Definition

NPE is a runtime exception that occurs when trying to use a null reference. The JVM throws this exception to protect against dereferencing null pointers, which could cause program crashes.

Common Causes

Basic Example

String name = null;
int length = name.length(); // NPE thrown here

Enter fullscreen mode Exit fullscreen mode

Here, we’re trying to call a method on a null reference. The variable name contains no object, so we can’t invoke length().

Nested Objects Example

class User {
    Address address;
}
class Address {
    String street;
}

User user = new User();
String street = user.address.street; // NPE: address is null

Enter fullscreen mode Exit fullscreen mode

This shows how nested object access can fail. While user exists, address is null because we haven’t initialized it.

Collection Example

List<String> items = null;
items.add("item"); // NPE

Enter fullscreen mode Exit fullscreen mode

Collections should be initialized before use. Better to initialize as empty: List<String> items = new ArrayList<>();

Optional Usage Explained

Basic Optional Chaining

Optional<User> user = Optional.of(new User());
String street = user
    .flatMap(User::getAddress)  // Converts User to Optional<Address>
    .map(Address::getStreet)    // Converts Address to Optional<String>
    .orElse("Unknown");         // Provides default if null

Enter fullscreen mode Exit fullscreen mode

This replaces nested null checks with a fluent API. Each step safely handles potential nulls.

Map and Filter

Optional<String> upperStreet = Optional.ofNullable(user)
    .filter(u -> u.getName() != null)  // Only proceed if name exists
    .map(User::getName)                // Get the name
    .map(String::toUpperCase);         // Transform it

Enter fullscreen mode Exit fullscreen mode

This shows how to transform values while handling nulls safely.

Default Values

String result = Optional.ofNullable(someString)
    .orElse("default");

Enter fullscreen mode Exit fullscreen mode

A clean way to provide fallback values instead of null checks.

Conditional Execution

Optional.ofNullable(user)
    .ifPresent(u -> System.out.println(u.getName()));

Enter fullscreen mode Exit fullscreen mode

Execute code only when the value exists, replacing if-not-null checks.

原文链接：Handling NullPointerException with Optional