When overriding the equals method one is required to also override
hashCode().
Check the JavaDoc for java.lang.Object... Much clearer than my
ramblings... :)
Here's some example code that produces the result you require...
The implementations of equals and hashcode is not null safe at all...
this is quick and dirty...
Cheers,
Renier
package sandbox;
import java.util.HashSet;
public class Test {
public static void main(String[] args) {
HashSet<Proposition> testSet1 = new HashSet<Proposition>();
HashSet<Proposition> testSet2 = new HashSet<Proposition>();
Proposition prop1 = new Proposition("test");
Proposition prop2 = new Proposition("goneElement");
Proposition prop3 = new Proposition("test");
testSet1.add(prop1);
testSet1.add(prop2);
testSet2.add(prop3);
testSet1.retainAll(testSet2);
System.out.println(testSet1.size());
}
private static class Proposition {
private String name = null;
public Proposition( String name ) {
this.name = name;
}
public boolean equals( Object o ) {
if ( o == null ) {
return false;
}
if ( ! ( o instanceof Proposition ) ) {
return false;
}
return this.name.equals(((Proposition)o).name);
}
public int hashCode() {
return name.hashCode();
}
}
}
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