Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>This is not how you create a generic class. If you use a raw type of your generic class, then all the parameterized type used inside the class, loose their type information. So, for <code>GenericTest</code> raw type, the <code>getKeys()</code> method signature changes to:</p> <pre><code>public Collection getKeys() { return null; } </code></pre> <p>So, if you iterate over <code>getKeys()</code> method of <code>GenericTest</code> raw type, you will get <code>Object</code>, and not <code>String</code>, which I don't see why you expect.</p> <p>From <a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-4.html#jls-4.8" rel="nofollow">JLS Section 4.8 - Raw Types</a>:</p> <blockquote> <p>The type of a constructor (§8.8), instance method (§8.4, §9.4), or non-static field (§8.3) M of a raw type C that is not inherited from its superclasses or superinterfaces is the raw type that corresponds to the erasure of its type in the generic declaration corresponding to C.</p> </blockquote> <hr /> <p>You should really use <code>GenericTest&lt;T&gt;</code> as parameter type in your method, instead of raw type. And change the return type of <code>getKeys</code> to <code>Collection&lt;T&gt;</code>.</p> <p>Change your class to:</p> <pre><code>public class GenericTest&lt;T&gt; { public Collection&lt;T&gt; getKeys() { return null; } public void copy(GenericTest&lt;T&gt; a_from) { for (T x : a_from.getKeys()) { } } } </code></pre> <p>The type <code>T</code> is infered from the parameterized type you create for this generic class. For <code>GenericTest&lt;String&gt;</code>, <code>T</code> will be infered as <code>String</code>, in your class.</p> <hr /> <p><strong>Reference:</strong></p> <ul> <li><a href="http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html" rel="nofollow">Angelika Langer - Java Generics FAQ</a> <ul> <li><a href="http://www.angelikalanger.com/GenericsFAQ/FAQSections/ParameterizedTypes.html#FAQ002" rel="nofollow">How to define Generic Type</a></li> </ul></li> </ul>
    singulars
    1. This table or related slice is empty.
    1. POGenerics compile issue: incompatible types
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USRohit Jain
    1. USRohit Jain
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. POGenerics compile issue: incompatible types
      singulars
      1. PTQuestion
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTAcceptedByOriginator
    3. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    1. COBut the getKeys() method really does return a Collection of String, not T. (The example here is pared down from my actual class, just to show the error condition.)
      singulars
      1. PO
      1. USMitch
    2. CO@Mitch. In that case, use `GenericTest<String>` as method parameter. But I don't see any reason you would make your class generic at all.
      singulars
      1. PO
      1. USRohit Jain
    3. COMy confusion was that the compiler changes the return type of the getKeys() method, even though I want it to be String. Just goes to show there is always more to learn!
      singulars
      1. PO
      1. USMitch
 

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