Apache Commons Collections - Filtering Objects

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Filtering a list

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

Usage

CollectionUtils.filter(integerList, new Predicate<Integer>() {
   @Override
   public boolean evaluate(Integer input) {
      if(input.intValue() % 2 == 0) {
         return true;
      }
      return false;
   }
});

Declaration

Following is the declaration for

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

public 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 - Filtering out odd numbers from a List of integers

The following example shows the usage of filter() method. We'll filter a list of integer to get even numbers only.

CommonCollectionsTester.java

package com.tutorialspoint;

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

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

public class CommonCollectionsTester {
   public 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
         public 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 list to remove objects, which satisfy condition provided by predicate passed.

Usage

CollectionUtils.filterInverse(integerList, new Predicate<Integer>() {
   @Override
   public boolean evaluate(Integer input) {
      if(input.intValue() % 2 == 0) {
         return true;
      }
      return false;
   }
});

Declaration

Following is the declaration for

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

public 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 - Filtering out even numbers from a List of integers

The following example shows the usage of filterInverse() method. We'll filter a list of integer to get odd numbers only.

CommonCollectionsTester.java

package com.tutorialspoint;

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

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

public class CommonCollectionsTester {
   public 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
         public 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]