hashmap - "double, Double, HashSet" in Java -

please see code below.

"double"(upper d) used in hashset, , "double" used x1, x2 , x3. after x1 added hashset, x2 cannot added, x3 can! why??????

thanks in advance :)

hashset<double> hs = new hashset<double>(); double x1, x2, x3;  x1 = (double)0/1; system.out.println(hs.add(x1)); //true  x2 = (double)0/2; system.out.println(hs.add(x2)); //false  x3 = (double)0/-1; system.out.println(hs.add(x3)); //true 

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and if add "0.0 +" x1, x2 , x3, result follows.

x1 = 0.0 + (double)0/1; system.out.println(hs.add(x1)); //true  x2 = 0.0 + (double)0/2; system.out.println(hs.add(x2)); //false  x3 = 0.0 + (double)0/-1; system.out.println(hs.add(x3)); //false 

try understand difference:

hashset<double> hs = new hashset<double>(); double x1, x2, x3;  x1 = (double)0/1; system.out.println(x1 + " "+ hs.add(x1)); //true  x2 = (double)0/2; system.out.println(x2 + " " + hs.add(x2)); //false  x3 = (double)0/-1; system.out.println(x3 + " " + hs.add(x3)); //true 

basically doubles signed , 0/-1 evaluated -0.0 instead of 0.0 x1 or x2.