How to resolve 'cannot find symbol' error in Java

Introduction

As a Java developer, you may encounter the frustrating 'cannot find symbol' error during compilation. This error indicates that the Java compiler cannot locate a specific variable, method, or class referenced in your code. Understanding the causes and solutions for this common issue is essential for writing correct and efficient Java programs. In this lab, you will learn how to identify the reasons behind the 'cannot find symbol' error and apply effective techniques to resolve it.

Skills Graph

Understanding the 'cannot find symbol' Error

The 'cannot find symbol' error is a compilation error in Java. It occurs when the Java compiler is unable to find the definition of a symbol (like a variable, method, or class) that is being used in your code. This means the compiler doesn't know what that symbol refers to.

Let's look at a simple example that will cause this error.

Open the integrated terminal in the WebIDE by clicking on Terminal > New Terminal.

In the terminal, navigate to the project directory if you are not already there. The default directory is /home/labex/project.

Create a new Java file named SymbolErrorExample.java in the /home/labex/project directory using the WebIDE file explorer or the command line.

touch /home/labex/project/SymbolErrorExample.java

Now, open /home/labex/project/SymbolErrorExample.java in the WebIDE editor and add the following code:

public class SymbolErrorExample {
    public static void main(String[] args) {
        int myVariable = 10;
        System.out.println(myVariabel); // Intentional typo
    }
}

Save the file by pressing Ctrl + S or using File > Save.

Now, compile the code using the javac command in the terminal:

javac /home/labex/project/SymbolErrorExample.java

You will see an error message similar to this:

/home/labex/project/SymbolErrorExample.java:4: error: cannot find symbol
        System.out.println(myVariabel); // Intentional typo
                           ^
  symbol:   variable myVariabel
  location: class SymbolErrorExample
1 error

This error message clearly indicates that the compiler cannot find the symbol myVariabel. The location of the error is also provided, pointing to the line where the misspelled variable is used.

This is a classic example of the 'cannot find symbol' error caused by a simple typo. In the next steps, we will explore other common causes and how to fix them.

Cause 1: Misspelled or Incorrect Identifier

As seen in the previous step, a common reason for the 'cannot find symbol' error is a typo or incorrect spelling of a variable, method, or class name. The compiler looks for an exact match for the identifier you use.

Let's fix the typo in our SymbolErrorExample.java file.

Open /home/labex/project/SymbolErrorExample.java in the WebIDE editor.

Change the misspelled variable name myVariabel to the correct name myVariable:

public class SymbolErrorExample {
    public static void main(String[] args) {
        int myVariable = 10;
        System.out.println(myVariable); // Corrected spelling
    }
}

Save the file by pressing Ctrl + S or using File > Save.

Now, compile the code again in the terminal:

javac /home/labex/project/SymbolErrorExample.java

This time, the compilation should be successful, and no error message will be displayed. A SymbolErrorExample.class file will be generated in the same directory.

You can then run the compiled code using the java command:

java SymbolErrorExample

The output will be:

10

This demonstrates how correcting a simple typo resolves the 'cannot find symbol' error. Always double-check the spelling of your identifiers.

Cause 2: Incorrect Package or Missing Import

Another frequent cause of the 'cannot find symbol' error is trying to use a class that is not in the current package and has not been explicitly imported. Java organizes classes into packages. To use a class from a different package, you need to either use its fully qualified name (including the package) or import the class.

Let's create a new example to demonstrate this.

Create a new Java file named MissingImportExample.java in the /home/labex/project directory.

touch /home/labex/project/MissingImportExample.java

Open /home/labex/project/MissingImportExample.java in the WebIDE editor and add the following code:

public class MissingImportExample {
    public static void main(String[] args) {
        ArrayList<String> myList = new ArrayList<>(); // Missing import for ArrayList
        myList.add("Hello");
        System.out.println(myList);
    }
}

Save the file by pressing Ctrl + S or using File > Save.

Now, compile the code:

javac /home/labex/project/MissingImportExample.java

You will get an error similar to this:

/home/labex/project/MissingImportExample.java:3: error: cannot find symbol
        ArrayList<String> myList = new ArrayList<>(); // Missing import for ArrayList
        ^
  symbol:   class ArrayList
  location: class MissingImportExample
/home/labex/project/MissingImportExample.java:3: error: cannot find symbol
        ArrayList<String> myList = new ArrayList<>(); // Missing import for ArrayList
                                       ^
  symbol:   class ArrayList
  location: class MissingImportExample
2 errors

The compiler cannot find the ArrayList symbol because it belongs to the java.util package and has not been imported.

To fix this, we need to add an import statement at the beginning of the file.

Open /home/labex/project/MissingImportExample.java again and add the following line at the top:

import java.util.ArrayList;

public class MissingImportExample {
    public static void main(String[] args) {
        ArrayList<String> myList = new ArrayList<>();
        myList.add("Hello");
        System.out.println(myList);
    }
}

Save the file by pressing Ctrl + S or using File > Save.

Compile the code again:

javac /home/labex/project/MissingImportExample.java

The compilation should now be successful.

Run the code:

java MissingImportExample

The output will be:

[Hello]

This shows that adding the correct import statement resolves the 'cannot find symbol' error when using classes from other packages. Alternatively, you could use the fully qualified name java.util.ArrayList instead of importing, but importing is generally preferred for readability.

Cause 3: Uninitialized Variable

In Java, you must declare and initialize a local variable before you use it. If you try to use a local variable that has been declared but not assigned a value, the compiler will report a 'cannot find symbol' error, even though the variable name exists. This is because the compiler doesn't know what value the symbol represents yet.

Let's create an example.

Create a new Java file named UninitializedVariableExample.java in the /home/labex/project directory.

touch /home/labex/project/UninitializedVariableExample.java

Open /home/labex/project/UninitializedVariableExample.java in the WebIDE editor and add the following code:

public class UninitializedVariableExample {
    public static void main(String[] args) {
        int uninitializedVariable;
        System.out.println(uninitializedVariable); // Using an uninitialized variable
    }
}

Save the file by pressing Ctrl + S or using File > Save.

Compile the code:

javac /home/labex/project/UninitializedVariableExample.java

You will see an error message like this:

/home/labex/project/UninitializedVariableExample.java:4: error: variable uninitializedVariable might not have been initialized
        System.out.println(uninitializedVariable); // Using an uninitialized variable
                           ^
1 error

While the error message here is slightly different ("variable might not have been initialized"), it's closely related to the 'cannot find symbol' concept in that the compiler cannot determine the value associated with the symbol. In some contexts or older Java versions, this might manifest as a 'cannot find symbol' error. The core issue is using a variable before it has a defined value.

To fix this, initialize the variable before using it.

Open /home/labex/project/UninitializedVariableExample.java again and modify the code:

public class UninitializedVariableExample {
    public static void main(String[] args) {
        int initializedVariable = 0; // Initialize the variable
        System.out.println(initializedVariable);
    }
}

Save the file by pressing Ctrl + S or using File > Save.

Compile the code again:

javac /home/labex/project/UninitializedVariableExample.java

The compilation should now be successful.

Run the code:

java UninitializedVariableExample

The output will be:

0

This demonstrates that initializing local variables before use is crucial to avoid compilation errors related to undefined symbols or uninitialized variables.

Cause 4: Incorrect Method Signature

When you call a method, the compiler checks if a method with that name and the correct number and types of arguments (the method signature) exists. If it cannot find a method that matches the call, you will get a 'cannot find symbol' error.

Let's create an example.

Create a new Java file named IncorrectMethodCallExample.java in the /home/labex/project directory.

touch /home/labex/project/IncorrectMethodCallExample.java

Open /home/labex/project/IncorrectMethodCallExample.java in the WebIDE editor and add the following code:

public class IncorrectMethodCallExample {

    public static void greet(String name) {
        System.out.println("Hello, " + name);
    }

    public static void main(String[] args) {
        greet("Alice", 25); // Calling greet with incorrect arguments
    }
}

Save the file by pressing Ctrl + S or using File > Save.

Compile the code:

javac /home/labex/project/IncorrectMethodCallExample.java

You will see an error message similar to this:

/home/labex/project/IncorrectMethodCallExample.java:8: error: cannot find symbol
        greet("Alice", 25); // Calling greet with incorrect arguments
        ^
  symbol:   method greet(String,int)
  location: class IncorrectMethodCallExample
1 error

The error message indicates that the compiler cannot find a method named greet that accepts a String and an int as arguments. It knows there's a greet method, but the signature greet(String,int) doesn't match the defined greet(String) method.

To fix this, ensure your method call matches the method's signature.

Open /home/labex/project/IncorrectMethodCallExample.java again and modify the main method:

public class IncorrectMethodCallExample {

    public static void greet(String name) {
        System.out.println("Hello, " + name);
    }

    public static void main(String[] args) {
        greet("Alice"); // Calling greet with the correct argument
    }
}

Save the file by pressing Ctrl + S or using File > Save.

Compile the code again:

javac /home/labex/project/IncorrectMethodCallExample.java

The compilation should now be successful.

Run the code:

java IncorrectMethodCallExample

The output will be:

Hello, Alice

This demonstrates that the number and types of arguments in a method call must match the method's definition to avoid the 'cannot find symbol' error related to method signatures.

Summary

In this lab, you have learned how to identify and resolve the common 'cannot find symbol' error in Java. You explored the main causes, including misspelled identifiers, missing imports, uninitialized variables, and incorrect method signatures. By understanding these issues and practicing the solutions, you can effectively troubleshoot and fix this error, leading to more robust and error-free Java programs. Remember to carefully check your code for these common pitfalls when you encounter the 'cannot find symbol' error.