Java: Logical operation of boolean with null

Question

Lets say I have a hash map like this:

private Map<someObject, Boolean> hashMap = new HashMap<>();

And I have two methods like this:

public void methodOne(key) {
    if (hashMap.get(key) == null || !hashMap.get(key)) {
       //do something
       hashMap.put(key, true);
    }
}

and

public void methodtwo(key) {
    if (hashMap.get(key) != null && hashMap.get(key)) {
       //do something else
       hashMap.put(key, false);
    }
}

My code seems to be working, but I am wondering the logical operation that I am doing in the if statements are valid and wether there is a better way to do it?

The other thing I am wondering is, what would be the result for the following logical operations:

null || true
null || false
null && true
null && false

The usual incorporation of null into boolean logic has null || true == true, null || false == null, null && true == null, null && false == false — khelwood
– khelwood, Commented Apr 16, 2020 at 23:36
Nothing wrong with your code, "better way" in this case is a matter of opinion. The result of the logical operations at the end of your question is a compilation error. — Oleg
– Oleg, Commented Apr 16, 2020 at 23:50
You cannot combine null with true or false. null can have many meanings, and such operations should not be performed. Java explicitly removed such operations from C/C++. — Maarten Bodewes
– Maarten Bodewes, Commented Apr 16, 2020 at 23:55
@Oleg I was describing the rules I know according to three-valued logic. If you know a better set of defined rules, go ahead. — khelwood
– khelwood, Commented Apr 17, 2020 at 7:20

Stepan Kolesnik · Accepted Answer · 2020-04-16 23:36:53Z

5

You could use Map::getOrDefault. See below:

public void methodOne(key) {
    if (!hashMap.getOrDefault(key, false)) {
       //do something
       hashMap.put(key, true);
    }
}

and

public void methodtwo(key) {
    if (hashMap.getOrDefault(key, false)) {
       //do something else
       hashMap.put(key, false);
    }
}

answered Apr 16, 2020 at 23:36

Stepan Kolesnik

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1 Comment

Maarten Bodewes Over a year ago

Great answer because it doesn't assume any property of null, it simply replaces the missing value with a default one, in this case false.

RaffleBuffle · Accepted Answer · 2020-04-16 23:56:20Z

0

How about this:

public void methodOne(key) {
  if (!Boolean.TRUE.equals(hashMap.get(key))) {
     //do something
     hashMap.put(key, true);
  }
}

public void methodtwo(key) {
  if (Boolean.TRUE.equals(hashMap.get(key))) {
     //do something
     hashMap.put(key, false);
  }
}

answered Apr 16, 2020 at 23:56

RaffleBuffle

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1 Comment

Maarten Bodewes Over a year ago

That hides the fact that null can be returned and completely goes against the principle of least surprise. It's a ugly hack and the next programmer may try and replace it with Boolean.FALSE.equals(...) and see the code fail horribly, usually in the field after testing.

Oleg · Accepted Answer · 2020-04-17 00:24:33Z

0

Another option is to use optional

public void methodOne(key) {
    if (!Optional.ofNullable(hashMap.get(key)).orElse(false)) {
       //do something
       hashMap.put(key, true);
    }
}

public void methodtwo(key) {
    if (Optional.ofNullable(hashMap.get(key)).orElse(false)) {
       //do something else
       hashMap.put(key, false);
    }
}

answered Apr 17, 2020 at 0:24

Oleg

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Comments

Community · Accepted Answer · 2020-06-20 09:12:55Z

-1

Are the if statements valid and is there a better way to do it?

Yes and yes. See answer by Stepan Kolesnik.

What would be the result for the following logical operations:

null || true

null || false

null && true

null && false

Truth table

                  null    false    true
null || true      yes     no       yes
null || false     yes     yes      no       methodOne: set true
null && true      ERROR   no       no
null && false     ERROR   no       no
!null && true     no      no       yes      methodtwo: set false
!null && false    no      yes      no

As you can see, in essence, methodOne is an unconditional key = true, while methodtwo leaves null unchanged, otherwise sets key = false.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Apr 16, 2020 at 23:57

Andreas

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