Bean列表根据Bean的某个属性去重

妖狐艹你老母 2024-03-22 21:16 91阅读 0赞

/**
     * Bean列表根据Bean的某个属性去重
     */
    public static List<Person> distinctByName(List<Person> list) {
        
        return list.stream().filter(distinctByKey(Person::getName)).toList();
    }
    
    /**
     * Bean列表根据Bean的某个属性去重
     */
    public static <T> Predicate<T> distinctByKey(Function<? super T, ?> function) {
        
        Map<Object, Boolean> map = new ConcurrentHashMap<>();
        return item -> map.putIfAbsent(function.apply(item), Boolean.TRUE) == null;
    }

List<Person> persons = new ArrayList<Person>();
    persons.add(new Person("John", 11, "boy"));
    persons.add(new Person("John1", 11, "boy"));
    persons.add(new Person("John1", 11, "boy"));
    persons.add(new Person("John1", 11, "boy"));
    persons.add(new Person("John1", 11, "boy"));
    persons.add(new Person("John1", 11, "boy"));
    persons.add(new Person("John1", 11, "boy"));
    persons.add(new Person("John1", 11, "boy "));
    List<Person> people = distinctByName(persons);
    System.out.println(people);
    
    [Person(name=John, age=11, number=boy), Person(name=John1, age=11, number=boy)]

--------------------

> **putIfAbsent**  
> 1、map不存在key，故返回null，并把当前key，value存储到Map中  
> 2、map存在key，故返回对应value，并忽略当前key，value  
>  问题:  
> 1、当key对应的值为null时，调用putIfAbsent返回null无法判断是存在键(key,null)，还是key不存在；

Map<Object, Boolean> map = new HashMap<>();
    System.out.println(map);    // {}
    
    Boolean value1 = map.putIfAbsent("key", true);
    System.out.println(value1); // map不存在key,故返回null
    System.out.println(map);    // {key1=true}
    
    Boolean value2 = map.putIfAbsent("key", false);
    System.out.println(value2); // map存在key,故返回value
    System.out.println(map);    // {key1=true}

> **平时都使用Map的put方法，但是put方法返回值是啥？**  
> key对应存储的上个值

Map<Object, Boolean> map = new HashMap<>();
    Boolean value1 = map.put("key", true);
    System.out.println(value1); // null key上个值为null，故返回null
    
    Boolean value2 = map.put("key", false);
    System.out.println(value2); // null key上个值为true，故返回null