Why can't I compare two numbers Double class Java using operator "=="?

Question

I have tried to make a simple code to compare numbers if is equal using Double class. This is my simple code:

BufferedReader bfr= new BufferedReader(new InputStreamReader(System.in));
    Double number1;
    Double number2;
    System.out.println("Ask for first number:");
    number1 = Double.parseDouble(bfr.readLine());
    System.out.println("Ask for second number:");
    number2 =  Double.parseDouble(bfr.readLine());
    
    if(number1==number2) {
        System.out.println("These numbers are equals!");
        System.out.println(number1);
        System.out.println(number2);
    }else {
        System.out.println("They're differents!");
        System.out.println(number1);
        System.out.println(number2);
    }

When I use "int" instead Double and the operator "==" it works fine but when I use "Double" it shows me they are different numbers even the same numbers are entered. I've tried to use Object.equals() method it works fine but I want to know why "Double" class does not admit operator "==".

Decimal processors cannot do exact representations of floating point numbers but rather only close approximations — Hovercraft Full Of Eels, Sep 25 '20 at 20:19
I seem to recall there's a compare() function for comparing doubles — Sélim Achour, Sep 25 '20 at 20:20
@HovercraftFullOfEels: this is not actually a problem about the numerical approximation. If you parse `0.1` twice you'll get the same `double` value. But OP is comparing two `Double` wrapper objects for object identity! — Joachim Sauer, Sep 25 '20 at 20:27
@JoachimSauer: ayyyy, my bad. Then `Double.compare(...)` or `Double#equals(...)` would be needed — Hovercraft Full Of Eels, Sep 25 '20 at 20:30
@JoachimSauer: a better duplicate: https://stackoverflow.com/questions/12226757/difference-between-double-and-double-in-comparison — Hovercraft Full Of Eels, Sep 25 '20 at 20:32

score 2 · Answer 1 · answered Sep 25 '20 at 20:30

The important difference between int and Double here is not the floating point nature (though that can make computations behave in unexpected ways), but the fact that int is a primitive type and Double is a reference type (a wrapper around double).

When you compare reference type values using == you check for object identity. I.e. the expression number1 == number2 means "do the variables number1 and number2 reference the exact same object", which is almost certainly not what you want.

The solution, as you found is to use number1.equals(number2).

Another workaround would be to switch from Double (the wrapper) to double the primitive type. Usually you can just assign one to the other and the appropriate autoboxing/autounboxing will handle the conversion.

Why can't I compare two numbers Double class Java using operator "=="?

1 Answers1