Interview question: do you know the difference between deep copy and shallow copy? What is a reference copy?

• Shallow copy ： Shallow copy creates a new object on the heap , however , If the attribute inside the original object is a reference type , Shallow copy will directly copy the reference address of the internal object , That is, the copy object and the original object share the same internal object .
• Deep copy ： Deep copy completely copies the entire object , Include the internal objects contained in this object

Icon ：

Shallow copy

The sample code for a shallow copy is as follows , We have achieved Cloneable Interface , Rewrite the clone() Method .

clone() The implementation of this method is very simple , What is called directly is the parent class Object Of clone() Method .

public class Address implements Cloneable{
    
    private String name;
    //  Omit constructor 、Getter&Setter Method 
    @Override
    public Address clone() {
    
        try {
    
            return (Address) super.clone();
        } catch (CloneNotSupportedException e) {
    
            throw new AssertionError();
        }
    }
}

public class Person implements Cloneable {
    
    private Address address;
    private String name;
    //  Omit constructor 、Getter&Setter Method 
    @Override
    public Person clone() {
    
        try {
    
            Person person = (Person) super.clone();
            return person;
        } catch (CloneNotSupportedException e) {
    
            throw new AssertionError();
        }
    }
}

test ：

Person person1 = new Person(new Address(" Guangzhou "));
Person person1Copy = person1.clone();
// true
System.out.println(person1.getAddress() == person1Copy.getAddress());

As can be seen from the output structure , person1 Clone objects and person1 The same... Is still used Address object .

String The type is very special , First ,String Type is a reference data type , Not a basic data type , however String Data of type is stored in the constant pool , That is, it cannot be modified ！ in other words , When person1 object person1.setName(" Xiaohong "); Revised name Type value , It's not changing the value of this data , It's a reference to this data from to Xiao Ming This constant points to Xiaohong This constant . under these circumstances , Another object person1Copy Of name Property values still point to Xiao Ming Not affected .

Person person1 = new Person(" Xiao Ming ");
Person person1Copy = person1.clone();
// Xiao Ming 
System.out.println(person1.getName());
person1.setName(" Xiaohong ");
// Xiao Ming 
System.out.println(person1Copy.getName());

Deep copy

Here we are simply right Person Class clone() Method to modify , With the intention of Person Inside the object Address Objects are copied together .

@Override
public Person clone() {
    
    try {
    
        Person person = (Person) super.clone();
        person.setAddress(person.getAddress().clone());
        return person;
    } catch (CloneNotSupportedException e) {
    
        throw new AssertionError();
    }
}

test ：

Person person1 = new Person(new Address(" Guangzhou "));
Person person1Copy = person1.clone();
// false
System.out.println(person1.getAddress() == person1Copy.getAddress());

As can be seen from the output structure , although person1 Clone objects and person1 Contains Address The object is already different .

So what is a reference copy ？ Simply speaking , Reference copy means that two different references point to the same object .

Object a = {
    ...}
Object b = a

Equal to multiple object names pointing to the same object