1Structured Programming

Structured Programming

1401104-3

Dr. Atif Alhejali

Lecture 4

ModifiersParameters passing

Structured Programming 2

Packages• A package is a container that contains a group of

classes• In real world, the package is the folder that

contains the classes (*.java files)• All classes within the same package are visible to

each other• To use a class form another package it should be

imported using the keyword “import”.

Structured Programming 3

Packages• Lets assume the following example where the

classes have the following hierarchy:

Structured Programming 4

Packages• In the previous example, Class1 can be used

directly while Class2 and Class3 must be imported

Structured Programming 5

Modifiers• Modifiers are keywords used before the

declaration of a variable or method to control who can see and use it

• For example :o public int x;o protected double y;o Public void print(){…}

Structured Programming 6

Modifiers• There are 4 modifiers (private, protected, public

and using no modifiers)• The next table shows the visibility of each

modifier.

Modifier Class Package Subclass World

public Y Y Y Y

protected Y Y Y N

no modifier Y Y N N

private Y N N N

Structured Programming 7

Modifiers• Lets assume the following example

Structured Programming 8

Modifiers• The following table shows where the members of

the Alpha class are visible for each of the access modifiers that can be applied to them

Modifier Alpha Beta Alphasub Gamma

public Y Y Y Y

protected Y Y Y N

no modifier Y Y N N

private Y N N N

Structured Programming 9

static• “static” is a special modifier• Usually variables and methods belongs to the

class instance (object). Hence, objects are independent from each other

• However, A variable or method that is defined static belongs to the class itself and shared by all its instances.

• In this case if a static variable has been changed in any object this change will be reflected on all working objects.

Structured Programming 10

staticpublic class A { int x; static int y; }

Structured Programming 11

staticpublic class Main { public static void main(String[] args) { A a1 = new A(); a1.x = 5; a1.y = 10; A a2 = new A(); a2.x = 6; a2.y = 11; A a3 = new A(); a3.x = 7; a3.y = 12; System.out.println("a1.x : " +a1.x + "a1.y : " +a1.y); System.out.println("a2.x : " +a2.x + "a2.y : " +a2.y); System.out.println("a3.x : " +a3.x + "a3.y : " +a3.y); } }

Output:a1.x : 5 - a1.y : 12a2.x : 6 - a3.y : 12a3.x : 7 - a3.y : 12

Structured Programming 12

static• In addition, a static variable can be

reached directly from the class without the need of an instance (object)

Structured Programming 13

staticpublic class Main { public static void main(String[] args) { A a1 = new A(); a1.x = 5; A.y = 10; A a2 = new A(); a2.x = 6; A.y = 11; A a3 = new A(); a3.x = 7; A.y = 12; System.out.println("a1.x : " +a1.x + " - a1.y : " +A.y); System.out.println("a2.x : " +a2.x + " – a2.y : " +A.y); System.out.println("a3.x : " +a3.x + " - a3.y : " +A.y); } }

Output:a1.x : 5 - a1.y : 12a2.x : 6 - a3.y : 12a3.x : 7 - a3.y : 12

Structured Programming 14

Constant• In java, a variable can be defined “final” which

means that once it is initialized it cannot be changed

• For example the following global variable is a constant:o public static final double pi=3.14;

• Constants must be declared with static final and must have a n initial value at declaration which cannot be changed.

• It may have any other modifier(public, private, protected or no modifier)

Structured Programming 15

How objects are stored

• When an object is created it will be stored in the computer memory

• However, its name will contain only a reference to where it has been stored.

• This different to the primitive data types (int, double, float, long, short, boolean and char) where the data itself is associated with the name.

Structured Programming 16

How objects are stored

public class A { int x; public A(int x) { this.x = x; }}

a1 data

a2 data

1001

1002

1003

1004

1005

memory

dataaddress

1001

a1

1002

a2public class Main { public static void main(String[] args) { A a1 = new A(5); A a2 = new A(10);

int i = 20; }}

20

i

Structured Programming 17

How objects are stored

• Because of this, programmers must be careful when creating, assigning and comparing objects as we will see in the following examples.

Structured Programming 18

Assigning objects to objects

public class A { int x; public A(int x) { this.x = x; }}

a1 data

a2 data

1001

1002

1003

1004

1005

memorydataaddress

1001

a1

public class Main { public static void main(String[] args) { A a1 = new A(5); A a2 = new A(10); a1.x = 15; a2.x = 20; System.out.println(a1.x); System.out.println(a2.x); }}

1002

a2

Output:1520

Structured Programming 19

Assigning objects to objects

public class A { int x; public A(int x) { this.x = x; }}

a1 & a2 data

1001

1002

1003

1004

1005

memorydataaddress

1001

a1

public class Main { public static void main(String[] args) { A a1 = new A(5); A a2 = a1; a1.x = 15; a2.x = 20; System.out.println(a1.x); System.out.println(a2.x); }}

1001

a2

Output:2020

Structured Programming 20

Assigning objects to objects

• In the first example creating 2 different objects by calling the constructor of each one of them meant that they are completely independent of each other.

• In the second example, however, by assigning the first object to the second we really copied the reference of the first object to the second which meant that any changes to either of them is reflected on the other because they are in fact the same.

Structured Programming 21

Comparing objectspublic class A { int x; public A(int x) { this.x = x; }}

a1 data

a2 data

1001

1002

1003

1004

1005

memorydataaddress

1001

a1

public class Main { public static void main(String[] args) { A a1 = new A(5); A a2 = new A(5); if(a1 = = a2)

System.out.println(“true”); else if(a1 != a2)

System.out.println(“false”); }}

1002

a2

Output:false

Structured Programming 22

Comparing objectspublic class A { int x; public A(int x) { this.x = x; }}

a1 & a2 data

1001

1002

1003

1004

1005

memorydataaddress

1001

a1

public class Main { public static void main(String[] args) { A a1 = new A(5); A a2 = a1; a1.x = 10 if(a1 = = a2)

System.out.println(“true”); else if(a1 != a2)

System.out.println(“false”); }}

1001

a2

Output:true

Structured Programming 23

Comparing objects• When comparing two objects using “= = ” the

compiler will compare if the reference of the two objects are the same regardless of the actual data inside it.

• For that, the result of comparison of the first example was “false” because a1 and a2 have different references (1001 and 1002)

• In the second example assigning a1 to a2 resulted in both objects having the same reference (and as a result the same data) which meant the result of the comparison is “true”

Structured Programming 24

Passing parameters• Similarly, when an object is passed into a method

as a parameter, its reference is sent which means that any change to the object in the called method will be reflected in the original object in the calling methods.

• This is called “call by reference” while using primitive data types is called “call by value” because the actual value is sent to the called method and hence any changes does not affect the original variable

Structured Programming 25

Passing parameterspublic class Main { public static void main(String[] args) { A a = new A(10); int y = 5; changeObject(a,y); System.out.println(a.x); System.out.println(y); } public static void changeObject(A a, int y){ a.x = 100; y = 50; }}

Output:1005