How to get a particular attribute of my object?

java, reflection

Solution

You can do the exact same thing you were trying to do with Java 8 using an anonymous class with Java 7

public static <X, Y> List<Y> processElements(Iterable<X> source, Function <X, Y> mapper) {
    List<Y> l = new ArrayList<>();
    for (X p : source) 
        l.add(mapper.apply(p));
    return l;
}

List<String> listNames = processElements(people, new Function<People, String>() {
    public String apply(People person) {
        return person.getName();
    }
});

Problem

Let's say I have this class. ``` class People { private String name; private int age; /** other stuff constructor / getters / setters / etc. **/ } ``` And let's say I have this list : ``` List<People> people = Arrays.asList(new People("Mark", 21), new People("Bob",17), new People("Alex", 22)); ``` Now I want to be able to get a `List` for all users of a particular attribute. Using Java 8, I can create a method that accepts the function I want like : ``` public static <X, Y> List<Y> processElements(Iterable<X> source, Function <X, Y> mapper) { List<Y> l = new ArrayList<>(); for (X p : source) l.add(mapper.apply(p)); return l; } ``` Then I will able to perform some calls like : ``` List<String> listNames = processElements(people, p -> p.getName()); List<Integer> listAges = processElements(people, p -> p.getAge()); ``` However this is not possible in Java 7 (I know that it exists in Guava, but I would come up with a solution only using the JDK). This is with what I come up using reflection : ``` public static List<Object> processElements(List<People> l, String fieldName) throws NoSuchFieldException, SecurityException, IllegalArgumentException, IllegalAccessException{ Field field = People.class.getDeclaredField(fieldName); field.setAccessible(true); List<Object> list = new ArrayList<>(); for(People p : l){ list.add(field.get(p)); } field.setAccessible(false); return list; } ``` Now I can do something like `List<Object> listNames = processElements(people, "name");` which works quite well. But I don't feel comfortable with it : - I'm getting a `List<Object>` - Whenever I use reflection, I always feel like it exists a better implementation not using it - Using `field.setAccessible(true);` is not very secure So my questions are : - Is it possible to do the same thing without using reflection with the JDK (not using Guava or Functional Java) ? - How can I get a List of String or Integer according to the type of the field I pass as parameter to my method ? Thanks

Original source