Var keyword in Java

Question

With Java 10 or +, we can use var keyword for declaration. At initialization, a type is going to be inferred by the compiler.

What happens when the class I instantiate and assign to the variable declared with var, is the implementation of the interface? which type is it going to be inferred, Interface or the implementation?

Answer 1

My 2 cents to correct the question and answers:

1. The var is NOT a Java keyword. It's a reserved type name. It seems not a big difference but in fact, it IS:

var var = 0;

Here var is a variable name too, so the var can be used as a type name, but there is no restriction like for regular keyword (i.e. we can have a variable named var too).

2. The actual type of the variable IS decided by the Java compiler on the line where the variable declared, at compile-time, but the type is NOT actually the exact implementation you use on the right side of the expression. See this code:

var i = true ? Integer.valueOf(1) : "ABC";

The Java compiler needs to pick a type for variable i which will satisfy both branches. It could be a) Object, b) Serializable, c) Comparable, or combinatioin, or all three. We don't care and don't know.