1. When the object of the String type uses the PrintLn () method, if the object is NULL, NULL is printed instead of the NullPointerexception, the resulting problem is easier to cause an illusion, and for future operations of the string. Problem. 2. Running NullPointerException is abnormal, the main reason is that there is no verification of the existence of the object, often appears in JSP programming: if (Request.getParameter ("UserName"). Equals ("xxx"), Out.println (session. GetAttribute ("Record")) The method of solving this problem is to determine the comparison before use: if (Request.GetParameter ("UserName")! = Null) {if if ("UserName"). Equals ("xxx")) ...} . 3. Running Numberformatexception: The main reason is that the content submitted by the user into an integer or the floating point number is input. Other illegal characters. The main mode of processing is to capture this exception using the TRY / CATCH block where the transition is required and then prompted to enter legitimate data. 4. StringIndexoutofboundsexception: The main reason is to use string substring (), charat (), etc., and the length of the string is not enough, this exception will cause this exception; nullpointerException will also be triggered when the string is NULL. The method of solving is to determine, determine the length or convert to byte arrays. Considering that there are many more operations, it can be encapsulated into the JavaBean. 5. Running NoclassDeffoundError errors, main reasons are class paths or class files to place errors, and the placement of class files should meet the requirements of the server. 6. Turn java.lang.Error error, main reason is that the external resources accessed, no shutdown operations, resulting in a lot of waste, which can eventually result in the system unable to run properly; there is too much external resource access to the system The external system does not work properly; the external resources accessed have an abnormal situation. The solution is: Access to external resources, first check if the resource (such as database) can be connected or operated; if you access external resources, if you have a connection, you will be closed, and only one shutdown operation is performed; try to be in the same Shared external resources in operation to reduce the operation of the operation to the resource, and improve the performance efficiency of the program.
