Quick Answer

Fix undefined variable/class errors.

Understanding the Issue

Compile-time error when referencing undefined variables, methods, or classes. Typically caused by typos, missing imports, or scope issues.

The Problem

This code demonstrates the issue:

Java Error 
public class Test {
    public static void main(String[] args) {
        System.out.println(UNDEFINED); // Error
    }
}

The Solution

Here's the corrected code:

Java Fixed 
// Solution 1: Declare variable
private static final String MESSAGE = "Hello";

// Solution 2: Add missing import
import java.util.List; // For List usage

// Solution 3: Check scope
public class Test {
    private String name; // Instance field
    
    public void print() {
        System.out.println(name); // Access instance field
    }
}

Key Takeaways

Verify all identifiers are properly declared and in scope.