Why ternary operation gives nullpointer while its ifelse counterpart doesn’t? [duplicate]

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Here’s the relevant quote from the spec (§15.25.2):

Boolean conditional expressions are standalone expressions (§15.2).

The type of a boolean conditional expression is determined as follows:

  • If the second and third operands are both of type Boolean, the conditional expression has type Boolean.

  • Otherwise, the conditional expression has type boolean.

Therefore, the overall expression’s type is considered to be boolean, and the Boolean value is autounboxed, causing a NullPointerException.

As mentioned in the comments, why doesn’t the following raise an exception?

return val == null ? null : "true".equalsIgnoreCase(val);

Well, the above excerpt from the spec specifically only applies to boolean conditional expressions, which are specified here (§15.25):

If both the second and the third operand expressions are boolean expressions, the conditional expression is a boolean conditional expression.

For the purpose of classifying a conditional, the following expressions are boolean expressions:

  • An expression of a standalone form (§15.2) that has type boolean or Boolean.

  • A parenthesized boolean expression (§15.8.5).

  • A class instance creation expression (§15.9) for class Boolean.

  • A method invocation expression (§15.12) for which the chosen most specific method (§15.12.2.5) has return type boolean or Boolean.
    (Note that, for a generic method, this is the type before instantiating the method’s type arguments.)

  • A boolean conditional expression.

Since null is not a boolean expression, the overall conditional expression is not a boolean conditional expression. Referring to Table 15.2 (later in the same section), we can see that such an expression is considered to have a Boolean type, so no unboxing occurs, and no exception is raised.

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