Java中List集合去重方法以及效率对比

public static List repeatListWayOne(List<String> list){
 for(int i = 0;i < list.size();i++){
  for(int j = i+1;j < list.size();j++){
   if(list.get(i).equals(list.get(j))){
    list.remove(j);
   }
  }
 }
 return list;
}

public static List repeatListWayTwo(List<String> list){
 //初始化HashSet对象，并把list对象元素赋值给HashSet对象
 HashSet set = new HashSet(list);
 //把List集合所有元素清空
 list.clear();
 //把HashSet对象添加至List集合
 list.addAll(set);
 return list;
}

public static List repeatListWayThird(List<String> list){
 //初始化TreeSet对象，并把list对象元素赋值给TreeSet对象
 TreeSet set = new TreeSet(list);
 //把List集合所有元素清空
 list.clear();
 //把TreeSet对象添加至List集合
 list.addAll(set);
 return list;
}

public static List repeatListWayFourth(List<String> list){
  //新建新List集合，用于存放去重后的元素
  List<String> newList = new ArrayList<String>();
  //循环遍历旧集合元素
  for(int i = 0; i < list.size(); i++ ){
    //判断新集合是否包含有，如果不包含有，则存入新集合中
    boolean isContains = newList.contains(list.get(i));
    if(!isContains){
      newList.add(list.get(i));
    }
  }
  //把List集合所有元素清空
  list.clear();
  //把新集合元素添加至List集合
  list.addAll(newList);
  return list;
}

public static List<String> getRandomList(){
  List<String> list = new ArrayList<String>();
  //随机生成20000个整数字符串
  for(int i = 1; i <= 20000; i++){
    //任意取[0,500)之间整数，其中0可以取到，500取不到
    int number = new Random().nextInt(500);
    String number_str = "geshan"+number;
    list.add(number_str);
  }
  return list;
}

public static void main(String[] args){
  //随机生成0-500之间的1000个整数字符串List集合
  List<String> list = getRandomList();
 
  //为了演示四种方式效率，创建四个List集合，保证List集合元素一致
  //方式一List集合
  List<String> oneList = new ArrayList<>();
  oneList.addAll(list);
  //方式二List集合
  List<String> twoList = new ArrayList<>();
  twoList.addAll(list);
  //方式三List集合
  List<String> thirdList = new ArrayList<>();
  thirdList.addAll(list);
  //方式四List集合
  List<String> fourthList = new ArrayList<>();
  fourthList.addAll(list);
 
  System.out.println("方式一：使用两个for循环实现List去重");
  System.out.println("原来集合大小:"+oneList.size()+"，集合元素>>"+oneList);
  Date oneDateBegin = new Date();
  repeatListWayOne(oneList);
  System.out.println("集合去重大小:"+oneList.size()+"，集合元素>>"+oneList);
  Date oneDateEnd = new Date();
  System.out.println("去重所需时间:"+(oneDateEnd.getTime()-oneDateBegin.getTime())+"毫秒");
 
  System.out.println("方式二：使用HashSet实现List去重");
  System.out.println("原来集合大小:"+twoList.size()+"，集合元素>>"+twoList);
  Date twoDateBegin = new Date();
  repeatListWayTwo(twoList);
  System.out.println("集合去重大小:"+twoList.size()+"，集合元素>>"+twoList);
  Date twoDateEnd = new Date();
  System.out.println("去重所需时间:"+(twoDateEnd.getTime()-twoDateBegin.getTime())+"毫秒");
 
  System.out.println("方式三：使用TreeSet实现List去重");
  System.out.println("原来集合大小:"+thirdList.size()+"，集合元素>>"+thirdList);
  Date thirdDateBegin = new Date();
  repeatListWayThird(thirdList);
  System.out.println("集合去重大小:"+thirdList.size()+"，集合元素>>"+thirdList);
  Date thirdDateEnd = new Date();
  System.out.println("去重所需时间:"+(thirdDateEnd.getTime()-thirdDateBegin.getTime())+"毫秒");
 
  System.out.println("方式四：利用List集合contains方法循环遍历去重");
  System.out.println("原来集合大小:"+fourthList.size()+"，集合元素>>"+fourthList);
  Date fourthDateBegin = new Date();
  repeatListWayFourth(fourthList);
  System.out.println("集合去重大小:"+fourthList.size()+"，集合元素>>"+fourthList);
  Date fourthDateEnd = new Date();
  System.out.println("去重所需时间:"+(fourthDateEnd.getTime()-fourthDateBegin.getTime())+"毫秒");
}