Return boolean in non boolean method

Question

I am trying to return False if result = false. I can't change the return type. linkList needs to return false if result = false. Thanks

private static class FilteringFlatApplyFunction<T> implements FlatApplyFunction<T,T> {        
    private final Predicate pred; 
    LinkedList<T> linkList = new LinkedList<T>();

    public FilteringFlatApplyFunction(Predicate<T> p) {                              
        this.pred = p;
        this.linkList = linkList;
    }

    @Override
    public List<T> apply(T x){               
        boolean result = pred.check(x);              
        if (result = true) {
            linkList.add(x);
            return linkList;
        }                
        if (result = false) {
             linkList = false;              
        }               
        return linkList;                              
    }
}

Re `if (result = true)` and `if (result = false)`: `=` is **assignment**, not comparison; comparison is `==`: http://stackoverflow.com/questions/39385382/why-doesnt-my-equality-comparison-using-a-single-equals-work-correctly-in-j (Also: Don't use `==` or `!=` with booleans [partially for this reason], you already have a boolean. Just `if (result)` and `if (!result)`.) — T.J. Crowder, Mar 07 '17 at 06:41

score 6 · Answer 1 · answered Mar 07 '17 at 06:41

6

Quite simply, you can't. The method signature says quite clearly that it is to return a List<T>, and a List<T> is what you shall return.

You could return null instead.

answered Mar 07 '17 at 06:41

Joe C

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score 0 · Answer 2 · edited Mar 07 '17 at 07:03

You can't return boolean to a List, what you can do is return null.

While you're checking the condition, what you can do is, create another method that encapsulate your apply method and return if you're list is null then it's false. Like below:

if(linkList == null)
   return false;
else
   return true;

Besides, Use == for comparison.

As below:

@Override
public List<T> apply(T x){               
    boolean result = pred.check(x);              
    if(result == true) {
        linkList.add(x);
        return linkList;
    }
    if(result == false){
         return null;              
    }
    return linkList;
}

OR in case of booleans you need not to use ==, like below

@Override
public List<T> apply(T x){               
    boolean result = pred.check(x);              
    if(result){
        linkList.add(x);
        return linkList;
    }
    if(result){
         linkList = false;              
    }
    return linkList;
}

I cannot set linkList = false, it will return an error saying boolean cannot be converted to LinkedList — Joe Van Horn, Mar 07 '17 at 06:43

score 0 · Answer 3 · edited Mar 07 '17 at 07:02

0

You can't return false if the method specifically returns a List. What you can do is return `null':

@Override
public List<T> apply(T x){               
    boolean result = pred.check(x);              
    if (result == true) {
        linkList.add(x);
        return linkList;
    }
    if (result == false) {
         linkList = null;              
    }
    return linkList;
}

Then in the calling method do the following:

List<T> list = apply(x);
boolean result;
if (list == null){
   result = false;
}

edited Mar 07 '17 at 07:02

Wasi Ahmad

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answered Mar 07 '17 at 06:43

Luke Bajada

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what is the difference between your answer and @JoeC answer? – Wasi Ahmad Mar 07 '17 at 06:44
This one will always return the non-null list. – Andy Turner Mar 07 '17 at 06:51
You are right Andy, it was a mistake in @Joe Van Horn' code that I copied over :) - updated my answer – Luke Bajada Mar 07 '17 at 06:53

score 0 · Answer 4 · answered Mar 07 '17 at 09:10

0

Sorry bud. You can't return different types. Returning NULL or maybe returning an object are your best chances.

answered Mar 07 '17 at 09:10

Hamster

1

score 0 · Answer 5 · answered Mar 07 '17 at 10:51

if result is false you return null so i think you dont need to do the test if result is false!!!

private static class FilteringFlatApplyFunction<T> implements FlatApplyFunction<T,T> {        
    private final Predicate pred; 
    LinkedList<T> linkList = new LinkedList<T>();

    public FilteringFlatApplyFunction(Predicate<T> p) {                              
        this.pred = p;
        this.linkList = linkList;
    }

    @Override
    public List<T> apply(T x){               
        boolean result = pred.check(x);              
        if (result) {
            linkList.add(x);  
        }                         
        return linkList;                              
    }
}

Return boolean in non boolean method

5 Answers5