2. Java Basics
Class – why?
Class Description
Class Fields
Defining Methods
Return Type
Parameters
Constructors
Objects
Using the this Keyword
Complex Numbers (1 of 4)
Complex Numbers (2 of 4)
Complex Numbers (3 of 4)
Complex Numbers (4 of 4)
Exercise 2.4.1.
Step by Step Solution
Step by Step Solution
Class for Complex Numbers
Step by Step Solution
Class for Complex Numbers
Step by Step Solution
Class for Complex Numbers
Class for Complex Numbers
Class for Complex Numbers
Accessors in Eclipse
Step by Step Solution
Class for Complex Numbers
Step by Step Solution
Unit Test for getModule I
Unit Test for getModule II
Step by Step Solution
Class for Complex Numbers
Class for Complex Numbers
Unit Test for getModule
Class for Complex Numbers
Exercise 2.4.1.
Exercise 2.4.2 (1 of 2)
Exercise 2.4.2 (2 of 2)
239.00K
Category: programmingprogramming

2. Java Basics. 4. Java Classes

1. 2. Java Basics

4. Java Classes

2. Class – why?

• Classes split application code to parts
(from sophisticated to simple)
• Very often class is a model of an object
from the real world
• Java says: Everything is an object
• Class describes object behaviour
• Class is a type
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3. Class Description

class name {
// field declarations
// method declarations
}
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4. Class Fields

• Class fields should be declared inside
class out of all class methods
• Fields can have primitive type, or
reference type such as array or object
• Fields are visible to all instance methods
• Fields are automatically initialized
(reference types with null, number types
with zero, boolean – with false)
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5. Defining Methods

return_type method_name (parameter_list){
// method body
}
Example:
int getFinalData(int a, int r){
int b = r % 18;
return a * 2 + b;
}
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6. Return Type

• The return type describes the value that
comes back from the method
• A method can have void return type
• Any method that is not declared void must
contain a return statement with a
corresponding return value
• Return statements for void return type is
not necessary
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7. Parameters

• Any data type is possible for a parameter of a
method
• Construct varargs is used to pass an arbitrary
number of values (e.g. type... args)
• Varargs can be used only in the final argument
position
• Parameters are passed into methods by value.
• The values of the object's fields can be
changed in the method
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8. Constructors

• Constructor name should be the same as class
name
• Constructor has no return type
• The compiler automatically provides a noargument, default constructor for any class
without parameters – don’t use this possibility,
declare such constructor explicitly
• A class can have several constructors (with
different sets of parameters)
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9. Objects

• Creating Object:
class_name object_variable = new construtor_call;
• Declaring a Variable to Refer to an Object:
class_name object_variable;
• Calling an Object's Methods:
object_variable.methodName(argumentList);
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10. Using the this Keyword

• this is a reference to the current object
• The most common example:
class Point {
int x = 0;
int y = 0;
//constructor
Point(int x, int y) {
this.x = x;
this.y = y;
}
}
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11. Complex Numbers (1 of 4)

• Is it always possible to solve square
2
ax
bx c 0 within real
equation
numbers set?
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12. Complex Numbers (2 of 4)

• Is it always possible to solve square
2
equation ax bx c 0 within real numbers
set?
• No, if b 2 4ac 0 it is impossible.
• We can expand real number set to complex
number set introducing new number type complex unit i - in such a way:
i * i = -1
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13. Complex Numbers (3 of 4)

• Number of a + b * i type where a and b are
real is called complex number.
• Every square equation can be solved
within complex numbers set.
• Moreover, every algebraic equation (with
arbitrary power) always can be solved
within complex numbers set.
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14. Complex Numbers (4 of 4)

• To add complex numbers use formula
(a1 b1i) (a2 b2i) (a1 a2 ) (b1 b2 )i
• To multiply complex numbers use formula
(a1 b1i) * (a2 b2i) (a1a2 b1b2 ) (a1b2 a2b1 )i
• To find absolute value of complex number
use formula r a 2 b 2
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15. Exercise 2.4.1.

• Create a class for saving and manipulating
complex numbers.
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16. Step by Step Solution

1. Check problem definition. If it is clear go
to step 2
2. Create class
3. Describe class fields
4. Create constructors and accessors
5. Create method prototypes
6. Create unit tests
7. Create method bodies
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17. Step by Step Solution

1. Check problem definition. If it is clear go
to step 2
2. Create class
3. Describe class fields
4. Create constructors and getters/setters
5. Create method prototypes
6. Create unit tests
7. Create method bodies
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18. Class for Complex Numbers

/**
* Represents complex numbers
*/
class Complex {
}
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19. Step by Step Solution

1. Check problem definition. If it is clear go
to step 2
2. Create class
3. Describe class fields
4. Create constructors and getters/setters
5. Create method prototypes
6. Create unit tests
7. Create method bodies
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20. Class for Complex Numbers

class Complex {
/**
* Real part of a complex number
*/
double r;
/**
* Imaginary part of a complex number
*/
double im;
}
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21. Step by Step Solution

1. Check problem definition. If it is clear go
to step 2
2. Create class
3. Describe class fields
4. Create constructors and getters/setters
5. Create method prototypes
6. Create unit tests
7. Create method bodies
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22. Class for Complex Numbers

/**
* Default constructor sets complex zero
*/
Complex(){
r = 0.0;
im = 0.0;
}
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23. Class for Complex Numbers

/**
* Initializes a complex number
* @param r - real part of a complex number
* @param im - imaginary part of a complex
number
*/
Complex(double r, double im){
this.r = r;
this.im = im;
}
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24. Class for Complex Numbers

double getR() { return r; }
void setR(double value){ r = value; }
double getIm() { return im; }
void setIm(double value){ im = value; }
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25. Accessors in Eclipse

• Right click in text editor, and select Source >
Generate Getters and Setters .. Menu item
• Check necessary boxes for creating getters
and / or setters, select access modifiers and
click Ok button.
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26. Step by Step Solution

1. Check problem definition. If it is clear go
to step 2
2. Create class
3. Describe class fields
4. Create constructors and getters/setters
5. Create method prototypes
6. Create unit tests
7. Create method bodies
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27. Class for Complex Numbers

/**
* Returns module of the complex number
*/
double getModule(){
}
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28. Step by Step Solution

1. Check problem definition. If it is clear go
to step 2
2. Create class
3. Describe class fields
4. Create constructors and getters/setters
5. Create method prototypes
6. Create unit tests
7. Create method bodies
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29. Unit Test for getModule I

public class E241TestComplex {
public static void main(String[] args) {
Complex test1 = new Complex();
test1.setR(3.0);
test1.setIm(4.0);
System.out.println("module = " +
test1.getModule());
}
}
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30. Unit Test for getModule II

public class E241TestComplex {
public static void main(String[] args) {
Complex test1 = new Complex();
test1.setR(3.0);
test1.setIm(4.0);
double res = test1.getModule();
if (res == 5.0){
System.out.println("getModule test is true");}
else{
System.out.println("getModule test failed");}
}
}
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31. Step by Step Solution

1. Check problem definition. If it is clear go
to step 2
2. Create class
3. Describe class fields
4. Create constructors and getters/setters
5. Create method prototypes
6. Create unit tests
7. Create method bodies
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32. Class for Complex Numbers

/**
* Returns module of the complex number
*/
double getModule(){
return Math.sqrt(r * r + im * im);
}
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33. Class for Complex Numbers

Complex add(Complex value){
}
Complex multiply(Complex value){
}
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34. Unit Test for getModule

public class E241TestComplex {
public static void main(String[] args) {
Complex conjugate1 = new Complex(3.0, 2.0);
Complex conjugate2 = new Complex(3.0, -2.0);
Complex result = conjugate1.add(conjugate2);
r = result.getR(); im = result.getIm();
if ((r == 13.0) && (im == 0.0)){
System.out.println("multiply test 1 is true");}
else{ System.out.println("multiply test 1 failed");}
}
}
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35. Class for Complex Numbers

Complex add(Complex value){
return new Complex(this.r + value.getR(), this.im + value.getIm());
}
Complex multiply(Complex value){
double rr = this.r * value.getR() - this.im *
value.getIm();
double rim = this.r * value.getIm() + this.im * value.getR();
return new Complex(rr, rim);
}
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36. Exercise 2.4.1.

• See 241Complex project for the full text
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37. Exercise 2.4.2 (1 of 2)

• 3D Vector is ordered sequence of 3
numbers (vector’s coordinates):
r ( x1 , x2 , x3 )
• Sum of two vectors
r 1 ( x11 , x12 , x13 )
r 2 ( x21 , x22 , x23 )
is a vector
r r1 r2 ( x11 x21 , x12 x22 , x13 x23 )
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38. Exercise 2.4.2 (2 of 2)

• Scalar product of two vectors
r 1 ( x11 , x12 , x13 )
r 2 ( x21 , x22 , x23 )
p x11 x21 x12 x22 x13 x23
is a number
• Vector product of two vectors is a vector
r r1 r2 ( x12 x23 x13 x22 , x13 x21 x11 x23 , x11 x22 x12 x21 )
• Vector’s module is a number m x12 x22 x32
• You should create a class Vector3D for
vector saving and manipulating
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