Cannot add an integer to ArrayList [duplicate]

Answer 1

A boxing conversion needs to be applied here, because lists contain boxed primitives, not primitives.

This conversion is described in JLS Sec 5.1.7:

Boxing conversion converts expressions of primitive type to corresponding expressions of reference type. Specifically, the following nine conversions are called the boxing conversions:

• ...
• From type int to type Integer
• From type long to type Long
• ...

But there is no boxing conversion "from int to type Long". As such, this method invocation cannot be applied, and so it's a compiler error.

Either explicitly cast to long, explicitly box using Long.valueOf(intValue) (in which a widening conversion converts the int parameter to a long), or use the L suffix on a literal.

Answer 2

When you try the following code:

fibo_list.add(0);
fibo_list.add(1);
fibo_list.add(1);

you are trying to add primitive integer values to a collection of Long objects. This doesn't work, and neither does boxing these values, which would yield an Integer. This also fails, because an Integer cannot be stored inside a collection of Long. However, the following code would work:

fibo_list.add(0L);
fibo_list.add(1L);
fibo_list.add(1L);

Here we are passing primitive long values, which can then be boxed to Long.

Answer 3

You are getting confused between casting and boxing.

When int gets converted to Integer - it's called boxing and revrese process is called unboxing.

JAVA supports autoboxing, which means your primitive will eb automatically converted to it's wrapper class when required & vice versa. E.g int -> Integer, long -> Long etc.

Java also supports casting among primitives which means int primitive can be implicitly casted to long primitive.

What you are trying to achieve is a combination of above to operations. You want to convert int to Long which will require 2 steps, which may be achieved in 2 different ways(let's assume for a moment following are possible):

Way 1:

1. casting int to long
2. Boxing long to Long

Way 2:

1. boxing int to Integer
2. casting Integer to Long

Since there is no clarity which way to choose, therefore JAVA does not allow allow any of the 2 step process as an automatic conversion.