Controller Hang whenever execute both functions(Parsing File and Writing File)

Using Arduino Programming Questions
9 
    String str = String(n) + ", " + String(u) + ", " + String(v);
    str        = str + String(n) + ", " + String(u) + ", ";        //coment this line and next indicates test 1 
    str        = str + String(n) + ", " + String(u);                //coment only this line indicates test 2.
    str        = str + ", " + String(FreeRam(),DEC);            // execute all lines indicate test3
    Serial.println(str);

Do not use the String class. Is there something about that simple statement that escapes you?