Databases
StackOverflow2013
Postsdbo
POJava - generics & wildcards & interface versus implementation
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POJava - generics & wildcards & interface versus implementation
    primarykey
    data
    text
    <p>I have a question about Java generics. Say I have the following interface:</p> <pre><code>public static class Something&lt;T&gt; { public void set(T t) { } } public static interface Manager&lt;T&gt; { public void add(final String key, final Something&lt;T&gt; o); public Something&lt;T&gt; get(final String key); } </code></pre> <p>An example of usage:</p> <pre><code>final Manager&lt;Number&gt; m = ...; m.add("key", new Something&lt;Number&gt;()); m.get("key").set(new Integer(5)); </code></pre> <p>I would also like to be able to add <code>Something&lt;Integer&gt;</code>, <code>Something&lt;Double&gt;</code>, ... to the a <code>Manager&lt;Number&gt;</code>. I would say I have to change the signature of the add-function:</p> <pre><code>public static interface Manager&lt;T&gt; { public void add(final String key, final Something&lt;? extends T&gt; o); public Something&lt;T&gt; get(final String key); } final Manager&lt;Number&gt; m = ...; m.add("key", new Something&lt;Integer&gt;()); m.get("key").set(new Integer(5)); </code></pre> <p>So far, so good. Let's look at a possible implementation of the manager:</p> <pre><code>public static class ManagerImplementation&lt;T&gt; implements Manager&lt;T&gt; { protected HashMap&lt;String, Something&lt;T&gt;&gt; map = new HashMap&lt;String, Something&lt;T&gt;&gt;(); public void add(final String key, final Something&lt;? extends T&gt; o) { map.put(key, o); // &lt;--- here } public Something&lt;T&gt; get(final String key) { return map.get(key); } } </code></pre> <p>This fails, since you cannot add a <code>Something&lt;? extends T&gt;</code> to a <code>Map&lt;X, Something&lt;T&gt;&gt;</code>. So let's change this:</p> <pre><code>public static class ManagerImplementation&lt;T&gt; implements Manager&lt;T&gt; { protected HashMap&lt;String, Something&lt;? extends T&gt;&gt; map = new HashMap&lt;String, Something&lt;? extends T&gt;&gt;(); public void add(final String key, final Something&lt;? extends T&gt; o) { map.put(key, o); } public Something&lt;T&gt; get(final String key) { return map.get(key); // &lt;--- here } } </code></pre> <p>This fails since <code>map.get(key)</code> returns <code>Something&lt;? extends T&gt;</code> while the get-function is required to return <code>Something&lt;T&gt;</code>, as defined in the interface.</p> <p>What is the common approach to solve this?</p> <p>Thanks!</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. This table or related slice is empty.
    1. USuser1284566
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POJava - generics & wildcards & interface versus implementation
      1. USuser1284566
      1. VTFavorite
    2. VO
      singulars
      1. POJava - generics & wildcards & interface versus implementation
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. POJava - generics & wildcards & interface versus implementation
      1. This table or related slice is empty.
      1. VTUpMod
    1. COWhy can't you change the get method in the interface to `public Something<? extends T> get(final String key);` as well?
      singulars
      1. POJava - generics & wildcards & interface versus implementation
      1. USHari Menon
    2. CO@HariShankar this will return `Something<? extends T>` so OP will not be able to `set` anything via such reference (except `null`).
      singulars
      1. POJava - generics & wildcards & interface versus implementation
      1. USPshemo
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload