Wrappers and Auto-boxing

Question

There is the following code:

Integer time = 12;
Double lateTime = 12.30;
Boolean late = false;
Double result = late ? lateTime : time;  //Why here can I assign an Integer to a Double?
System.out.println(result);

It prints:

12.0

This one doesn't compile. Why?

Integer time = 12;
Double lateTime = 12.30;
Double result = time;      //Integer cannot be converted to Double
System.out.println(result);

Answer 1

The differences are due to the ternary operator behaviour in Java.

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The ternary conditional case:

In the expression late ? lateTime : time, Java will auto-unbox exactly one of the arguments (according to the value of late) to its respective primitive type. (You can observe this by setting time to null and late to true: a NullPointerException is not thrown. The same applies when setting lastTime to null and late to false.)

If the value of the expression will be time, then this is widened to a double.

In either case, the resulting double is auto-boxed to a Double in assigning it to result.

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The simple assignment case:

In writing Double result = time;, Java disallows this as it expects you to be more explicit.

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^{Personally I find the mechanism of the Java ternary conditional operator with respect to the boxed primitive types to be one of the most pernicious parts of the language.}