There are three ways to copy the reference of one object to another object. The first method is direct assignment, the second method is shallow copy, and the third method is deep copy.

1. Direct assignment

In Java, A a1 = a2, this actually copies the reference, that is, a1 and a2 points to the same object. Therefore, when a1 changes, the member variables in a2 will also change.

2. Shallow copy (copy the reference but not the referenced object)

Shallow copy, also known as shallow copy. Create a new object, and then copy the non-static fields of the current object to the new object. If the field is of value type, then copy the field; if the field is of reference type, copy the reference but not the referenced object. Therefore, the original object and its copy refer to the same object.

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class Resume implements Cloneable{ 
     public Object clone() { 
         try { 
             return (Resume)super.clone(); 
         } catch (Exception e) { 
             e.printStackTrace(); 
         return null; 
         } 
     } 
}

3. Deep copy (copy the object and its reference object)

Also known as deep copy, not only copies the object itself, but also copies all objects pointed to by the references contained in the object.

class Student implements Cloneable {
         String name;
         int age;
         Professor p;
     Student(String name, int age, Professor p) {
         this.name = name;
         this.age = age;
         this.p = p;
     }
     public Object clone() {
         Student o = null;
         try {
             o = (Student) super.clone();
         } catch (CloneNotSupportedException e) {
             System.out.println(e.toString());
         }
     o.p = (Professor) p.clone();
     return o;
     }
}

Steps to clone an object using the clone() method:

1) The cloned class must implement the Cloneable interface.

2) The cloned class must override the clone() method.

So how to choose which copy method to use when programming? First, the class is checked for data members that are non-basic types (i.e., objects). If not, just return super.clone(). If so, ensure that all non-basic type member variables contained in the class implement deep copying.

Extension: What is the difference between shallow copy and deep copy?

Shallow Clone: ??All variables of the copied object contain the same values ??as the original object, and references to all other objects still point to the original object. In other words, a shallow copy only copies the object in question, not the objects it refers to.

Deep Clone: ??All variables of the copied object contain the same values ??as the original object, except those variables that refer to other objects. Variables that reference other objects will point to the new object being copied, not the original referenced objects. In other words, deep copy copies all the objects referenced by the copied object.

Extension:

The prototype mode is mainly used for copying objects, implements the interface (implements the Cloneable interface), and overrides a method (overrides clone in the Object class () method), that is, the prototype mode is completed.

Copy in prototype mode is divided into "shallow copy" and "deep copy":

Shallow copy: copy the value of the member variable of the value type, Member variables of reference types only copy the reference, not the referenced object.

Copy: Copy the value of the member variable of the value type, and copy the reference object of the member variable of the reference type.

(The clone method of the Object class will only copy the basic data type value in the object. Arrays, container objects, reference objects, etc. will not be copied. This is a shallow copy. If you want to implement a deep copy, you must Copy arrays, container objects, reference objects, etc. in prototype mode separately.)

Advantages of prototype mode:

1. If creating a new object is more complicated , you can use the prototype pattern to simplify the object creation process.

2. It is always the mode to create an object, which is much better in performance than directly new an object, because the clone method of the Object class is a local method, which directly operates the binary stream in the memory, especially copying large When using objects, the performance difference is very obvious.

Usage scenarios of prototype pattern:

Because of the above advantages, you can consider using prototype pattern when you need to repeatedly create similar objects. For example, if you need to create an object within a loop, if the object creation process is complex or there are many loops, using the prototype pattern can not only simplify the creation process, but also greatly improve the overall performance of the system.

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