Please explain the following about "Cannot find symbol", "Cannot resolve symbol" or "Symbol not found" errors (in Java):
What do they mean?
What things can cause them?
How does the programmer go about fixing them?
This question is designed to seed a comprehensive Q&A about these common compilation errors in Java.
0. Is there any difference between these errors?
Not really. "Cannot find symbol", "Cannot resolve symbol" and "Symbol not found" all mean the same thing. (Different Java compilers are written by different people, and different people use different phraseology to say the same thing.)
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 class, while, and so on.
Literals: like true, false, 42, 'X' and "Hi mum!".
Operators and other non-alphanumeric tokens: like +, =, {, and so on.
Identifiers: like Reader, i, toString, processEquibalancedElephants, 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. StringBiulder instead of StringBuilder. Java cannot and will not attempt to compensate for bad spelling or typing errors.
Perhaps you got the case wrong; i.e. stringBuilder instead of StringBuilder. All Java identifiers are case sensitive.
Perhaps you used underscores inappropriately; i.e. mystring and my_string 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. "rope".push()2.
Perhaps you are trying to use a method as a field, or vice versa; e.g. "rope".length or someArray.length().
Perhaps you are mistakenly operating on an array rather than array element; e.g.
String strings[] = ...
if (strings.charAt(3)) { ... }
// maybe that should be 'strings[0].charAt(3)'
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 new as in:
String s = String(); // should be 'new String()'
Perhaps you are trying to import or otherwise use a class that has been declared in the default package; i.e. the one where classes with no package statements go.
Hint: learn about packages. You should only use the default package for simple applications that consist of one class ... or at a stretch, one Java source file.
For cases where type or instance doesn't appear to have the member (e.g. method or field) 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 may have suggested java.awt.List rather than java.util.List.
Perhaps you are using (compiling against) the wrong version of an API.
Perhaps you forgot to cast your object to an appropriate subclass.
Perhaps you have declared the variable's type to be a supertype of the one with the member you are looking for.
The problem is often a combination of the above. For example, maybe you "star" imported java.io.* and then tried to use the Files class ... which is in java.nio not java.io. Or maybe you meant to write File ... which is a class in java.io.
Here is an example of how incorrect variable scoping can lead to a "Cannot find symbol" error:
List<String> strings = ...
for (int i = 0; i < strings.size(); i++) {
if (strings.get(i).equalsIgnoreCase("fnord")) {
break;
}
}
if (i < strings.size()) {
...
}
This will give a "Cannot find symbol" error for i in the if statement. Though we previously declared i, that declaration is only in scope for the for statement and its body. The reference to i in the if statement cannot see that declaration of i. It is out of scope.
(An appropriate correction here might be to move the if statement inside the loop, or to declare i 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:
for (int i = 0; i < 100; i++); {
System.out.println("i is " + i);
}
This will give you a compilation error in the println call saying that i 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 for loop. So that code actually means this:
for (int i = 0; i < 100; i++);
// The previous and following are separate statements!!
{
System.out.println("i is " + i);
}
The { ... } block is NOT the body of the for loop, and therefore the previous declaration of i in the for statement is out of scope in the block.
Here is another example of "Cannot find symbol" error that is caused by a typo.
int tmp = ...
int res = tmp(a + b);
Despite the previous declaration, the tmp in the tmp(...) expression is erroneous. The compiler will look for a method called tmp, and won't find one. The previously declared tmp 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:
int res = tmp * (a + b);
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 Foo and Bar where Foo uses Bar. If you have never compiled Bar and you run javac Foo.java, you are liable to find that the compiler can't find the symbol Bar. The simple answer is to compile Foo and Bar together; e.g. javac Foo.java Bar.java or javac *.java. 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:
for (int i = 1; i < 10; i++) {
for (j = 1; j < 10; j++) {
...
}
}
Suppose that the compiler says "Cannot find symbol" for j. There are many ways I could "fix" that:
I could change the inner for to for (int j = 1; j < 10; j++) - probably correct.
I could add a declaration for j before the inner for loop, or the outer for loop - possibly correct.
I could change j to i in the inner for 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.
Cannot find symbol 'var': You are probably trying to compile source code that uses local variable type inference (i.e. a var declaration) with an older compiler or older --source level. The var was introduced in Java 10. Check your JDK version and your build files, and (if this occurs in an IDE), the IDE settings.
You are not compiling / 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.
Another example of this is when you use (Java 9+) java SomeClass.java to compile and run a class. If the class depends on another class that you haven't compiled (or recompiled), you are liable to get "Cannot resolve symbol" errors referring to the 2nd class. The other source file(s) are not automatically compiled. The java command's new "compile and run" mode is not designed for running programs with multiple source code files.
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 tar -tvf 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 described a scenario where Eclipse did not handle a Maven "test" tree correctly: see this answer. (Apparently that particular bug was been fixed a long time ago.)
Android issues: When you are programming for Android, and you have "Cannot find symbol" errors related to R, be aware that the R symbols are defined by the context.xml file. Check that your context.xml file is correct and in the correct place, and that the corresponding R 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.
Hiding system classes: I've seen cases where the compiler complains that substring is an unknown symbol in something like the following
String s = ...
String s1 = s.substring(1);
It turned out that the programmer had created their own version of String and that his version of the class didn't define a substring methods. I've seen people do this with System, Scanner and other classes.
Lesson: Don't define your own classes with the same names as common library classes!
The problem can also be solved by using the fully qualified names. For example, in the example above, the programmer could have written:
java.lang.String s = ...
java.lang.String s1 = s.substring(1);
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 \uxxxx 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.
2 - The three basic principles of Civil Engineering: water doesn't flow uphill, a plank is stronger on its side, and you can't push on a rope.
You'll also get this error if you forget a new:
String s = String();
versus
String s = new String();
because the call without the new keyword will try and look for a (local) method called String without arguments - and that method signature is likely not defined.
One more example of 'Variable is out of scope'
As I've seen that kind of questions a few times already, maybe one more example to what's illegal even if it might feel okay.
Consider this code:
if(somethingIsTrue()) {
String message = "Everything is fine";
} else {
String message = "We have an error";
}
System.out.println(message);
That's invalid code. Because neither of the variables named message is visible outside of their respective scope - which would be the surrounding brackets {} in this case.
You might say: "But a variable named message is defined either way - so message is defined after the if".
But you'd be wrong.
Java has no free() or delete operators, so it has to rely on tracking variable scope to find out when variables are no longer used (together with references to these variables of cause).
It's especially bad if you thought you did something good. I've seen this kind of error after "optimizing" code like this:
if(somethingIsTrue()) {
String message = "Everything is fine";
System.out.println(message);
} else {
String message = "We have an error";
System.out.println(message);
}
"Oh, there's duplicated code, let's pull that common line out" -> and there it it.
The most common way to deal with this kind of scope-trouble would be to pre-assign the else-values to the variable names in the outside scope and then reassign in if:
String message = "We have an error";
if(somethingIsTrue()) {
message = "Everything is fine";
}
System.out.println(message);
SOLVED
Using IntelliJ
Select Build->Rebuild Project will solve it
One way to get this error in Eclipse :
Define a class A in src/test/java.
Define another class B in src/main/java that uses class A.
Result : Eclipse will compile the code, but maven will give "Cannot find symbol".
Underlying cause : Eclipse is using a combined build path for the main and test trees. Unfortunately, it does not support using different build paths for different parts of an Eclipse project, which is what Maven requires.
Solution :
Don't define your dependencies that way; i.e. don't make this mistake.
Regularly build your codebase using Maven so that you pick up this mistake early. One way to do that is to use a CI server.
"Can not find " means that , compiler who can't find appropriate variable, method ,class etc...if you got that error massage , first of all you want to find code line where get error massage..And then you will able to find which variable , method or class have not define before using it.After confirmation initialize that variable ,method or class can be used for later require...Consider the following example.
I'll create a demo class and print a name...
class demo{
public static void main(String a[]){
System.out.print(name);
}
}
Now look at the result..
That error says, "variable name can not find"..Defining and initializing value for 'name' variable can be abolished that error..Actually like this,
class demo{
public static void main(String a[]){
String name="smith";
System.out.print(name);
}
}
Now look at the new output...
Ok Successfully solved that error..At the same time , if you could get "can not find method " or "can not find class" something , At first,define a class or method and after use that..
If you're getting this error in the build somewhere else, while your IDE says everything is perfectly fine, then check that you are using the same Java versions in both places.
For example, Java 7 and Java 8 have different APIs, so calling a non-existent API in an older Java version would cause this error.
There can be various scenarios as people have mentioned above. A couple of things which have helped me resolve this.
If you are using IntelliJ
File -> 'Invalidate Caches/Restart'
OR
The class being referenced was in another project and that dependency was not added to the Gradle build file of my project. So I added the dependency using
compile project(':anotherProject')
and it worked. HTH!
If eclipse Java build path is mapped to 7, 8 and in Project pom.xml Maven properties java.version is mentioned higher Java version(9,10,11, etc..,) than 7,8 you need to update in pom.xml file.
In Eclipse if Java is mapped to Java version 11 and in pom.xml it is mapped to Java version 8. Update Eclipse support to Java 11 by go through below steps in eclipse IDE
Help -> Install New Software ->
Paste following link http://download.eclipse.org/eclipse/updates/4.9-P-builds at Work With
or
Add (Popup window will open) ->
Name: Java 11 support
Location: http://download.eclipse.org/eclipse/updates/4.9-P-builds
then update Java version in Maven properties of pom.xml file as below
<java.version>11</java.version>
<maven.compiler.source>${java.version}</maven.compiler.source>
<maven.compiler.target>${java.version}</maven.compiler.target>
Finally do right click on project Debug as -> Maven clean, Maven build steps
I too was getting this error. (for which I googled and I was directed to this page)
Problem: I was calling a static method defined in the class of a project A from a class defined in another project B.
I was getting the following error:
error: cannot find symbol
Solution: I resolved this by first building the project where the method is defined then the project where the method was being called from.
you compiled your code using maven compile and then used maven test to run it worked fine. Now if you changed something in your code and then without compiling you are running it, you will get this error.
Solution: Again compile it and then run test. For me it worked this way.
In my case - I had to perform below operations:
Move context.xml file from src/java/package to the resource directory (IntelliJ
IDE)
Clean target directory.
For hints, look closer at the class name name that throws an error and the line number, example:
Compilation failure
[ERROR] \applications\xxxxx.java:[44,30] error: cannot find symbol
One other cause is unsupported method of for java version say jdk7 vs 8.
Check your %JAVA_HOME%
We got the error in a Java project that is set up as a Gradle multi-project build. It turned out that one of the sub-projects was missing the Gradle Java Library plugin.
This prevented the sub-project's class files from being visible to other projects in the build.
After adding the Java library plugin to the sub-project's build.gradle in the following way, the error went away:
plugins {
...
id 'java-library'
}
Re: 4.4: An earlier build problem in Stephen C's excellent answer:
I encountered this scenario when developing an osgi application.
I had a java project A that was a dependency of B.
When building B, there was the error:
Compilation failure: org.company.projectA.bar.xyz does not exist
But in eclipse, there was no compile problem at all.
Investigation
When i looked in A.jar, there were classes for org.company.projectA.foo.abc but none for org.company.projectA.bar.xyz.
The reason for the missing classes, was that in the A/pom.xml, was an entry to export the relevant packages.
<plugin>
<groupId>org.apache.felix</groupId>
<artifactId>maven-bundle-plugin</artifactId>
...
<configuration>
<instructions>
....
<Export-Package>org.company.projectA.foo.*</Export-Package>
</instructions>
</configuration>
</plugin>
Solution
Add the missing packages like so:
<Export-Package>org.company.projectA.foo.*,org.company.projectA.bar.*</Export-Package>
and rebuild everything.
Now the A.jar includes all the expected classes, and everything compiles.
I was getting below error
java: cannot find symbol
symbol: class __
To fix this
I tried enabling lambok, restarted intellij, etc but below worked for me.
Intellij Preferences ->Compiler -> Shared Build process VM Options and set it to
-Djps.track.ap.dependencies=false
than run
mvn clean install
Optional.isEmpty()
I was happily using !Optional.isEmpty() in my IDE, and it works fine, as i was compiling/running my project with >= JDK11. Now, when i use Gradle on the command line (running on JDK8), i got the nasty error in the compile task.
Why?
From the docs (Pay attention to the last line):
boolean java.util.Optional.isEmpty()
If a value is not present, returns true, otherwise false.
Returns:true if a value is not present, otherwise false
Since:11
I solved this error like this... The craziness of android. I had the package name as Adapter and the I refactor the name to adapter with an "a" instead of "A" and solved the error.
I have an error type that impls the Error trait, and it wraps an underlying error cause, so the source method returns Some(source). I want to know whether the Display impl on my error type should include a description of that source error, or not.
I can see two options:
Yes, include source in Display output, e.g. "Error opening database: No such file"
This makes it easy to print a whole error chain just by formatting with "{}" but impossible to only display the error itself without the underlying chain of source errors. Also it makes the source method a bit pointless and gives client code no choice on how to format separation between each error in the chain. Nevertheless this choice seems common enough in example code I have found.
No, just print the error itself e.g. "Error opening database" and leave it to client code to traverse and display source if it wants to include that in the output.
This gives client code the choice of whether to display just the surface error or the whole chain, and in the latter case how to format separation between each error in the chain. It leaves client code with the burden of iterating through the chain, and I haven't yet fallen upon a canonical utility for conveniently formatting an error chain from errors that each only Display themselves excluding source. (So of course I have my own.)
The snafu crate (which I really like) seems to hint at favoring option 2, in that an error variant with a source field but no display attribute defaults to formatting Display output that does not include source.
Maybe my real question here is: What is the purpose of the source method? Is it to make formatting error chains more flexible? Or should Display really output everything that should be user-visible about an error, and source is just there for developer-visible purposes?
I would love to see some definitive guidance on this, ideally in the documentation of the Error trait.
#[derive(Debug)]
enum DatabaseError {
Opening { source: io::Error },
}
impl Error for DatabaseError {
fn source(&self) -> Option<&(dyn Error + 'static)> {
match self {
DataBaseError::Opening { source } => Some(source),
}
}
}
impl fmt::Display for DatabaseError {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
match self {
DatabaseError::Opening { source } => {
// ??? Should we include the source?
write!(f, "Error opening database: {}", source)
// ??? Or should we leave it to the caller to call .source()
// if they want to include that in the error description?
write!(f, "Error opening database")
}
}
}
}
The two options of whether to print the source error on a Display implementation creates two schools of design. This answer will explain the two while objectively stating their key differences and clarifying a few possible misconceptions in the way.
Design 1: Yes, include source on your Display impl
Example with SNAFU:
#[derive(Debug, Snafu)]
enum Error {
#[snafu(display("Could not read data set token: {}", source))]
ReadToken {
#[snafu(backtrace)]
source: ReadDataSetError,
},
}
The key advantage, as already mentioned in the question, is that providing the full amount of information is as simple as just printing the error value.
eprintln!("[ERROR] {}", err);
It is simple and easy, requiring no helper functions for reporting the error, albeit with the lack of presentation flexibility. Without string manipulation, a chain of colon-separated errors is what you will always get.
[ERROR] Could not read data set token: Could not read item value: Undefined value length of element tagged (5533,5533) at position 3548
Design 2: No, leave out source on your Display impl
#[derive(Debug, Snafu)]
enum Error {
#[snafu(display("Could not read data set token"))]
ReadToken {
#[snafu(backtrace)]
source: ReadDataSetError,
},
}
While this will not give you the full information with a single line print like before, you can leave that task to a project-wide error reporter. This also grants the consumer of the API greater flexibility on error presentation.
A simple example follows. Additional logic would be required for presenting the error's backtrace.
fn report<E: 'static>(err: E)
where
E: std::error::Error,
E: Send + Sync,
{
eprintln!("[ERROR] {}", err);
if let Some(cause) = err.source() {
eprintln!();
eprintln!("Caused by:");
for (i, e) in std::iter::successors(Some(cause), |e| e.source()).enumerate() {
eprintln!(" {}: {}", i, e);
}
}
}
Playground
It's also worth considering the interest of integrating with opinionated libraries. That is, certain crates in the ecosystem may have already made an assumption about which option to choose. In anyhow, error reports will already traverse the error's source chain by default. When using anyhow for error reporting, you should not be adding source, otherwise you may come up with an irritating list of repeated messages:
[ERROR] Could not read data set token: Could not read item value: Undefined value length of element tagged (5533,5533) at position 3548
Caused by:
0: Could not read item value: Undefined value length of element tagged (5533,5533) at position 3548
1: Undefined value length of element tagged (5533,5533) at position 3548
Likewise, the eyre library provides a customizable error reporting abstraction, but existing error reporters in the eyre crate ecosystem also assume that the source is not printed by the error's Display implementation.
So, which one?
Thanks to the efforts of the Error Handling project group, a key guideline regarding the implementation of Display was proposed in early 2021:
An error type with a source error should either return that error via source or include that source's error message in its own Display output, but never both.
That would be the second design: avoid appending the source's error message in your Display implementation. For SNAFU users, this currently means that applications will need to bring in an error reporter until one is made available directly in the snafu crate. As the ecosystem is yet to mature around this guideline, one may still find error utility crates lacking support for error reporting in this manner.
In either case...
This decision only plays a role in error reporting, not in error matching or error handling in some other manner. The existence of the source method establishes a chain-like structure on all error types, which can be exploited in pattern matching and subsequent flow control of the program.
The Error::source method has a purpose in the ecosystem, regardless of how errors are reported.
In addition, it's ultimately up to the developers to choose how to design their errors and respective Display implementations, although once it starts integrating with other components, following the guideline will be the right way towards consistent error reporting.
What about Rust API guidelines?
The Rust API guidelines do not present an opinion about Display in errors, other than C-GOOD-ERR, which only states that the error type's Display message should be "lowercase without trailing punctuation, and typically concise". There is a pending proposal to update this guideline, instructing developers to exclude source in their Display impl. However, the pull request was created before the guideline was proposed, and has not been updated since then (at the time of writing).
See also:
The error handling protect group (GitHub)
Inside Rust Blog post 2021-07-01: What the error handling project group is working towards
I'm creating my first grammar with ANTLR and ANTLRWorks 2. I have mostly finished the grammar itself (it recognizes the code written in the described language and builds correct parse trees), but I haven't started anything beyond that.
What worries me is that every first occurrence of a token in a parser rule is underlined with a yellow squiggle saying "Implicit token definition in parser rule".
For example, in this rule, the 'var' has that squiggle:
variableDeclaration: 'var' IDENTIFIER ('=' expression)?;
How it looks exactly:
The odd thing is that ANTLR itself doesn't seem to mind these rules (when doing test rig test, I can't see any of these warning in the parser generator output, just something about incorrect Java version being installed on my machine), so it's just ANTLRWorks complaining.
Is it something to worry about or should I ignore these warnings? Should I declare all the tokens explicitly in lexer rules? Most exaples in the official bible The Defintive ANTLR Reference seem to be done exactly the way I write the code.
I highly recommend correcting all instances of this warning in code of any importance.
This warning was created (by me actually) to alert you to situations like the following:
shiftExpr : ID (('<<' | '>>') ID)?;
Since ANTLR 4 encourages action code be written in separate files in the target language instead of embedding them directly in the grammar, it's important to be able to distinguish between << and >>. If tokens were not explicitly created for these operators, they will be assigned arbitrary types and no named constants will be available for referencing them.
This warning also helps avoid the following problematic situations:
A parser rule contains a misspelled token reference. Without the warning, this could lead to silent creation of an additional token that may never be matched.
A parser rule contains an unintentional token reference, such as the following:
number : zero | INTEGER;
zero : '0'; // <-- this implicit definition causes 0 to get its own token
If you're writing lexer grammar which wouldn't be used across multiple parser grammmar(s) then you can ignore this warning shown by ANTLRWorks2.
I have an antlr generated Java parser that uses the C target and it works quite well. The problem is I also want it to parse erroneous code and produce a meaningful AST. If I feed it a minimal Java class with one import after which a semicolon is missing it produces two "Tree Error Node" objects where the "import" token and the tokens for the imported class should be.
But since it parses the following code correctly and produces the correct nodes for this code it must recover from the error by adding the semicolon or by resyncing. Is there a way to make antlr reflect this fixed input it produces internally in the AST? Or can I at least get the tokens/text that produced the "Tree Node Errors" somehow?
In the C targets
antlr3commontreeadaptor.c around line 200 the following fragment indicates that the C target only creates dummy error nodes so far:
static pANTLR3_BASE_TREE
errorNode (pANTLR3_BASE_TREE_ADAPTOR adaptor, pANTLR3_TOKEN_STREAM ctnstream, pANTLR3_COMMON_TOKEN startToken, pANTLR3_COMMON_TOKEN stopToken, pANTLR3_EXCEPTION e)
{
// Use the supplied common tree node stream to get another tree from the factory
// TODO: Look at creating the erronode as in Java, but this is complicated by the
// need to track and free the memory allocated to it, so for now, we just
// want something in the tree that isn't a NULL pointer.
//
return adaptor->createTypeText(adaptor, ANTLR3_TOKEN_INVALID, (pANTLR3_UINT8)"Tree Error Node");
}
Am I out of luck here and only the error nodes the Java target produces would allow me to retrieve the text of the erroneous nodes?
I haven't used antlr much, but typically the way you handle this type of error is to add rules for matching wrong syntax, make them produce error nodes, and try to fix up after errors so that you can keep parsing. Fixing up afterwards is the problem because you don't want one error to trigger more and more errors for each new token until the end.
I solved the problem by adding new alternate rules to the grammer for all possible erroneous statements.
Each Java import statement gets translated to an AST subtree with the artificial symbol IMPORT as the root for example. To make sure that I can differentiate between ASTs from correct and erroneous code the rules for the erroneous statements rewrite them to an AST with a root symbol with the prefix ERR_, so in the example of the import statement the artifical root symbol would be ERR_IMPORT.
More different root symbols could be used to encode more detailed information about the parse error.
My parser is now as error tolerant as I need it to be and it's very easy to add rules for new kinds of erroneous input whenever I need to do so. You have to watch out to not introduce any ambiguities into your grammar, though.