I am NOT doing test driven development and I write my test classes after the actual code is written. In my current project I have a test coverage of(Line coverage) %70 for 3000 lines of Java code.(Using JUnit, Mockito and Sonar for testing) But while I feel actually I am not covering and catching %70 of the problems that can occur.

So my question is in theory is that possible to have a %100 Line coverage but in reality it is meaningless because of low quality of the test code and maybe a %40 well written test code is much better than a bad %100 coverage? or we can always say line coverage more or less gives the percentage of all covered issues?