Difference between double and Double in comparison

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?

Answer 2

When applied to expressions of a class type, == will always perform a reference comparison (JLS section 15.21.3). So this line:

System.out.println(c == d);  

is checking whether c and d refer to the same objects. Auto-boxing in Java always (I believe) creates a new object for float and double (the situation is more complicated for integral types¹). Therefore c and d refer to different objects, and so it prints false.

If you want to compare objects for equality, you need to call equals explicitly:

System.out.println(c.equals(d)); 

With double, it's using numeric equality instead - as specified in section 15.21.1. Hence the difference in behaviour.

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¹ For integral autoboxing, "small" values are cached - so autoboxing 5 (say) will return the same reference every time. The definition of "small" is implementation-specific, but it's guaranteed within the range -128 to 127. See the bottom of section 5.1.7 for details.