Explains runtime errors and compilation failures with actionable debugging guidance. Use when Python or Java code throws runtime exceptions (NullPointerException, TypeError, AttributeError, etc.), compilation errors (syntax errors, type mismatches, import failures), or dependency issues. Analyzes error messages, stack traces, and code context to identify root causes and provide concrete fixes with examples. Distinct from test-related errors - focuses on errors during normal code execution and build processes.
Explains runtime errors and compilation failures with actionable debugging guidance. Use when Python or Java code throws runtime exceptions (NullPointerException, TypeError, AttributeError, etc.), compilation errors (syntax errors, type mismatches, import failures), or dependency issues. Analyzes error messages, stack traces, and code context to identify root causes and provide concrete fixes with examples. Distinct from test-related errors - focuses on errors during normal code execution and build processes.
Runtime Error Explainer
Analyze runtime and compilation errors with clear explanations and actionable fixes.
Core Capabilities
This skill helps debug runtime and compilation errors by:
Parsing error output - Extract key information from error messages and stack traces
Identifying root causes - Determine why errors occur during execution or compilation
Explaining clearly - Translate technical errors into understandable language
Providing fixes - Suggest concrete code changes with before/after examples
Preventing recurrence - Recommend best practices to avoid similar errors
Error Analysis Workflow
Step 1: Gather Error Context
Collect all relevant information about the error.
Essential Information:
Complete error message and stack trace
Programming language and version
Code that triggered the error
How the error was triggered (command, action, input)
Additional Context (if available):
Recent code changes
Environment details (OS, IDE, dependencies)
Input data or parameters that caused error
Configuration files
How to Gather:
# Python - Run with full traceback
python script.py
# Python - More verbose error output
python -v script.py
# Java - Compile with verbose output
javac -verbose MyClass.java
# Java - Run with stack trace
java -XX:+ShowCodeDetailsInExceptionMessages MyClass
Step 2: Categorize the Error
Identify the error type using references/error_catalog.md:
Error Categories:
Syntax/Compilation Errors
Python: SyntaxError, IndentationError
Java: Compilation errors (missing semicolons, type errors)
Java: Variable not initialized, cannot find symbol
Step 3: Extract Key Information
Pull out critical details from the error.
From Error Message:
Error type and name
Error description
Line number where error occurred
File name
Variable names or values involved
From Stack Trace:
Execution path leading to error
Function/method call chain
Origin of error in your code vs. library code
Example Python Error:
Traceback (most recent call last):
File "app.py", line 45, in <module>
result = process_user(None)
File "app.py", line 12, in process_user
return user.name.upper()
AttributeError: 'NoneType' object has no attribute 'name'
Extracted Information:
Error type: AttributeError
Problem: Tried to access .name on None
Location: app.py line 12, in process_user function
Root cause: user parameter is None
Step 4: Identify Root Cause
Analyze why the error occurred.
Common Root Causes:
Syntax/Compilation:
Missing or mismatched parentheses/brackets
Incorrect indentation (Python)
Missing semicolons (Java)
Typos in keywords or variable names
Runtime:
Null/None values where objects expected
Wrong data types in operations
Index out of bounds
Missing dictionary keys
Division by zero
File not found
Import/Dependency:
Module/package not installed
Incorrect import path
Circular imports
Classpath issues (Java)
See references/error_catalog.md for detailed root cause analysis for each error type.
Step 5: Provide Explanation
Structure the explanation with clear sections.
Explanation Template:
## Error Summary
[One-sentence description of what went wrong]
## What Happened
[Explain the error in plain language]
**Error Type:** [Error name and category]
**Location:** [File and line number]
**Problem:** [What specifically failed]
## Root Cause
[Explain why this happened - the underlying issue]
## How to Fix### Solution 1: [Most common fix] ⭐
[Specific code change or action]
```[language]
# Before
[problematic code]
# After
[fixed code]
Why this works: [Brief explanation]
Solution 2: [Alternative approach]
[Another fix if Solution 1 doesn't apply]
Verification
[How to verify the fix works]
[command to test]
Prevention
[How to avoid this error in future]
[Best practice 1]
[Best practice 2]
[Tool or technique recommendation]
### Step 6: Suggest Next Steps
Provide actionable debugging guidance.
**If fix is straightforward:**
- Show exact code change
- Provide command to verify
- Mention related code to review
**If more investigation needed:**
- Suggest adding print/logging statements
- Recommend debugging tools
- Propose simplification to isolate issue
**If environmental issue:**
- Suggest checking installation/versions
- Recommend environment verification
- Propose configuration review
## Common Error Patterns
### Python: AttributeError - NoneType
**Error:**
AttributeError: 'NoneType' object has no attribute 'method_name'
**Explanation:**
```markdown
## Error Summary
Attempted to call a method on None instead of an object instance.
## What Happened
**Error Type:** AttributeError
**Problem:** Code tried to access an attribute or method on `None`
Python functions return `None` by default if no explicit return statement is
used. When you try to call a method on `None`, you get this error.
## Root Cause
Common causes:
1. Function didn't return a value but you expected an object
2. Variable not initialized before use
3. Database/API query returned no result
4. Method called before object creation
## How to Fix
### Solution 1: Add None check ⭐
```python
# Before
result = get_user(user_id)
name = result.name # Crashes if result is None
# After
result = get_user(user_id)
if result is not None:
name = result.name
else:
name = "Unknown"
Why this works: Defensively handles the case where get_user returns None.
Solution 2: Fix the source function to always return a value
# Beforedefget_user(user_id):
if user_id in users:
return users[user_id]
# Implicitly returns None# Afterdefget_user(user_id):
if user_id in users:
return users[user_id]
return User(id=user_id, name="Guest") # Return default user
Solution 3: Use walrus operator with conditional (Python 3.8+)
# Assign and check in one lineif (result := get_user(user_id)) isnotNone:
name = result.name
Verification
python script.py
Prevention
Always check return values before using them
Use type hints: def get_user(user_id: int) -> Optional[User]:
Add assertions: assert result is not None, "User not found"
**Explanation:**
```markdown
## Error Summary
Python cannot find the module you're trying to import.
## What Happened
**Error Type:** ModuleNotFoundError (Python 3) or ImportError (Python 2)
**Problem:** The module is not installed or not in Python's path
## Root Cause
1. Package not installed via pip
2. Wrong virtual environment activated
3. Typo in module name
4. Package installed in different Python version
## How to Fix
### Solution 1: Install the missing package ⭐
```bash
# Install package
pip install requests
# Or if using Python 3 explicitly
pip3 install requests
# Or with specific Python version
python -m pip install requests
Why this works: Makes the package available to your Python environment.
Use requirements.txt: pip install -r requirements.txt
Use virtual environments for project isolation
Document all dependencies
Use pip freeze > requirements.txt to capture dependencies
### Java: NullPointerException
**Error:**
Exception in thread "main" java.lang.NullPointerException
at com.example.MyClass.process(MyClass.java:25)
**Explanation:**
```markdown
## Error Summary
Attempted to use an object reference that is null.
## What Happened
**Error Type:** NullPointerException
**Location:** MyClass.java, line 25
**Problem:** Tried to access a method or field on a null object
## Root Cause
Common causes:
1. Object not initialized before use
2. Method returned null unexpectedly
3. Array element is null
4. Forgotten to instantiate object
## How to Fix
### Solution 1: Add null check ⭐
```java
// Before
String name = user.getName().toUpperCase();
// After
if (user != null && user.getName() != null) {
String name = user.getName().toUpperCase();
} else {
String name = "UNKNOWN";
}
Why this works: Prevents accessing methods on null references.
Solution 2: Use Optional (Java 8+)
// Beforepublic User getUser(int id) {
return userMap.get(id); // Returns null if not found
}
Stringname= getUser(123).getName(); // NPE if user not found// Afterpublic Optional<User> getUser(int id) {
return Optional.ofNullable(userMap.get(id));
}
Stringname= getUser(123)
.map(User::getName)
.orElse("Unknown");
Solution 3: Initialize objects properly
// Beforeprivate User user; // Field is null by defaultpublicvoidprocess() {
user.getName(); // NullPointerException
}
// AfterprivateUseruser=newUser(); // Initialize at declaration// Or in constructorpublicMyClass() {
this.user = newUser();
}
Solution 4: Use Objects utility (Java 7+)
import java.util.Objects;
// Throw meaningful exception if null
Objects.requireNonNull(user, "User must not be null");
Stringname= user.getName();
Verification
javac MyClass.java && java MyClass
Prevention
Initialize all object fields
Use Optional for potentially null values
Add @Nullable/@NonNull annotations
Enable null-safety warnings in IDE
Consider using Objects.requireNonNull() for critical parameters
### Java: ClassNotFoundException
**Error:**
java.lang.ClassNotFoundException: com.example.MyClass
at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
**Explanation:**
```markdown
## Error Summary
Java cannot find the specified class at runtime.
## What Happened
**Error Type:** ClassNotFoundException
**Problem:** Class exists in source code but not found when running
## Root Cause
1. Class not in classpath
2. Package name doesn't match directory structure
3. .class file not compiled
4. Typo in class name
5. JAR file not included
## How to Fix
### Solution 1: Check classpath ⭐
```bash
# Compile with source files in correct package structure
javac -d bin src/com/example/MyClass.java
# Run with classpath pointing to bin directory
java -cp bin com.example.MyClass
Why this works: Ensures compiled .class files are in classpath.
# Check if class file existsls bin/com/example/MyClass.class
# Verify package declaration matcheshead -1 src/com/example/MyClass.java
Prevention
Use build tools (Maven, Gradle) to manage dependencies
Follow Java package naming conventions
Keep package declarations synchronized with directory structure
Use IDE features to auto-manage imports and packages
### Python: SyntaxError
**Error:**
File "script.py", line 10
if x == 5
^
SyntaxError: invalid syntax
**Explanation:**
```markdown
## Error Summary
Code has syntax error preventing Python from parsing it.
## What Happened
**Error Type:** SyntaxError
**Location:** Line 10
**Problem:** Missing colon at end of if statement
## Root Cause
Python syntax violated - common causes:
- Missing colons after if/for/while/def/class
- Unmatched parentheses/brackets
- Incorrect indentation
- Invalid operator usage
## How to Fix
### Solution 1: Add missing colon ⭐
```python
# Before
if x == 5
print("Five")
# After
if x == 5:
print("Five")
Why this works: Python requires colon to start code blocks.
Solution 2: Check for unmatched brackets
# Before
result = calculate(a, b, (c + d)
print(result)
# After
result = calculate(a, b, (c + d))
print(result)
Solution 3: Fix indentation
# Before (mixing tabs and spaces)defmy_function():
x = 5return x # Indentation error# After (consistent spaces)defmy_function():
x = 5return x
Verification
# Python will parse file without running
python -m py_compile script.py
# Or just run it
python script.py
Prevention
Use IDE with syntax highlighting
Enable linting (pylint, flake8)
Use auto-formatters (black, autopep8)
Configure editor to show whitespace characters
### Java: Type Mismatch Compilation Error
**Error:**
MyClass.java:15: error: incompatible types: int cannot be converted to String
String result = 42;
^
**Explanation:**
```markdown
## Error Summary
Trying to assign value of wrong type to variable.
## What Happened
**Error Type:** Compilation error - type mismatch
**Location:** Line 15
**Problem:** Assigning int (42) to String variable
## Root Cause
Java is statically typed - variables can only hold values of declared type
or compatible types.
## How to Fix
### Solution 1: Convert type explicitly ⭐
```java
// Before
String result = 42;
// After
String result = String.valueOf(42);
// or
String result = Integer.toString(42);
// or
String result = "" + 42; // Concatenation with empty string
Why this works: Explicitly converts int to String.
Solution 2: Change variable type
// BeforeStringresult=42;
// Afterintresult=42;
Solution 3: Use correct literal type
// Before (assigning int to double)doubleprice=10; // Works but not ideal// After (use double literal)doubleprice=10.0;
Verification
javac MyClass.java
Prevention
Use proper type conversions
Be aware of implicit type casting rules
Use IDE suggestions for quick fixes
Understand primitive types vs object types (int vs Integer)
## Language-Specific Notes
### Python Error Characteristics
**Common Patterns:**
- Runtime errors only (no compilation phase)
- Clear stack traces with file/line numbers
- Descriptive error messages
- Exception hierarchy (all inherit from BaseException)
**Key Files to Check:**
- Module `__init__.py` for package imports
- `sys.path` for import resolution
- Virtual environment activation
**Debugging Tools:**
```bash
# Interactive debugging
python -m pdb script.py
# Print traceback programmatically
import traceback
traceback.print_exc()
