Databases
StackOverflow2013
Postsdbo
POHow to avoid unchecked cast warnings with Java Generics
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POHow to avoid unchecked cast warnings with Java Generics
    primarykey
    data
    text
    <p>Somehow my old question was closed, so I open a new one:</p> <p>I am using Java Generics to implement a generic bidirectional Hash Map out of an SQL Query. It should be able to map any combination of String, Integer pairs back and forth. It should be used like this:</p> <pre><code>String sql = "SELECT string_val, int_val FROM map_table"; PickMap&lt;String, Integer&gt; pm1 = new PickMap&lt;String, Integer&gt;(sql); String key1 = "seven"; Integer value1 = pm1.getLeft2Right(key1); Integer key2 = 7; String value2 = pm1.getRightToLeft(key2); </code></pre> <p>Of course it should be possible to create an pm (Integer, Integer) and so on...</p> <p>My implementation of Pick Map looks like this (without the getter...):</p> <pre><code>public class PickMap&lt;L, R&gt; { private final HashMap&lt;L, R&gt; left2Right = new HashMap&lt;L, R&gt;(); private final HashMap&lt;R, L&gt; right2Left = new HashMap&lt;R, L&gt;(); public PickMap(String sql) throws OException { DTable d = new DTable(sql); int colTypeL = d.t.getColType(1); int colTypeR = d.t.getColType(2); Extractor&lt;L&gt; extLeft = (Extractor&lt;L&gt;) getInstance(colTypeL); Extractor&lt;R&gt; extRight = (Extractor&lt;R&gt;) getInstance(colTypeR); int numRows = d.t.getNumRows(); for(int i=1;i&lt;=numRows;i++) { L leftVal = extLeft.extract(d, i); R rightVal = extRight.extract(d, i); this.left2Right.put(leftVal, rightVal); this.right2Left.put(rightVal, leftVal); } } private Extractor&lt;?&gt; getInstance(int type) { if(type == 1) return new IntExtractor(); else return new StringExtractor(); } } interface Extractor&lt;E&gt; { E extract(DTable source, int row); } class IntExtractor implements Extractor&lt;Integer&gt; { @Override public Integer extract(DTable source, int row) { int value = 5; return new Integer(value); } } class StringExtractor implements Extractor&lt;String&gt; { @Override public String extract(DTable source, int row) { String retVal = "hello"; return retVal; } } </code></pre> <p>I have no compiler errors and I'm pretty sure, that it will work this way. BUT I'm getting unchecked cast warnings on the "getInstance" methods Where I cast Extractor(E) to Extractor(L)...</p> <p>How should I cast properly? Or what am I missing? Or should I just suppress those warnings? </p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USMr_and_Mrs_D
    1. USSauer
    plurals
    1. PL
      singulars
      1. LTLinked
    1. PL
      singulars
      1. LTLinked
    2. PL
      singulars
      1. LTLinked
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POHow to avoid unchecked cast warnings with Java Generics
      1. This table or related slice is empty.
      1. VTApproveEditSuggestion
    1. This table or related slice is empty.
 

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