Java switch case statement is returning different type of values(different data-type) and is assigne-CodePudding

Following is my code snippet:

Scanner sc = new Scanner(System.in);
int input = sc.nextInt();

var output = switch (input){
  case 1 -> "one";
  case 2 -> '2';
  case 3 -> 3.14;
  default -> 10;
};

System.out.println(output);

Now if I enter 1 as input, it prints "one", for 2 it prints '2', for 3 it prints 3.14 and for the rest it prints 10.

Does this mean datatype of output is decided at runtime according to the return type?

CodePudding user response：

No, the output variable is always determined at compile time as explained

Note that this is not a generic Object. Infact you can't reassign the output variable to a generic Object like an ArrayList.

output = new ArrayList<String>());    // It doesn't compile

while you can reassign to any Comparable Constable and Serializable, for example to a Long

output = 10L;    // It compiles!

CodePudding user response：

output is inferred to the closest common superclass of all of those types, which is the intersection of Comparable & Serializable. That's slightly more specific than Object, but probably not in a meaningful way in most cases.

Luckily System.out.println accepts any old Object, since it can toString() any possible arguments.

'2' is autoboxed to Character, 3.14 to Double and 10 to Integer.

So output.getClass() would return one of four distinct things.

CodePudding user response：

The type of output is determined at compile time as something that is Serializable, Comparable and Constable, because that's the closest common ancestor to all of the result.

The object assigned to the variable is determined at runtime is one of String, Character, Double or Integer.

The actual compile type is Serializable & Comparable<? extends Serializable & Comparable<?> & Constable> & Constable (My IDE told me that!).