Gracefully find the duplicate elements in the ArrayList

Method 1

/**


 
 
 

  
   
   

    
    
Created by cxh on 17/1/9.

*/

public class Main {

public static void main(String[] args){

List
      
      
      list = new ArrayList
     
      
      
        (); 
       
 list.add("aaa"); 
       
 list.add("bbb"); 
       
 list.add("ccc"); 
       
 list.add("ddd"); 
       
 list.add("aaa"); 
       
 list.add("aaaa"); 
       
 list.add("eee"); 
       
 list.add("bbb"); 
       
 list.add("ccc"); 
       
 StringBuilder builder = new StringBuilder(); 
       
 for(String str : list) { 
       
 // 如果不存在返回 -1。 
       
 if(builder.indexOf(","+str+",") > -1) { 
       
 Sy stem.out.println("重复的有："+str); 
       
 } else { 
       
 builder.append(",").append(str).append(","); 
       
 } 
       
 } 
       
 } 
       
 } 
      
    
     
     
   
   
 
 
 

Method 2

import java.util.List;

import java.util.ArrayList;

import java.util.HashMap;

/**

 
 
 

  
   
   

    
    
Created by cxh on 17/1/9.

*/

public class Main {

public static void main(String[] args){

List
      
      
      list=new ArrayList
     
      
      
        (); 
       
 list.add("string1"); 
       
 list.add("string2"); 
       
 list.add("string3"); 
       
 list.add("string1"); 
       
 list.add("string1"); 
       
 list.add("string1"); 
       
 list.add("string2"); 
       
 //list.add("string3"); 
       
 HashMap<String,Integer> hashMap=new HashMap<String,Integer>(); 
       
 for(String string:list){ 
       
 if(hashMap.get(string)!=null){ //hashMap包含遍历list中的当前元素 
       
 Integer integer=hashMap.get(string); 
       
 hashMap.put(string,integer+1); 
       
 Sy stem.out.println("the element:"+string+" is repeat"); 
       
 } 
       
 else{ 
       
 hashMap.put(string,1); 
       
 } 
       
 } 
       
 } 
       
 } 
      
    
     
     
   
   
 
 
 

Method 3

public class Main {

public static <E> List<E> getDuplicateElements(List<E> list) {
    return list.stream() // list 对应的 Stream
            .collect(Collectors.toMap(e -> e,e -> 1,(a,b) -> a + b)) // 获得元素出现频率的 Map，键为元素，值为元素出现的<a href="https://www.jb51.cc/tag/cishu/" target="_blank" class="keywords">次数</a>
            .entrySet().stream() // 所有 entry 对应的 Stream
            .filter(entry -> entry.getValue() > 1) // 过滤出元素出现<a href="https://www.jb51.cc/tag/cishu/" target="_blank" class="keywords">次数</a>大于 1 的 entry
            .map(entry -> entry.getKey()) // 获得 entry 的键（重复元素）对应的 Stream
            .collect(Collectors.toList());  // 转化为 List
}

public static void main(String[] args) throws Exception {
    List<String> list = Arrays.asList("a","b","c","d","a","d");
    List<String> duplicateElements = getDuplicateElements(list);

    Sy<a href="https://www.jb51.cc/tag/stem/" target="_blank" class="keywords">stem</a>.out.println("list 中重复的元素：" + duplicateElements);
}



    }