Use Java lambda instead of 'if else'

Question

With Java 8, I have this code:

if(element.exist()){
    // Do something
}

I want to convert to lambda style,

element.ifExist(el -> {
    // Do something
});

with an ifExist method like this:

public void ifExist(Consumer<Element> consumer) {
    if (exist()) {
        consumer.accept(this);
    }
}

But now I have else cases to call:

element.ifExist(el -> {
    // Do something
}).ifNotExist(el -> {
    // Do something
});

I can write a similar ifNotExist, and I want they are mutually exclusive (if the exist condition is true, there is no need to check ifNotExist, because sometimes, the exist() method takes so much workload to check), but I always have to check two times. How can I avoid that?

Maybe the "exist" word make someone misunderstand my idea. You can imagine that I also need some methods:

 ifVisible()
 ifEmpty()
 ifHasAttribute()

Many people said that this is bad idea, but:

In Java 8 we can use lambda forEach instead of a traditional for loop. In programming for and if are two basic flow controls. If we can use lambda for a for loop, why is using lambda for if bad idea?

for (Element element : list) {
    element.doSomething();
}

list.forEach(Element::doSomething);

In Java 8, there's Optional with ifPresent, similar to my idea of ifExist:

Optional<Elem> element = ...
element.ifPresent(el -> System.out.println("Present " + el);

And about code maintenance and readability, what do you think if I have the following code with many repeating simple if clauses?

    if (e0.exist()) {
        e0.actionA();
    } else {
        e0.actionB();
    }

    if (e1.exist()) {
        e0.actionC();
    }

    if (e2.exist()) {
        e2.actionD();
    }

    if (e3.exist()) {
        e3.actionB();
    }

Compare to:

    e0.ifExist(Element::actionA).ifNotExist(Element::actionB);
    e1.ifExist(Element::actionC);
    e2.ifExist(Element::actionD);
    e3.ifExist(Element::actionB);

Which is better? And, oops, do you notice that in the traditional if clause code, there's a mistake in:

if (e1.exist()) {
    e0.actionC(); // Actually e1
}

I think if we use lambda, we can avoid this mistake!

Answer 1

The problem

(1) You seem to mix up different aspects - control flow and domain logic.

element.ifExist(() -> { ... }).otherElementMethod();
          ^                      ^
        control flow method     business logic method

(2) It is unclear how methods after a control flow method (like ifExist, ifNotExist) should behave. Should they be always executed or be called only under the condition (similar to ifExist)?

(3) The name ifExist implies a terminal operation, so there is nothing to return - void. A good example is void ifPresent(Consumer) from Optional.

The solution

I would write a fully separated class that would be independent of any concrete class and any specific condition.

The interface is simple, and consists of two contextless control flow methods - ifTrue and ifFalse.

There can be a few ways to create a Condition object. I wrote a static factory method for your instance (e.g. element) and condition (e.g. Element::exist).

public class Condition<E> {

    private final Predicate<E> condition;
    private final E operand;

    private Boolean result;

    private Condition(E operand, Predicate<E> condition) {
        this.condition = condition;
        this.operand = operand;
    }

    public static <E> Condition<E> of(E element, Predicate<E> condition) {
        return new Condition<>(element, condition);
    }

    public Condition<E> ifTrue(Consumer<E> consumer) {
        if (result == null)
            result = condition.test(operand);

        if (result)
            consumer.accept(operand);

        return this;
    }

    public Condition<E> ifFalse(Consumer<E> consumer) {
        if (result == null)
            result = condition.test(operand);

        if (!result)
            consumer.accept(operand);

        return this;
    }

    public E getOperand() {
        return operand;
    }

}

Moreover, we can integrate Condition into Element:

class Element {

    ...

    public Condition<Element> formCondition(Predicate<Element> condition) {
        return Condition.of(this, condition);
    }

}

The pattern I am promoting is:

• work with an Element;
• obtain a Condition;
• control the flow by the Condition;
• switch back to the Element;
• continue working with the Element.

The result

Obtaining a Condition by Condition.of:

Element element = new Element();

Condition.of(element, Element::exist)
             .ifTrue(e -> { ... })
             .ifFalse(e -> { ... })
         .getOperand()
             .otherElementMethod();

Obtaining a Condition by Element#formCondition:

Element element = new Element();

element.formCondition(Element::exist)
           .ifTrue(e -> { ... })
           .ifFalse(e -> { ... })
       .getOperand()
           .otherElementMethod();

Update 1:

For other test methods, the idea remains the same.

Element element = new Element();

element.formCondition(Element::isVisible);
element.formCondition(Element::isEmpty);
element.formCondition(e -> e.hasAttribute(ATTRIBUTE));

Update 2:

It is a good reason to rethink the code design. Neither of 2 snippets is great.

Imagine you need actionC within e0.exist(). How would the method reference Element::actionA be changed?

It would be turned back into a lambda:

e0.ifExist(e -> { e.actionA(); e.actionC(); });

unless you wrap actionA and actionC in a single method (which sounds awful):

e0.ifExist(Element::actionAAndC);

The lambda now is even less 'readable' then the if was.

e0.ifExist(e -> {
    e0.actionA();
    e0.actionC();
});

But how much effort would we make to do that? And how much effort will we put into maintaining it all?

if(e0.exist()) {
    e0.actionA();
    e0.actionC();
}

Answer 2

As it almost but not really matches Optional, maybe you might reconsider the logic:

Java 8 has a limited expressiveness:

Optional<Elem> element = ...
element.ifPresent(el -> System.out.println("Present " + el);
System.out.println(element.orElse(DEFAULT_ELEM));

Here the map might restrict the view on the element:

element.map(el -> el.mySpecialView()).ifPresent(System.out::println);

Java 9:

element.ifPresentOrElse(el -> System.out.println("Present " + el,
                        () -> System.out.println("Not present"));

In general the two branches are asymmetric.

Answer 3

It's called a 'fluent interface'. Simply change the return type and return this; to allow you to chain the methods:

public MyClass ifExist(Consumer<Element> consumer) {
    if (exist()) {
        consumer.accept(this);
    }
    return this;
}

public MyClass ifNotExist(Consumer<Element> consumer) {
    if (!exist()) {
        consumer.accept(this);
    }
    return this;
}

You could get a bit fancier and return an intermediate type:

interface Else<T>
{
    public void otherwise(Consumer<T> consumer); // 'else' is a keyword
}

class DefaultElse<T> implements Else<T>
{
    private final T item;

    DefaultElse(final T item) { this.item = item; }

    public void otherwise(Consumer<T> consumer)
    {
        consumer.accept(item);
    }
}

class NoopElse<T> implements Else<T>
{
    public void otherwise(Consumer<T> consumer) { }
}

public Else<MyClass> ifExist(Consumer<Element> consumer) {
    if (exist()) {
        consumer.accept(this);
        return new NoopElse<>();
    }
    return new DefaultElse<>(this);
}

Sample usage:

element.ifExist(el -> {
    //do something
})
.otherwise(el -> {
    //do something else
});

Answer 4

You can use a single method that takes two consumers:

public void ifExistOrElse(Consumer<Element> ifExist, Consumer<Element> orElse) {
    if (exist()) {
        ifExist.accept(this);
    } else {
        orElse.accept(this);
    }
}

Then call it with:

element.ifExistOrElse(
  el -> {
    // Do something
  },
  el -> {
    // Do something else
  });

Answer 5

If you are performing a simple check on an object and then executing some statements based on the condition then one approach would be to have a Map with a Predicate as key and desired expression as value for example.

Map<Predicate<Integer>,Supplier<String>> ruleMap = new HashMap<Predicate<Integer>,Supplier<String>>(){{
put((i)-> i<10,()->"Less than 10!");
put((i)-> i<100,()->"Less than 100!");
put((i)-> i<1000,()->"Less than 1000!");
}};

We could later stream the following Map to get the value when the Predicate returns true which could replace all the if/else code

        ruleMap.keySet()
               .stream()
               .filter((keyCondition)->keyCondition.test(numItems,version))
               .findFirst()
               .ifPresent((e)-> System.out.print(ruleMap.get(e).get()));

Since we are using findFirst() it is equivalent to if/else if /else if ......