Why does addition of long variables cause concatenation?

Vector speeds = ... //whatever, speeds.size() returns 2
long estimated = 1l;
long time = speeds.size() + estimated; // time = 21; string concatenation??

Vector speeds = ... //whatever, speeds.size() returns 2
long estimated = 1l;
long time = estimated + speeds.size(); // time = 12; string concatenation??

Vector speeds = ... //whatever, speeds.size() returns 2
long estimated = 1l;
long size = speeds.size();
long time = size + estimated; // time = 3; correct

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
    }
}

System.out.println("" + size + estimated);

"" + size        <--- string concatenation, so if size is 3, will produce "3"
"3" + estimated  <--- string concatenation, so if estimated is 2, will produce "32"

System.out.println("" + (size + estimated));

"" + (expression) <-- string concatenation - need to evaluate expression first
(3 + 2)           <-- 5
Hence:
"" + 5            <-- string concatenation - will produce "5"