Set in set (III)

Catalog

One 、 frame

Two 、set Self defined methods in

3、 ... and 、set: Storage out of order 、 Non repeatable data in HashSet For example, :

Four 、 The process of adding elements : With Hashset For example :（ The underlying logic ）

5、 ... and 、LinkedHashSet Use

6、 ... and 、TreeSet Use  

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One 、 frame

/----Collection Interface ： Single column set , Objects used to store one by one 
      /----Set Interface ： Disordered storage , Non repeatable data （ Similar to the collection of high school ）
          /----HashSet :  As set The main implementation class of the interface , Thread unsafe , Can be stored null value 
          /----LinkedHashSet： As HashSet Subclasses of , When traversing its internal data , You can traverse it in the order you add it 
               advantage ： For frequent traversal operations ,LinkedHashSet Efficiency is higher than Hashset
          /----TreeSet： You can add the specified properties of the object according to , Sort .

Two 、set Self defined methods in

 1. Set There are no additional new methods defined in the interface , All of them are collection The method declared in .
 2． requirement : towards Set Data added in , Its class must be overridden hashcode( ) and equals()
     requirement : Rewrite the hashCode() and equals() Be as consistent as possible : Equal objects must have equal hash codes , Generally, it is good to adjust it directly 

3、 ... and 、set: Storage out of order 、 Non repeatable data in HashSet For example, :

1． Disorder : It's not equal to randomness . The data stored in the underlying array is not added in the order of the array index , It is sorted according to the hash value of the data .
2． Non repeatability : Make sure that the added elements follow equals() When judging , Can't return true. namely : Only one can be added to the same element .

Four 、 The process of adding elements : With Hashset For example :（ The underlying logic ）

 We ask HashSet Add elements to it a, First call the element a Of the class hashCode() Method , Calculated element a Hash value of ,
 This hash value is then calculated by some algorithm in HashSet The storage location in the underlying array （ That is to say : Index position ）, Determine whether there are elements in this position of the array :
       If there are no other elements in this position , The element α Add success .---> situation 1
       If there are other elements in this position b( Or multiple elements in the form of a linked list ）, Compare elements a And element b Of hash value :
           If hash The value is different , The element α Add success .---> situation 2
           If hash Same value , In turn, you need to call the element α Of the class equals() Method :
              equals() return true, Elements α Add failure 
              equals() return faLse, The element α Add success .---> situation 3

  For successful additions 2 And circumstances 3 for : Elements α And the existing data at the specified index position are stored in the form of linked list .
 jdk 7 ∶ Elements a Put it in the array , Point to the original element .
 jdk 8 ∶ The original element is in the array , Point to elements d

 HashSet Bottom ：  Array + The structure of the list 

5、 ... and 、LinkedHashSet Use

    LinkedHashSet As HashSet Subclasses of , While adding data , Each data also maintains two indexes , Record the previous data and the next data of this data 
     advantage : For frequent traversal operations ,LinkedHashSet Efficiency is higher than Hashset

6、 ... and 、TreeSet Use  

1. towards Treeset Data added in , The requirement is  Objects of the same class .
2. Two ways of sorting : Natural ordering （  Realization Comparable Interface ） and  Custom sort （ Realization Comparator）
3. In natural order , The criterion for comparing whether two objects are the same is : compareTo() Return value , If it is 0, Then judge that the two objects are the same . without equals().
4. Custom sorting , The criterion for comparing whether two objects are the same is : compare() Return value , If it is 0, Then judge that the two objects are the same . without equals().

Customized sorting code display ：

    @Test
    public void test2() {
        Comparator comparator = new Comparator() {
            @Override
//             Sort by age 
            public int compare(Object o, Object t1) {
                if (o instanceof Person && t1 instanceof Person) {
                    Person p1 = (Person) o;
                    Person p2 = (Person) t1;
                    return Integer.compare(p1.getAge(), p2.getAge());
                } else {
                    throw new RuntimeException(" Data type mismatch ");
                }
            }
        };

        TreeSet treeSet = new TreeSet(comparator);// Here is the constructor with parameters 

        treeSet.add(new Person(12, "q"));
        treeSet.add(new Person(5, "a"));
        treeSet.add(new Person(88, "l"));
        treeSet.add(new Person(43, "b"));
        treeSet.add(new Person(2, "d"));
        treeSet.add(new Person(9, "d"));

        Iterator iterator = treeSet.iterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.next());
        }
    }
}