Session04-More on Classes and Nested Classes

• Use this refer to any member of the current object from within an instance method or a constructor by using this... The Access Modifiers1Access modifiers control which classes may use

Session 04 More on Classes and Nested Classes

(http://docs.oracle.com/javase/tutorial/java/javaOO/more.html)

• Returning a Value from a Method

• Using the this Keyword

• Controlling Access to Members of a Class

• Understanding Class Members

Returning a Value from a Method

• A method returns to the code that invoked it when it:

– completes all the statements in the method,

– reaches a return statement, or

– throws an exception (covered later),

• A method can return a primitive values or

reference value such as class or interface.

Using the this Keyword

• Within an instance method or a constructor, this is

a reference to the current object.

• Use this refer to any member of the current object from within an instance method or a constructor by using this.

Point(int x, int y)

{ this.x = x; this.y =

y; }

public Point() { this(0, 0);

}

The Access Modifiers(1)

Access modifiers control which classes may use a feature:

Features:

• The class itself

• Its member variables

• Its methods and constructors

• Its nested classes

The Access Modifiers(2)

• The access modifiers are

non-The Access Modifiers(3)

• A feature may have at most one access

modifier If a feature has no access modifier

we call default access modifier.

A Sample of Access Level

Tips on Choosing an Access Level

• Use the most restrictive access level that

makes sense for a particular member

Use private unless you have a good reason not to.

• Avoid public fields except for constants Public fields tend to link you to a particular

implementation and limit your flexibility in

changing your code.

Understanding Class Members (1)

• Class Variables

• Fields that have the static modifier in their

declaration

• References by class name itself.

public class Bicycle {

// add an instance variable for the object ID

private int id;

Understanding Class Members (2)

• Class Methods:

– Static methods: will be invoked by using class name

• ClassName.methodName(args) – Can access class variables and class methods directly.

– cannot access instance variables or instance methods

directly—they must use an object reference

– cannot use the this keyword as there is no instance for this to refer to.

Understanding Class Members (3)

– The static modifier, in combination with

the final modifier, is also used to define

constants The final modifier indicates that the value of this field cannot change.

static final double PI = 3.141592653589793;

Initializing Fields (2)

Initializing Fields (3)

• Initializing Instance Members

– Will be copied initializer blocks into every constructor – Can be used to share a block of code between multiple constructors.

– Initializer blocks for instance variables look just like static initializer blocks, but without the static keyword:

{

// whatever code is needed for initialization goes here }

Nested Classes

• Define a class within another class Such a

class is called a nested class:

Why Use Nested Classes?

• It is a way of logically grouping classes that are only used in one place.

• It increases encapsulation.

• It can lead to more readable and maintainable code.

public void makeInner() {

InnerOne anInner = new InnerOne();

anInner.innerMethod();

}

Create an instance of an inner class in

an unrelated class.

InnerOne();

i.innerMethod();

public static void main(String args[]) {

OuterOne o = new OuterOne();

OuterOne.InnerOne i = o.new InnerOne();

i.innerMethod();

}

Static Inner Class Sample

public class MyOuter {

public static class MyInner {

}

public static void main(String [] args) {

MyInner aMyInner = new

MyOuter.MyInner();

}

}

Shadowing (2)

• Declaration of a type in a particular scope has the same name as another declaration in the enclosing scope, then the

declaration shadows the declaration of the

enclosing scope.

}

public static void main(String args) {

ShadowTest st = new ShadowTest();

ShadowTest.FirstLevel fl = st.new FirstLevel();

Anonymous Classes (1)

• Enable you to make your code more concise

• Enable you to declare and instantiate a class at the same time

• They are like local classes except that they do not have a name

• Use them if you need to use a local class only once.

Anonymous Classes (2)

HelloWorld englishGreeting = new EnglishGreeting();

HelloWorld frenchGreeting = new HelloWorld() {

String name = "tout le monde";

public void greet() {

Anonymous Classes (3)

public class HelloWorldAnonymousClasses {

interface HelloWorld {

public void greet();

public void greetSomeone(String someone);

}

public void sayHello() {

class EnglishGreeting implements HelloWorld {

String name = "world";

public void greet() {

Enum Types (1)

• An enum type is a special data type that

enables for a variable to be a set of

predefined constants.

• We use enum types any time you need to represent a fixed set of constants.

case SATURDAY: case SUNDAY:

System.out.println("Weekends are best."); break;

public static void main(String[] args) {

EnumTest firstDay = new EnumTest(Day.MONDAY); firstDay.tellItLikeItIs();

}

}

• Returning a Value from a Method

• Using the this Keyword

• Controlling Access to Members of a Class

• Understanding Class Members

• https://

docs.oracle.com/javase/tutorial/java/javaOO/ QandE/nested-questions.html

programming.guide/java/overloading-overridi ng-shadowing-hiding-obscuring.html

www.math.uni-hamburg.de/doc/java/tutorial/ java/javaOO/QandE/nested-answers.html

• Hãy giải thích 5 khái niệm sau đây trong java: Overloading, overriding, shadowing, hiding, and obscuring

• Dùng ví dụ để giải thích

• Gửi mail trước 23pm về hunghh12@fe.edu.vn

• (Dùng email fpt của SV)