Why does addition of long variables cause concatenation?
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04-07-2019 - |
Question
What does Java do with long variables while performing addition?
Wrong version 1:
Vector speeds = ... //whatever, speeds.size() returns 2
long estimated = 1l;
long time = speeds.size() + estimated; // time = 21; string concatenation??
Wrong version 2:
Vector speeds = ... //whatever, speeds.size() returns 2
long estimated = 1l;
long time = estimated + speeds.size(); // time = 12; string concatenation??
Correct version:
Vector speeds = ... //whatever, speeds.size() returns 2
long estimated = 1l;
long size = speeds.size();
long time = size + estimated; // time = 3; correct
I don't get it, why Java concatenate them.
Can anybody help me, why two primitive variables are concatenated?
Greetings, guerda
Solution
I suspect you're not seeing what you think you're seeing. Java doesn't do this.
Please try to provide a short but complete program which demonstrates this. Here's a short but complete program which demonstrates correct behaviour, but with your "wrong" code (i.e. a counterexample).
import java.util.*;
public class Test
{
public static void main(String[] args)
{
Vector speeds = new Vector();
speeds.add("x");
speeds.add("y");
long estimated = 1l;
long time = speeds.size() + estimated;
System.out.println(time); // Prints out 3
}
}
OTHER TIPS
My guess is you are actually doing something like:
System.out.println("" + size + estimated);
This expression is evaluated left to right:
"" + size <--- string concatenation, so if size is 3, will produce "3"
"3" + estimated <--- string concatenation, so if estimated is 2, will produce "32"
To get this to work, you should do:
System.out.println("" + (size + estimated));
Again this is evaluated left to right:
"" + (expression) <-- string concatenation - need to evaluate expression first
(3 + 2) <-- 5
Hence:
"" + 5 <-- string concatenation - will produce "5"
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