java functional interface and lambda expression

Question

functional interface

import java.util.ArrayList;
public interface EmployeeAudit {

    public ArrayList<String> fetchEmployeeDetails (double salary);
}

in public class Main

public static EmployeeAudit findEmployee(){
    ArrayList<String> name=new ArrayList<>();
    return  (sal) -> {
        employeeMap.forEach((key,value) -> {
            if(value<=sal) 
                name.add(key);
        });
        return name;
    };
}    

In main function:

ArrayList<String> str = findEmployee().fetchEmployeeDetails(sal);

---

can anyone help me to understand how value of sal is transferred to findEmployee(), as findEmployee() is called first as per the chaining. And how these calls are working.

Answer 1

Your method findEmployee() does in fact instanciate an EmployeeAudit object. EmployeeAudit is an interface so it needs to define its method, as their is only one, it is a functionnal interface and can be done with a lambda but thta is equivalent to

public static EmployeeAudit findEmployee() {
    ArrayList<String> name = new ArrayList<>();
    return new EmployeeAudit() {
        @Override
        public ArrayList<String> fetchEmployeeDetails(double sal) {
            employeeMap.forEach((key, value) -> {
                if (value <= sal)
                    name.add(key);
            });
            return name;
        }
    };
}

Then, on that instance, you call the fetchEmployeeDetails method, and that is maybe easier to see with splitting the code

EmployeeAudit ea = findEmployee();
ArrayList<String> str = ea.fetchEmployeeDetails(10);

---

You could even imagine create the class implementing EmployeeAudit and use very easily

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map;

interface EmployeeAudit {
    ArrayList<String> fetchEmployeeDetails(double salary);
}

public class Test {
    static Map<String, Integer> employeeMap = new HashMap<>();

    public static void main(String[] args) {
        employeeMap.put("Jean", 10);
        employeeMap.put("Jean2", 100);
        employeeMap.put("Jean3", 100);
        EmployeeAudit ea = new EmployeeAuditImpl();
        ArrayList<String> str = ea.fetchEmployeeDetails(10);
        System.out.println(str);
    }

    static class EmployeeAuditImpl implements EmployeeAudit {
        @Override
        public ArrayList<String> fetchEmployeeDetails(double sal) {
            ArrayList<String> name = new ArrayList<>();
            employeeMap.forEach((key, value) -> {
                if (value <= sal)
                    name.add(key);
            });
            return name;
        }
    }
}

Answer 2

findEmployee() returns an anonymous function (actually a EmployeeAudit defined as a function because EmployeeAudit is a functional interface).

This function is defined as taking an input parameter called sal but this is not the sal of your main call: you could rename sal to anything else in findEmployee and the code would work the same.

The inner sal is the name of a parameter like you could use in any other regular function:

void someFunction(double sal) { ??? }