How to call method from parameter JAVA

Question

my problem is duplicit code. I have 4 methods like below but one is for Name, other for Login, etc. So the only thing that changes are getters (p.getName(), p.getLogin()...).

private Set<Person> filterByName(Set<String> values, Set<Person> peeps){
    Set<Person> filtered = new HashSet<>();
    for (Person p : peeps){
        if(isIn(p.getName(), values)){
            filtered.add(p);
        }
    }
    return filtered;
}

I call these methods in switch:

switch(var){
     case NAME:
          filtered = filterByName(arg1, arg2);
     case ...

How can I do this without implementing a method for each case block?

My idea is to somehow tell the method to use the right getter in case blocks and therefore have only one method for all filtering, e.g.

private Set<Person> universalFilter(Set<String> values, Set<Person> peeps) {
Set<Person> filtered = new HashSet<>();
    for (Person p : peeps){
        if(isIn(***SOMETHING CLEVER***, values)){
            filtered.add(p);
        }
    }
    return filtered;
}

Which version of Java? If 8, that sounds like a job for a Predicate... Or if less than 8, then Guava also has Predicate. — fge, Dec 03 '15 at 17:25

score 3 · Answer 1 · answered Dec 03 '15 at 17:53

If you are using Java 8, you can use method references to do exactly what you want. The smallest change is to use a Function object. Your filterByName method takes an additional argument which maps each Person object to the property you want to test:

private Set<Person> filter(Function<Person,String> filter, Set<String> values, Set<Person> peeps) {
    Set<Person> filtered = new HashSet<>();
    for (Person p : peeps){
        if(isIn(filter.apply(p), values)){
            filtered.add(p);
        }
    }
    return filtered;
}

and then you call the method as follows:

filter(Person::getName, values, peeps);

If you want to allow for more complex filters, you can specify the entire filter as a predicate:

private Set<Person> filter(Predicate<Person,String> filter, Set<Person> peeps) {
    Set<Person> filtered = new HashSet<>();
    for (Person p : peeps){
        if (filter.test(p)){
            filtered.add(p);
        }
    }
    return filtered;
}

which can be called with:

filter(p -> isIn(p.getName(), values), peeps);

In fact, Java 8 provides filtering functionality in the run-time library, either by the removeIf method:

private Set<Person> filter(Predicate<Person,String> filter, Set<Person> peeps) {
    Set<Person> filtered = new HashSet<>();
    filtered.removeIf(filter);
    return filtered;
}

or by using streams:

private Set<Person> filter(Predicate<Person,String> filter, Set<Person> peeps) {
    return peeps.stream().filter(filter).collect(Collectors.toSet());
}

score 1 · Answer 2 · answered Dec 03 '15 at 17:26

A simple way is to use the switch variable to pass in as an argument to a new method get() on Person.

if(isIn(p.get(var), values)){

Then implement the new method in Person, eg:

public void get(String var) {
  if (var.equals("NAME")) return name;
  ...
}

Arnaud · Answer 3 · 2015-12-03T18:35:16.220

0

You could pass another String parameter, which would be the variable on which you want to filter ("Name", "Login"...)

Then with reflection, you could find and call the relevant getter method :

Method method = Person.class.getMethod("get"+ filter, null);

String returnValue = (String)(method.invoke(p, null));

if(isIn(returnValue , values))
...

edited Dec 03 '15 at 18:35

answered Dec 03 '15 at 17:27

Arnaud

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I think you mean "reflection", not "recursion" – arcy Dec 03 '15 at 17:37
I modified the first line to: Method method = Person.class.getMethod("get"+ filter, null); and it works. Thank you, this is what I wanted :) – Martin Dec 03 '15 at 17:56
@arcy , yes indeed, I just corrected it, thanks for pointing it ! – Arnaud Dec 03 '15 at 18:16

How to call method from parameter JAVA

3 Answers3