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Andrew (he/him)
Andrew (he/him)

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Java String Puzzle

Eclipse posted this Java puzzle to Twitter today:

...do you know what will be printed without coding it and trying it?

Top comments (11)

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James Perkins

that one should throw a NPE exception at line 10. Almost had me but then I read it again.

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Bertil Muth

b can‘t possibly be true. Can it? :-)

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Ben Lovy • Edited

I don't know anything about Java. Why not? To the completely untrained eye it sure looks true.

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Bertil Muth

Java has 2 ways of determining if two objects are “the same”. The == operator, and the equals method. Both work differently, and both are shown in the example.

The == operator compares object identity. Creating two String objects and comparing them with == would return false, even if the text is „ABC“ both times. Like: two persons can both be called „Martin Fowler“, but have distinct identities.

So, my first instinct told me: there are multiple occurrences of “ABC”. So multiple String objects. So the == returns false. Then, Java doesn’t even evaluate the second and operand (after &&). The result of the riddle: false is printed.

Then I looked at the riddle again. Java does heavy optimizing. Strings in Java are immutable, and in the example, “ABC” is not changed. So chances are that there only is exactly 1 “ABC” object.

In that case, the == yields true and the second and operand (with Y) is evaluated. And because Y is null, and calling any method (including equals) on null throws a NullPointerException...

The riddles answer is: nothing is printed. A NullPointerException is thrown.

I guess :-) Am I right?

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awwsmm profile image
Andrew (he/him)

The answer lies in the Java String pool!

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bertilmuth profile image
Bertil Muth

Interesting, thanks for sharing!

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Jason R Tibbetts

NullPointerException for Y.equals(null).

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Ildarov

You can't invoke any method on null reference object :)

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Fernando B 🚀

Gonna say True. Though not sure if b is True 100% lol

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Sam Rose

False. :)