Java Basic Knowledge Set

Characteristics

Duplicate elements are not allowed in set, according to what is the principle? The answer is: this is distinguished by the Equals () method. Then, if you want the custom class object instance to not recur in set, you need to override the Equals method:
Here we assume that the custom class is person:

 Packagepublic; Public  class person {    Private intAgePrivateString name; Public  Person() {Super(); } Public  Person(intAge, String name) {Super(); This. Age = Age; This. name = name; }//The same object returns True, and vice versa returns false    @Override     Public Boolean equals(Object obj) {//If the address is the same, it must be the same object        if( This= = obj) {return true; }//To compare objects that are not custom class objects (here is person), certainly not the same object        //Think about why not write as this.equals (obj)???         if(! (objinstanceofperson)) {return false; }//Use polymorphism for transformationPerson P = (person) obj;//Compare the values of each member variable for equality        if(P.age = = This. Age && P.name.equals ( This. Name)) {return true; }Else{return false; }    }@Override     PublicStringtoString() {return "person [age="+ Age +", name="+ name +"]"; }}

There's no need to be busy testing, set is an interface, it is this: public interface Set extends Collection, for which to instantiate the class to use to implement it, usually with treeset (with sort function), HashSet (stored according to the hash value).

Instantiate using TreeSet:

It says TreeSet has a sort function, which is based on what is implemented? The answer is: the CompareTo () method, which means that our custom classes implement the comparable interface, overwrite the CompareTo () method:
Let's build the person class:

Package public;//Focus here Public classPerson implements comparable<person>{Private intAgePrivateString name; Public  Person() {super (); } Public  Person(intAge, String name) {super (); This. Age = Age; This. name = name; }//Focus here@Override Public int CompareTo(Person O) {if( This. Age > O.age) {return 1; }Else if( This. Age < O.age) {return-1; }Else{return  This. Name.compareto (O.name); }} @Override PublicBooleanequals(Object obj) {if( This= = obj) {return true; }if(! (obj instanceof person)) {return false; } person P = (person) obj;if(P.age = = This. Age && P.name.equals ( This. Name)) {return true; }Else{return false; }} @Override PublicStringtoString() {return "person [age="+ Age +", name="+ name +"]"; }}

Under test:

 PackageSet;ImportJava.util.Set;ImportJava.util.TreeSet;ImportPublic.person; Public  class treesetdemo {     Public Static void Main(string[] args) {set<person> s =NewTreeset<person> (); S.add (NewPerson (3,"x"));//Add elementS.add (NewPerson (1,"x"));//Add elementS.add (NewPerson (1,"x"));//Add repeating elementsS.add (NewPerson (1,"Y"));//Add elementS.add (NewPerson (2,"x"));//Add elementSystem.out.println (s); }}/* Results: [person [Age=1, name=x], person [age=1, name=y], person [age=2, name=x] , person [age=3, name=x]]*/]

The result shows that the added elements are not duplicated and the output is orderly.

Instantiating using HashSet

HashSet does not have a sort, so can not implement the comparable interface, but to overwrite the hashcode () method, after all, people have their own things to mark it?
Simple modification of the person class:

 Packagepublic; Public  class person {    Private intAgePrivateString name; Public  Person() {Super(); } Public  Person(intAge, String name) {Super(); This. Age = Age; This. name = name; }@Override     Public int hashcode() {return  This. Name.hashcode () * This. Age; }@Override     Public Boolean equals(Object obj) {if( This= = obj) {return true; }if(! (objinstanceofperson)) {return false; } person P = (person) obj;if(P.age = = This. Age && P.name.equals ( This. Name)) {return true; }Else{return false; }    }@Override     PublicStringtoString() {return "person [age="+ Age +", name="+ name +"]"; }}

Test it:

 PackageSet;ImportJava.util.HashSet;ImportJava.util.Set;ImportPublic.person; Public  class hashsetdemo {     Public Static void Main(string[] args) {set<person> s =NewHashset<person> (); S.add (NewPerson (1,"x")); S.add (NewPerson (1,"x")); S.add (NewPerson (1,"Z"));    System.out.println (s); }}/* Results: [person [Age=1, name=z], person [age=1, name=x]]*/

All right, let's talk about it!

Java Basic Knowledge Set