95-712 object oriented programming java 1 some notes on cloning, packages and visibility notes from...

95-712 Object Oriented Programming Java

1

Some Notes on Cloning, Packages and Visibility

Notes from Bruce Eckel’s “Thinking in Java” and “Just Java” by Peter van der Linden


2

Review Passing Handles

• All arguments are passed as handles

• You never pass the object itself

• Very efficient if

– You’re only reading the object

– You want to modify the outside object

• Sometimes it’s necessary to create a “local”

– Changes only affect the local object, not the outside object

– Java doesn’t support this directly, but provides

“cloning”


3

Making Local Copies

In general, you call a method in order to produce a return value and/or a change of state in the object that the method is called for.

It is much less common to call a method in order to manipulate its arguments; this is referred to a “calling a method for its side effects”.

If you need to modify an argument during a method calland don’t intend to modify the outside argument, then youshould protect that argument by making a copy inside yourmethod.


4

Cloning Objects

• To produce pass- by- value effect, explicitly

clone the object that’s passed to your method.

class SomeClass {

void f( Sheep x) {

x = (Sheep) x. clone();

// Now we have a local sheep object

// Any changes to x are private changes

}

}


5

// A simple Int class

class Int {

private int i;

public Int( int i) { this.i = i; }

public void increment() { i++; }

public String toString() {

return Integer.toString( i);

}

}


6

public class Cloning {

public static void main( String args[]) {

Int v[] = new Int[10];

for( int i = 0; i < 10; i++ )

v[i] = new Int(i);

for(int i = 0; i < 10; i++)

System. out. print("v[" + i + "]=" + v[i]+ " ");

System.out.println();

Int v2[] = (Int[]) v. clone();


7

// Increment all v2's elements:

for(int i = 0; i < 10; i++)

v2[i].increment();

// See if it changed v's elements:

for(int i = 0; i < 10; i++)

System. out. print("v[" + i + "]=" + v[i]+ " ");

}

}


8

v[0]=0 v[1]=1 v[2]=2 v[3]=3 v[4]=4 v[5]=5 v[6]=6 v[7]=7 v[8]=8 v[9]=9

v[0]=1 v[1]=2 v[2]=3 v[3]=4 v[4]=5 v[5]=6 v[6]=7 v[7]=8 v[8]=9 v[9]=10

Only A Shallow Clone Is Available for Arrays.

• If you create a new class you must write code to make that class cloneable

• But why bother?


9

// Forgetting to clone may break encapsulation

class BankAccount {

private double balance;

public BankAccount() { balance = 0; }

public BankAccount(double initialBalance) { balance = initialBalance; }

public void deposit(double amount) { balance += amount; }

public void withdraw(double amount ) { balance = balance - amount; }

public double getBalance() { return balance; }

public String toString() { return Double.toString(balance); }


10

class Customer {

private BankAccount account;

private String name;

public Customer(String aName) {

name = aName;

account = new BankAccount(0);

}

public String getName() {

return name;

}


11

public BankAccount getAccount()

{

return account;

}

public void addToAccount(double amt) {

account.deposit(amt);

}


return name + account;

}


12

// Anyone can withdraw money!

public class CustomerTest {

public static void main(String args[]) {

Customer joe = new Customer("Joeseph Smith");

BankAccount sureItsPrivate = joe.getAccount();

joe.addToAccount(1000);

sureItsPrivate.withdraw(100);

System.out.println(joe);

}

}

Joeseph Smith900.0


13

Adding Clonability to a class

• Object. clone( ) is protected so you can’t normally access it

• Must override and make it public

• Must also implement the Cloneable interface

– A flag:

interface Cloneable {} // Empty!

– Allows you to check it:

if( myHandle instanceof Cloneable) // …

– Tested by Object. clone( )

• Virtually always call Object.clone( ) in your

new clone (via super. clone( ) )


14

// Allow others to clone objects of class BankAccount

class BankAccount implements Cloneable {

private double balance;

public BankAccount() { balance = 0; }

public BankAccount(double initialBalance) { balance = initialBalance; }

public void deposit(double amount) { balance += amount; }

public void withdraw(double amount ) { balance = balance - amount; }

public double getBalance() { return balance; }

public String toString() { return Double.toString(balance); }


15

public Object clone() {

try {

Object clonedAccount = super.clone();

return clonedAccount;

}

catch(CloneNotSupportedException e) {

// can't happen -- we implement cloneable

return null;

}

}

}


16

class Customer {




name = aName;


}


return name;

}


17

public BankAccount getAccount() {

return (BankAccount)account.clone();

}



}


return name + account;

}

}


18

// We can only review Joe's account.

// We can change it only through the proper interface.

public class CustomerTest {


Customer joe = new Customer("Joeseph Smith");

BankAccount sureItsPrivate = joe.getAccount();

joe.addToAccount(1000);

sureItsPrivate.withdraw(100);

System.out.println(joe);

}

}

Joeseph Smith1000.0


19

The Effect of Object. clone( )

• Copies the bits for the exact object (not just the root

class Object)

• Should always be the first part of your cloning process

(via super. clone( ) )

• Copies the handle values, but not what they’re

pointing to (a shallow copy)


20

• To perform a deep copy you must clone the contents

of the handles as well

• Once you make clone public, it remains public in derived

classes (the protected trick may only be used once).


21

Be Careful when cloning a class with references.

•We can provide a clone method for the Customer class.

•We have to remember to clone the account object.


22

class Customer implements Cloneable {




name = aName;


}


return name;

}


23

public BankAccount getAccount() {

return (BankAccount)account.clone();

}



}


return name + "\t" + account;

}


24


try {

Object clonedCustomer = super.clone();

((Customer)clonedCustomer).account =

(BankAccount)account.clone();

return clonedCustomer;

}


25

catch(CloneNotSupportedException e)

{

// can't happen we implement

// cloneable

return null;

}

}

}


26

public class CustomerTest2 {


Customer list[] = new Customer[3];

Customer list2[];

list[0] = new Customer("Mike");

list[1] = new Customer("Sue");

list[2] = new Customer("Bill");

list2 = (Customer[]) list.clone();


27

for(int j = 0; j < 3; j++) {

list2[j] = (Customer) list[j].clone();

list2[j].addToAccount(100);

}

for (int j = 0; j < 3; j++)

System.out.print(list[j] + "\t");

System.out.println();

for (int j = 0; j < 3; j++)

System.out.print(list2[j] + "\t");

}

}


28

Inserting Clonability in a derived class

class Person {}

class Hero extends Person {}

class Scientist extends Person implements Cloneable {


try {

return super. clone();

} catch (CloneNotSupportedException e) {


29

// This should never happen:

// It's Cloneable already!

throw new InternalError();

}

}

}

class MadScientist extends Scientist {}


30

public class HorrorFlick {

public static void main( String args[]) {

Person p = new Person();

Hero h = new Hero();

Scientist s = new Scientist();

MadScientist m = new MadScientist();

// p = (Person) p. clone(); // Compile error

// h = (Hero) h. clone(); // Compile error

s = (Scientist) s. clone();

m = (MadScientist) m. clone();

}


31

// Snake.java

public class Snake implements Cloneable {

private Snake next;

private char c;

// Value of i == number of segments

Snake(int i, char x) {

c = x;

if(--i > 0)

next = new Snake(i, (char)(x +1));

}


32

void increment() {

c++;

if(next != null)

next.increment();

}


String s = ":" + c;

if(next != null)

s += next.toString();

return s;

}


33


Object o = null;

try {

o = super.clone();

} catch (CloneNotSupportedException e) {}

return o;

}


34

public static void main(String[] args) {

Snake s = new Snake(5, 'a');

System.out.println("s = " + s);

Snake s2 = (Snake)s.clone();

System.out.println("s2 = " + s2);

s.increment();

System.out.println(

"after s.increment, s2 = " + s2);

}

}


35

C:\McCarthy\eckelbook\source\c12>java Snake

s = :a:b:c:d:e

s2 = :a:b:c:d:e

after s.increment, s2 = :a:c:d:e:f


36

Homework

Why doesn’t snake.java work properly?

What changes would you make to fix it?


37

Packages and Visibility


38

package A;

public class Foo { int x; void bar() { Example e = new Example(); x = e.i; x = e.j; x = e.k; x = e.l; }}

package A;

public class Example { public int i;

int j; //package

protected int k;

private int l;

}

Access Between Two Classes in the Same Package


39

package B;import A.*;public class Foo { int x; void bar() { Example e = new Example(); x = e.i; x = e.j; x = e.k; x = e.l; }}

package A;


int j; //package

protected int k;

private int l;

}

Access Between Two Unrelated Classes in Different Packages


40

package B;import A.Example;public class Fooextends A.Example { int x; void bar() { Example e = new Example(); x = e.i; x = e.j; x = e.k; x = e.l; x = this.k; //OK! }}

package A;


int j; //package

protected int k;

private int l;

}

Access Between A Parent and Child Class in Different Packages

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