使用Set的Java代码显示奇怪的输出？

Question

The output of the following code should be 1 according to my logic. But its showing 100 instead. Can anyone explain me where is the problem?

  /* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
    public static void main (String[] args) throws java.lang.Exception
    {
        Set<Short> s=new HashSet<Short>();
        for(Short i=0;i<100;i++){
            s.add(i);
            s.remove(i-1);
        }
        System.out.println(s.size());

    }
}

Answer 1

The result of

i-1

is of type int .

This is described in the Java Language Specification, here .

Binary numeric promotion is performed on the operands (§5.6.2).

This int value then gets boxed to Integer in order to be used as an argument to Set#remove(Object) .

The remove(Object) method then uses this Integer object to compare to the elements in the Set , using Object#equals(Object) . But no Short is equals to an Integer and no Integer is equal to a Short . And therefore nothing gets removed.

Answer 2

The answer lays in out- and autoboxing:

The statement i-1 is of type int not of type Short . The method remove() accepts a parameter of type Object (not necessarily the Generic type of the collection). Java will auto-box the numeric value as an Integer. The Integer cannot be found in the set and will thus not be removed.