Trouble in testing with invalid field - testing

Question

public with sharing class SobjectByParams {
public SObject createSObject(String sObjectName, Map<String, String> fields) {
String invalidSObjectError = System.Label.invalid_Sobject_Name;
String invalidFieldError = System.Label.Invalid_Sobject_Field;
SObject newObject;
try {
newObject = (SObject) Type.forName(sObjectName).newInstance();
} catch (NullPointerException ex) {
throw new InvalidTypeNameException(invalidSObjectError);
}
for (String field : fields.keySet()) {
try {
newObject.put(field, fields.get(field));
} catch (SObjectException ex) {
throw new InvalidTypeNameException(invalidFieldError);
}
}
insert newObject;
return newObject;
}
public class InvalidTypeNameException extends Exception {
}
}
#IsTest
public with sharing class SobjectByParamsTest {
SobjectByParams sobjectByParams;
private static final String TestName = 'TestName';
private static final String BCity = 'Lviv';
private static final String LastName = 'Kapo';
private static final String Email = 'email';
#IsTest
static void createSObject() {
SobjectByParams sobjectByParams = new SobjectByParams();
Map<String, String> fields = new Map<String, String>();
fields.put('BillingCity', BCity);
Test.startTest();
SObject result = sobjectByParams.createSObject(TestName, fields);
Test.stopTest();
System.assertEquals(BCity, result.get(BCity));
}
}
SobjectByParams.InvalidTypeNameException: invalidSobjectNameError - PROBLEM
TEST WORKING on 53.33% but I need min 80%
I don`t know how to fix my problem.

Answer 1

Shouldn't TestName be a sObject name like Contact? Does this code work when you run it normally, does it insert anything or throw? Check/fix that and then you probably need a negative test too.
Make second test method
#isTest
static void testPassingBadData() {
SobjectByParams sobjectByParams = new SobjectByParams();
Map<String, String> fields = new Map<String, String>();
fields.put('BillingCity', BCity);
try{
sobjectByParams.createSObject('NoSuchObjectInTheSystem', fields);
System.assert(false, 'This should have failed and thrown exception');
} catch(Exception e){
System.assert(e.getMessage().contains(Label.Invalid_Sobject_Field));
}
}