Common NullPointerException in java

The following lists the NullPointerException that often occurs when we learn java and its solutions.

1. Null Pointer Error java. lang. nullPointerException uses the Basic JAVA data type. The value of the variable is either the default value. If no value is assigned to the variable, the program cannot compile the variable. Therefore, the basic JAVA data type (double, float, boolean, char, int, long) generally does not cause NULL pointer exceptions. It can be seen that Null Pointer exceptions are mainly related to object operations.

The following lists the possible situations and solutions for NULL pointer exceptions:
You can directly start using an object regardless of whether it is null.

(JSP) code Segment 1: out. println (request. getParameter ("username "));

Description: The function of code segment 1 is very simple, that is, to output the value of the user input table field "username.
Note: It seems that no syntax error is found in the preceding statement, and in most cases, no problem occurs. However, if a user does not provide the value of the form field "username" when entering data, or directly enters the form by bypassing the form in some way, request. the value of getParameter ("username") is null (not an empty string, it is a null object .), The println method of the out object cannot directly operate on the null object. Therefore, the JSP page where code segment 1 is located will throw a "java. lang. NullPointerException" exception. Even if the Object may be empty, some methods of java. lang. Object or Object itself, such as toString () and equals (Object obj), are called.

(JSP) code Segment 2: String userName = request. getParameter ("username ");
If (userName. equals ("root ")){....}

Description: The function of code snippet 2 is to check the user name provided by the user. If the user name is "root", perform some special operations.
Note: In code segment 2, if a user does not provide a value for the form field "username", the string object userName is null, A null object cannot be directly compared with another object. Similarly, the JSP page where code snippet 2 is located will throw (java. lang. nullPointerException) NULL pointer error.

(JSP) code Segment 3: String userName = session. getAttribute ("session. username"). toString ();
Description: The function of code snippet 3 is to retrieve the value of session. username in the session and assign the value to the string object userName. Note: In general, if a user has already performed a session, no problem occurs. However, if the application server is restarted at this time, and the user has not logged on again, (The user may close the browser but still open the original page .) In this case, the session value becomes invalid, and the value of session. username in the session is empty. If you directly perform the toString () operation on a null object, the system throws a null pointer exception (java. lang. NullPointerException.
Solution: to ensure that the objects to be operated or referenced are not empty, if we want to operate or reference an object, we first check whether the object has been instantiated and is not empty; in addition, the system adds the processing of the condition when the object is null. For example, the String object is used to save the results submitted by the user. If operations involving objects are performed, the system first checks whether the object is empty and then checks whether the object is empty, you can choose one of the following processing methods:
Processing Method 1) when the object is null, set the object value to an empty string or a default value;
Processing Method 2) when the object is detected to be null, no operation is performed, and the operation is directly redirected to other processes.
Processing Method 3) when the object is null, the system prompts that the user operation has an error. Rewrite code snippet 2 in the above way To get:
Method 1: String userName = request. getParameter ("username"); // If (userName = null) userName = ""; If (userName. equals ("root ")){..........}
Method 2: String userName = request. getParameter ("username"); // If the variable value is null, it is converted to a default null String and no related operations are performed. If (usreName! = Null) {If (userName. equals ("root ")){..........}}
Method 3: String userName = request. getParameter ("username"); // If the variable value is null, it is converted to a default null String and no related operations are performed. If (usreName = null) {// prompt that the user input information is null}

In practice, the above three methods also apply to the handling of other exceptions:
Exception Handling Method 1) Check for exceptions and set the object value to an empty string or a default value;
Exception Handling Method 2) if an exception is detected and no operation is performed, the system jumps to another process.
Exception Handling Method 3) Check for exceptions and prompt users for Operation errors.

Author "Mo"