instanceOf operator in java

Question

Why is this line of code in Java not printing the message?

System.out.println("a instanceof String:"+a instanceof String);

Why is it just printing true and not the String before it???

@RC.: As it happens, it doesn't matter. But yes, a minimal reproducible example would be better. — Jon Skeet
– Jon Skeet, Commented Apr 1, 2017 at 11:43
@JonSkeet I saw your answer, so yes it doesn't matter in that case ;) — user180100
– user180100, Commented Apr 1, 2017 at 11:44

Jon Skeet · Accepted Answer · 2017-04-01 11:42:30Z

8

Ah, the joys of operator precedence. That line is effectively:

System.out.println(("a instanceof String:" + a) instanceof String);

... so you're doing string concatenation, and then checking whether the result is a string. So it will always print true, regardless of the value of a.

You just need parentheses

System.out.println("a instanceof String:" + (a instanceof String));

See the Java tutorial page on operators for a precedence list.

answered Apr 1, 2017 at 11:42

Jon Skeet

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Comments

Vasu · Accepted Answer · 2017-04-01 11:47:43Z

3

Your code will be executed as follows:

System.out.println(("a instanceof String:"+a) instanceof String);

So, the whole string i.e., ("a instanceof String:"+a) has been considered for the instanceof check.

In order to print, "a instanceof String:true", you need to use parenthesis at the right place:

System.out.println("a instanceof String:" + (a instanceof String));

edited Apr 1, 2017 at 11:47

answered Apr 1, 2017 at 11:42

Vasu

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Comments

Markus · Accepted Answer · 2017-04-01 11:43:04Z

1

You have to use brackets.

System.out.println(("a instanceof String:"+a) instanceof String);

answered Apr 1, 2017 at 11:43

Markus

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