Databases
StackOverflow2013
Postsdbo
POJava Higher Order Polymorphic Function
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POJava Higher Order Polymorphic Function
    primarykey
    data
    text
    <p>I am very familiar with functional langauges such as Scheme, and Haskell. I'm trying to solve a problem in Java and struggling, possibly because I'm still in a functional mindset.</p> <p>I want to write:</p> <pre><code>public void doQueryAndStoreData(String query, &lt;? extends Collection&gt; storeIn) { /* make a jdbc query, get ResultSet */ ResultSet rset = ...; ProcessResultSet proc = new ProcessResultSet(); proc.process(rset, storeIn); /* clean up */ } </code></pre> <p>with an interface like:</p> <pre><code>private interface IProcessResultSet&lt;C&gt; { public void process(ResultSet rset, C storeIn); } </code></pre> <p>and a class implementing the interface like:</p> <pre><code>private class ProcessResultSet implements IProcessResultSet&lt;? extends Collection&gt; { public void process(ResultSet rset, Map storeIn) { /* do something */ } public void process(ResultSet rset, List storeIn) { /* do something else */ } } </code></pre> <p>so that the first method can call the appropriate <code>process</code> based on what type of <code>storeIn</code> it's given.</p> <p>In Haskell I could write</p> <pre><code>class Storeable c a where store :: a -&gt; c a -&gt; c a doQueryAndStoreData :: Storeable c a =&gt; ResultSet a -&gt; c a -&gt; c a doQueryAndStoreData (ResultSet rs) coll = foldr store coll rs </code></pre> <p>and provide <code>Storeable</code> instances for whatever collection type I want to store my <code>ResultSet</code> in.</p> <p>Is this the correct approach in Java? Because I feel like I'm somewhat fighting the langauge to accomplish this.</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. UScdk
    1. UScdk
    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
    2. PO
      singulars
      1. PTAnswer
    3. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POJava Higher Order Polymorphic Function
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POJava Higher Order Polymorphic Function
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. POJava Higher Order Polymorphic Function
      1. This table or related slice is empty.
      1. VTUpMod
    1. COI think what you need can be solved using the visitor pattern - double dispatch.
      singulars
      1. POJava Higher Order Polymorphic Function
      1. USScorpion
    2. COPersonally, I would be more apt to return something from a function. Tends to make the code more readable. Can't think of the closest supertype to Map and List, but that would be the return value, and then cast to Map/List in the assignment statement, i.e., `Map<something> myMap = new Map<something>; myMap = (Map<something>) process(rset, myMap);"
      singulars
      1. POJava Higher Order Polymorphic Function
      1. USanthropomo
 

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