在Java中从ArrayList获取唯一值

ArrayList is a class of Java Collection Framework that implements List Interface. It is a linear structure that stores and accesses each element sequentially. It allows the storage of duplicate elements however, there are a few approaches that may help to get unique values from an ArrayList. In this article, we are going to see the practical implementation 通过Java示例程序的方式之一。

Java Program to get Unique Values from ArrayList

Before jumping to the solution program for the given problem, let’s discuss the following Collection Interface的概念−

HashSet

它是Java Collection接口的一个类，实现了set接口。它与之相似 到数学集合中。其中，所有的元素都以排序的方式存储，以减少 the retrieval time and also, it doesn’t allow duplicate values. This is the reason for using HashSet to filter duplicates and to get unique values from a list.

语法

HashSet< Type >nameOfSet = new HashSet<>();

Here, Type is the wrapper class object, not a primitive datatype.

Syntax for ArrayList

ArrayList<Type> nameOflist = new ArrayList< Type >();

Approach 1

• Create an ArrayList object of type Integer and store some elements to it by using a built-in method named ‘add()’. This method takes an argument and adds it to the 指定的列表。

• 现在，创建一个相同类型的HashSet，并存储前一个元素的所有内容

使用内置方法‘addAll()’将ArrayList添加到其中。该方法接受一个ArrayList as an argument and adds all its elements to the new 指定的列表。

• 最后，通过调用列表来打印值。

Example 

下面的示例演示了使用HashSet类从ArrayList中获取唯一值的方法。

import java.util.*;
public class Unique {
   public static void main(String[] args) {
      // Creating arraylist
      ArrayList<Integer> araylist = new ArrayList<Integer>();
      
      // Adding elements in arraylist
      araylist.add(8);
      araylist.add(5);
      araylist.add(2);
      araylist.add(9);
      araylist.add(2);
      araylist.add(4);
      araylist.add(9);
      araylist.add(7);
      System.out.println("List with duplicate elements: ");
      
      // loop to iterate through elements
      for(int i = 0; i < araylist.size(); i++ ) {
         // to print the elements in the list
         System.out.println(araylist.get(i));
      }
      HashSet<Integer> set = new HashSet<>(); // creating hashset
      set.addAll(araylist); // adding all elements to set
      System.out.println("List with unique elements: " + set);
   }
}     

Output

List with duplicate elements:
8
5
2
9
2
4
9
7
List with unique elements: [2, 4, 5, 7, 8, 9]

Approach 2

• Create an ArrayList object of type Integer and store some elements to it by using a built-in method named ‘add()’. This method takes an argument and adds it to the 指定的列表。

• Now, create another ArrayList object of the same type.

  

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• Define a for loop that will iterate till the ArrayList size. Inside this loop, take an if 检查索引'i'处的值是否唯一的代码块。如果是唯一的 然后将该值添加到新的ArrayList中，否则将其丢弃。

• 结束时，通过调用列表打印值。

Example 

This is another example in which we will try to get unique values from an ArrayList.

import java.util.*;
public class Unique {
   public static void main(String[] args) {
      // Creating arraylist
      ArrayList<Integer> araylist1 = new ArrayList<Integer>();
      // Adding elements in arraylist
      araylist1.add(8);
      araylist1.add(5);
      araylist1.add(2);
      araylist1.add(9);
      araylist1.add(2);
      araylist1.add(4);
      araylist1.add(9);
      araylist1.add(7);
      System.out.println("List with duplicate elements: " + araylist1);
      ArrayList<Integer>araylist2 = new ArrayList<Integer>();
      for(int i = 0; i < araylist1.size(); i++ ) {
         int elem;
         // to filter the duplicate elements
         if(!araylist2.contains(araylist1.get(i))) {
            // storing unique elements to elem and adding to new list
            elem = araylist1.get(i);
            araylist2.add(elem);
         }
      }
      System.out.println("List with unique elements: " + araylist2);
   }
}

Output

List with duplicate elements: [8, 5, 2, 9, 2, 4, 9, 7]
List with unique elements: [8, 5, 2, 9, 4, 7]

Conclusion

We started this article by defining ArrayList and in the next section we understand the use of HashSet in finding unique values from a list. We discussed two approaches to get unique values from an ArrayList, first using the HashSet class and the second without using this class

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