Why Java’s Ternary Operator Can Trigger NullPointerException with Autoboxing

Recently Alibaba released the new "Taishan" version of its Java Development Manual, which mentions a common pitfall involving the ternary operator and autoboxing that can cause a NullPointerException (NPE).

1. Ternary Operator

The conditional expression b?x:y evaluates the condition b first; if true, it evaluates x, otherwise y. Only one of x or y is evaluated, and the operator is right‑associative.

2. Autoboxing and Unboxing

Since Java 5, primitive types can be automatically boxed into their wrapper classes and unboxed back. For example, Integer i = 100; is compiled to Integer i = Integer.valueOf(100);.

3. Problem Review

The following code often appears in projects:

Map<String, Boolean> map = new HashMap<>();
Boolean b = (map != null ? map.get("test") : false);

When map.get("test") returns null, the ternary expression forces unboxing of a

null

Boolean

, leading to an NPE.

4. Analysis

Decompiling the code shows the compiler generates:

HashMap hashmap = new HashMap();
Boolean b = Boolean.valueOf(hashmap == null ? false : ((Boolean)hashmap.get("test")).booleanValue());

The unboxing step ((Boolean)hashmap.get("test")).booleanValue() invokes booleanValue() on null, which throws the exception.

The Java Language Specification (JLS 15.25) states that if one operand is primitive and the other is its boxed type, the result type is the primitive, causing the automatic unboxing.

5. Solution

Ensure both operands of the ternary operator are reference types, e.g.:

Map<String, Boolean> map = new HashMap<>();
Boolean b = (map != null ? map.get("test") : Boolean.FALSE);

This prevents unboxing of a null wrapper and eliminates the NPE.