How to find elements in a collection by property?

Question

I have a list of items, and I want to find a list of items that have the boolean property (field variable) x=true.

I know that this can be accomplished by iteration, but I was looking for a common method to do that in commons library like Apache Commons.

Answer 1

You could use apache commons collection implementing a Predicate for it.

http://commons.apache.org/collections/apidocs/org/apache/commons/collections/CollectionUtils.html

Sample:

package snippet;

import java.util.Arrays;
import java.util.Collection;

import org.apache.commons.collections.CollectionUtils;
import org.apache.commons.collections.Predicate;

public class TestCollection {

    public static class User {

        private String name;

        public User(String name) {
            super();
            this.name = name;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        @Override
        public String toString() {
            return "User [name=" + name + "]";
        }

    }

    public static void main(String[] args) {
        Collection<User> users = Arrays.asList(new User("User Name 1"), new User("User Name 2"), new User("Another User"));
        Predicate predicate = new Predicate() {

            public boolean evaluate(Object object) {
                return ((User) object).getName().startsWith("User");
            }
        };
        Collection filtered = CollectionUtils.select(users, predicate);
        System.out.println(filtered);
    }
}

A few sample can be found here: http://apachecommonstipsandtricks.blogspot.de/2009/01/examples-of-functors-transformers.html

And if you need something more generic, like inspect a value of a specific field or property you could do something like:

public static class MyPredicate implements Predicate {

    private Object expected;
    private String propertyName;

    public MyPredicate(String propertyName, Object expected) {
        super();
        this.propertyName = propertyName;
        this.expected = expected;
    }

    public boolean evaluate(Object object) {
        try {
            return expected.equals(PropertyUtils.getProperty(object, propertyName));
        } catch (Exception e) {
            return false;
        }
    }

}

That could compare a specific property to a expected value, and the use would be something like:

Collection filtered = CollectionUtils.select(users, new MyPredicate("name", "User Name 2"));