0. Is there any difference between the two errors?
Not really. "Cannot find symbol" and "Cannot resolve symbol" mean the same thing. Some Java compilers use one phrase, and some the other one.
1. What does a "Cannot find symbol" error mean?
Firstly, it is a compilation error1. It means that either there is a problem in your Java source code, or there is a problem in the way that you are compiling it.
Your Java source code consists of the following things:
- Keywords: like , , , , and so on.
- Literals: like and and .
- Operators and other non-alphanumeric tokens: like , , , and so on.
- Identifiers: like , , , , and so on.
- Comments and whitespace.
A "Cannot find symbol" error is about the identifiers. When your code is compiled, the compiler needs to work out what each and every identifier in your code means.
A "Cannot find symbol" error means that the compiler cannot do this. Your code appears to be referring to something that the compiler doesn't understand.
2. What can cause a "Cannot find symbol" error?
As a first order, there is only one cause. The compiler looked in all of the places where the identifier should be defined, and it couldn't find the definition. This could be caused by a number of things. The common ones are as follows:
- For identifiers in general:
- Perhaps you spelled the name incorrectly; i.e. instead of . Java cannot and will not attempt to compensate for bad spelling or typing errors.
- Perhaps you got the case wrong; i.e. instead of . All Java identifiers are case sensitive.
- Perhaps you used underscores inappropriately; i.e. and are different. (If you stick to the Java style rules, you will be largely protected from this mistake ...)
- Perhaps you are trying to use something that was declared "somewhere else"; i.e. in a different context to where you have implicitly told the compiler to look. (A different class? A different scope? A different package? A different code-base?)
- For identifiers that should refer to variables:
- Perhaps you forgot to declare the variable.
- Perhaps the variable declaration is out of scope at the point you tried to use it. (See example below)
For identifiers that should be method or field names:
- Perhaps you are trying to refer to an inherited method or field that wasn't declared in the parent / ancestor classes or interfaces.
- Perhaps you are trying to refer to a method or field that does not exist (i.e. has not been declared) in the type you are using; e.g. 2.
- Perhaps you are trying to use a method as a field, or vice versa; e.g. or .
Perhaps you are mistakenly operating on an array rather than array element; e.g.
For identifiers that should be class names:
- Perhaps you forgot to import the class.
- Perhaps you used "star" imports, but the class isn't defined in any of the packages that you imported.
Perhaps you forgot a as in:
For cases where type or instance doesn't appear to have the member you were expecting it to have:
- Perhaps you have declared a nested class or a generic parameter that shadows the type you were meaning to use.
- Perhaps you are shadowing a static or instance variable.
- Perhaps you imported the wrong type; e.g. due to IDE completion or auto-correction.
- Perhaps you are using (compiling against) the wrong version of an API.
- Perhaps you forgot to cast your object to an appropriate subclass.
The problem is often a combination of the above. For example, maybe you "star" imported and then tried to use the class ... which is in not . Or maybe you meant to write ... which is a class in .
Here is an example of how incorrect variable scoping can lead to a "Cannot find symbol" error:
This will give a "Cannot find symbol" error for in the statement. Though we previously declared , that declaration is only in scope for the statement and its body. The reference to in the statement cannot see that declaration of . It is out of scope.
(An appropriate correction here might be to move the statement inside the loop, or to declare before the start of the loop.)
Here is an example that causes puzzlement where a typo leads to a seemingly inexplicable "Cannot find symbol" error:
This will give you a compilation error in the call saying that cannot be found. But (I hear you say) I did declare it!
The problem is the sneaky semicolon ( ) before the . The Java language syntax defines a semicolon in that context to be an empty statement. The empty statement then becomes the body of the loop. So that code actually means this:
The block is NOT the body of the loop, and therefore the previous declaration of in the statement is out of scope in the block.
Here is another example of "Cannot find symbol" error that is caused by a typo.
Despite the previous declaration, the in the expression is erroneous. The compiler will look for a method called , and won't find one. The previously declared is in the namespace for variables, not the namespace for methods.
In the example I came across, the programmer had actually left out an operator. What he meant to write was this:
There is another reason why the compiler might not find a symbol if you are compiling from the command line. You might simply have forgotten to compile or recompile some other class. For example, if you have classes and where uses . If you have never compiled and you run , you are liable to find that the compiler can't find the symbol . The simple answer is to compile and together; e.g. or . Or better still use a Java build tool; e.g. Ant, Maven, Gradle and so on.
There are some other more obscure causes too ... which I will deal with below.
3. How do I fix these errors ?
Generally speaking, you start out by figuring out what caused the compilation error.
- Look at the line in the file indicated by the compilation error message.
- Identify which symbol that the error message is talking about.
- Figure out why the compiler is saying that it cannot find the symbol; see above!
Then you think about what your code is supposed to be saying. Then finally you work out what correction you need to make to your source code to do what you want.
Note that not every "correction" is correct. Consider this:
Suppose that the compiler says "Cannot find symbol" for . There are many ways I could "fix" that:
- I could change the inner to - probably correct.
- I could add a declaration for before the inner loop, or the outer loop - possibly correct.
- I could change to in the inner loop - probably wrong!
- and so on.
The point is that you need to understand what your code is trying to do in order to find the right fix.
4. Obscure causes
Here are a couple of cases where the "Cannot find symbol" is seemingly inexplicable ... until you look closer.
Incorrect dependencies: If you are using an IDE or a build tool that manages the build path and project dependencies, you may have made a mistake with the dependencies; e.g. left out a dependency, or selected the wrong version. If you are using a build tool (Ant, Maven, Gradle, etc), check the project's build file. If you are using an IDE, check the project's build path configuration.
You are not recompiling: It sometimes happens that new Java programmers don't understand how the Java tool chain works, or haven't implemented a repeatable "build process"; e.g. using an IDE, Ant, Maven, Gradle and so on. In such a situation, the programmer can end up chasing his tail looking for an illusory error that is actually caused by not recompiling the code properly, and the like ...
An earlier build problem: It is possible that an earlier build failed in a way that gave a JAR file with missing classes. Such a failure would typically be noticed if you were using a build tool. However if you are getting JAR files from someone else, you are dependent on them building properly, and noticing errors. If you suspect this, use to list the contents of the suspect JAR file.
IDE issues: People have reported cases where their IDE gets confused and the compiler in the IDE cannot find a class that exists ... or the reverse situation.
This could happen if the IDE has been configured with the wrong JDK version.
This could happen if the IDE's caches get out of sync with the file system. There are IDE specific ways to fix that.
This could be an IDE bug. For instance @Joel Costigliola describes a scenario where Eclipse does not handle a Maven "test" tree correctly: see this answer.
Android issues: When you are programming for Android, and you have "Cannot find symbol" errors related to , be aware that the symbols are defined by the file. Check that your file is correct and in the correct place, and that the corresponding class file has been generated / compiled. Note that the Java symbols are case sensitive, so the corresponding XML ids are be case sensitive too.
Other symbol errors on Android are likely to be due to previously mention reasons; e.g. missing or incorrect dependencies, incorrect package names, method or fields that don't exist in a particular API version, spelling / typing errors, and so on.
Redefining system classes: I've seen cases where the compiler complains that is an unknown symbol in something like the following
It turned out that the programmer had created their own version of and that his version of the class didn't define a methods.
Lesson: Don't define your own classes with the same names as common library classes!
Homoglyphs: If you use UTF-8 encoding for your source files, it is possible to have identifiers that look the same, but are in fact different because they contain homoglyphs. See this page for more information.
You can avoid this by restricting yourself to ASCII or Latin-1 as the source file encoding, and using Java escapes for other characters.
1 - If, perchance, you do see this in a runtime exception or error message, then either you have configured your IDE to run code with compilation errors, or your application is generating and compiling code .. at runtime.
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