Why does adding a string to an Object change the type to a string?

Question

Say I have the following code

URI uri = URI.create("https://example.com");

String myString = uri + "/something"; // why is this allowed and compiler doesnt complain?

I would have thought compiler would complain about adding a string to URI type

Spectric · Accepted Answer · 2021-04-04 00:58:47Z

2

JVM calls the object's toString() method.

edited Apr 4, 2021 at 0:58

Spectric

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answered Apr 4, 2021 at 0:37

Joseph Giuliani

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1 Comment

Joseph Giuliani Over a year ago

Thanks for editing. I wasn't sure if it was a runtime or compiler thing.

Stephen C · Accepted Answer · 2021-04-04 01:28:00Z

1

String myString = uri + "/something"; 

// why is this allowed and compiler doesnt complain?

There are two meanings for + in Java: numeric addition and string concatenation; see Assignment, Arithmetic, and Unary Operators.

The string concatenation meaning is adopted if one or both of the operand expressions is a String. In that case, the other operand is converted to a string by calling toString() on it. Therefore ...

String myString = uri + "/something";

means the almost¹ the same thing as:

String myString = uri.toString().concat("/something");

^{1 - There is some special case handling for null ...}

edited Apr 4, 2021 at 1:28

answered Apr 4, 2021 at 1:22

Stephen C

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