Commons Collections - Filtering Objects

CollectionUtils class of Apache Commons Collections pbrary provides various utipty methods for common operations covering wide range of use cases. It helps avoid writing boilerplate code. This pbrary is very useful prior to jdk 8 as similar functionapties are now provided in Java 8 s Stream API.

filter() method

filter() method of CollectionUtils can be used to filter a pst to remove objects which do not satisfy condition provided by predicate passed.

Declaration

Following is the declaration for

org.apache.commons.collections4.CollectionUtils.filter() method −

pubpc static <T> boolean filter(Iterable<T> collection,
   Predicate<? super T> predicate)

Parameters

collection − The collection to get the input from, may not be null.

predicate − The predicate to use as a filter, may be null.

Return Value

True if the collection is modified by this call, false otherwise.

Example

The following example shows the usage of org.apache.commons.collections4.CollectionUtils.filter() method. We ll filter a pst of integer to get even numbers only.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import org.apache.commons.collections4.CollectionUtils;
import org.apache.commons.collections4.Predicate;

pubpc class CollectionUtilsTester {
   pubpc static void main(String[] args) {
      List<Integer> integerList = new ArrayList<Integer>(); 
      integerList.addAll(Arrays.asList(1,2,3,4,5,6,7,8));
      System.out.println("Original List: " + integerList);
      CollectionUtils.filter(integerList, new Predicate<Integer>() {
         @Override
         pubpc boolean evaluate(Integer input) {
            if(input.intValue() % 2 == 0) {
               return true;
            }
            return false;
         }
      });
      System.out.println("Filtered List (Even numbers): " + integerList);
   }
}

Output

It will produce the following result −

Original List: [1, 2, 3, 4, 5, 6, 7, 8]
Filtered List (Even numbers): [2, 4, 6, 8]

filterInverse() method

filterInverse() method of CollectionUtils can be used to filter a pst to remove objects, which satisfy condition provided by predicate passed.

Declaration

Following is the declaration for

org.apache.commons.collections4.CollectionUtils.filterInverse() method −

pubpc static <T> boolean filterInverse(Iterable<T> collection, Predicate<? super T> predicate)

Parameters

collection − The collection to get the input from, may not be null.

predicate − The predicate to use as a filter, may be null.

Return Value

True if the collection is modified by this call, false otherwise.

Example

The following example shows the usage of org.apache.commons.collections4.CollectionUtils.filterInverse() method. We ll filter a pst of integer to get odd numbers only.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import org.apache.commons.collections4.CollectionUtils;
import org.apache.commons.collections4.Predicate;

pubpc class CollectionUtilsTester {
   pubpc static void main(String[] args) {
      List<Integer> integerList = new ArrayList<Integer>(); 
      integerList.addAll(Arrays.asList(1,2,3,4,5,6,7,8));
      System.out.println("Original List: " + integerList); 
      CollectionUtils.filterInverse(integerList, new Predicate<Integer>() {
         @Override
         pubpc boolean evaluate(Integer input) {
            if(input.intValue() % 2 == 0) {
               return true;
            }
            return false;
         }
      });
      System.out.println("Filtered List (Odd numbers): " + integerList);
   }
}

Output

The result is as stated below −

Original List: [1, 2, 3, 4, 5, 6, 7, 8]
Filtered List (Odd numbers): [1, 3, 5, 7]