CODEWITHSUNDEEP
java
avalon
avalon

Reputation: 2291

Java boolean null cast

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?

Upvotes: 2

Views: 21690

Answers (5)

Ctx
Ctx

Reputation: 18420

Assigning null to a native type (like boolean, int, etc.) will result in a NullPointerException.

Upvotes: 9

kaushik
kaushik

Reputation: 312

  1. First of all your code will not compile. It will give you a compilation error on the if (null = var) { line.

  2. 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.

Upvotes: 1

yanghaogn
yanghaogn

Reputation: 863

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.

Upvotes: 0

luk2302
luk2302

Reputation: 57124

Regarding the first question Why does it compile?

Unedited version: It does not 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 null considered as false?

Not really. Only when parsing a String which is null this is treated as false. That is basically the only place / situation.

After the edit changing 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.

Upvotes: 8

kimixuchen
kimixuchen

Reputation: 155

At first, do you know the difference between Boolean and boolean? Because assign null to boolean will cause java.lang.NullPointerException.

Upvotes: 0

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