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13.6. Java Expressions

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Table 13.1. Java Expressions
Capability Example
You can use any Java expression that returns a boolean as a constraint inside the parentheses of a pattern. Java expressions can be mixed with other expression enhancements, such as property access. 
Person( age == 50 )
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You can change the evaluation priority by using parentheses, as in any logic or mathematical expression. 
Person( age > 100 && ( age % 10 == 0 ) )
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You can reuse Java methods. 
Person( Math.round( weight / ( height * height ) ) < 25.0 )
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Type coercion is always attempted if the field and the value are of different types; exceptions will be thrown if a bad coercion is attempted. 
Person( age == "10" ) // "10" is coerced to 10
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Warning

Methods must not change the state of the object in a way that may affect the rules. Any method executed on a fact in the LHS should be a read only method.

Warning

The state of a fact should not change between rule invocations (unless those facts are marked as updated to the working memory on every change): 
Person( System.currentTimeMillis() % 1000 == 0 ) // Do NOT do this
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Important

All operators have normal Java semantics except for == and !=.
The == operator has null-safe equals() semantics: 
// Similar to: java.util.Objects.equals(person.getFirstName(), "John")
// so (because "John" is not null) similar to:
// "John".equals(person.getFirstName())
Person( firstName == "John" )
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The != operator has null-safe !equals() semantics: 
// Similar to: !java.util.Objects.equals(person.getFirstName(), "John")
Person( firstName != "John" )
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