What is the difference between Double and Double in java?

Question

I was making a code and, by mistake, I put double and the IDE accepted, I always used with the D capital.

What’s the difference between the two?

Answer 1

The class Double involves a primitive type value double on an object. An object of the type Double contains a single field whose type is double.

In addition, this class provides several methods to convert a double in a String and a String in a double, as well as other constants and methods useful in dealing with a double.

Source: https://docs.oracle.com/javase/7/docs/api/java/lang/Double.html

In summary, the classe Double is a wrapper. See more about Wrappers here.

Answer 2

double is a type of primitive data (i.e., it is not created by reference, in other words, it is not an object). Not being an object, you don’t have access to a number of facilities that object manipulation offers, perhaps the most important being the conversion of a value double text-based (String) and the use of collections (ArrayList, Set etc.), since they only deal with objects and not primitive ones.

To make it possible for primitive types to make use of the resources that objects have at their disposal, in Java, for each primitive type there exists a class whose only function is to receive the value of it and to "pack" the value of that primitive with an object. These classes are called Wrappers (packers). As in Java, the default class nomenclature is uppercase first letter, the class wrapper of the primitive double is the Double.