Question

Recently I have got a compilation error in the code below:

import org.eclipse.swt.widgets.TreeItem;
Object parent; // can only be a Tree or a TreeItem    
...
TreeItem item = new TreeItem((parent instanceof TreeItem) ? (TreeItem) parent : (Tree) parent, SWT.NONE);

Compiler says: “The constructor TreeItem(Widget, int) is undefined”

Then I have tried it with another code:

Object x = new Integer(1);

Test t = new Test((x instanceof String) ? (String) x : (Integer) x);

class Test{
    public Test(String s){}
    public Test(Integer i){}
}

And got another error: “The constructor Test(Object&Serializable&Comparable) is undefined”

So I was forced to use the traditional if-else structure. Any ideas why the compiler behaves so?

Answer 1

JLS §15.25 describes the ternary operator.

Otherwise, the second and third operands are of types S1 and S2
respectively. Let T1 be the type that results from applying boxing
conversion to S1, and let T2 be the type that results from applying
boxing conversion to S2.

The type of the conditional expression is the result of applying
capture conversion (§5.1.10) to lub(T1, T2) (§15.12.2.7).

Basically, you can think of the ternary operator as a method: it can only have one “return type”. Since String and Integer are two different objects, it finds the common superclass and all interfaces implemented by both and creates a return type from that. (String and Integer both implement Serializable and Comparable and extend Object, so you get Object & Serializable & Comparable.)