Quick Answer

Avoid and handle null references.

Understanding the Issue

This runtime exception occurs when trying to dereference a null object, indicating missing object initialization.

The Problem

This code demonstrates the issue:

Java Error 
String str = null;
int length = str.length(); // Throws NPE

The Solution

Here's the corrected code:

Java Fixed 
// Solution 1: Defensive checks
if (str != null) {
    length = str.length();
}

// Solution 2: Use Optional (Java 8+)
Optional<String> opt = Optional.ofNullable(str);
length = opt.map(String::length).orElse(0);

// Solution 3: Validate parameters
public void process(String input) {
    Objects.requireNonNull(input, "Input cannot be null");
    // ...
}

// Solution 4: Annotations (@Nullable, @NonNull)
public void process(@NonNull String input) {
    // ...
}

Key Takeaways

Validate object references before use and leverage Optional for nullable values.