Code review comment: I initialized vals[1] with an empty by default so that the NPE wouldnt arise.   But ideally, the default value provided in the mock annotation should be applied in this case.

Code under review:
public void process(FunctionNode functionNode, List<AnnotationAttachmentNode> annotations) {

        parent = (BLangPackage) ((BLangFunction) functionNode).parent;
        String packageName = getPackageName(parent);
        annotations = annotations.stream().distinct().collect(Collectors.toList());

        
        for (AnnotationAttachmentNode attachmentNode : annotations) {
            String annotationName = attachmentNode.getAnnotationName().getValue();
            String functionName = functionNode.getName().getValue();

            if (MOCK_ANNOTATION_NAME.equals(annotationName)) {
                String[] vals = new String[2];
                
                vals[0] = packageName;

                if (attachmentNode.getExpression() instanceof BLangRecordLiteral) {
                    List<RecordLiteralNode.RecordField> attributes = ((BLangRecordLiteral) attachmentNode
                            .getExpression()).getFields();
                    attributes.forEach(field -> {
                        String name;
                        BLangExpression valueExpr;

                        if (field.isKeyValueField()) {
                            BLangRecordLiteral.BLangRecordKeyValueField attributeNode =
                                    (BLangRecordLiteral.BLangRecordKeyValueField) field;
                            name = attributeNode.getKey().toString();
                            valueExpr = attributeNode.getValue();
                        } else {
                            BLangRecordLiteral.BLangRecordVarNameField varNameField =
                                    (BLangRecordLiteral.BLangRecordVarNameField) field;
                            name = varNameField.variableName.value;
                            valueExpr = varNameField;
                        }

                        String value = valueExpr.toString();

                        if (MODULE.equals(name)) {
                            value = formatPackageName(value, parent);
                            vals[0] = value;
                        } else if (FUNCTION.equals(name)) {
                            vals[1] = value;
                        }
                    });

                    
                    if (vals[1].isEmpty()) {
                        diagnosticLog.logDiagnostic(DiagnosticSeverity.ERROR, attachmentNode.getPosition(),
                                "function name cannot be empty");
                        break;
                    }

                    
                    PackageID functionToMockID = getPackageID(vals[0]);
                    if (functionToMockID == null) {
                        diagnosticLog.logDiagnostic(DiagnosticSeverity.ERROR, attachmentNode.getPosition(),
                                "could not find module specified ");
                    }

                    BType functionToMockType = getFunctionType(packageEnvironmentMap, functionToMockID, vals[1]);
                    BType mockFunctionType = getFunctionType(packageEnvironmentMap, parent.packageID,
                            ((BLangFunction) functionNode).name.toString());

                    if (functionToMockType != null && mockFunctionType != null) {
                        if (!typeChecker.isAssignable(mockFunctionType, functionToMockType)) {
                            diagnosticLog.logDiagnostic(DiagnosticSeverity.ERROR, ((BLangFunction) functionNode).pos,
                                    "incompatible types: expected " + functionToMockType.toString()
                                            + " but found " + mockFunctionType.toString());
                        }
                    } else {
                        diagnosticLog.logDiagnostic(DiagnosticSeverity.ERROR, attachmentNode.getPosition(),
                                "could not find functions in module");
                    }

                    
                    BLangTestablePackage bLangTestablePackage =
                            (BLangTestablePackage) ((BLangFunction) functionNode).parent;
                    bLangTestablePackage.addMockFunction(functionToMockID + MOCK_FN_DELIMITER + vals[1],
                            functionName);

                }

            }

        }

    }


Revised code:
public void process(FunctionNode functionNode, List<AnnotationAttachmentNode> annotations) {
        BLangPackage parent = (BLangPackage) ((BLangFunction) functionNode).parent;
        String packageName = getPackageName(parent);
        annotations = annotations.stream().distinct().collect(Collectors.toList());

        
        for (AnnotationAttachmentNode attachmentNode : annotations) {
            String annotationName = attachmentNode.getAnnotationName().getValue();
            String functionName = functionNode.getName().getValue();

            if (MOCK_ANNOTATION_NAME.equals(annotationName)) {
                String[] vals = new String[2];
                
                vals[0] = packageName;
                vals[1] = "";

                if (attachmentNode.getExpression() instanceof BLangRecordLiteral) {
                    List<RecordLiteralNode.RecordField> attributes = ((BLangRecordLiteral) attachmentNode
                            .getExpression()).getFields();
                    attributes.forEach(field -> {
                        String name;
                        BLangExpression valueExpr;

                        if (field.isKeyValueField()) {
                            BLangRecordLiteral.BLangRecordKeyValueField attributeNode =
                                    (BLangRecordLiteral.BLangRecordKeyValueField) field;
                            name = attributeNode.getKey().toString();
                            valueExpr = attributeNode.getValue();
                        } else {
                            BLangRecordLiteral.BLangRecordVarNameField varNameField =
                                    (BLangRecordLiteral.BLangRecordVarNameField) field;
                            name = varNameField.variableName.value;
                            valueExpr = varNameField;
                        }

                        String value = valueExpr.toString();

                        if (MODULE.equals(name)) {
                            value = formatPackageName(value, parent);
                            vals[0] = value;
                        } else if (FUNCTION.equals(name)) {
                            vals[1] = value;
                        }
                    });

                    
                    if (vals[1].isEmpty()) {
                        diagnosticLog.logDiagnostic(DiagnosticSeverity.ERROR, attachmentNode.getPosition(),
                                "function name cannot be empty");
                        break;
                    }

                    
                    PackageID functionToMockID = getPackageID(vals[0]);
                    if (functionToMockID == null) {
                        diagnosticLog.logDiagnostic(DiagnosticSeverity.ERROR, attachmentNode.getPosition(),
                                "could not find module specified ");
                    }

                    BType functionToMockType = getFunctionType(packageEnvironmentMap, functionToMockID, vals[1]);
                    BType mockFunctionType = getFunctionType(packageEnvironmentMap, parent.packageID,
                            ((BLangFunction) functionNode).name.toString());

                    if (functionToMockType != null && mockFunctionType != null) {
                        if (!typeChecker.isAssignable(mockFunctionType, functionToMockType)) {
                            diagnosticLog.logDiagnostic(DiagnosticSeverity.ERROR, ((BLangFunction) functionNode).pos,
                                    "incompatible types: expected " + functionToMockType.toString()
                                            + " but found " + mockFunctionType.toString());
                        }
                    } else {
                        diagnosticLog.logDiagnostic(DiagnosticSeverity.ERROR, attachmentNode.getPosition(),
                                "could not find functions in module");
                    }

                    
                    BLangTestablePackage bLangTestablePackage =
                            (BLangTestablePackage) ((BLangFunction) functionNode).parent;
                    bLangTestablePackage.addMockFunction(functionToMockID + MOCK_FN_DELIMITER + vals[1],
                            functionName);
                }

            }

        }

    }