Cannot find symbol compilation error

Question

Please explain the following about the "Cannot find symbol" error:

• What does this error mean?
• What things can cause this error?
• How does the programmer go about fixing this error?

This question is designed to be a comprehensive question about "cannot find symbol" compilation errors in Java.

Answer 1

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);

Answer 2

The "Cannot find symbol" errors generally occur when you try to reference an undeclared variable in your code. 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.

When your code is compiled, the compiler needs to work out what each and every identifier in your code means. As the compiler is going through the code it will find something and know what to do with it or not. Your Cannot find symbol error relates to the identifiers and means that Java cannot figure out what the "symbol" means.

The general causes for a Cannot find symbol error are things like:

• Incorrect spelling.
• Wrong case. Halo is different from halo.
• Improper use of acceptable identifier values (letters, numbers, underscore, dollar sign), my-class is not the same as myclass.
• No variable declaration or variable is outside of the scope you are referencing it in.

Answer 3

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.

Answer 4

The "Cannot find symbol" errors generally occur when you try to reference an undeclared variable in your code. 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.

When your code is compiled, the compiler needs to work out what each and every identifier in your code means. As the compiler is going through the code it will find something and know what to do with it or not. Your Cannot find symbol error relates to the identifiers and means that Java cannot figure out what the "symbol" means.

Example

public class TestClass {
  public static void main(String[] args) {
    int x = 2;
    int y = 4;
    sum = x + y ;
    System.out.println(sum);
  }
}

output

TestClass.java:10: error: cannot find symbol
            sum = x + y ;
  symbol:   variable sum
  location: class TestClass
TestClass.java:11: error: cannot find symbol
            System.out.println(sum);
  symbol:   variable sum
  location: class TestClass
2 errors