Iterating over a Hashset in Java
author :Sai K
A HashSet is a collection that implements the Set interface, backed by a HashMap. It stores elements
in no
particular order and does not allow duplicates. This guide will provide examples of how to
iterate over a HashSet
using different methods, including detailed explanations and outputs.
Additionally, it will cover how to iterate over
a HashSet containing custom objects.
Table of Contents
- 1.Introduction
- 2.Using Enhanced For-Loop
- 3.Using Iterator
- 4.Using forEach Method (Java 8)
- 5.Using Stream API (Java 8)
- 6.Iterating Over a HashSet with Custom Objects
- 7.Conclusion
1. Introduction
A HashSet is a part of the Java Collections Framework and is an implementation of the Set interface
that does
not allow duplicate elements. It is useful for scenarios where you need a set that ensures
uniqueness without any
specific ordering.
2. Using Enhanced For-Loop
The enhanced for-loop (or for-each loop) provides a simple and readable way to iterate over a HashSet.
Example: Using Enhanced For-Loop
import java.util.HashSet;
import java.util.Set;
public class EnhancedForLoopExample {
public static void main(String[] args) {
Set fruits = new HashSet<>();
fruits.add("Apple");
fruits.add("Banana");
fruits.add("Orange");
for (String fruit : fruits) {
System.out.println(fruit);
}
}
}
Output:
Apple
Banana
Orange
3. Using Iterator
The Iterator provides a way to iterate over the elements and allows element removal during iteration if needed.
Example: Using Iterator
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;
public class IteratorExample {
public static void main(String[] args) {
Set fruits = new HashSet<>();
fruits.add("Apple");
fruits.add("Banana");
fruits.add("Orange");
Iterator iterator = fruits.iterator();
while (iterator.hasNext()) {
String fruit = iterator.next();
System.out.println(fruit);
}
}
} Output:
Apple
Banana
Orange
4. Using forEach Method (Java 8)
The forEach method is part of the Java 8 Stream API and provides a functional approach to iteration.
Example: Using forEach Method
import java.util.HashSet;
import java.util.Set;
public class ForEachMethodExample {
public static void main(String[] args) {
Set fruits = new HashSet<>();
fruits.add("Apple");
fruits.add("Banana");
fruits.add("Orange");
// Using forEach with lambda expression
fruits.forEach(fruit -> System.out.println(fruit));
// Using forEach with method reference
fruits.forEach(System.out::println);
}
}
Output:
Apple
Banana
Orange
5. Using Stream API (Java 8)
The Stream API provides a powerful way to process sequences of elements, including iteration.
Example: Using Stream API
import java.util.HashSet;
import java.util.Set;
public class StreamAPIExample {
public static void main(String[] args) {
Set fruits = new HashSet<>();
fruits.add("Apple");
fruits.add("Banana");
fruits.add("Orange");
// Using stream and forEach
fruits.stream().forEach(fruit -> System.out.println(fruit));
// Using parallel stream and forEach
fruits.parallelStream().forEach(fruit -> System.out.println("Parallel: " + fruit));
}
}
Output:
Apple
Banana
Orange
Parallel: Apple
Parallel: Banana
Parallel: Orange
6. Iterating Over a HashSet with Custom Objects
When dealing with custom objects in a HashSet, it is important to override the equals and hashCode methods to
ensure the uniqueness of the elements. Here is an example using a custom Person class.
Custom Object Example: Using Enhanced For-Loop
import java.util.HashSet;
import java.util.Set;
class Person {
private String name;
private int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public int getAge() {
return age;
}
@Override
public boolean equals(Object obj) {
if (this == obj) return true;
if (obj == null || getClass() != obj.getClass()) return false;
Person person = (Person) obj;
return age == person.age && name.equals(person.name);
}
@Override
public int hashCode() {
return name.hashCode() * 31 + age;
}
@Override
public String toString() {
return "Person{name='" + name + "', age=" + age + '}';
}
}
public class CustomObjectExample {
public static void main(String[] args) {
Set persons = new HashSet<>();
persons.add(new Person("Alice", 30));
persons.add(new Person("Bob", 25));
persons.add(new Person("Charlie", 35));
// Using Enhanced For-Loop
System.out.println("Using Enhanced For-Loop:");
for (Person person : persons) {
System.out.println(person);
}
// Using Iterator
System.out.println("\nUsing Iterator:");
Iterator iterator = persons.iterator();
while (iterator.hasNext()) {
Person person = iterator.next();
System.out.println(person);
}
// Using forEach Method (Java 8)
System.out.println("\nUsing forEach Method (Java 8):");
persons.forEach(person -> System.out.println(person));
// Using Stream API (Java 8)
System.out.println("\nUsing Stream API (Java 8):");
persons.stream().forEach(person -> System.out.println(person));
}
}
Output:
Using Enhanced For-Loop:
Person{name='Alice', age=30}
Person{name='Bob', age=25}
Person{name='Charlie', age=35}
Using Iterator:
Person{name='Alice', age=30}
Person{name='Bob', age=25}
Person{name='Charlie', age=35}
Using forEach Method (Java 8):
Person{name='Alice', age=30}
Person{name='Bob', age=25}
Person{name='Charlie', age=35}
Using Stream API (Java 8):
Person{name='Alice', age=30}
Person{name='Bob', age=25}
Person{name='Charlie', age=35}
7. Conclusion
In this guide, we covered various methods to iterate over a HashSet in Java:
- Using Enhanced For-Loop: Simplifies code and improves readability.
- Using Iterator: Allows element removal during iteration.
- Using forEach Method (Java 8): Provides a functional programming approach.
- Using Stream API (Java 8): Offers powerful operations for processing sequences of elements, including
parallel processing.
Additionally, we demonstrated how to iterate over a HashSet containing custom objects, ensuring the proper
implementation of equals and hashCode methods for correct behavior in a set.