generics - What is the difference between a compile time type vs run time type for any object in Java?

Question

What is the difference between compile time and run time type of any object in Java ? I am reading Effective Java book and Joshua Bloch mentions about compile time type and run time types of array instances in Item 26 many times mainly to describe that suppressing cast warnings can be safe sometimes.

// Appropriate suppression of unchecked warning
public E pop() {
 if (size == 0)
   throw new EmptyStackException();
   // push requires elements to be of type E, so cast is correct
   @SuppressWarnings("unchecked") E result = (E) elements[--size];
   elements[size] = null; // Eliminate obsolete reference
   return result;
}

Here the author is talking about these different types of types in context of arrays . But through this question I would like to understand the difference between compile time types vs run time types for any type of object .

Answer 1

Java is a statically typed language, so the compiler will attempt to determine the types of everything and make sure that everything is type safe. Unfortunately static type inference is inherently limited. The compiler has to be conservative, and is also unable to see runtime information. Therefore, it will be unable to prove that certain code is typesafe, even if it really is.

The run time type refers to the actual type of the variable at runtime. As the programmer, you hopefully know this better than the compiler, so you can suppress warnings when you know that it is safe to do so.

For example, consider the following code (which will not compile)

public class typetest{
    public static void main(String[] args){
        Object x = args;
        String[] y = x;

        System.out.println(y[0])
    }
}

The variable x will always have type String[], but the compiler isn't able to figure this out. Therefore, you need an explicit cast when assigning it to y.