if statement - 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

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.