java - Comparison of two null objects from two different types -

public void m1(integer f) {     ... }  public void m1(float f) {     ... }  public void main() {     m1(null); // error: method m1(integer) ambiguous type main     m1((integer) null); // success } 

given above example, can admit in ways null typed. why following lines print true? sure o1 , o2 both have no value (i.e. null), aren't same type (integer vs float). firstly thought false have been printed.

integer = null; object o1 = (object) i; float f = null; object o2 = (object) f; system.out.println(o1 == o2); // prints true  // in short: system.out.println(((object) ((integer) null)) == ((object) ((float) null))); // prints true 

all null values untyped , equal. can pass different reference types makes no difference comparison purposes.

it not null value typed reference null can typed.

a common question happens here

class {     public static void hello() { system.out.println("hello world"); }      public static void main(string... args) {         a = null;         a.hello();         system.out.println("a " + (a instanceof a)); // prints false.     } } 

the compiler sees type of a a static method called. value referenced null , untyped.

the operations can perform null without causing nullpointerexception assign or pass without examining or comparing reference.

btw

in short: compiler select method based on type of reference, @ runtime execution based on class of object referenced. @ runtime null treated type or no type or nullpointerexception if try dereference it.