java - When is generic return value of function casted after type erasure?

Question

This question was inducted by this StackOverflow question about unsafe casts: Java Casting method without knowing what to cast to. While answering the question I encountered this behaviour I couldn't explain based on purely the specification

I found the following statement in The Java Tutorials at the Oracle docs:

• Insert type casts if necessary to preserve type safety. The Java Tutorials: Type Erasure

It is not explained what "if necessary" means exactly, and I've found no mention about these casts in the Java Language Specification at all, so I started to experiment.

Let's look at the following piece of code:

// Java source
public static <T> T identity(T x) {
    return x;
}
public static void main(String args[]) {
    String a = identity("foo");
    System.out.println(a.getClass().getName());
    // Prints 'java.lang.String'

    Object b = identity("foo");
    System.out.println(b.getClass().getName());
    // Prints 'java.lang.String'
}

Compiled with javac and decompiled with the Java Decompiler:

// Decompiled code
public static void main(String[] paramArrayOfString)
{
    // The compiler inserted a cast to String to ensure type safety
    String str = (String)identity("foo");
    System.out.println(str.getClass().getName());

    // The compiler omitted the cast, as it is not needed
    // in terms of runtime type safety, but it actually could
    // do an additional check. Is it some kind of optimization
    // to decrease overhead? Where is this behaviour specified?
    Object localObject1 = identity("foo");
    System.out.println(localObject1.getClass().getName());
}

I can see that there is a cast which ensures type safety in the first case, but in the second case it is omitted. It is fine of course, because I want to store the return value in an Object typed variable, so the cast is not strictly necessary as per type safety. However it leads to an interesting behaviour with unsafe casts:

public class Erasure {
    public static <T> T unsafeIdentity(Object x) {
        return (T) x;
    }

    public static void main(String args[]) {
        // I would expect c to be either an Integer after this
        // call, or a ClassCastException to be thrown when the
        // return value is not Integer
        Object c = Erasure.<Integer>unsafeIdentity("foo");
        System.out.println(c.getClass().getName());
        // but Prints 'java.lang.String'
    }
}

Compiled and decompiled, I see no type cast to ensure correct return type at runtime:

// The type of the return value of unsafeIdentity is not checked,
// just as in the second example.
Object localObject2 = unsafeIdentity("foo");
System.out.println(localObject2.getClass().getName());

This means that if a generic function should return an object of a given type, it is not guaranteed it will return that type ultimately. An application using the above code will fail at the first point where it tries to cast the return value to an Integer if it does so at all, so I feel like it breaks the fail-fast principle.

What are the exact rules of the compiler inserting this cast during compilation that ensures type safety and where are those rules specified?

EDIT:

I see that the compiler will not dig into the code and try to prove that the generic code really returns what it should, but it could insert an assertation, or at least a type cast (which it already does in specific cases, as seen in the first example) to ensure correct return type, so the latter would throw a ClassCastException:

// It could compile to this, throwing ClassCastException:
Object localObject2 = (Integer)unsafeIdentity("foo");

Answer 1

If you can't find it in the specification, that means it's not specified, and it is up to the compiler implementation to decide where to insert casts or not, as long as the erased code meets the type safety rules of non-generic code.

In this case, the compiler's erased code looks like this:

public static Object identity(Object x) {
    return x;
}
public static void main(String args[]) {
    String a = (String)identity("foo");
    System.out.println(a.getClass().getName());

    Object b = identity("foo");
    System.out.println(b.getClass().getName());
}

In the first case, the cast is necessary in the erased code, because if you removed it, the erased code wouldn't compile. This is because Java guarantees that what is held at runtime in a reference variable of reifiable type must be instanceOf that reifiable type, so a runtime check is necessary here.

In the second case, the erased code compiles without a cast. Yes, it will also compile if you added a cast. So the compiler can decide either way. In this case, the compiler decided not to insert a cast. That is a perfectly valid choice. You should not rely on the compiler to decide either way.