Where does this Java function infer its generic type from?

Question

I found a bit of generic code and it has stumped me as to how it actually works. I don't understand where it gets the generic type that is used for T. This is an oversimplified example but I still don't understand how this is valid Java code.

public static void main(String[] args) {
  System.out.print(get());
}

public static <T> T get()
{
  return (T) getObj();
}

public static Object getObj() 
{
  return Boolean.FALSE;
}

Answer 1

Type inference happens based on the callsite.

However, type inference only applies to return types if the return value is assigned to a variable. This is written in the spec

If the method result occurs in a context where it will be subject to assignment conversion, then [...]

Otherwise, any unresolved type parameters always become Object.
In your example, that will work, because there is a print(Object) overload.

On the other hand, look at this code:

print(get());

public void print(Boolean x) { }

public <T> T get() {
    return (T) Boolean.FALSE;
}

This will give a compilation error, because the compiler infers get() as returning Object before looking at print().

Answer 2

In the code you wrote T is not bound to anything. Java will assign a type to T whenever you invoke the get method and it will look at the type that you are expecting to get back... but that seems to be working only if you declare explicitly which type are you waiting for. Look at the following example:

public static void jump(String a) {
}

public static <T> T get() {
    return (T) null;
}

public static void main(String[] args) {
    //This works quite well, generic parameter T is inferred to be String
    String blah = get();
    jump(blah);
}

On the other hand

public static void jump(String a) {
}

public static <T> T get() {
    return (T) null;
}

public static void main(String[] args) {
    //This doesn't work, Java does not bind T to anything hence Object is used... and no jump for object is found
    jump(get());
}

At least this is how it works on Java 6.