Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>If you are mixing many different concrete types in the same map's key set, you can still use <code>Objects.hashCode()</code> and minimize collisions by xoring the output with a different value per concrete type.</p> <pre><code>class Class1 { public int hashCode() { return Object.hashCode(...) ^ 0x12b7eff8; } } class Class2 { public int hashCode() { return Object.hashCode(...) ^ 0xe800792b; } } </code></pre> <p>By xoring with a value that is randomly chosen, but stable per-concrete class, you eliminate the chance of collisions that might occur solely because the arguments to <code>Object.hashCode</code> are equivalent.</p> <blockquote> <blockquote> <p>Warning: When a single object is supplied, the returned hash code does not equal the hash code of that object.</p> </blockquote> <p>Is this why the javadoc warns about only supplying a single object to this method? From the javadoc,</p> </blockquote> <p>No. This warning is not about the chance of collisions between instances of different concrete classes that have the same members. It is instead warning about false negatives in hashcode matches due to an assumption that the hash of a single value is the same as that of the <code>singleValue.hashCode()</code>.</p> <p>For example, look at the assumption made below in the <em>incorrect</em> fast track code that tries to avoid an equality check by using cached hash codes:</p> <pre><code>class Name { int cachedHashCode; ... } class Person { int cachedHashCode; // 0 if not computed private final Name name; public boolean hasName(Name n) { return ((cachedHashCode != 0 &amp;&amp; n.cachedHashCode != 0) &amp;&amp; cachedHashCode == n.cachedHashCode) || n.equals(name); } public int hashCode() { if (cachedHashCode == 0) { cachedHashCode = Object.hashCode(name); } return cachedHashCode; } } </code></pre>
    singulars
    1. This table or related slice is empty.
    1. POguava-libraries: Is Objects.hashCode(Object[]) collision safe?
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USMike Samuel
    1. USMike Samuel
    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. This table or related slice is empty.
    1. This table or related slice is empty.
    1. COI suppose that `Object.hashCode(...) ^ 0x12b7eff8` destroys the distribution of values for one type. `Object.hashCode(0x12b7eff8, ...)` should be okay. `Object.hashCode(..., getClass().getName())` may be a bit more readable.
      singulars
      1. PO
      1. USThomas Jung
    2. CO@Thomas, What do you mean by "destroys the distribution?"
      singulars
      1. PO
      1. USMike Samuel
    3. COI thought that all bit operations change the distribution (simplest example hashCode | Integer.MAX_VALUE). Well, exor is the one that does not change the distribution of values. They are still equality distributed.
      singulars
      1. PO
      1. USThomas Jung
 

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