Your problem

Your current problem is that you use directly a lambda expression. Lambdas are instances of functional interfaces. Your lambda does not have the boolean type, that's why your if does not accept it.

This special case's solution

You can use a stream from your collections of booleans here.

if (bools.stream().allMatch((Boolean b)->b)) {
    // do something
}

It is actually much more powerful than this, but this does the trick I believe.

General hint

Basically, since you want an if condition, you want a boolean result. Since your result depends on a collection, you can use Java 8 streams on collections.

Java 8 streams allow you to do many operations on a collection, and finish with a terminal operation. You can do whatever complicated stuff you want with Stream's non-terminal operations. In the end you need one of 2 things:

• use a terminal operation that returns a boolean (such as allMatch, anyMatch...), and you're done
• use any terminal operation, but use it in a boolean expression, such as myStream.filter(...).limit(...).count() > 2

You should have a look at your possibilities in this Stream documentation or this one.

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
});

You could declare a body for a lambda. For example:

Runnable run = () -> System.out.println("Hey");

Could be

Runnable run = () -> {
    System.out.println("Hey");
};

Within that body, you can create nested statements:

Runnable run = () -> {
    int num = 5;

    if(num == 5) {
        System.out.println("Hey");
    }
};

For that particular function, a ternary would be possible.

(int x) -> x % 2 == 0 ?  x/2 : (x+1)/2;

Otherwise, make a block

(int x) -> {
    // if... else 
} 

Inside of which, you return the value

You could group by condition and then remap:

public void booleanGrouping() throws Exception {
    List<String> strings = new ArrayList<>();
    strings.add("ala");
    strings.add("ela");
    strings.add("jan");

    strings.stream()
            .collect(
                    Collectors.groupingBy(s -> s.endsWith("a")) // using function Obj -> Bool not predicate
            ).entrySet()
            .stream()
            .collect(
                    Collectors.toMap(
                            e -> e.getKey() ? "Present" : "Past",
                            e -> e.getValue().stream().collect(Collectors.joining(""))
                    )
            );
}

First stream group by condition, you should use equivalentCourse.getNcourse() != null second remap collections from value to string. You could introduce:

enum PresentPast{
    Present, Past
    PresentPast is(boolean v){
         return v ? Present : Past
    }
}

and change e -> e.getKey() ? "Present" : "Past" to enum based solution.

Edit:

Solution for else if:

public Map<Classifier, String> booleanGrouping() throws Exception {
    List<String> strings = new ArrayList<>();
    strings.add("ala");
    strings.add("ela");
    strings.add("jan");
    // our ifs:
    /*
        if(!string.endsWith("n")){
        }else if(string.startsWith("e")){}

        final map should contains two elements
        endsWithN -> ["jan"]
        startsWithE -> ["ela"]
        NOT_MATCH -> ["ala"]

     */
    return strings.stream()
            .collect(
                    Collectors.groupingBy(Classifier::apply) // using function Obj -> Bool not predicate
            ).entrySet()
            .stream()
            .collect(
                    Collectors.toMap(
                            e -> e.getKey(),
                            e -> e.getValue().stream().collect(Collectors.joining(""))
                    )
            );
}

enum Classifier implements Predicate<String> {
    ENDS_WITH_N {
        @Override
        public boolean test(String s) {
            return s.endsWith("n");
        }
    },
    STARTS_WITH_E {
        @Override
        public boolean test(String s) {
            return s.startsWith("e");
        }
    }, NOT_MATCH {
        @Override
        public boolean test(String s) {
            return false;
        }
    };

    public static Classifier apply(String s) {
        return Arrays.stream(Classifier.values())
                .filter(c -> c.test(s))
                .findFirst().orElse(NOT_MATCH);
    }
}

Maybe I hadn't fully explained my problem, but my colleague proposed another solution that I really like. He suggested I make the if-else block a java.util.Supplier, and invoke it in the Entity class where I need it.

So my code went from this block of logic sprinkled everywhere:

public class ServiceImpl {
  ...
  if (entity.getStatus) == 'A' then
    finalStatus = handleA();
  else if (entity.getStatus() == 'B') then
    finalStatus = handleB();
  else
    finalStatus = handleEverythingElse();
  ...
}

To this nicely compacted form:

public class ServiceImpl {
  finalStatus = entity.getFinalStatus(this::handleStatus);

  public int handleStatus() {
    return dao.getStatus();
  }
}

With the implementation in my Entity class:

public class Entity {
  public int handleStatus(Supplier<Integer> s) {
    int finalStatus;
    if (status) == 'A' then
      finalStatus = handleA();
    else if (status() == 'B') then
      finalStatus = handleB();
    else
      finalStatus = supplier.get();
    return status;
  }
}

I hope this make sense...