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When programming with an object-oriented programming language like Java, it is important to understand what is going on when you assign the value of one variable to another With variables of standard primitive types such as int, it is easy Take, for example: int a, b; a = 1; b = a; It is clear that the value of b will be 1, just like a When the variable is an object reference, however, the situation is not so clear For example:
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Circle c1, c2; c1 = new Circle(05); c2 = c1; In this case, c1 is a reference to a Circle object with a radius of 05 The important thing to note is that c1 is a reference to a Circle, and not a Circle object Thus, the assignment c2 = c1 makes c2 a reference to the same Circle object that c1 refers to Thus, c2 is a reference copy of c1 While you are programming, it is easy to think of c1 as a particular Circle object, but as soon as you start assigning one object variable to another, you must remember that you are really using references to objects Consider the following code fragment: Circle c1, c2; c1 = new Circle(05); c2 = c1; c1setRadius(10); // Change the radius of c1 int r1 = c1getRadius(); int r2 = c2getRadius(); What are the values of r1 and r2 Both are 10 because both c1 and c2 are references to the same object If the goal of having the c2 variable was to keep a copy of the original value of the circle, then this code does not accomplish that task There are plenty of times you will want and need a copy of an object reference, but there are times when you want and need a copy of an entire object, not just its reference Java provides a mechanism for creating copies of objects called cloning However, just to confuse matters even more, there are two different ways to make a copy of an object called shallow copy and deep copy Depending on just how you are using a copy of an object, sometimes shallow copy will be all you need, and sometimes you will have to use deep copy
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reference copy A reference copy of an object is simply another reference variable that is a duplicate reference to the same object shallow copy A bit-wise copy of an object A new object is created that has an
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exact copy of the values in the original object If any of the fields of the object are references to other objects, just the references are copied
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deep copy A complete duplicate copy of an object If an object has references to
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A shallow copy makes a bit by bit copy of your object, including references to other objects Thus, if the object you are copying contains references to yet other objects, a shallow copy refers to the same subobjects Sometimes this is OK, especially if the values of the subobjects won't change A deep copy generates a copy not only of the primitive values of the original object, but copies of all subobjects as well, all the way to the bottom If you need a true, complete copy of the original object, then you will need to implement a full deep copy for the object Java supports shallow and deep copy with the Cloneable5 interface to create copies of objects To make a clone of a Java object, you declare that an object implements Cloneable, and then provide an override of the clone method of the standard Java Object base class When making a clone of an object, the assignment statement looks like: MyObject copy = (MyObject)originalclone(); (Note: clone always returns an Object, thus the need for the cast) Implementing Cloneable simply tells the Java compiler that your object is Cloneable The cloning is actually done by the clone method The default behavior of the clone method provided by the standard Java Object class is to make a shallow copy of the object To build a deep copy, you override clone with a version that calls the standard clone method to first create a shallow copy, and then explicitly create the copies of the subobjects used by the class If all the subobjects are Cloneable, then you can simply clone each subobject It is good programming practice to make all subobjects of a class Cloneable if you need to implement deep copy The examples presented in Listing 3-15 should help clarify the whole issue Whether you need a reference copy, a shallow copy, or a deep copy of an object will usually be clear from the context of your program Many Java programs never need to use anything other than a reference copy of an object But it is important to understand what it means to be using a reference to an object rather than either a shallow or deep copy (clone) of the object The following examples should make the difference between the three types of object copies clear We will use a small console based Java program to demonstrate this These are the two classes in this example program It requires two classes to demonstrate
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