NPE when trying to return null

Question

This program supposed to avoid null when calling toFloat with null. but I'm still getting NPE .. any help

System.out.println(toFloat(null, null));

private static Float toFloat(Float def, String str) {
    try {
        return str != null ? Float.parseFloat(str) : def;
    } catch (NumberFormatException e) {
        return def;
    }
}

Answer 1

It's very subtle. Float.parseFloat returns float, not Float. The second two operands of a conditional operator must be of the same type, but you're giving it float (the result of Float.parseFloat) and Float (def). The compiler picks float because Float can be coerced to float through auto-unboxing.

So what the compiler outputs is as though you'd written this:

private static Float toFloat(Float def, String str) {
    try {
        return str != null ? Float.parseFloat(str) : def.floatValue();
        // Note ----------------------------------------^^^^^^^^^^^^^
    } catch (NumberFormatException e) {
        return def;
    }
}

...and of course, calling floatValue on null throws an NPE.

You can fix it by making sure the second operand's type is Float, not float. Lots of ways to do that, but as Zefick points out, the simplest is Float.valueOf(String):

private static Float toFloat(Float def, String str) {
    try {
        return str != null ? Float.valueOf(str) : def;
    } catch (NumberFormatException e) {
        return def;
    }
}