ArrayList去重

1.利用HashSet（不保证元素顺序一致）

　　HashSet不会存在相同的元素，可以利用这一点去除List中的重复元素

　　　　 List<String> beforeList = new ArrayList<String>();
        
        beforeList.add("sun");
        beforeList.add("star");
        beforeList.add("moon");
        beforeList.add("earth");
        beforeList.add("sun");
        beforeList.add("earth");
        
        Set<String> middleHashSet = new HashSet<String>(beforeList);
        
        List<String> afterHashSetList = new ArrayList<String>(middleHashSet);
        
        System.out.println(beforeList);
        System.out.println(afterHashSetList);

但是HashSet不保证顺序，如果要按照原来的顺序，用第二种方法

2.利用LinkedHashSet (去重后顺序一致)

　　　　 List<String> beforeList = new ArrayList<String>();
        
        beforeList.add("sun");
        beforeList.add("star");
        beforeList.add("moon");
        beforeList.add("earth");
        beforeList.add("sun");
        beforeList.add("earth");
        
        Set<String> middleLinkedHashSet = new LinkedHashSet<String>(beforeList);
        
        List<String> afterHashSetList = new ArrayList<String>(middleLinkedHashSet);
        
        System.out.println(beforeList);
        System.out.println(afterHashSetList);

去重后元素顺序不变