Class and Method Modifiers

Roderick Rodriguez, Dianne Musciano, Sukumar Simhadri,

George Blank

Modifiers

• Modifiers are Java keywords that extend or place limits on defined classes, fields, methods, and sub-classes.

• Imagine that we are creating a game and want to simulate a pair of dice. We might define a die class, shown graphically on the next slide.

Class Example: Die

• A class has a name and can contain data declarations and/or method declarations

Data declarations

Method declarations

Class NameDie- faceValue: integer

+ roll() : integer

Definitions for Die

• class – a specification of all the data, and behavior for a single die.

• object – a die class that has been instantiated in a running program

• field – a data attribute or variable like faceValue used by the class

• method – a specification of behavior like roll() used by the class

Object & Class

• Object– Abstracts away (data, algorithm) details– Encapsulates data – Instances exist at run time

• Class– Blueprint for objects (of same type)– Exists at compile time

How die works

• The values of the data define the state of an object created from the class

• The functionality of the methods define the behaviors of the object

• For our Die class, we might declare an integer that represents the current value showing on the face

• One of the methods would allow us to “roll” the die by setting its face value to a random number between one and six

The extends clause specifies the super class of this class. The implements clause specifies the interfaces being implemented by this class. Member declarations can be declarations of fields, methods, and nested classes. The field, method, and nested class declarations can be intermixed. The order of declarations is immaterial to the compiler. Thus, they should be ordered in a way that is most logical and comprehensible.

Super and Sub Classes

Modifiers• Modifiers in Java can define accessibility

and can be applied to both classes and class members (sub-classes, fields and methods).

• Some of the modifier keywords are shared by both classes and methods.

• When using access modifiers only one can be used at a time.

Scope, Visibility, Accessibility

• Scope refers to the boundaries of programs and components of programs.

• Visibility refers to portions of the scope that can be known to a component of a program.

• Accessibility refers to portions of the scope that a component can interact with.

Scope• Scope

– Part of program where a variable may be referenced

– Determined by location of variable declaration• Boundary usually demarcated by { }

• Example public MyMethod1() {

int myVar; myVar accessible in ... method between { } }

Scope – Example

package edu.umd.cs ;public class MyClass1 { public void MyMethod1() { ... } public void MyMethod2() { ... }}public class MyClass2 {}

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Scope

Accessibility/Visibility Modifiers

• Modifiers can control visibility/accessibility. • These modifiers are used to implement the

object-oriented principles of abstraction and encapsulation.

• Visibility and Accessibility are normally used interchangeably, since visible components are normally accessible and accessible components are normally visible.

Other Modifiers

• In addition to scope, modifiers can refer to related concepts, such as limiting access to a thread in a multithreaded environment (synchronized) or indicating that a method or field belongs to the whole class instead of a single instance (static).

Modifier Examples

• public class MyApplet extends java.applet.Applet{…}

• private boolean killJabberwork;• static final double weeks = 9.5;• protected static final int MEANINGOFLIFE = 42

• public static void main(String arguments[]{…}

Class Modifiers

• Class modifiers include the following:– <none>  - When no modifier is present, by

default the class is accessible by all the classes within the same package.

– public - A public class is accessible by any class.     

– abstract - An abstract class contains abstract methods.

– final - A final class may not be extended, that is, have subclasses.

Class Modifiers

• Syntax for class modifiers:[ClassModifier] class ClassName {…}

• Example:public class Employee {…}

Class Modifiers

• Important points for class modifiers:– A single Java file can only contain one class that

is declared public.– An abstract class cannot be instantiated.– A static nested class does not have an implicit

reference to the enclosing class.

Method Modifiers

• Method Modifiers include the following:– <none> - When no modifier is present, by

default the method is accessible by all the classes within the same package.

– public - A public method is accessible by any class.

– protected -A protected method is accessible by the class itself, all its subclasses.

– private - A private method is accessible only by the class itself.

Method Modifiers– static - accesses only static fields.  – final - may not be overridden in subclasses. – abstract - defers implementation to its subclasses.– synchronized - atomic in a multithreaded environment– native - compiled to machine code by Assembler, C or

C++.

Method Modifiers

• Syntax for method modifiers:[MethodModifers] ReturnType MethodName

([ParameterList) {}

• Example:private void GetEmployee(int ID) { }

Method Modifiers

• Important points for method modifiers:– Abstract methods must be contained in an

abstract class.– Static methods cannot be overridden in a child

class but they can be hidden. – Methods may not be overridden to be more

private. For example, a protected method can not be overridden by a default one. A default method can be overridden by a protected or public method.

Field (variable) Only Modifiers

• volatile: may be modified by nonsynchronized methods in a multithread environment

• transient: not part of the persistent state of instantiated objects

Visibility Modifier

• Properties– Controls access to class members– Applied to instance variables & methods

• Four types of access in Java– Public Most

visible– Protected– Package

• Default if no modifier specified

– Private Least visible

Visibility Modifier – Where Visible

• “public”– Referenced anywhere (i.e., outside package)

• “protected”– Referenced within package, or by subclasses outside

package• None specified (package)

– Referenced only within package• “private”

– Referenced only within class definition– Applicable to class fields & methods

Member Visibility Modifiers

• Members of a class that are declared with public visibility can be referenced anywhere

• Members of a class that are declared with private visibility can be referenced only within that class

• Members declared without a visibility modifier have default visibility and can be referenced by any class in the same package

Field Visibility Modifiers

• For instance variables– Should usually be private to enforce

encapsulation– Sometimes may be protected for subclass

access

Field Visibility Modifiers• Public variables violate the spirit of encapsulation

because they allow the client to “reach in” and modify the object’s internal values directly

• Therefore, instance variables should not be declared with public visibility

• It is acceptable to give a constant public visibility, which allows it to be used outside of the class

• Public constants do not violate encapsulation because, although the client can access it, its value cannot be changed

Method Visibility Modifiers

• For methods– Public methods – provide services to clients– Private methods – provide support other

methods– Protected methods – provide support for

subclass

Method Visibility Modifiers

• Methods that provide the object's services are declared with public visibility so that they can be invoked by clients

• Public methods are also called service methods

• A method created simply to assist a service method is called a support method

• Since a support method is not intended to be called by a client, it should be declared with private - not with public visibility

Modifier - Private

Private methods and variables are particularly useful in two circumstances:– When other classes have no reason to use that

variable or method– When another class could wreak havoc by

changing a variable in an inappropriate way

Visibility Modifiers - Summary

public private

Variables

Methods Provide servicesto clients

Support othermethods in the

class

Enforceencapsulation

Violateencapsulation

Static Methods

• Methods declared with static modifier• Can be defined in any class• Not associated with an object• Is not invoked by sending it as a message to

an object• The class method foo in the class C is invoked

using the notation foo.C( … )• It has no receiver and cannot refer to instance

variables• It can refer to and manipulate class variables

Modifier – Static

• Static variable– Single copy for class– Shared among all objects of class

• Static method– Can be invoked through class name– Does not need to be invoked through object– Can be used even if no objects of class exist– Can not reference instance variables

Modifier – Final

• Final variable– Value can not be changed– Must be initialized in every constructor– Attempts to modify final are caught at compile

time

• Final static variable– Used for constants– Example

final static int Increment = 5;

Modifier – Final

• Final method– Method can not be overloaded by subclass– Private methods are implicitly final

• Final class– Class can not be a superclass (extended)– Methods in final class are implicitly final

Modifier – Final• Final classes

– Prevent inheritance / polymorphism– May be useful for

• Security• Object oriented design

• Example – class String is final– Programs can depend on properties specified in Java

library API– Prevents subclass from bypassing security restrictions

Modifier – Abstract

• Description– Represents generic concept– Can not be instantiated

• Abstract class– Placeholder in class hierarchy– Can be partial description of class– Can contain non-abstract methods– Required if any method in class is abstract

All possible combinations of Features and Modifiers

Modifier Class Variable Method Constructor Free-Floating Block

Public y y y y n

Protected n y y y n

Default y y y y y

Private n y y y n

Final y y y n n

Abstract y n y n n

Static n y y n y

All possible combinations of Features and Modifiers

Modifier Class Variable Method Constructor Free-Floating Block

Native n n y n n

Transient n y n n n

Volatile n y n n n

Synchro- n n y n y

nized

A variation of Murphy’s Law

• One of the statements of Murphy’s law, as applied to programming, is:

All variables are constant, andAll constants are variable

• The final key word is concerned with immutability. This final section of the lecture is based on a Wikipedia discussion of immutability.

Introduction to Immutability

• Immutable means not changeable– All Java variables are by default mutable. You can make them

immutable by using the final keyword.– The wrapper classes, Byte, Character, Short, Integer, Long, Float

and Double are all immutable.– Strings are immutable( StringBuffers are mutable ). The only way

to change the value of the number inside the object wrapper is to create a new object and point to that instead.

Properties of Immutable Objects

• Immutable objects have a number of properties that make working with them easier, including relaxed synchronization requirements and the freedom to share and cache object references without concern for data corruption.

• An immutable object is one whose externally visible state cannot change after it is instantiated. The String, Integer, and BigDecimal classes in the Java class library are examples of immutable objects -- they represent a single value that cannot change over the lifetime of the object.

Creating an Immutable class

• Writing immutable classes is easy. A class will be immutable if all of the following are true:

1. All of its fields are final 2. The class is declared final 3. The this reference is not allowed to escape

during construction

References to mutable objects in immutable classes

Any fields that contain references to mutable objects, such as arrays, collections, or mutable classes like Date: – Are private – Are never returned or otherwise exposed to callers – Are the only reference to the objects that they

reference – Do not change the state of the referenced objects

after construction

String example

• String s = "ABC"; s.toLower();

// The method toLower() will not change the data "ABC" that s contains. Instead, a new String object is instantiated and given the data "abc" during its construction. A reference to this String object is returned by the toLower() method. To make the String s contain the data "abc", a different approach is needed.

• s = s.toLower(); // Now the String s references a new String object that

contains "abc". The String class's methods never affect the data that a String object contains.

Are constants still variable?• Immutability does not guarantee that the object

as stored in the computer's memory is unwriteable. Rather, immutability is a compile time construct that indicates what a programmer should do, not necessarily what he can do (for instance, by circumventing the type system or violating constant corrections in C or C++). Java has more protection against violating constraints than C or C++, but circumvention may still be possible.

Copy on Write• A technique which blends the advantages of mutable

and immutable objects, and is supported directly in almost all modern hardware, is copy-on-write (COW). Using this technique, when a user asks the system to copy an object, it will instead merely create a new reference which still points to the same object. As soon as a user modifies the object through a particular reference, the system makes a real copy and sets the reference to refer to the new copy. The other users are unaffected, because they still refer to the original object.

Benefit of Copy on Write

• Under COW, all users appear to have a mutable version of their objects, although if users do not modify their objects, the space-saving and speed advantages of immutable objects are preserved. Copy-on-write is popular in virtual memory systems because it saves memory space in the core while still correctly handling anything an application program might do.

Immutable Colors

• An example of immutability in the Java class library is java.awt.Color. While colors are generally represented as an ordered set of numeric values in some color representation (such as RGB, HSB, or CMYK), it makes more sense to think of a color as a distinguished value in a color space, rather than an ordered set of individually addressable values, and therefore it makes sense to implement Color as an immutable class.

Immutable Events

• Events are another example of good candidates for implementation with immutable classes. Events are short-lived, and are often consumed in a different thread than they were created in, so making them immutable has more advantages than disadvantages.

Where to use it ?• Should containers for multiple primitive values,

such as points, vectors, matrices, or RGB colors, be mutable or immutable objects? It depends on how are they going to be used. Are they used primarily to represent multi-dimensional values (like the color of a pixel), or simply as containers for a collection of related properties of some other object (like the height and width of a window)? How often are these properties going to be changed? If they are changed, do the individual component values have meaning within the application on their own?

Benefits of immutability• Immutable classes, when used properly, can greatly

simplify programming. They can only be in one state, so as long as they are properly constructed, they can never get into an inconsistent state.

• You can freely share and cache references to immutable objects without having to copy or clone them.

• Immutable classes generally make the best map keys. And they are inherently thread-safe, so you don't have to synchronize access to them across threads.

Freedom to cache• Because immutable objects do not change their

value, you can freely cache references to them and be confident that the reference will refer to the same value later. Similarly, because their properties cannot change, you can cache their fields and the results of their methods.

• If an object is mutable, you have to exercise some care when storing a reference to it. Consider the code in Listing 1, which queues two tasks for execution by a scheduler. The intent is that the first task would start now and the second task would start in one day.

Potential problem with a mutable Date object

• Date d = new Date(); Scheduler.scheduleTask(task1, d); d.setTime(d.getTime() + ONE_DAY); scheduler.scheduleTask(task2, d);

Because Date is mutable, the scheduleTask method must defensively copy the date parameter (clone()) into its internal data structure. Otherwise, task1 and task2 might both execute tomorrow. Worse, the internal data structure could become corrupt.

Debugging the mutable Date example (previous slide)

• It is very easy to forget to defensively copy the date parameter when writing a method like scheduleTask(). If you do forget, you've created a subtle bug that's not going to show up for a while, and one that will take a long time to track down when it does. An immutable Date class would have made this sort of bug impossible.

Benefits of Immutability• The freedom to share references to immutable

objects across threads without synchronization can greatly simplify the process of writing concurrent programs and reduces the number of potential concurrency errors a program could have.

Safety with ill-behaved code• Methods that take objects as arguments should not

mutate the state of those objects. When we pass an object to an ordinary method, we do not expect that the object will come back changed. If we pass a java.awt.Point to a method such as Component.setLocation(), setLocation can modify the location of the Point we pass in or store a reference to that point and change it later in another method. Now, the state of the Point has changed without our knowledge, with potentially hazardous results.

Immutable Keys• Immutable objects make the best HashMap or

HashSet keys. Some mutable objects will change their hashCode() value depending on their state. If you use a mutable object as a HashSet key, and then the object changes its state, the HashSet implementation will become confused -- the object will still be present if you enumerate the set, but it may not appear to be present if you query the set with contains().

Shopping Cart example• class Cart {

private final List items; public Cart(List items) {

this.items = items; }

public List getItems() { return items; }

public int total() { /* return sum of the prices */ } }

Shopping Cart explanation• An instance of this class is not immutable: one

can add or remove items either by obtaining the field items by calling getItems() or by retaining a reference to the List object passed when an object of this class is created. The following code partially solves this problem. In the ImmutableCart class, the list is immutable: you cannot add or remove items.

Shopping Cart solution• The decorator pattern used as a wrapper around

each of the list's items makes them immutable.• class ImmutableCart {

private final List items;public ImmutableCart(List items) {

this.items = Arrays.asList(items.toArray()); }public List getItems() { return Collections.unmodifiableList(items);

} public int total() { /* return sum of the prices */

} }

Summary of Immutability

• You can share immutable objects between threads without danger of changes confusing the other thread.

• Once you check the value, you know it has to stay safe. No one can pass you a value, then behind your back swap it to an unsafe one. This is particularly important in high security situations where allowing an invalid value to sneak in could compromise system integrity, e.g. a filename.

Summary

• You can share duplicate objects by pointing them to a single instance.

• You can create substrings without copying. Immutability is the secret behind Java's very fast substring implementation.

• Immutable objects are much better suited to be Hashtable keys. If you change the value of an object that is used as a hash table key without removing it and re-adding it you lose the mapping.

Returning an Immutable Result

• How would you return some data from a method without changing the original. Here are four ways:

1. Wrap the reference in an immutable wrapper class and return that.

2. Give the caller a private copy of the data.3. Request that the user not to modify the data.4. Return an immutable interface to the original data. You

can then change fields in the object, but the caller cannot unless he cheats by casting. Expose only the methods you want the user to have. Doing this with classes is harder since a subclass must expose everything its superclass does.

Bibliography• Jia, Xiaoping, Object Oriented Software Development,

using Java. Addison Wesley, 2003.• Declarations and access control. Certkey for Java2.

26 April 2006 http://www.jchq.net/certkey/0102certkey.htm

• Java 2 Basics: Modifiers. GARBA.ORG. 26 April 2006 http://www.garba.org/documents/java2/modifiers.html

• http://www.answers.com/topic/immutable-object