Runtime Error Explainer

# 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

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'

## 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]

[command to test]

### 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:**

**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"

# Before
def get_user(user_id):
    if user_id in users:
        return users[user_id]
    # Implicitly returns None

# After
def get_user(user_id):
    if user_id in users:
        return users[user_id]
    return User(id=user_id, name="Guest")  # Return default user

# Assign and check in one line
if (result := get_user(user_id)) is not None:
    name = result.name

python script.py

### Python: ImportError / ModuleNotFoundError

**Error:**

**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

# Check current environment
which python

# Activate virtual environment
source venv/bin/activate  # Linux/Mac
venv\Scripts\activate     # Windows

# Install package in virtual environment
pip install requests

# Wrong
import reqests  # Typo

# Correct
import requests

python -c "import requests; print('Success')"

### Java: NullPointerException

**Error:**

**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";
}

// Before
public User getUser(int id) {
    return userMap.get(id);  // Returns null if not found
}

String name = getUser(123).getName();  // NPE if user not found

// After
public Optional<User> getUser(int id) {
    return Optional.ofNullable(userMap.get(id));
}

String name = getUser(123)
    .map(User::getName)
    .orElse("Unknown");

// Before
private User user;  // Field is null by default

public void process() {
    user.getName();  // NullPointerException
}

// After
private User user = new User();  // Initialize at declaration

// Or in constructor
public MyClass() {
    this.user = new User();
}

import java.util.Objects;

// Throw meaningful exception if null
Objects.requireNonNull(user, "User must not be null");
String name = user.getName();

javac MyClass.java && java MyClass

### Java: ClassNotFoundException

**Error:**

**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