How does the object class work in this code?

Question

Java

How does the object class work? I don't understand why he's here.

public class Main {
public static void main(String[] args) { //throws - exception
Date x,y;
x=new Date();
Object clone = x.clone();
System.out.println(clone);
}
}

Answer 1

Object clone = x.clone();
This is a clone (copy) of an object of the Date class.
Then it needs to be cast to the Date type
. Why does clone() return an Object ? - because this method is inherited from the Object class, and since all types in Java inherit the Object class, therefore clone() must return a common type for all types - Object
First you need to figure out why you still need to override the clone () method, and not use the inherited native implementation of this method. The point is that the basic clone method only performs a shallow copy. Only the values ​​of the class fields will be copied. For primitive types (int, byte, char), everything will be fine, but if they are arrays, collections, objects (for example Date), or your types, then only references to them will be copied. As a result of cloning, we will have two objects whose fields refer to the same fields, and through these fields we can influence all objects that were cloned:

class TestClone implements Cloneable {
    		
    		private Date date = new Date();

      public Date getDate() {
        return date;
      }

      public void setDate(Date date) {
        this.date = date;
      }

      @Override
      protected Object clone() throws CloneNotSupportedException {
        return super.clone();
      }
    	}
    	
    	TestClone a = new TestClone();
    	TestClone b = (TestClone) a.clone();
    	System.out.println(a.getDate());
    	System.out.println(b.getDate());
    	b.getDate().setYear(0);
    	System.out.println(a.getDate());
    	System.out.println(b.getDate());



// Fri Jul 10 23:13:47 EEST 2015
// Fri Jul 10 23:13:47 EEST 2015
// изменили один, а поменялось два
// Tue Jul 10 23:13:47 EET 1900
// Tue Jul 10 23:13:47 EET 1900

This will not work if there are immutable classes, the same String. It has no methods for changing the state.
That's why you need to override the clone method and perform a deep copy of all mutable objects referenced by fields (if any).

class TestClone implements Cloneable {
    		
    		private Date date = new Date();
    		private String s = "string"; // этот не нужно клонировать
    		
      public Date getDate() {
        return date;
      }

      public void setDate(Date date) {
        this.date = date;
      }

      @Override
      protected Object clone() throws CloneNotSupportedException {
        TestClone clone = (TestClone) super.clone();
        if (this.date != null) {
          clone.date = (Date) this.date.clone();
        }
        return clone;
      }
    	}


Fri Jul 10 23:25:16 EEST 2015
Fri Jul 10 23:25:16 EEST 2015
// теперь это действительно клоны
Fri Jul 10 23:25:16 EEST 2015
Tue Jul 10 23:25:16 EET 1900

Why can't the JVM itself perform a deep clone? - Because the JVM is not aware of how deep the cloning should be :) And how to copy the fields correctly, with all the nuances that the programmer intended :)
IMHO: the Cloneable marker interface is required so that you can understand whether the clone method is overridden in the class or not .

TestClone с = new TestClone();
    	
    	if (с instanceof Cloneable) {
    		System.out.println("ок, меня можно клонировать");
    	}

There is a ready solution from google for deep cloning.
Great article: Deep Cloning in Java Without Bicycles