JavaServer Pages, 3rd Edition

Coercion Rules

The EL automatically converts, or coerces, variable values and the result of an expression to the type required by an attribute or an operator:

To Java type	Conversion rule
String	null: to empty string (“”). All other types: to the corresponding String value.
Primitive number or Number	null or empty string: 0. Character or char: to the value represented by the character code. String: parse as an Integer or Floating point literal. Numeric types: coerce to the requested precision.
boolean or Boolean	null: to false. String: to true if the value is “true”, ignoring case, otherwise false.
Other type	null: keep as null. String: use the PropertyEditor for the requested type, if any, otherwise null if the string is empty. Other: type cast, if possible.

In all cases, the EL evaluator throws an exception for attempts to convert between types not defined in the table or if the defined conversion rule fails.