Null Can Not Be Compared to Double - Why

How to check if a double is null?

I would recommend using a Double not a double as your type then you check against null.

Comparing two nulls, getting NullPointerException

Here's what new Double(null) does...

The compiler has to find a constructor that matches. There are two constructors for the Double class (according to the javadoc):

Double(double value)
    // Constructs a newly allocated Double object that represents the primitive double argument.
Double(String s)
    // Constructs a newly allocated Double object that represents the floating-point value of type double represented by the string.

null is not a valid value for a primitive type (i.e. small-d double), so this results in an attempt to call the second constructor, that takes a String. At runtime, it then tries to parse the string in the format of a double. And this results in a NullPointerException because the String parameter is null. [To be more precise, I think at some point there will be a call to s.trim() from the class sun.misc.FloatingDecimal, which throws an exception if s is null.]

Inconsistent null equality check scala 2.11.7

Edit: This issue no longer exists in Scala 2.12.6. See pull-request with explanation.

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Original answer (for Scala 2.11.7):

null.asInstanceOf[Double] == null compiles to the:

aconst_null
ifnonnull

The val-version compiles to the:

aconst_null
invokestatic unboxToDouble
putfield
aload_0
invokevirtual а
invokestatic boxToDouble
ifnonnull

So compiler just forgets to add unbox/box in the first case

Comparison of two null objects from two different types

All null values are untyped and are equal. You can pass it to different reference types but it makes no difference for comparison purposes.

It is not the null value which is typed but the reference to the null which can be typed.

A common question is what happens here

class A {
    public static void hello() { System.out.println("Hello World"); }

    public static void main(String... args) {
        A a = null;
        a.hello();
        System.out.println("a is an A is " + (a instanceof A)); // prints false.
    }
}

The compiler sees the type of a as an A so the static method is called. But the value referenced is null and untyped.

The only operations you can perform with null without causing a NullPointerException is to assign or pass it without examining it or comparing it with another reference.

BTW

In short: The compiler will select a method based on the type of the reference, at runtime the execution is based on the class of the object referenced. At runtime null is treated as any type or no type or you get a NullPointerException if you try to dereference it.

how to check double value is empty or not

In java double is a primitive type which can not be null, There is Double wrapper class in java which can be checked as null.

Declare your coords of type Double not double and check if Double value is null.

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