Question

I know that Double is a a wrapper class, and it wraps double number. Today, I have seen another main difference :

double a = 1.0;
double b = 1.0;
Double c = 1.0;
Double d = 1.0;
System.out.println(a == b);  // true
System.out.println(c == d);  // false

So strange with me !!!

So, if we use Double, each time, we must do something like this :

private static final double delta = 0.0001;
System.out.println(Math.abs(c-d) < delta); 

I cannot explain why Double make directly comparison wrong. Please explain for me.

Answer 1

c and d are technically two different objects and == operator compares only references.

c.equals(d)

is better as it compares values, not references. But still not ideal. Comparing floating-point values directly should always take some error (epsilon) into account (Math.abs(c - d) < epsilon).

Note that:

Integer c = 1;
Integer d = 1;

here comparison would yield true, but that’s more complicated (Integer internal caching, described in JavaDoc of Integer.valueOf()):

This method will always cache values in the range -128 to 127, inclusive, and may cache other values outside of this range.

Why valueOf()? Because this method is implicitly used to implement autoboxing:

Integer c = Integer.valueOf(1);
Integer d = Integer.valueOf(1);

See also

• Weird Integer boxing in Java
• How to properly compare two Integers in Java?