I have always had this one issue with arrays of ArrayLists. Maybe you can help.
//declare in class
private ArrayList<Integer>[] x;
//in constructor
x=new ArrayList[n];
This generates a warning about unchecked conversion.
But
x=new ArrayList<Integer>[n];
is a compiler error.
Any idea?
Thanks!
I think you cannot make array of generic arraylist because no generic information will be available at runtime.Instead you can do like this:
List<Integer>[] arr=new ArrayList[30];
arr[0]=new ArrayList<Integer>();//create new arraylist for every index.
You can't make a array of generics lists. Fortunately, there are workarounds. And even more fortunately, there is a nice site about Generics with more information than you'd ever want to know. The link goes straight to the Arrays in Java Generics part.
ArrayList<?>[] x;
x=(ArrayList<? extends Integer>[]) new ArrayList<?>[10];
x[0] = new ArrayList(1);
Run the flowing code:
public class Test {
ArrayList<Long>[] f0;
ArrayList<Long> f1;
ArrayList[] f2;
public static void main(String[] args) {
Test t = new Test();
Field[] fs = t.getClass().getDeclaredFields();
for(Field f: fs ){
System.out.println(f.getType().getName());
}
}
}
You will get:
[Ljava.util.ArrayList;
java.util.ArrayList
[Ljava.util.ArrayList;
Because Java don't support generic array. When you declare：
private ArrayList<Integer>[] x;
The compiler will think it is :
private ArrayList[] x;
So, you should do like that:
int n = 10;
ArrayList<Long>[] f = new ArrayList[n];
for(int i=0;i<n;i++){
f[i] = new ArrayList<Long>();
}
It shouldn't have been an error. A warning is enough. If nobody can create an ArrayList<Integer>[], there is really no point to allow the type.
Since javac doesn't do us the favor, we can create the array ourselves:
@SuppressWarnings("unchecked")
<E> E[] newArray(Class<?> classE, int length)
{
return (E[])java.lang.reflect.Array.newInstance(classE, length);
}
void test()
ArrayList<Integer>[] x;
x = newArray(ArrayList.class, 10);
The type constraint isn't perfect, caller should make sure the exact class is passed in. The good news is if a wrong class is passed in, a runtime error occurs immediately when assigning the result to x, so it's fail fast.
Well, it seems you want to avoid the warning about Generics when you simply write
ArrayList[] x=new ArrayList[n];
as well as remove the compilation error, of x=new ArrayList<Integer>[n];
You can use the generic type <?> to satisfy both the conditions, something like:
ArrayList<?> x[]=new ArrayList[n];
You might want to refer to the link below for knowing more about generics: https://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html
