I need more clarification about lambda expression. How 'p' represents List<Person> people? Could you explain clear to me
List<Person> people = new ArrayList<>();
people.add(new Person("Mohamed", 69));
people.add(new Person("Doaa", 25));
people.add(new Person("Malik", 6));
Predicate<Person> pred = (p) -> p.getAge() > 65;
No, p is not a List<Person> but a Person.
Predicate<Person> pred = p -> p.getAge() > 65;
This lambda expression declares 1 formal parameter and returns a boolean. As such, it can be represented as a Predicate (because the Predicate interface has a single functional method with this exact signature, called test). The type of p will be determined by the type of the Predicate you are creating.
For example, in the following code, p will be a List<Person>:
Predicate<List<Person>> predicate = p -> p.isEmpty();
(p), the same as p, (the parenthesis are not needed in this case) is the formal parameter of the lambda expression. I suggest you read this Oracle tutorial on lambda expressions.Lambdas in java it is just a syntax shugar for anonymous classes
Code from you example is equal to
List<person> people = new ArrayList<>();
people.add(new Person("Mohamed", 69));
people.add(new Person("Doaa", 25));
people.add(new Person("Malik", 6));
Predicate<person> pred = new Predicate<person>() {
public boolean test(person p) {
return p.getAge() > 65;
}
}
To simplify syntax in java you can skip type declaration in lambda expression and add only name of value, like you did.
Predicate<person> pred = (p) -> p.getAge() > 65;
Or if you want, you can write something like this
Predicate<person> pred = (person p) -> p.getAge() > 65;
Its to be noted you can skip type declaration only if it can be counted from lambda code somehow. For example
Comparator<String> comp
= (firstStr, secondStr) // Same as (String firstStr, String secondStr)
-> Integer.compare(firstStr.length(),secondStr.length());
this and super have a different meaning. Further, there are some practical differences at runtime and big differences on the byte code level