In Java, what is a shallow copy?

Question

java.util.Calendar.clone() returns "...a new Calendar with the same properties" and returns "a shallow copy of this Calendar".

This does not appear to be a shallow copy as answered here on SO. That question is tagged language-agnostic, Java does not seem to follow the language agnostic definition. As I step through the code I notice that the structure and the elements are copied to this new object, more than the language agnostic structure only.

In Java, what is a shallow copy?

How does it differ from a Java deep copy (if that exists)?

Answer 1

A shallow copy just copies the values of the references in the class. A deep copy copies the values. given:

class Foo {   private Bar myBar;   ...   public Foo shallowCopy() {     Foo newFoo = new Foo();     newFoo.myBar = myBar;     return newFoo;   }    public Foo deepCopy() {     Foo newFoo = new Foo();     newFoo.myBar = myBar.clone(); //or new Bar(myBar) or myBar.deepCopy or ...     return newFoo;   } }  Foo myFoo = new Foo();   Foo sFoo = myFoo.shallowCopy();   Foo dFoo = myFoo.deepCopy();    myFoo.myBar == sFoo.myBar => true   myFoo.myBar.equals(sFoo.myBar) => true   myFoo.myBar == dFoo.myBar => **false**   myFoo.myBar.equals(dFoo.myBar) => true   

In this case the shallow copy has the same reference (==) and the deep copy only has an equivalent reference (.equals()).

If a change is made to the value of a shallowly copied reference, then the copy reflects that change because it shares the same reference. If a change is made to the value of a deeply copied reference, then the copy does not reflect that change because it does not share the same reference.

C-ism

int a = 10; //init int& b = a; //shallow - copies REFERENCE int c = a;  //deep - copies VALUE ++a; 

Result:

a is 11   *b is 11   c is 10