compare -999 in type double in Java with == and .equals()

Question

Edit : Some people stated that this is possible duplicate of one of question asking how to compare string values. I was asking about comparing double with int, that is why .equal() did not work.

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First, I set a LinkedList of doubles with first element = -999

then if the comparison is

return listOfNumber.getFirst().equals(-999);

the outcome will be false.

However if the comparison is written as

return listOfNumber.getFirst()== -999;

Then the outcome will be true.

I thought .equals() compares the value and == compare the object and == can only take -128 to 127. So why exactly I cannot use .equals to compare doubles?

Also if the LinkedList is String with first element = "a" if I use

return ListOfString.getFirst() == "a";

then the outcome is false but if I use

return ListOfString.getFirst().equals("a");

then the outcome is true.

I am very confused why the comparison of double and string need to be different to yield correct result?

Answer 1

Your problem is caused by autoboxing in Java.

Assuming (as you stated listOfNumber is a list of Double`):

ArrayList<Double> listOfNumber = new ArrayList<>();
listOfNumber.add(-999);

Since Java knows the signature of ... add(T) is ... add(Double) , your add statement gets translated to:

listOfNumber.add(Double.valueOf(-999));

When you are executing

return listOfNumber.getFirst().equals(-999);

Java knows that Object.equals accepts a Object , and -999 fits an integer (the default type for a number), so it translates it to:

return listOfNumber.getFirst().equals(Integer.valueOf(-999));

Since a Integer and a Double aren't the same, it returns false, but for the == case, the java compilers gets smarter, and knows (double)-999 == (integer)-999