the type of String.class is Class<String>.
So, whenever you call the constructor for GenSet, you pass in a class literal for the first argument representing an array of the GenSet instance's declared type (e.g. String[].class for GenSet<String>). Note that you won't be able to get an array of primitives, since primitives can't be used for type variables.
Inside the constructor, calling the method cast returns the passed Object argument cast to the class represented by the Class object on which the method was called. Calling the static method newInstance in java.lang.reflect.Array returns as an Object an array of the type represented by the Class object passed as the first argument and of the length specified by the int passed as the second argument. Calling the method getComponentType returns a Class object representing the component type of the array represented by the Class object on which the method was called (e.g. String.class for String[].class, null if the Class object doesn't represent an array).
That last sentence isn't entirely accurate. Calling String[].class.getComponentType() returns a Class object representing the class String, but its type is Class<?>, not Class<String>, which is why you can't do something like the following.
String foo = String[].class.getComponentType().cast("bar"); // won't compile
Same goes for every method in Class that returns a Class object.
Regarding Joachim Sauer's comment on this answer (I don't have enough reputation to comment on it myself), the example using the cast to T[] will result in a warning because the compiler can't guarantee type safety in that case.
Edit regarding Ingo's comments:
public static <T> T[] newArray(Class<T[]> type, int size) {
return type.cast(Array.newInstance(type.getComponentType(), size));
}
This is the only answer that is type safe
E[] a;
a = newArray(size);
#SafeVarargs
static <E> E[] newArray(int length, E... array)
{
return Arrays.copyOf(array, length);
}
To extend to more dimensions, just add []'s and dimension parameters to newInstance() (T is a type parameter, cls is a Class<T>, d1 through d5 are integers):
T[] array = (T[])Array.newInstance(cls, d1);
T[][] array = (T[][])Array.newInstance(cls, d1, d2);
T[][][] array = (T[][][])Array.newInstance(cls, d1, d2, d3);
T[][][][] array = (T[][][][])Array.newInstance(cls, d1, d2, d3, d4);
T[][][][][] array = (T[][][][][])Array.newInstance(cls, d1, d2, d3, d4, d5);
See Array.newInstance() for details.
In Java 8, we can do a kind of generic array creation using a lambda or method reference. This is similar to the reflective approach (which passes a Class), but here we aren't using reflection.
#FunctionalInterface
interface ArraySupplier<E> {
E[] get(int length);
}
class GenericSet<E> {
private final ArraySupplier<E> supplier;
private E[] array;
GenericSet(ArraySupplier<E> supplier) {
this.supplier = supplier;
this.array = supplier.get(10);
}
public static void main(String[] args) {
GenericSet<String> ofString =
new GenericSet<>(String[]::new);
GenericSet<Double> ofDouble =
new GenericSet<>(Double[]::new);
}
}
For example, this is used by <A> A[] Stream.toArray(IntFunction<A[]>).
This could also be done pre-Java 8 using anonymous classes but it's more cumbersome.
You do not need to pass the Class argument to the constructor.
Try this.
public class GenSet<T> {
private final T[] array;
#SafeVarargs
public GenSet(int capacity, T... dummy) {
if (dummy.length > 0)
throw new IllegalArgumentException(
"Do not provide values for dummy argument.");
this.array = Arrays.copyOf(dummy, capacity);
}
#Override
public String toString() {
return "GenSet of " + array.getClass().getComponentType().getName()
+ "[" + array.length + "]";
}
}
and
GenSet<Integer> intSet = new GenSet<>(3);
System.out.println(intSet);
System.out.println(new GenSet<String>(2));
result:
GenSet of java.lang.Integer[3]
GenSet of java.lang.String[2]
This is covered in Chapter 5 (Generics) of Effective Java, 2nd Edition, item 25...Prefer lists to arrays
Your code will work, although it will generate an unchecked warning (which you could suppress with the following annotation:
#SuppressWarnings({"unchecked"})
However, it would probably be better to use a List instead of an Array.
There's an interesting discussion of this bug/feature on the OpenJDK project site.
Java generics work by checking types at compile time and inserting appropriate casts, but erasing the types in the compiled files. This makes generic libraries usable by code which doesn't understand generics (which was a deliberate design decision) but which means you can't normally find out what the type is at run time.
The public Stack(Class<T> clazz,int capacity) constructor requires you to pass a Class object at run time, which means class information is available at runtime to code that needs it. And the Class<T> form means that the compiler will check that the Class object you pass is precisely the Class object for type T. Not a subclass of T, not a superclass of T, but precisely T.
This then means that you can create an array object of the appropriate type in your constructor, which means that the type of the objects you store in your collection will have their types checked at the point they are added to the collection.
Although the thread is dead, I would like to draw your attention to this.
Generics are used for type checking during compile time. Therefore, the purpose is to check
What comes in is what you need.
What you return is what the consumer needs.
Check this:
Don't worry about typecasting warnings when you are writing a generic class; worry when you are using it.
What about this solution?
#SafeVarargs
public static <T> T[] toGenericArray(T ... elems) {
return elems;
}
It works and looks too simple to be true. Is there any drawback?
The example is using Java reflection to create an array. Doing this is generally not recommended, since it isn't typesafe. Instead, what you should do is just use an internal List, and avoid the array at all.
Look also to this code:
public static <T> T[] toArray(final List<T> obj) {
if (obj == null || obj.isEmpty()) {
return null;
}
final T t = obj.get(0);
final T[] res = (T[]) Array.newInstance(t.getClass(), obj.size());
for (int i = 0; i < obj.size(); i++) {
res[i] = obj.get(i);
}
return res;
}
It converts a list of any kind of object to an array of the same type.
I have found a quick and easy way that works for me. Note that i have only used this on Java JDK 8. I don't know if it will work with previous versions.
Although we cannot instantiate a generic array of a specific type parameter, we can pass an already created array to a generic class constructor.
class GenArray <T> {
private T theArray[]; // reference array
// ...
GenArray(T[] arr) {
theArray = arr;
}
// Do whatever with the array...
}
Now in main we can create the array like so:
class GenArrayDemo {
public static void main(String[] args) {
int size = 10; // array size
// Here we can instantiate the array of the type we want, say Character (no primitive types allowed in generics)
Character[] ar = new Character[size];
GenArray<Character> = new Character<>(ar); // create the generic Array
// ...
}
}
For more flexibility with your arrays you can use a linked list eg. the ArrayList and other methods found in the Java.util.ArrayList class.
Passing a list of values...
public <T> T[] array(T... values) {
return values;
}
I made this code snippet to reflectively instantiate a class which is passed for a simple automated test utility.
Object attributeValue = null;
try {
if(clazz.isArray()){
Class<?> arrayType = clazz.getComponentType();
attributeValue = Array.newInstance(arrayType, 0);
}
else if(!clazz.isInterface()){
attributeValue = BeanUtils.instantiateClass(clazz);
}
} catch (Exception e) {
logger.debug("Cannot instanciate \"{}\"", new Object[]{clazz});
}
Note this segment:
if(clazz.isArray()){
Class<?> arrayType = clazz.getComponentType();
attributeValue = Array.newInstance(arrayType, 0);
}
for array initiating where Array.newInstance(class of array, size of array). Class can be both primitive (int.class) and object (Integer.class).
BeanUtils is part of Spring.
The forced cast suggested by other people did not work for me, throwing an exception of illegal casting.
However, this implicit cast worked fine:
Item<K>[] array = new Item[SIZE];
where Item is a class I defined containing the member:
private K value;
This way you get an array of type K (if the item only has the value) or any generic type you want defined in the class Item.
Actually an easier way to do so, is to create an array of objects and cast it to your desired type like the following example:
T[] array = (T[])new Object[SIZE];
where SIZE is a constant and T is a type identifier
No one else has answered the question of what is going on in the example you posted.
import java.lang.reflect.Array;
class Stack<T> {
public Stack(Class<T> clazz, int capacity) {
array = (T[])Array.newInstance(clazz, capacity);
}
private final T[] array;
}
As others have said generics are "erased" during compilation. So at runtime an instance of a generic doesn't know what its component type is. The reason for this is historical, Sun wanted to add generics without breaking the existing interface (both source and binary).
Arrays on the other hand do know their component type at runtime.
This example works around the problem by having the code that calls the constructor (which does know the type) pass a parameter telling the class the required type.
So the application would construct the class with something like
Stack<foo> = new Stack<foo>(foo.class,50)
and the constructor now knows (at runtime) what the component type is and can use that information to construct the array through the reflection API.
Array.newInstance(clazz, capacity);
Finally we have a type cast because the compiler has no way of knowing that the array returned by Array#newInstance() is the correct type (even though we know).
This style is a bit ugly but it can sometimes be the least bad solution to creating generic types that do need to know their component type at runtime for whatever reason (creating arrays, or creating instances of their component type, etc.).
I found a sort of a work around to this problem.
The line below throws generic array creation error
List<Person>[] personLists=new ArrayList<Person>()[10];
However if I encapsulate List<Person> in a separate class, it works.
import java.util.ArrayList;
import java.util.List;
public class PersonList {
List<Person> people;
public PersonList()
{
people=new ArrayList<Person>();
}
}
You can expose people in the class PersonList thru a getter. The line below will give you an array, that has a List<Person> in every element. In other words array of List<Person>.
PersonList[] personLists=new PersonList[10];
I needed something like this in some code I was working on and this is what I did to get it to work. So far no problems.
Generic array creation is disallowed in java but you can do it like
class Stack<T> {
private final T[] array;
public Stack(int capacity) {
array = (T[]) new Object[capacity];
}
}
According to vnportnoy the syntax
GenSet<Integer> intSet[] = new GenSet[3];
creates an array of null references, to be filled as
for (int i = 0; i < 3; i++)
{
intSet[i] = new GenSet<Integer>();
}
which is type safe.
You could create an Object array and cast it to E everywhere. Yeah, it's not very clean way to do it but it should at least work.
try this.
private int m = 0;
private int n = 0;
private Element<T>[][] elements = null;
public MatrixData(int m, int n)
{
this.m = m;
this.n = n;
this.elements = new Element[m][n];
for (int i = 0; i < m; i++)
{
for (int j = 0; j < n; j++)
{
this.elements[i][j] = new Element<T>();
}
}
}
An easy, albeit messy workaround to this would be to nest a second "holder" class inside of your main class, and use it to hold your data.
public class Whatever<Thing>{
private class Holder<OtherThing>{
OtherThing thing;
}
public Holder<Thing>[] arrayOfHolders = new Holder<Thing>[10]
}
Maybe unrelated to this question but while I was getting the "generic array creation" error for using
Tuple<Long,String>[] tupleArray = new Tuple<Long,String>[10];
I find out the following works (and worked for me) with #SuppressWarnings({"unchecked"}):
Tuple<Long, String>[] tupleArray = new Tuple[10];
I'm wondering if this code would create an effective generic array?
public T [] createArray(int desiredSize){
ArrayList<T> builder = new ArrayList<T>();
for(int x=0;x<desiredSize;x++){
builder.add(null);
}
return builder.toArray(zeroArray());
}
//zeroArray should, in theory, create a zero-sized array of T
//when it is not given any parameters.
private T [] zeroArray(T... i){
return i;
}
Edit: Perhaps an alternate way of creating such an array, if the size you required was known and small, would be to simply feed the required number of "null"s into the zeroArray command?
Though obviously this isn't as versatile as using the createArray code.
You could use a cast:
public class GenSet<Item> {
private Item[] a;
public GenSet(int s) {
a = (Item[]) new Object[s];
}
}
I actually found a pretty unique solution to bypass the inability to initiate a generic array. What you have to do is create a class that takes in the generic variable T like so:
class GenericInvoker <T> {
T variable;
public GenericInvoker(T variable){
this.variable = variable;
}
}
and then in your array class just have it start like so:
GenericInvoker<T>[] array;
public MyArray(){
array = new GenericInvoker[];
}
starting a new Generic Invoker[] will cause an issue with unchecked but there shouldn't actually be any issues.
To get from the array you should call the array[i].variable like so:
public T get(int index){
return array[index].variable;
}
The rest, such as resizing the array can be done with Arrays.copyOf() like so:
public void resize(int newSize){
array = Arrays.copyOf(array, newSize);
}
And the add function can be added like so:
public boolean add(T element){
// the variable size below is equal to how many times the add function has been called
// and is used to keep track of where to put the next variable in the array
arrays[size] = new GenericInvoker(element);
size++;
}
If you really want to wrap a generic array of fixed size you will have a method to add data to that array, hence you can initialize properly the array there doing something like this:
import java.lang.reflect.Array;
class Stack<T> {
private T[] array = null;
private final int capacity = 10; // fixed or pass it in the constructor
private int pos = 0;
public void push(T value) {
if (value == null)
throw new IllegalArgumentException("Stack does not accept nulls");
if (array == null)
array = (T[]) Array.newInstance(value.getClass(), capacity);
// put logic: e.g.
if(pos == capacity)
throw new IllegalStateException("push on full stack");
array[pos++] = value;
}
public T pop() throws IllegalStateException {
if (pos == 0)
throw new IllegalStateException("pop on empty stack");
return array[--pos];
}
}
in this case you use a java.lang.reflect.Array.newInstance to create the array, and it will not be an Object[], but a real T[].
You should not worry of it not being final, since it is managed inside your class.
Note that you need a non null object on the push() to be able to get the type to use, so I added a check on the data you push and throw an exception there.
Still this is somewhat pointless: you store data via push and it is the signature of the method that guarantees only T elements will enter. So it is more or less irrelevant that the array is Object[] or T[].
for performance reasons I need to use arrays to store data. I implemented this in a generic fashion like this (see this answer):
import java.lang.reflect.Array;
public class SimpleArray<T> {
private T[] data;
#SuppressWarnings("unchecked")
public SimpleArray(Class<T> cls, int size) {
this.data = (T[]) Array.newInstance(cls, size);
}
public T get(int i) {
return data[i];
}
}
The problem is that I need the involved Class<?>es. However, I might have a more complex class hierarchy containing generics:
public class Outer<T> {
public class Inner {
}
}
I would like to initialize the array as I would with an ordinary class:
SimpleArray<Integer> intArray = new SimpleArray<>(Integer.class, 10);
intArray.get(0);
SimpleArray<Outer<Integer>> outerArray;
// how to initialize this?
SimpleArray<Outer<String>.Inner> innerArray;
// how to initialize this?
I read the post on how to (not) get the Class of something generic (here) but the bottom-line seems to be that everything is type-safety related syntactic sugar.
My question is the following: How can I create instances of the SimpleArray classes above while avoiding as much ugliness as possible?
There are two issues here.
Do you really need to pass in a Class? In this case, no. Your class does not actually need to know the element type at runtime to do its job. For example, you can just do:
public class SimpleArray<T> {
private Object[] data;
public SimpleArray(int size) {
this.data = new Object[size];
}
#SuppressWarnings("unchecked")
public T get(int i) {
return (T)data[i];
}
}
If you really needed a Class<T>, how would you get one? Well, first you need to ask yourself, what are you going to use this for? There will never be a "true" Class<T> for a non-reifiable type T because with a Class<T> you can do things like .isInstance() to check whether something is an instance of T at runtime; but of course it's not possible to check instance-of with non-reifiable types at runtime.
In this case, you're only going to pass it to Array.newInstance(), and Array.newInstance() uses the raw type anyway (it does not care about the compile-time type of the Class parameter -- the parameter type is Class<?> -- it only uses the runtime value of the Class object), it is sufficient to simply coerce a Class object representing the raw type to the appropriately-parameterized Class type:
(Class<Outer<Integer>>)(Class<?>)Outer.class
You seem to be trying to make a class that wraps an array and provides a method to get elements. The class Arrays.ArrayList does exactly that already, so there is no need to reinvent the wheel. It works as follows:
List<String> list = Arrays.asList(new String[30]);
list.set(3, "foo");
System.out.println(list.get(3));
You can't use Arrays.asList to produce a List<T> if the type T is generic without suppressing a warning because it is not possible to create a generic array. You can write a helper method to do this for you though.
#SuppressWarnings("unchecked")
public static <T> List<T> newArray(int size) {
return (List<T>) Arrays.asList(new Object[size]);
}
You can use the returned List to get and set elements without having to cast, even if the type T is generic. For example:
List<List<String>> list = newArray(30);
list.set(4, Arrays.asList("A", "B", "C"));
System.out.println(list.get(4));
I want to create a generic array in java maintaining the type safety usually offered by Java.
I am using this code :
class Stack<T> {
private T[] array = null;
public Stack(Class<T> tClass, int size) {
maximumSize = size;
// type unsafe
//array = (T[])new Object[maximumSize];
this.array = (T[])java.lang.reflect.Array.newInstance(tClass,maximumSize);
}
is this code type safe? ans if so, why? why if it is type safe I need a cast?
The Array.newInstance(..) method has a return type of Object. As such, you cannot directly assign it to anything other than Object. You therefore need a cast.
The method delegates to a native method which
Creates a new array with the specified component type and length
Therefore it is creating an array of type T.
The type safety, assuming array is declared as
T[] array;
, is guaranteed by the Class<T> parameter and the cast using the same type variable.
You should add the
#SuppressWarnings("unchecked")
with a comment explaining the above reason in your source code. Always comment why a cast whose warning you are suppressing is safe.
It's not type safe because of the primitive Class objects. For example I can create a new Stack in the following manner:
new Stack<Boolean>(boolean.class, 10);
Which is OK with the compiler but throws an exception because boolean.class is a Class<Boolean> and boolean[] cannot be cast to Boolean[].
The alternative you show commented out:
array = (T[])new Object[size];
Is actually somewhat type safe but for a different reason: it is due to erasure. You cannot, for example, cast a new Object[size] to a Number[], but the cast never happens on the array. It happens some time later, like when you return an element of the array from a method (in which case the element is casted). If you tried to do something like return the array to outside the object it will throw an exception.
Usually the solution is not to generically type the array. Instead, do something like this:
class Stack<E> {
Object[] array = new Object[10];
int top;
void push(E elem) {
if(top == array.length)
array = Arrays.copyOf(array, array.length * 2);
array[top++] = elem;
}
E pop() {
#SuppressWarnings("unchecked")
E elem = (E)array[--top]; // type safe cast
array[top] = null;
return elem;
}
}
The above cast is type safe because you can only push an E in to the array. Both the JDK Stack (which extends Vector) and ArrayList work this way.
If you want to use newInstance, you would have to reject primitives as there is no way to represent them generically:
Stack(Class<T> tClass, int size) {
if(tClass.isPrimitive())
throw new IllegalArgumentException();
// ...
}
Since Array.newInstance returns an Object it needs a cast. The compiler will always give warning in such cases. This is the limitation of generics.
As we know that generics are checked at the compile time so, my friend
java.lang.reflect.Array.newInstance(tClass,size); returns you the object and you are type casting it, and if array is not of type T[] then there can be compile time error
Here is the code I'm using
public class aClass<T> {
private T[] elements;
public aClass(T[] elements) {
this.elements = elements;
}
public void doSomething() {
T[] newArray = (T[]) new Object[5];
...
}
}
I've seen people saying that creating an array like this is a bad idea, due to it being not type safe. However, every time I use it, I have no problems with it. When would creating an array like this cause a problem?
Thanks
Here is an example that causes issues:
public class Main {
public static class List<E extends Number> {
private E[] myE;
public void hello(E e) {
E[] newArray = (E[]) new Object[5];
for (int i = 0; i < newArray.length; i++) {
newArray[i] = e;
}
myE = newArray;
}
}
public static <T> T[] createArray(int size) {
return (T[]) new Object[size];
}
public static void main(String[] args) throws IOException {
List<Integer> integers = new List<Integer>();
integers.hello(5);
}
}
Your code works because when you declare your generic parameter <T> it is unbound, meaning that it extends Object. When you cast your array to (T[])you are actually casting to (Object[]) because that is the best the compiler can do. Now, if you keep your array inside your code, you should not have too many problems. But if somebody outside your code can retrieve that array and has instantiated your class with a type other than object, he will have ClassCastException.
You cannot create an array of T because Java does not know, at run time what is the type of T. This is due to the fact that in Java generics is implemented with type erasure. This means that the compiler discards most of the generic type information after ensuring everything is Ok.
With arrays the story is different, because Java needs to know the exact type of T in order to create the given array, and since such thing cannot be determined you cannot create an array of a generic type.
What you can do is to provide a instance of the actual array that you want to use, and the Java compiler can ensure it is of the appropriate type:
public static <T> void fillWith(T[] destiny, List<? extends T> source){
for(int i=0; i<= destiny.length; i++){
destiny[i] = source.get(i);
}
}
The java.utils.Arrays.copy method offers an alternative carefully using generics and reflections that you can use as a reference for what you want to do.
public static <T> T[] copyOf(T[] original, int newLength) {
return (T[]) copyOf(original, newLength, original.getClass());
}
public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
T[] copy = ((Object)newType == (Object)Object[].class)
? (T[]) new Object[newLength]
: (T[]) Array.newInstance(newType.getComponentType(), newLength);
System.arraycopy(original, 0, copy, 0,
Math.min(original.length, newLength));
return copy;
}
Something that is not type safe doesn't create problems in itself. But it can hide problems at compile time that won't popup until the right moment.
You would be able to fully work in a not type safe environment without having problems, but it's a bad habit just because a type safe environment does guarantee you that you won't have a set of common runtime errors, you don't have any at the moment but you don't have any guarantee, and these are really important, any safety you can trust into means less effort.
The other people are wrong. There is no other way to create the array unless you have an instance of the type at creation time. See also How to create a generic array in Java?
If you have the type instance (= something that has the type Class<T>), you can call java.lang.reflect.Array.newInstance(Class<?>, int) but even then, you'd need the cast, so why bother?
I'd say things were different if the type of elements was Object instead of T but since that's not the case, the code is perfectly OK.
If you want to hide this further, you can write a helper method:
#SuppressWarnings( "unchecked" )
public static <T> T[] createArray( int size ) {
return (T[]) new Object[size];
}
That creates an array without needing a cast when you call it.
It's safer to use a Collection in this cases. Just make something like
...
Collection<T> newElements = new ArrayList<T>(5);
...
Anyway, from what I known creating a generic array like this won't give you real problems, but "smells bad", as it requires explicit type casting. Do you really need an array in this case?
I've seen people saying that creating an array like this is a bad
idea, due to it being not type safe. However, every time I use it, I
have no problems with it. When would creating an array like this cause
a problem?
People say it is a bad idea because, logically, it is not correct to cast an object whose runtime type is Object[] to type T[] if T is not Object.
However, you do not see any immediate problems because, inside the class (within the scope of T), T is erased. So the cast from Object[] to T[] is not checked.
If all you ever do is use this variable inside your class (inside the scope of the type variable T), and you never return it to people outside of the class and there is no way people outside of the class can get access to it, then it will not cause any problems.
In fact, there are benefits to the way you're doing it -- you get all the benefits of generic type checking when getting and setting elements of the array, which you would not get if you simply followed the completely "type-safe" solution of using a variable of type Object[], in which case you would have to manually cast things you get out of it.
You will get a problem if you ever return this array variable to the outside (or otherwise allow the outside to access it) as type T[], because the calling code will expect a certain type of array, and the generics will insert a cast in the calling code when it receives this array, and the cast will fail at runtime (since e.g. a Object[] is not a String[]; they are different types at runtime). Here is a simple example:
public class aClass<T> {
private T[] elements;
public aClass(T[] elements) {
this.elements = elements;
}
public void doSomething() {
elements = (T[]) new Object[5];
...
}
public T[] getArray() {
return elements;
}
}
// some other code...
aClass<String> foo = new aClass<String>(new String[2]);
foo.doSomething();
String[] bar = foo.getArray(); // ClassCastException here
You may have problems with comparissions and printing.. basically any time that knowing the type is important for formatting. the way you have it the system have no clue what data is in the array