What is a Null. Pointer. Exception, and how do I fix itQuestion What causes a Null. Pointer. Exception As you should know, Java types are divided into primitive types boolean, int etc and reference types. Reference types in Java allow you to use the special value null which is the Java way of saying no object. A Null. Pointer. Exception is thrown at runtime whenever your program attempts to use a null as if it was a real reference. For example, if you write this public class Test. String args. String foo null. HERE. the statement labelled HERE is going to attempt to run the length method on a null reference, and this will throw a Null. Using a Serial Terminal at PCMac. So far so good. You have made the hardware connections. Now what How do you send serial data from your PC via USB to any other. Ad blocker interference detected Wikia is a freetouse site that makes money from advertising. We have a modified experience for viewers using ad blockers. This tutorial describes the procedure to connect an Arduino UNO board to MATLAB using the USB port. For more details visit matlabarduino. Configuration. Pointer. Exception. There are many ways that you could use a null value that will result in a Null. Pointer. Exception. Hi there. First off this worked first time using my nano v3, thanks. I was wondering how you would go about programming the z axes into this sketch, would this be in. Reading Data From Serial Port Using Matlab To Find' title='Reading Data From Serial Port Using Matlab To Find' />If fact, the only things that you can do with a null without causing an NPE are assign it to a reference variable or read it from a reference variable,assign it to an array element or read it from an array element provided that array reference itself is non null,pass it as a parameter or return it as a result, ortest it using the or operators, or instanceof. Question How do I read the NPE stacktrace Suppose that I compile and run the program above javac Test. Exception in thread main java. Null. Pointer. Exception. Test. mainTest. java 4. First observation the compilation succeeds The problem in the program is NOT a compilation error. Free Cooking Games Without Adobe Flash Player. It is a runtime error. Some IDEs may warn your program will always throw an exception. Second observation when I run the program, it outputs two lines of gobbledy gook. WRONG Thats not gobbledy gook. It is a stacktrace. So lets look at what is says Exception in thread main java. Null. Pointer. Exception. The first line of the stack trace tells you a number of things It tells you the name of the Java thread in which the exception was thrown. For a simple program with one thread like this one, it will be main. Lets move on. It tells you the full name of the exception that was thrown i. Null. Pointer. Exception. If the exception has an associated error message, that will be output after the exception name. Null. Pointer. Exception is unusual in this respect because it rarely has an error message. The second line is the most important one in diagnosing an NPE. Test. mainTest. java 4. This tells us a number of things at Test. Test class. Test. AND it tells us that the statement where this occurred is in line 4 of the file. And if you count the lines in the file above, line 4 is the one that I labelled with the HERE comment. Note that in a more complicated example, there will be lots of lines in the NPE stack trace. But you can be sure that the second line the first at line will tell you where the NPE was thrown. In short the stacktrace will tell us unambiguously which statement of the program has thrown the NPE. Not quite true. There are things called nested exceptions. Question How do I track down the cause of the NPE exception in my codeThis is the hard part. The short answer is to apply logical inference to the evidence provided by the stack trace, the source code and the relevant API documentation. Lets illustrate with the simple example above first. We start by looking at the line that the stacktrace has told us is where the NPE happened int length foo. HERE. How can that throw an NPE In fact there is only one way it can only happen if foo has the value null. We then try to run the length method on null and. BANG But I hear you say what if the NPE was thrown inside the length method call Well if that happened, the stacktrace would look different. The first at line would say that the exception was thrown in some line in the java. String class, and line 4 of Test. So where did that null come from In this case it is obvious and it is obvious what we need to do to fix it. Assign a non null value to fooOK, so lets try a slightly more tricky example. This will require some logical deduction. Test. private static String foo new String2. String bar, int pos. String args. Test. Exception in thread main java. Null. Pointer. Exception. Test. testTest. java 6. Test. mainTest. java 1. So now we have 2 at lines. The first one is for this line return argspos. So looking at the first line, how could that throw an NPE In fact, there are two ways If the value of bar is null then barpos will throw an NPE. If the value of barpos is null then calling length on it will throw an NPE. So next we need to figure out which of those scenarios explains what is actually happening. Lets start by exploring the first one Where does bar come from It is a parameter to the test method call, and if we look at how test was called, we can see that it comes from the foo static variable. And we can see clearly that we initialized foo to a non null value. That is sufficient to tentatively dismiss this explanation. In theory, something else could changefoo to null. So what about our 2nd scenario Well we can see that pos is 1, so that means that foo1 must be null. Is that possible Indeed it is And that is the problem. When we initialize like this private static String foo new String2. String with two elements that are initialized to null. And then we didnt change the contents of foo. Read RB7 port from Pic F1. There would appear to be a number of things wrong with your code and possibly your circuit. You say that the LEDs are all on Port B but your first code lines set up port D. Im not sure what you are attempting to do with the interrupt you have set up your LEDs on bits 4, 5 and 6 which are correctly set as outputs. The RBIE bit enables an interrupt if any port B input changes. However, you have set these as outputs Im not sure that the point of your code after the test for the RBIF value. If this flag is set, then it will be because of the preceding statement that sets the bottom 4 bits high. I note that these are NOT the bits you have the LEDs connected to, and yet you say that they are being turned on. Perhaps a circuit diagram and not a photo which is very hard to see what is really going on. Within the body of the if statement you have a series of other if statements that test the state of some Port B bits however these are outputs that you have only just set their value. Your logic is a bit unusual here in that you dont need to test the value of a bit you have only just set yourself you already know its value. Also within the outer if statement, it would appear that you have a while1 statement this will effectively stop anyu further processing of your program as it ias a tight loop and a common construct in some situations generally when your entire program is interrupt driven. I suspect that you really meant to have the outer if statement that tests for the RBIF flag being set as the body of the while statement that way you would loop until the flag is set and then do some processing. You say that you have the LDR attached to Port B1. As outlined about, you have all of Port B set as outputs. I would have expected to see you set the TRISB bit for B1 so that it is an input and also see some testing again within a loop or whatever of the Port. B1 value. An LDR generally provides an analogue value by measuring the voltage across it or current through it etc. Going only by your picture, I cant see where you are providing the voltage source for the LDR it looks like you have simply connected it between the B1 pin and ground again a circuit diagram would help here. Generally you set the LDR up as a voltage divider with another resistor and a known stable voltage source and then use the Analogue capabilities of the MCU to measure that value. This brings up another point with your program. I know that RB1 is also AN1. I cant see any place in your code where you set up the AD converter.