I have the following code snippet:
Boolean var = false;
boolean var1 = (var = null);
if (var1) {
// It compiles
}
if (var = null) {
// It compiles
}
Why does it compile?
In the Boolean class I found the following:
public static boolean parseBoolean(String s) {
return ((s != null) && s.equalsIgnoreCase("true"));
}
Does it mean that is null considered as false? Why the result of = operation on boolean is false? What the practical reason of this behavior?
Assigning null to a native type (like boolean, int, etc.) will result in a NullPointerException.
Regarding the first question Why does it compile?
It will give a compile error since if (null = var) is not valid java code. You cannot assign something to null, you can only ever assign null to something.
You might want to use the == instead comparing var and null for equality.
Beyond that, at runtime you will get an NPE as @Ctx already correctly mentioned. The line boolean var1 = (var = null); will firstly assign null to var, then the assignment operator = will return what it has just assigned (null) and try to assign that to the boolean var1, which throws a NullPointerException.
Does it mean that is
nullconsidered asfalse?
Not really. Only when parsing a String which is null this is treated as false. That is basically the only place / situation.
null = var to var = null:Now your code will actually compile and crash with a NullPointerException. Let's go through what happens here step by step:
Boolean var = false; — A Boolean object is created by autoboxing the boolean value false.
boolean var1 = (var = null); — The first operation is (var = null) which assigns null to var. = therefore returns null. The statement then is "equivalent" to boolean var1 = null which the compiler would reject.
Unfortunately, the compiler is not capable of deducing that the statement boolean var1 = (var = null); would always lead to the invalid assignment boolean var1 = null. The code therefore compiles fine, but crashes at runtime.
String which is null this is treated as false" — Do you mean when using Boolean.valueOf(null)?
Boolean.parseBoolean, not sure what valueOf does exactly.First of all your code will not compile. It will give you a compilation error on the if (null = var) { line.
Second thing is that while initializing a primitive data type, you can't assign a null value. If you assigning it on compile time, it will give you a compilation error. But if you doing it on run time, it gives a NullPointerException. Just like that boolean b = null; is not valid in Java. But Boolean bObj = null; is valid. While in your code when your are doing (var = null) it actually returns null. Hence, boolean var1 = (var = null) becomes boolean var1 = null. Here, the variable var1 is assigned with null value with a run time basis. So, it will give a NullPointerException.
At first, do you know the difference between Boolean and boolean? Because assign null to boolean will cause java.lang.NullPointerException.
Code would be written like this:
Boolean var = false;
boolean var1 = (var == null);
if (var1) {
// It compiles
}
if (var == null) {
// It compiles
}
The reason of var != null is that null and Boolean.FALSE do not share the same memory.
boolean var1=(var=null); compiles fine, but will cause an exception during runtime.
=int a, b, c; a = b = c = 1;var1 = (var = null) have an exception during runtime. The complier don't know the return value of (var = null), so it can compile, but when running, for var = null will return null, and an error will happen when executing expression boolean var1 = nullvar, but fails, because it can't unbox null into a meaningful value for a primitive type.
if(null=var)won't compile...if(null=var)never compile.