The problem of storing elements in TreeSet collection

TreeSet Problems with collection storage elements

Use TreeSet Set to test

/** * @Author Handsome * @Date 2022/6/20 17:24 * @Version 1.0. * * TreeSet Problems with collection storage elements  */
class Person{
    
    String name;
    public Person(String name) {
    
        this.name = name;
    }
}
public class Test {
    
    public static void main(String[] args) {
    
        TreeSet<String> set1 = new TreeSet<>();
        set1.add(new String("hjj"));
        set1.add(new String("hjj"));
        System.out.println("set1 The size is ："+set1.size());
        
        TreeSet<Person> set2 = new TreeSet<>();
        set2.add(new Person("hjj"));
        set2.add(new Person("hjj"));
        System.out.println("set2 The size is ："+set1.size());
    }
}

Running results

set1 The size is ：1
Exception in thread "main" java.lang.ClassCastException: class com.question.Person cannot be cast to class java.lang.Comparable (com.question.Person is in unnamed module of loader 'app'; java.lang.Comparable is in module java.base of loader 'bootstrap')
	at java.base/java.util.TreeMap.compare(TreeMap.java:1291)
	at java.base/java.util.TreeMap.put(TreeMap.java:536)
	at java.base/java.util.TreeSet.add(TreeSet.java:255)
	at com.question.Test.main(Test.java:24)

set1 The size is 1 Easier to understand
because TreeSet It's disordered and not repetitive , Cannot store the same element , namely set1 The size is 1

What then? set2 Will report a mistake ？
because compareTo yes TreeSet The underlying comparison rules , From Compareable Interface .
Not every object has compareTo Method , Need to achieve Compareable It's the interface compareTo Method . Want to save objects into Tree Collection , This object needs to be implemented Compareable Interface , rewrite compareTo The way to do it . Otherwise, put the object directly into the interface ,Tree The collection will call the object compareTo Method , It turns out that there's no way , Will appear

java.lang.ClassCastException

Rethinking : If you will TreeSet Replace with HashSet, What is the result ？

Use HashSet Set to test

/** * @Author Handsome * @Date 2022/6/20 17:24 * @Version 1.0 * * HashSet Problems with collection storage elements  */
class Person{
    
    String name;
    public Person(String name) {
    
        this.name = name;
    }
}
public class Test {
    
    public static void main(String[] args) {
    
        HashSet<String> set1 = new HashSet<>();
        set1.add(new String("hjj"));
        set1.add(new String("hjj"));
        System.out.println("set1 The size is ："+set1.size());

        HashSet<Person> set2 = new HashSet<>();
        set2.add(new Person("hjj"));
        set2.add(new Person("hjj"));
        System.out.println("set2 The size is ："+set2.size());
    }
}

Running results

set1 The size is ：1
set2 The size is ：2

set1 Cannot store the same element , therefore set1 The size is 1

set2 Why not report an error and the result is 2？
because HashSet Based on the hashCode() and equals() Method to determine whether the object is the same element ,String Rewrote hashCode() and equals() Method , That is to compare whether the strings are the same ,Pserson Not rewritten hashCode() and equals() Method , That is, the address is compared , Address different , So it's not the same object .

summary

HashSet Based on the hashCode() and equals() Method de duplication 
TreeSet According to the object compareTo() Method de duplication 

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The article links :http://huangjunjie.vip:66/blog/read/oviupvehzj312b0n8n