[rules-users] Problem with "!=" field constraint

[Markus Reitz]

Hi,

I have a problem with a rule which uses a "!=" field
constraint. The following code illustrates the
problem.

import org.drools.*;
import org.drools.compiler.*;
import org.drools.rule.Package;

import java.io.*;

public class DroolsTest {
  private RuleBase        rules;
  private StatefulSession memory;

  public DroolsTest(File file) throws Exception {
    rules =RuleBaseFactory.newRuleBase();
    memory=rules.newStatefulSession();
    
    rules.addPackage(loadPackage(file));
  }

  protected Package loadPackage(File file) throws
IOException {
    FileInputStream stream=null;
    
    try {
      stream=new FileInputStream(file);
    
      return(loadPackage(stream));
    }
    finally {
      if (stream!=null)
        stream.close();
    }
  }

  protected Package loadPackage(InputStream stream)
throws IOException {
    try {
      PackageBuilder builder=new PackageBuilder();

      builder.addPackageFromDrl(new
InputStreamReader(stream));

      return(builder.getPackage());
    }
    catch(Exception ex) {
      throw new IOException();
    }
  }
  
  public StatefulSession getSession() {
    return(memory);
  }
  
  public static void main(String ... args) {
    try {
      DroolsTest test=new DroolsTest(new
File(args[0]));
      
      SpecialString first42 =new SpecialString("42");
      SpecialString second42=new SpecialString("42");
      
      test.getSession().insert(new
SpecialString("World"));
      test.getSession().insert(first42);
      test.getSession().insert(second42);
      
      System.out.println("Fact handle:
"+test.getSession().getFactHandle(first42));
      System.out.println("Fact handle:
"+test.getSession().getFactHandle(second42));
      
      System.out.println("Firing rules ...");
      
      test.getSession().fireAllRules();
    }
    catch (Exception ex) {
      ex.printStackTrace();
    }
  }
}

I'm inserting three different objects in the working
memory of the Drools 4 Engine. Two of the three
objects have the same text - "42". When I'm using the
following rule

  package test

  rule "A test"

    when
      x : SpecialString()
      y : SpecialString(this!=x)

    then
      System.out.println(x+"/"+y);
  end

I get the output

  Fact handle: [fid:2:2:42[SpecialString at fde8da]]
  Fact handle: [fid:3:3:42[SpecialString at e4d6ef]]
  Firing rules ...
  42[SpecialString at e4d6ef]/World[SpecialString at faa824]
  World[SpecialString at faa824]/42[SpecialString at e4d6ef]
  42[SpecialString at fde8da]/World[SpecialString at faa824]
  World[SpecialString at faa824]/42[SpecialString at fde8da]

which is not what I expect. Why are combinations of x
and y with the two "42"-instances not listed, i.e.

Why doesn't the output contain

  42[SpecialString at e4d6ef]/42[SpecialString at fde8da]
  42[SpecialString at fde8da]/42[SpecialString at e4d6ef]

The class SpecialString uses the standard equals(...)
and hashCode(...) implementation, so these should be
based on the object references, not the content:

public class SpecialString {
  private String text;
  
  public SpecialString(String text) {
    this.text=text;
  }
  
  public String getText() {
    return(text);
  }
  
  @Override public String toString() {
    return(getText()+"["+super.toString()+"]");
  }
}

Best regards

Markus


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