Java HashSet 类是 Java 集合框架的成员。它实现了 Set
接口。 HashSet 用于存储唯一 元素的集合。
以下是 Java 中关于 HashSet 需要注意的几个关键点 -
HashSet 不能包含重复值。
*
HashSet 允许 null
值。
*
HashSet 是一个无序集合。它不保持元素插入的顺序。
*
HashSet 内部使用 HashMap 来存储其元素。
*
HashSet 不是线程安全的。如果多个线程同时尝试修改一个 HashSet,那么最终结果是不确定的。您必须在多线程环境中显式同步对 HashSet 的并发访问。
下面的示例展示了如何使用 HashSet() 构造函数创建 HashSet,并使用 add() 方法向其添加新元素。
import java.util.HashSet;
import java.util.Set;
public class CreateHashSetExample {
public static void main(String[] args) {
// Creating a HashSet
Set<String> daysOfWeek = new HashSet<>();
// Adding new elements to the HashSet
daysOfWeek.add("Monday");
daysOfWeek.add("Tuesday");
daysOfWeek.add("Wednesday");
daysOfWeek.add("Thursday");
daysOfWeek.add("Friday");
daysOfWeek.add("Saturday");
daysOfWeek.add("Sunday");
// Adding duplicate elements will be ignored
daysOfWeek.add("Monday");
System.out.println(daysOfWeek);
}
}
# Output
[Monday, Thursday, Friday, Sunday, Wednesday, Tuesday, Saturday]
以下示例显示了如何
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
public class CreateHashSetFromCollectionExample {
public static void main(String[] args) {
List<Integer> numbersDivisibleBy5 = new ArrayList<>();
numbersDivisibleBy5.add(5);
numbersDivisibleBy5.add(10);
numbersDivisibleBy5.add(15);
numbersDivisibleBy5.add(20);
numbersDivisibleBy5.add(25);
List<Integer> numbersDivisibleBy3 = new ArrayList<>();
numbersDivisibleBy3.add(3);
numbersDivisibleBy3.add(6);
numbersDivisibleBy3.add(9);
numbersDivisibleBy3.add(12);
numbersDivisibleBy3.add(15);
// Creating a HashSet from another collection (ArrayList)
Set<Integer> numbersDivisibleBy5Or3 = new HashSet<>(numbersDivisibleBy5);
// Adding all the elements from an existing collection to a HashSet
numbersDivisibleBy5Or3.addAll(numbersDivisibleBy3);
System.out.println(numbersDivisibleBy5Or3);
}
}
# Output
[3, 20, 5, 6, 25, 9, 10, 12, 15]
import java.util.HashSet;
import java.util.Set;
public class HashSetSimpleOperationsExample {
public static void main(String[] args) {
Set<String> popularCities = new HashSet<>();
// Check if a HashSet is empty
System.out.println("Is popularCities set empty? : " + popularCities.isEmpty());
popularCities.add("London");
popularCities.add("New York");
popularCities.add("Paris");
popularCities.add("Dubai");
// Find the size of a HashSet
System.out.println("Number of cities in the HashSet " + popularCities.size());
// Check if the HashSet contains an element
String cityName = "Paris";
if(popularCities.contains(cityName)) {
System.out.println(cityName + " is in the popular cities set.");
} else {
System.out.println(cityName + " is not in the popular cities set.");
}
}
}
# Output
Is popularCities set empty? : true
Number of cities in the HashSet 4
Paris is in the popular cities set.
此示例显示如何:
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
public class HashSetRemoveExample {
public static void main(String[] args) {
Set<Integer> numbers = new HashSet<>();
numbers.add(2);
numbers.add(3);
numbers.add(4);
numbers.add(5);
numbers.add(6);
numbers.add(7);
numbers.add(8);
numbers.add(9);
numbers.add(10);
System.out.println("numbers : " + numbers);
// Remove an element from a HashSet (The remove() method returns false if the element does not exist in the HashSet)
boolean isRemoved = numbers.remove(10);
System.out.println("After remove(10) => " + numbers);
// Remove all elements belonging to a given collection from a HashSet
List<Integer> perfectSquares = new ArrayList<>();
perfectSquares.add(4);
perfectSquares.add(9);
numbers.removeAll(perfectSquares);
System.out.println("After removeAll(perfectSquares) => " + numbers);
// Remove all elements matching a given predicate
numbers.removeIf(num -> num % 2 == 0);
System.out.println("After removeIf() => " + numbers);
// Remove all elements from HashSet (clear it completely)
numbers.clear();
System.out.println("After clear() => " + numbers);
}
}
# Output
numbers : [2, 3, 4, 5, 6, 7, 8, 9, 10]
After remove(10) => [2, 3, 4, 5, 6, 7, 8, 9]
After removeAll(perfectSquares) => [2, 3, 5, 6, 7, 8]
After removeIf() => [3, 5, 7]
After clear() => []
以下示例显示了迭代 HashSet 的不同方式
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;
public class IterateOverHashSetExample {
public static void main(String[] args) {
Set<String> programmingLanguages = new HashSet<>();
programmingLanguages.add("C");
programmingLanguages.add("C++");
programmingLanguages.add("Java");
programmingLanguages.add("Python");
programmingLanguages.add("PHP");
programmingLanguages.add("Ruby");
System.out.println("=== Iterate over a HashSet using Java 8 forEach and lambda ===");
programmingLanguages.forEach(programmingLanguage -> {
System.out.println(programmingLanguage);
});
System.out.println("=== Iterate over a HashSet using iterator() ===");
Iterator<String> programmingLanguageIterator = programmingLanguages.iterator();
while (programmingLanguageIterator.hasNext()) {
String programmingLanguage = programmingLanguageIterator.next();
System.out.println(programmingLanguage);
}
System.out.println("=== Iterate over a HashSet using iterator() and Java 8 forEachRemaining() method ===");
programmingLanguageIterator = programmingLanguages.iterator();
programmingLanguageIterator.forEachRemaining(programmingLanguage -> {
System.out.println(programmingLanguage);
});
System.out.println("=== Iterate over a HashSet using simple for-each loop ===");
for(String programmingLanguage: programmingLanguages) {
System.out.println(programmingLanguage);
}
}
}
# Output
=== Iterate over a HashSet using Java 8 forEach and lambda ===
Java
C++
C
PHP
Ruby
Python
=== Iterate over a HashSet using iterator() ===
Java
C++
C
PHP
Ruby
Python
=== Iterate over a HashSet using iterator() and Java 8 forEachRemaining() method ===
Java
C++
C
PHP
Ruby
Python
=== Iterate over a HashSet using simple for-each loop ===
Java
C++
C
PHP
Ruby
Python
此示例显示如何创建用户定义对象的 HashSet。
import java.util.HashSet;
import java.util.Objects;
import java.util.Set;
class Customer {
private long id;
private String name;
public Customer(long id, String name) {
this.id = id;
this.name = name;
}
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
// Two customers are equal if their IDs are equal
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Customer customer = (Customer) o;
return id == customer.id;
}
@Override
public int hashCode() {
return Objects.hash(id);
}
@Override
public String toString() {
return "Customer{" +
"id=" + id +
", name='" + name + '\'' +
'}';
}
}
public class HashSetUserDefinedObjectExample {
public static void main(String[] args) {
Set<Customer> customers = new HashSet<>();
customers.add(new Customer(101, "Rajeev"));
customers.add(new Customer(102, "Sachin"));
customers.add(new Customer(103, "Chris"));
/* HashSet will use the `equals()` & `hashCode()` implementations of the Customer class to check for duplicates and ignore them */
customers.add(new Customer(101, "Rajeev"));
System.out.println(customers);
}
}
# Output
[Customer{id=101, name='Rajeev'}, Customer{id=102, name='Sachin'}, Customer{id=103, name='Chris'}]
恭喜各位!在本文中,您了解了什么是 HashSet、如何创建 HashSet、如何向 HashSet 添加元素、如何从 HashSet 中删除元素、如何检查 HashSet 中是否存在元素、如何迭代 HashSet ,以及如何创建用户定义对象的 HashSet。