Rewrite of the "Java" Clone Clone () method and the equal Equals () method

1. Why rewrite the Clone () method?

The shallow copy in Java does not re-create a new reference space for the object that you want to copy, and we'll rewrite the Clone () method when we need a deep copy.

2. Example of the Equals () and Clone () method overloads

Hourse class:

Importjava.util.Date; Public classHouseImplementscloneable {Private intID; Private DoubleArea ; PrivateDate Whenbulit; House () {} house (intNewId,DoubleNewarea) {ID=newId; Area=Newarea; Whenbulit=NewDate (); }         Public intgetId () {returnID; }         Public voidSetId (intID) { This. ID =ID; }         Public voidSetarea (DoubleArea ) {             This. Area =Area ; }          Public voidSetwhenbulit (Longwhenbulit) {            This. Whenbulit.settime (Whenbulit); }             Public DoubleGetarea () {returnArea ; }         PublicDate Getwhenbulit () {returnWhenbulit; }         Public LongGetwhenbulittime () {returnWhenbulit.gettime (); }       //override the Equals method because we are comparing the time in the date class to the reference address of the object         Public Booleanequals (House obj) {//Obj.getarea () ==this.getarea ( )//Obj.getid () ==this.getid (); //obj.getwhenbuildtime==this.getwhenbuildtime ()            if(Obj.getarea () = = This. Getarea () &&obj.getid () = = This. GetId () &&obj.getwhenbulittime () = = This. Getwhenbulittime ())return true; Else            {                return false; }        }        //if the Equals method is overloaded, hashcode () should also be overloaded by specification, which simply associates hashcode with the passed-in ID.@Override Public inthashcode () {returngetId (); }         PublicObject Clone ()throwsclonenotsupportedexception{//constructs an object house, and uses the ID and areaHouse House =NewHouse ( This. GetId (), This. Getarea ()); //call the Setwhenbuild method in the object, and use GetTime () to get a number of milliseconds. House.setwhenbulit ( This. Getwhenbulit (). GetTime ()); returnHouse ; }    }

Testor class:

 public  class   Testor { static  void  main (string[] args) throws   clonenotsupportedexception{house h1  = new  House (1,200.0 = ==H2);                        System.out.println (H1.equals (H2));                        System.out.println (H1.getwhenbulit ()  == H2.getwhenbulit ());        System.out.println (H1.getwhenbulit (). Equals (H2.getwhenbulit ())); }}

3, the API in the Clone () method introduced the doubts

In the API1.6 saw such a sentence:

the case:     true, but this is not a requirement that must be met.

It is important to note that if we call the code test like this:

 Public classTestcloneImplementscloneable { Public Static voidMain (String []args)throwsclonenotsupportedexception{Testclone TC=NewTestclone (); Try{Object obj=Tc.clone (); System.out.println (obj==TC);//The result is falseSystem.out.println (Obj.equals (TC));//The result is false}Catch(Exception e) {System.out.println ("Exception:" +e.getmessage ()); }    }}

Then the result is false. Because the default condition for equals is the value of the reference address in the comparison variable, the API means that if we need to use the Clone () method, the Equals () method should be overridden in general when overloaded, without the need to compare the object's references, You only need to write the rewritten code as the value of the comparison object.

Rewrite of the "Java" Clone Clone () method and the equal Equals () method