TreeSet and Java custom type sorting

• Demonstrates that TreeSet can sort String
• TreeSet Unable to sort custom types
• How to write comparison rules
• Self-balancing binary tree structure
• Implementing the comparator interface
• Collections tool class

(Free learning recommendation: java basic tutorial)

Demonstrates TreeSet pairing String is sortable

1. The bottom layer of the TreeMap collection is actually a TreeMap
2. The bottom layer of the TreeMap collection is a binary tree
3. The elements placed in the TreeSet collection , which is equivalent to being placed in the key part of the TreeMap collection.
4. The elements in the TreeSet collection are unordered and cannot be repeated, but they can be automatically sorted according to the size of the elements.
is called: a sortable collection
For example: write a program to retrieve data from the database, and display user information on the page in ascending or descending order by birthday.
You can use the TreeSet collection at this time, because the TreeSet collection is put in and taken out in order. of.

//創(chuàng)建一個TreeSet集合
??TreeSet<string>?ts=new?TreeSet();
??//添加Stringts.add("zhangsan");ts.add("lisi");ts.add("wangwu");ts.add("zhangsi");ts.add("wangliu");for(String?s:ts){
??????//按照字典順序排序
??????System.out.print(s+"?");
??}
??TreeSet<integer>?ts2=new?TreeSet();ts2.add(100);ts2.add(200);ts2.add(900);ts2.add(800);??ts2.add(600);ts2.add(10);for(Integer?i:ts2){
??????//按照升序排序
??????System.out.print(i+"?");}</integer></string>

TreeSet cannot sort custom types

Can TreeSet sort custom types?
In the following program, the Person class cannot be sorted because the comparison rules between Person objects are not specified. It is not stated who is older and who is younger.

public?class?TreeSetTest02?{
????public?static?void?main(String[]?args)?{
????????Person?p1=new?Person(50);
????????Person?p2=new?Person(10);
????????Person?p3=new?Person(20);
????????Person?p4=new?Person(60);
????????Person?p5=new?Person(40);
????????Person?p6=new?Person(30);
????????TreeSet<person>?persons=new?TreeSet();
????????persons.add(p1);
????????persons.add(p2);
????????persons.add(p3);
????????persons.add(p4);
????????persons.add(p5);
????????persons.add(p6);
????????for(Person?p:persons){
????????????System.out.println(p);
????????}
????}}class?Person{
????int?age;
????public?Person(int?age){
????????this.age=age;
????}
????@Override
????public?String?toString()?{
????????return?"Person?[age="?+?age?+?"]";
????}}</person>

Exception?in?thread?"main"?java.lang.ClassCastException:?testCollection.Person?cannot?be?cast?to?java.lang.Comparable

The reason for this error is that the
Person class does not implement the java.lang, Comparable interface

//Put it in the TreeSet collection The elements in need to implement the java.lang.Comparable interface
//And implement the compareTo method, equals does not need to be written

??public?class?TreeSetTest04?{
????????public?static?void?main(String[]?args)?{
????????????Customer?p1=new?Customer(50);
????????????Customer?p2=new?Customer(10);
????????????Customer?p3=new?Customer(20);
????????????Customer?p4=new?Customer(60);
????????????Customer?p5=new?Customer(40);
????????????Customer?p6=new?Customer(30);
????????????TreeSet<customer>?customers=new?TreeSet();
????????????customers.add(p1);
????????????customers.add(p2);
????????????customers.add(p3);
????????????customers.add(p4);
????????????customers.add(p5);
????????????customers.add(p6);
????????????for(Customer?p:customers){
????????????????System.out.println(p);
????????????}
????????}
????}
????//放在TreeSet集合中的元素需要實現(xiàn)java.lang.Comparable接口//并且實現(xiàn)compareTo方法，equals可以不寫
????class?Customer?implements?Comparable<customer>{
????????int?age;
????????public?Customer(int?age){
????????????this.age=age;
????????}
????????@Override
????????public?String?toString()?{
????????????return?"Customer?[age="?+?age?+?"]";
????????}
????????//需要在這個方法中編寫比較的邏輯，或者說比較的規(guī)則，按照什么進行比較。
????????//k.compareTo(t.key)
????????//拿著參數(shù)k和集合中的每個k進行比較，返回值可能是>0,age2){//???????return?1;//????}else{//???????return?-1;//????}
????????????return?this.age-c.age;????//>,<p>//You need to write the comparison logic in this method, or compare The rules according to which comparisons are made. <br> //k.compareTo(t.key)<br> //Compare parameter k with each k in the set. The return value may be >0, / /<strong>Comparison rules are ultimately implemented by programmers: for example, in ascending order by age, or in descending order by age</strong></p>
<p><strong>How to write comparison rules</strong></p>
<p>First sort by age Ascending order, if the age is the same, then order by name in ascending order </p>
<pre class="brush:php;toolbar:false">public?class?TreeSetTest05?{
????public?static?void?main(String[]?args)?{
????????TreeSet<vip>?vips=new?TreeSet();
????????vips.add(new?Vip("zhangsi",20));
????????vips.add(new?Vip("zhangsan",20));
????????vips.add(new?Vip("king",18));
????????vips.add(new?Vip("soft",17));
????????for(Vip?vip:vips){
????????????System.out.println(vip);
????????}
????}}class?Vip?implements?Comparable<vip>{
????String?name;
????int?age;
????public?Vip(String?name,int?age){
????????this.name=name;
????????this.age=age;
????}
????@Override
????public?String?toString()?{
????????return?"Vip?[name="?+?name?+?",?age="?+?age?+?"]";
????}
????//compareTo方法的返回值很重要：
????//返回0表示相同，value會覆蓋
????//>0,會繼續(xù)在右子樹上找
????//<p><strong>Self-balancing binary tree structure</strong></p>
<p>1.<strong>Self-balancing binary tree, follow the principle of small left and large right </strong><br> 2. There are three ways to traverse a binary tree <br> Pre-order traversal: left and right roots <br> In-order traversal: left and right roots <br> Post-order traversal: left and right roots <br> Note: front and center What I will talk about later is the location of the root<br> 3.<strong>TreeSet collection and TreeMap collection use in-order traversal, that is, left root and right. They are self-balancing binary trees</strong><br> 100 200 50 60 80 120 140 130 135 180 666</p>
<p><strong>Implement the comparator interface</strong></p>
<p>The elements in the TreeSet collection can be sorted The second way is to use a comparator</p>
<pre class="brush:php;toolbar:false">public?class?TreeSetTest06?{
????public?static?void?main(String[]?args)?{
????????//創(chuàng)建TreeSet集合的時候，需要使用比較器
????????//TreeSet<wugui>?wuGuis=new?TreeSet();???//這樣不行，沒有通過構造方法傳遞一個比較器進去
????????TreeSet<wugui>?wuGuis=new?TreeSet(new?WuguiComparator());
????????wuGuis.add(new?Wugui(1000));
????????wuGuis.add(new?Wugui(800));
????????wuGuis.add(new?Wugui(900));
????????wuGuis.add(new?Wugui(300));
????????wuGuis.add(new?Wugui(60));
????????for(Wugui?wugui:wuGuis){
????????????System.out.println(wugui);
????????}

????}}class?Wugui{
????int?age;

????public?Wugui(int?age)?{
????????super();
????????this.age?=?age;
????}

????@Override
????public?String?toString()?{
????????return?"Wugui?[age="?+?age?+?"]";
????}}//單獨再這里編寫一個比較器//比較器實現(xiàn)java.util.Comparator接口（Comparable是java.lang包下的）class?WuguiComparator?implements?Comparator<wugui>{
????public?int?compare(Wugui?o1,Wugui?o2){
????????//指定比較規(guī)則
????????//按照年齡排序
????????return?o1.age-o2.age;
????}}</wugui></wugui></wugui>

We can use the anonymous inner class method
We can use the anonymous inner class method (this class has no name , direct new interface)

TreeSet<wugui>?wuGuis=new?TreeSet(new?Comparator<wugui>(){public?int?compare(Wugui?o1,Wugui?o2){
????????//指定比較規(guī)則
????????//按照年齡排序
????????return?o1.age-o2.age;
????????}});</wugui></wugui>

Final conclusion, if you want to sort the elements placed in the key part of the TreeSet or TreeMap collection, there are two ways
The first one: put The elements in the collection implement the java.lang.Comparable interface
The second method: pass a comparator object to it when constructing the TreeSet or TreeMap collection.
How to choose between Comparable and Comparator?
When the comparison rules will not change, or when there is only one comparison rule, it is recommended to implement the Comparable interface
If there are multiple comparison rules and multiple comparison rules are needed For frequent switching between comparison rules, it is recommended to use the comparator interface
The design of the comparator interface complies with OCP principles.

Collections tool class

java.util.Collections collection tool class, which facilitates the operation of collections

public?class?CollectionsTest?{
????static?class?Wugui2?implements?Comparable<wugui2>{
????????int?age;

????????public?Wugui2(int?age)?{
????????????super();
????????????this.age?=?age;
????????}

????????@Override
????????public?String?toString()?{
????????????return?"Wugui2?[age="?+?age?+?"]";
????????}

????????@Override
????????public?int?compareTo(Wugui2?o)?{
????????????//?TODO?Auto-generated?method?stub
????????????return?this.age-o.age;
????????}
????}
????public?static?void?main(String[]?args)?{
????????//ArrayList集合不是線程安全的
????????List<string>?list=new?ArrayList<string>();
????????//變成線程安全的
????????Collections.synchronizedList(list);
????????//排序
????????list.add("abc");
????????list.add("abe");
????????list.add("abd");
????????list.add("abf");
????????list.add("abn");
????????list.add("abm");
????????Collections.sort(list);
????????for(String?s:list){
????????????System.out.println(s);
????????}
????????List<wugui2>?wuguis=new?ArrayList();
????????wuguis.add(new?Wugui2(1000));
????????wuguis.add(new?Wugui2(8000));
????????wuguis.add(new?Wugui2(4000));
????????wuguis.add(new?Wugui2(6000));
????????//注意：對list集合中元素排序，需要保證list集合中元素實現(xiàn)了Comparable接口
????????Collections.sort(wuguis);
????????for(Wugui2?wugui:wuguis){
????????????System.out.println(wugui);
????????}
????????//對set集合怎么排序呢
????????Set<string>?set=new?HashSet();
????????set.add("king");
????????set.add("kingsoft");
????????set.add("king2");
????????set.add("king1");
????????//將set集合轉(zhuǎn)換成list集合
????????List<string>?myList=new?ArrayList(set);
????????Collections.sort(myList);
????????for(String?s:myList){
????????????System.out.println(s);
????????}
????????//這種方式也可以排序
????????//Collections.sort(list集合，比較器對象)
????}}</string></string></wugui2></string></string></wugui2>

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