How does Apex handle null values, and what patterns help avoid NullPointerException?

Apex's null semantics are mostly Java-like, with some Salesforce-specific quirks.

What is null:

• Variables declared but uninitialised: String s; is null.
• sObject fields not selected in SOQL come back as null — even if they have values in the DB.
• Empty collections are NOT null — new List<Account>() is empty but non-null.
• null returns true for == against another null.

NullPointerException sources:

• Calling a method on null: acc.Name.toLowerCase() when acc or acc.Name is null.
• Dereferencing a map's value when key not present: myMap.get(badKey).field — get() returns null, then dereferencing fails.
• SOQL returning empty list, then .get(0) on it — IndexOutOfBoundsException, not NPE, but similar pattern.

Patterns to avoid NPEs:

1. Safe navigation operator (Apex equivalent: ?.):

apex String name = acc?.Account?.Name;

1. Null check before access:

apex if (acc != null && acc.Name != null) { ... }

1. Default values via `.get(key, defaultValue)` — Map's get with default.

1. String.isBlank() / String.isEmpty() — check string-ness without NPE.

1. `List.isEmpty()` before accessing element by index.

1. Null-coalescing for primitives:

apex Decimal amount = opp.Amount == null ? 0 : opp.Amount;

The single biggest source of NPEs in Apex is querying with LIMIT 1 and then accessing the result without checking if the list is empty.