Java: Generic Array Creation

Question

I'm writing my custom Map, it have custom Pair array and Map uses the pair for operations.

They are generic, I don't know their's type it can be integer, string or double. So I can not use ArrayList, It's forbidden for me.

public class FMap<K, V> {
   private FPair<K, V>[] data;
   int capacity=23;
   int used=0;

   public FMap(int cap){
      super();
      capacity=cap;
      used =0;
      data = new FPair[ capacity];
      for(int i=0; i< data.length; ++i)
          data[i] = new FPair<K, V>();
}

But compiler saying:

javac -g -Xlint BigramDyn.java
./TemplateLib/FMap.java:23: warning: [rawtypes] found raw type: FPair
        data = new FPair[capacity];
                   ^
  missing type arguments for generic class FPair<A,B>
  where A,B are type-variables:
    A extends Object declared in class FPair
    B extends Object declared in class FPair
./TemplateLib/FMap.java:23: warning: [unchecked] unchecked conversion
        data = new FPair[capacity];
               ^
  required: FPair<K,V>[]
  found:    FPair[]
  where K,V are type-variables:
    K extends Object declared in class FMap
    V extends Object declared in class FMap
2 warnings

if I use data = new FPair<K, V>[capacity] instead of data = new FPair[capacity]

Compiler is saying :

TemplateLib/FMap.java:23: error: generic array creation
        data = new FPair<K,V>[capacity];
               ^
1 error

--

And in equal function of map: I'm doing: FMap

FMap<K,V> otherPair = (FMap<K,V>) other;

But compiler saying:

./TemplateLib/FMap.java:34: warning: [unchecked] unchecked cast
            FMap<A,B> otherPair = (FMap<A,B>) other;
                                                ^
  required: FMap<A,B>
  found:    Object
  where A,B are type-variables:
    A extends Object declared in class FMap
    B extends Object declared in class FMap
1 warning

Possible duplicate of Compilation error: Generic array creation — Seelenvirtuose
– Seelenvirtuose, Commented Jan 6, 2017 at 12:46

Joop Eggen · Accepted Answer · 2017-01-06 17:36:37Z

0

Use an ArrayList as such:

List<FPair<K, V>> data = new ArrayList<>(capacity);

As ArrayList wraps an array you have all the comfort of List and the functionality of an array.

Only fill in an item when there is data, no new FPair<K, V>();.

As ArrayList is not allowed:

    data = (FPair<K, V>[]) new FPair<?, ?>[10];

The <?, ?> or <Object, Object> will satisfy the compiler (no longer a raw type FPair used). The abuse/cast is unlucky. But as types are stripped, there is no actual difference.

If you want to fill every item (not advisable):

    Arrays.fill(data, new FPair<K, V>());

    Arrays.setAll(data, ix -> new FPair<K,V>());

The first fills the same element at every positon, usefull when FPair is not changed but can be shared: when it is "immutable".

The second is just a fancy way to loop.

edited Jan 6, 2017 at 17:36

answered Jan 6, 2017 at 13:33

Joop Eggen

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2 Comments

Jonathan Cedric Over a year ago

very thanks but I can not use this because it is forbidden for me. My code is running like I want but it gives only some warnings, That's means, my code is not right, can not I use generic array like it?

Salem Over a year ago

@FurkanYıldız the warning doesn't necessarily mean your code is wrong, @SuppressWarnings is perfectly acceptable in some cases (mostly with generics)

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