Search code examples
javagenericsvarecj

What does var<T> do in Java?

A friend of mine noticed that 

var<Integer> list = new ArrayList<Double>();

was valid in Java. It turns out that the type of list is evaluated to ArrayList<Double>.

When using var<Integer> list = new ArrayList<>();, list is just ArrayList<Object>.

Both of us were not able to figure out, what the generic type of var does, as it seems to be ignored. But if so, why is this even syntactically correct in the first place?

Solution

  • This is indeed a bug, but the proof lies in the Java Language Specification § 14.4 Local Variable Declaration Statements:

    LocalVariableType:
    UnannType
    var

    Ad you can see, the restricted identifier var is listed without any other token. Also, UnannType eventually resolves to the token TypeIdentifier which explicitly forbids var.

    So no, var<Integer> is not valid.