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Copying an object

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• What does it mean to copy an object
  • As we saw in the previous webpage, an object has

    state information (stored in its instance variables) operations that can change the state information (i.e., that can update the instance variables)

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  • The operations are computer instructions and these instructions are unchanged (always the same to do a specific task).

    Therefore:

    We can use the one (= the same) copy of the computer instructions to perform the specified task on any object ! Therefore, we do not need to make multiple copies of the operations (that are defined on the object)

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  • The state information stored in different objects can be different !!!

    Example:

    The balance of my Bank Account is certainly not the same as the balance of your Bank Account

    Therefore:

    When we want to make a copy of an object, we must make a copy of the instance variables of the object !!!

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• Reminder: What is a copy         (See: click here)
  • Definition: copy of a variable

    Copy of a variable = an independent variable that contains the exact value of the original variable Two variables are independent of each other if their values can change independently of each other

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• Observation: objects and arrays in Java have similar access patterns
  • We just saw that:

    the process of accessing the instance variables in an object is identical to the process of accessing the elements in an array You can review the notes on creating/accessing an object: click here You can review the notes on creating/accessing an array: click here

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  • Both an object and an array are accessed through a reference variable:

    Accessing the (instance) variables in an object through the reference variable stu1: Accessing the array elements in an array through the reference variable a:

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• Reminder: how not to copy an array         (See: click here)
  • The following statement does not make a true copy of an array:

    b = a;

    All it does is: make the reference variable b point to the same array:

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• Copying an object: how not to copy an object
  • Consider the following Java program that tries to copy the object a to the object b:

    public class CopyObject1 { public static void main(String[] args) { BankAccount a = new BankAccount( ); // Object a a.accNum = 123; a.name = "John"; a.balance = 1000; BankAccount b; // The copy b = a; // is b a copy of a ?? /* ==================================== Let's do an experiment.... ==================================== */ /* ---------------------------------- Print the values before the update ---------------------------------- */ System.out.println("Object a: " + a.convToString()); System.out.println("Object b: " + b.convToString()); b.accNum = 789; // Update the COPY b.name = "Jess"; // (must NOT change the original) b.balance = 5000; // /* ---------------------------------- Print the values after the update ---------------------------------- */ System.out.println("Values after updating the copy b.balance:"); System.out.println("Object a: " + a.convToString()); System.out.println("Object b: " + b.convToString()); } }

    Output of this program:

    Object a: Account number: 123, Name: John, Balance: 1000.0 Object b: Account number: 123, Name: John, Balance: 1000.0 Values after updating the copy b.balance: Object a: Account number: 789, Name: Jess, Balance: 5000.0 Object b: Account number: 789, Name: Jess, Balance: 5000.0

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  • Example Program: (Demo above code)                                                
    • The BankAccount.java class file: click here
    • The CopyObject1.java Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac CopyObject1.java To run:          java CopyObject1

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    Observation (conclusion from this experiment):

    The values of the instance variables in the original object a are changed !!! In fact: the object a is the same as the object b Conclusions: The object identified by the name a is the same set of instance variables that is identified by the name b The statement b = a; did not make a (true) copy of an object

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  • We have seen this phenomenon in copying an array before:

    The statement b = a; does not create a copy Instead, the statement b = a; creates an alias

    The solution is the same as the case of copying an attay:

    Create a new object first Then copy the value from the old object into the newly created object

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• Copying an object: how to do it for real...

  • How to copy an object in Java:

    public class CopyObject2 { public static void main(String[] args) { BankAccount a = new BankAccount( ); // Object a a.accNum = 123; a.name = "John"; a.balance = 1000; BankAccount b; // The copy /* ================================== How to copy an object in Java ================================== */ b = new BankAccount( ); // Create a new object b.accNum = a.accNum; // Copy old values over b.name = a.name; // to the new object b.balance = a.balance; /* ==================================== Let's do the experiment again.... ==================================== */ /* ---------------------------------- Print the values before the update ---------------------------------- */ System.out.println("Object a: " + a.convToString()); System.out.println("Object b: " + b.convToString()); b.accNum = 789; // Update the COPY b.name = "Jess"; // (must NOT change the original) b.balance = 5000; // /* ---------------------------------- Print the values after the update ---------------------------------- */ System.out.println("Values after updating the copy b.balance:"); System.out.println("Object a: " + a.convToString()); System.out.println("Object b: " + b.convToString()); } }

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  • Example Program: (Demo above code)                                                
    • The BankAccount.java class file: click here
    • The CopyObject2.java Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac CopyObject2.java To run:          java CopyObject2