XbaseInterpreter.java:_doEvaluate(XCastedExpression - `throw new EvaluationException(new ClassCastException(typeName))` - not enough information to show to the user what cannot be casted to what

[dilyanpalauzov]

class A {
    public static void main(String[] arg) {
        try {
            Object x = Integer.valueOf(0);
            System.out.println("A" + (String)x);
        } catch (ClassCastException e) {
            System.out.println(e.getMessage());
        }
    }
}

produces with Java 21

$ java A.java
class java.lang.Integer cannot be cast to class java.lang.String (java.lang.Integer and java.lang.String are in module java.base of loader 'bootstrap')

That is: ClassCastException.getMessage() is expected to spell both source and target classes of casting.

When XbaseInterpreter.java:protected Object _doEvaluate(XCastedExpression castedExpression, IEvaluationContext, CancelIndicator) creates a ClassCastException, it puts as detailed message the typeName - the target type.

To print a casting error to the user, containing both source and target classes of casting, one has in a derived class to:

override Object _doEvaluate(XCastedExpression castedExpression, IEvaluationContext context, CancelIndicator indicator) {
    try {
        return super._doEvaluate(castedExpression, context, indicator)
    } catch (RuntimeException e) {
        if (e.cause instanceof ClassCastException) {

and then constructs a message, containing both source and target types of the casting. Obtaining the target type is easy, it is a matter of calling castedExpression.getType().getType().getQualifiedName(). In fact this is the value of the parameter passed to the constructor of ClassCastException in XbaseInterpreter.java:_doEvaluate(XCastedExpression, IEvaluationContext, CancelIndicator).

However there is no way to know, when the EvaluationException(new ClassCastException()) is catched, what was the source type of the expression: In a as B the class of a.

In openHAB to get the source type is re-executed val Object result = internalEvaluate(castedExpression.getTarget(), context, indicator);. The problem is that calling once val Object result = internalEvaluate(castedExpression.getTarget(), context, indicator); in the super class, and calling again val Object result = internalEvaluate(castedExpression.getTarget(), context, indicator); in the derived class might not necessary have the same result: states might have changed in the meantime, so that re-evaluating the same expression, for the sake of logging an error, might produce no more errors. This in fact does happen, as described at openhab/openhab-core#5323 .

That said in

protected Object _doEvaluate(XCastedExpression castedExpression, IEvaluationContext context, CancelIndic
ator indicator) {
…
                try {
                        expectedType.cast(result);
                } catch (ClassCastException e) {
                        throw new EvaluationException(new ClassCastException(typeName));
                }

• spell in the ClassCastException constructor parameter as sentence the target and source types of the casting, as java does in the example of the beginning, or
• instead of passing the target class name (which can be implied when overriding the method by calling castedExpression.getType().getType().getQualifiedName() , pass the name of the source class.

In both cases the aim is to present to the user a message telling what cannot be casted to what, and currently this is not possible.

[dilyanpalauzov]

For instance

--- a/org.eclipse.xtext.xbase/src/org/eclipse/xtext/xbase/interpreter/impl/XbaseInterpreter.java
+++ b/org.eclipse.xtext.xbase/src/org/eclipse/xtext/xbase/interpreter/impl/XbaseInterpreter.java
@@ -547,7 +547,7 @@ public class XbaseInterpreter implements IExpressionInterpreter {
                        try {
                                expectedType.cast(result);
                        } catch (ClassCastException e) {
-                               throw new EvaluationException(new ClassCastException(typeName));
+                               throw new EvaluationException(new ClassCastException("Could not cast (" + result.class + ")" + result + " to " + typeName));
                        }
                        return result;
                }

[LorenzoBettini]

You can create a PR with an updated (or new) test

[szarnekow]

Since the original ClassCastException nowadays has enough information, we can probably just use that one instead of making a new instance.

[dilyanpalauzov]

With invalid casts the interpreter currently aborts: the XBase language cannot try a cast and catch (ClassCastException). It would be good, if in XBase this could be written:

try {
  val a = x as Number; 
  …
} catch (ClassCastException e) {
  …
} finally {
  …
}

[szarnekow]

What if you try this?

--- a/org.eclipse.xtext.xbase/src/org/eclipse/xtext/xbase/interpreter/impl/XbaseInterpreter.java
+++ b/org.eclipse.xtext.xbase/src/org/eclipse/xtext/xbase/interpreter/impl/XbaseInterpreter.java
@@ -547,7 +547,7 @@ public class XbaseInterpreter implements IExpressionInterpreter {
                        try {
                                expectedType.cast(result);
                        } catch (ClassCastException e) {
-                               throw new EvaluationException(new ClassCastException(typeName));
+                               throw new EvaluationException(e);
                        }
                        return result;
                }

[dilyanpalauzov]

The above will include the source and destination type, eventually solving the initial problem, reported here, and is an improvement.

[dilyanpalauzov]

} catch (ClassCastException e) {
    throw new EvaluationException(new ClassCastException(
        "Could not cast (" + result.class + ")" + result + " to " + typeName
        ))
}

emits also the value, which cannot be casted, apart from its class:

Could not cast (class org.openhab.core.types.UnDefType)NULL to java.lang.Number in <filename>

while

} catch (ClassCastException e) {
    throw new EvaluationException(e)
}

prints:

Cannot cast org.openhab.core.types.UnDefType to java.lang.Number in <filename>

This text from above by me is not correct. Correct is that the ClassCastException can be caught by the XBase code.

With invalid casts the interpreter currently aborts: the XBase language cannot try a cast and catch (ClassCastException).

Line and column numbers in errors from XbaseInterpreter

In openHAB in the method override protected _doEvaluate(XCastedExpression castedExpression, IEvaluationContext, CancelIndicator), if on ClassClassException is executed throw new org.openhab.core.model.script.engine.ScriptExecutionException(new org.openhab.core.model.script.engine.ScriptError(String, castedExpression)) then the constructor of ScriptError does:

public ScriptError(final String message, final EObject atEObject) {
    this.message = message;
    INode node = NodeModelUtils.getNode(atEObject);
    if (node == null) { … } else
    LineAndColumn lac = NodeModelUtils.getLineAndColumn(node, node.getOffset());
    this.line = lac.getLine();
    this.column = lac.getColumn();
    this.length = node.getEndOffset() - node.getOffset();
}

and the logged error message contains the line and column, where it happened.

When instead _doEvaluate(XCastedExpression, IEvaluationContext, CancelIndicator) does throw new throw new EvaluationException(new ClassCastException(…)) then the error message, if ClassCastException is not caught, does not include the line and column number, where it happened.

What should be altered, so that runtime errors from XbaseInterpreter include the line and column numbers, where the errors happened?