Collections.frequency in Java
February 01, 2022
On this page we will learn Java Collections.frequency
method.
Find the Java doc.
static int frequency(Collection<?> c, Object o)
2. Same elements in the collection are found using
Objects.equals(o, e)
method.
3. The
frequency
method is a static method and hence we can call by class directly.
4. If specified collection is
null
, it throws NullPointerException
.
5. The
frequency
method is introduced in java.util.Collections
since Java 5.
Example-1
We have a collection of numbers and we are finding the frequency of specified number.CollectionsFrequency1.java
package com.concretepage; import java.util.ArrayList; import java.util.Collections; import java.util.List; public class CollectionsFrequency1 { public static void main(String[] args) { List<Integer> ageList = new ArrayList<>(); ageList.add(10); ageList.add(20); ageList.add(30); ageList.add(20); int freq1 = Collections.frequency(ageList, 20); System.out.println(freq1); // Output 2 int freq2 = Collections.frequency(ageList, 30); System.out.println(freq2); // Output 1 int freq3 = Collections.frequency(ageList, 50); System.out.println(freq3); // Output 0 } }
Example-2
We have a collection of strings and we are finding the frequency of specified string.CollectionsFrequency2.java
package com.concretepage; import java.util.ArrayList; import java.util.Collections; import java.util.List; public class CollectionsFrequency2 { public static void main(String[] args) { List<String> cityList = new ArrayList<>(); cityList.add("Varanasi"); cityList.add("Noida"); cityList.add("Noida"); cityList.add("Prayag"); int freq1 = Collections.frequency(cityList, "Noida"); System.out.println(freq1); // Output 2 int freq2 = Collections.frequency(cityList, "Varanasi"); System.out.println(freq2); // Output 1 } }
Example-3
We have a collection of objects and we are finding the frequency of specified object.CollectionsFrequency3.java
package com.concretepage; import java.util.ArrayList; import java.util.Collections; import java.util.List; public class CollectionsFrequency3 { public static void main(String[] args) { Person p1 = new Person("Ram"); Person p2 = new Person("Shyam"); Person p3 = new Person("Mahesh"); Person p4 = new Person("Shyam"); List<Person> list = new ArrayList<Person>(); list.add(p1); list.add(p2); list.add(p3); list.add(p4); Person p = new Person("Shyam"); int freq = Collections.frequency(list, p); System.out.println(freq); // Output 2 } } class Person { private String name; public Person(String name) { this.name = name; } public String getName() { return name; } @Override public boolean equals(Object o) { return name.equals(((Person)o).name); } @Override public int hashCode() { int hash = 13; hash = (31 * hash) + (null == name ? 0 : name.hashCode()); return hash; } }