Download + (int)

Lesson 2
The Java Prog Lan
AUBG ICoSCIS Team
Assoc. Prof. Stoyan Bonev
March, 23 - 24, 2013
SWU, Blagoevgrad
Time Schedule
March 23, Saturday – 10:00 – 10:45
– 11:00 – 11:45
– 12:00 – 12:45
Lunch Break (45’)
– 13:30 – 14:15
– 14:30 – 15:15
– 15:30 – 16:15
March 24, Sunday
09:00 – 09:45
10:00 – 10:45
11:00 – 11:45
Lunch Break (45’)
12:30 – 13:15
13:30 – 14:15
14:30 – 15:15
Lesson Contents
 Overview of Java Programming Language
 Anatomy of a Java program





Sample source text
Packages
Comments
Names – identifiers, reserved words, modifiers
Classes – data fields, methods (statements, blocks of statements )
 Components of a Java program







Data – by category
Data – by type
Expressions (incl. operands & operators)
Statements
Routines (member functions - methods)
I/O facilities
Data collections – arrays
 Demo programs - examples
The Basics of a Java Program
 Java program: collection of classes. Classes
capsulated as packages – named or unnamed
 Package: collection of related classes
 Class: collection of data items and methods
 Method: designed to accomplish a specific task
 There is a main method public static void
main()in every Java program. The main method
provides the control of program flow. The JVM
executes application by invoking the main method
4
4
The package






Logical entity describes user defined or API predefined package
Really located as one file or as many files
Syntax to mark package start: package <name>;
Always first line in the source code
May be omitted: program without explicit package name
Reference to predefined package: import <package name>;
 import java.lang.*;

This statement makes the methods in the library
package java.lang available to the class following it.
 java.lang is just 1 package in the Java library or API.
 Primitive data types and the class String:

Part of the Java language. They Don’t need to be imported
Java Programming: From Problem Analysis to Program Design,
5
4e
5
Anatomy of a Java program: Sample source text
Sample Run:
Java Programming: From Problem Analysis to Program Design, 4e
3e
6
Comments
Three types of comments in Java.
Line comment: A line comment is preceded by two slashes
(//) in a line.
Paragraph comment: A paragraph comment is enclosed
between /* and */ in one or multiple lines.
javadoc comment: javadoc comments begin with /** and
end with */. Used for documenting classes, data, methods.
They can be extracted into HTML file using JDK's
javadoc command.
5/3/2017
Assoc. Prof. Stoyan 7
Bonev
Names - Identifiers
• Names of entities (like variables, named constants,
methods, classes, objects etc.)
• An identifier is a sequence of characters that consist
of letters, digits, underscores (_), and dollar signs ($).
• An identifier must start with a letter, an underscore
(_), or a dollar sign ($). It cannot start with a digit.
• An identifier cannot be a reserved word.
• An identifier can be of any length.
• Java is case sensitive – ion, Ion, IOn, ION
Java Programming: From Problem Analysis to Program Design, 3e
8
Naming Conventions
• Choose meaningful and descriptive names.
• Class names:
– Capitalize the first letter of each word in the name. For example, the
class name ComputeArea.
• Named Constants:
– Capitalize all letters in constants, and use underscores to connect
words. For example, PI and MAX_VALUE
• Variables and method names:
– Use lowercase. If the name consists of several words, concatenate all
in one, use lowercase for the first word, and capitalize the first letter
of each subsequent word in the name. For example, the variables
radius and area, and the method computeArea.
Java Programming: From Problem Analysis to Program Design, 3e
9
Reserved Words
Reserved words or keywords are words that have a specific
meaning to the compiler and cannot be used for other
purposes in the program. For example, class, void. are
reserved words.
Modifiers
Java uses certain reserved words called modifiers that
specify the properties of the data, methods, and classes and
how they can be used. Examples of modifiers are public
and static. Other modifiers are private, final,
abstract, and protected.
Java Programming: From Problem Analysis to Program Design, 3e
10
Data classified by Type
• Data Types Classified:
– 8 Primitive Data Types
– boolean, byte, short, int, long, float, double, char
– Abstract/Reference Data Types (classes, objects)
– String, Object, Frame, Person, Animal, …
– Array types (also reference types)
Java Programming: From Problem Analysis to Program Design, 4e
3e
11
Primitive Data Types – 3 subgroups
• Integral, which is a data type that deals with integers (or
numbers without a decimal part) and characters
• char – Unicode – 2 bytes/char, to present symbolic data
• byte, short, int, long, to present numeric data fixed point
• Floating-point, a data type that deals with decimal numbers
• float: precision = 6 or 7 decimal places, Single (4 bytes)
• double: precision = 15 decimal places, Double (8 bytes)
• Boolean, a data type that deals with logical values: true, false
• Memory allocated to boolean data type is 1 bit
Java Programming: From Problem Analysis to Program Design, 4e
3e
12
Values and Memory Allocation for
Integral Data Types
Java Programming: From Problem Analysis to Program Design, 4e
3e
13
Primitive Data Types
• Classified by Category as:
• Literals
• Named Constants
• Variables
Java Programming: From Problem Analysis to Program Design, 4e
3e
14
Literals (Constants)
• Integer literals, integer constants, or integers:
23
-67
+447
0177
0x1a
• Floating-point literals, floating-point constants, floatingpoint numbers:
12.34
+25.60
-3.15
5.
.166
3.14e-2
2.78e+3
5.23e4
• Character literals, character constants, or characters:
'a'
'5'
'+'
'\n'
Java Programming: From Problem Analysis to Program Design, 4e
3e
15
Named Constants
•
•
Named constant
–
–
Cannot be changed during program execution
Declared by using the reserved word final
–
Initialized when it is declared
Example:
final
final
final
final
double CENTIMETERS_PER_INCH = 2.54;
int NO_OF_STUDENTS = 20;
char BLANK = ' ';
double PAY_RATE = 15.75;
Java Programming: From Problem Analysis to Program Design, 4e
3e
16
Variables
•
Variable (name, value, data type, size)
–
–
–
–
–
•
Content may change during program execution
Must be declared before it can be used
May not be automatically initialized
If new value is assigned, old one is destroyed
Value can only be changed by an assignment statement
or an input (read) statement
Example:
double
int
char
int
amountDue;
counter;
ch;
num1, num2;
Java Programming: From Problem Analysis to Program Design, 4e
3e
17
Declaring and Initializing Variables
int first;
first = 13;
OR
int first = 13;
char ch;
ch = ‘E’;
OR
char ch = ‘E’;
double value;
value = 35.68;
OR
double value = 35.68;
-------------------------------------------------------------
Java Programming: From Problem Analysis to Program Design, 3e
18
Unicode Format
Java characters use Unicode, a 16-bit encoding scheme
established by the Unicode Consortium to support the
interchange, processing, and display of written texts in the
world’s diverse languages. Unicode takes two bytes, preceded
by \u, expressed in four hexadecimal numbers that run from
'\u0000' to '\uFFFF'. So, Unicode can represent 65535 + 1
characters.
char letter = 'A'; (ASCII)
char letter = '\u0041'; (Unicode)
char numChar = '4';
char numChar = '\u0034';
Unicode \u03b1 \u03b2 \u03b3 for three Greek letters
Java Programming: From Problem Analysis to Program Design, 3e
19
(Non-)Pointers in Java
• In Java, all primitive variables are value variables. (Real, actual)
– it is impossible to have a pointer to any variable of one of the
primitive data types
• All object variables are actually reference variables (pointers,
memory addresses) to objects.
– it is impossible to have anything but references to objects. You
can never have a plain object variable
Java Programming: From Problem Analysis to Program Design, 3e
20
Expressions - operands
• Operands
– Literals
– Named Constants
– Variables – regular(scalar) or indexed
– Method call
– Sub expression like ( … )
Java Programming: From Problem Analysis to Program Design, 3e
21
Arithmetic Operators and Operator
Precedence
•
•
1.
2.
•
Operators in 1 have a higher precedence than
operators in 2
When operators have the same level of precedence,
operations are performed from left to right
Unary operator: operator that has one operand
Binary operator: operator that has two operands
•
•
•
*
+
/
-
%
(same precedence)
(same precedence)
Java Programming: From Problem Analysis to Program Design, 4e
3e
22
Mixed Arithmetic Expressions
• Operands of different types
• Examples
2 + 3.5
6 / 4 + 3.9
• Integer operands yield an integer result; floating-point
numbers yield floating-point results
• If both types of operands are present, the result is a
floating-point number
• Precedence rules are followed
Java Programming: From Problem Analysis to Program Design, 4e
3e
23
Type Conversion (Casting)
• Used to avoid implicit type coercion
• Syntax
(dataTypeName)expression
• Expression evaluated first, then type converted
to dataTypeName
• Examples
(int)(7.9 + 6.7)
=> 14
(int)(7.9) + (int)(6.7) => 13
Java Programming: From Problem Analysis to Program Design, 4e
3e
24
Conversion Rules
When performing a binary operation involving two
operands of different types, Java automatically converts
the operand based on the following rules:
1. If one of the operands is double, the other is converted into
double.
2. Otherwise, if one of the operands is float, the other is
converted into float.
3. Otherwise, if one of the operands is long, the other is
converted into long.
4. Otherwise, both operands are converted into int.
Java Programming: From Problem Analysis to Program Design, 3e
25
Type Casting
Implicit casting
double d = 3; (type widening permitted)
Explicit casting
int i = (int)3.0; (type narrowing)
int i = (int)3.9; (Fraction part is truncated)
What is wrong? int x = 5 / 2.0;
range increases
byte, short, int, long, float, double
Java Programming: From Problem Analysis to Program Design, 3e
26
Comparison Operators
Logical Operators
Operator Name
Operator Name
<
less than
!
not
<=
less than or equal to
&&
and
>
greater than
||
or
>=
greater than or equal to ^
==
equal to
!=
not equal to
Java Programming: From Problem Analysis to Program Design, 3e
exclusive or
Conditional Operator
y = (x > 0) ? 1 : -1;27
The class String
• Used to manipulate strings
• String
–
–
–
–
Sequence of zero or more characters
Enclosed in double quotation marks
Null or empty strings have no characters
Numeric strings consist of integers or decimal
numbers
– Length is the number of characters in a string
Java Programming: From Problem Analysis to Program Design, 4e
3e
28
Strings and the Operator +
• Operator + can be used to concatenate two strings or
a string and a numeric value or character
• Example
"The sum = " + 12 + 26
-After this statement executes, the string assigned to str is:
"The sum = 1226";
• Consider the following statement:
"The sum = " + (12 + 26)
• In this statement, because of the parentheses, you first evaluate
num1 + num2
– Because num1 and num2 are both int variables, num1 + num2 =
12 + 26 = 38
– After this statement executes, the string assigned to str is:
"The sum = 38";
Java Programming: From Problem Analysis to Program Design, 4e
3e
29
Types of statements
• Declaration/definition statements
• Executable statements
• Every statement in Java ends with a semicolon (;).
Java Programming: From Problem Analysis to Program Design, 4e
3e
30
Blocks
A pair of braces in a program forms a block that groups
components of a program.
public class Test {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
}
}
Class block
Method block
Use end-of-line style for braces.
Java Programming: From Problem Analysis to Program Design, 3e
31
Executable Statements
•
Assignment stmts
–
•
To implement linear algorithms
Selection/Decision stmts
– To implement branch algorithmsif, if-else,
if-else if, switch
•
Repetition/Iteration stmts
– To implement loop algorithms for, while, dowhile, for(version foreach)
•
Input/Output
– Other break, continue
Java Programming: From Problem Analysis to Program Design, 4e
3e
32
Assignment Statements
x = 1;
// Assign 1 to x;
radius = 1.0;
// Assign 1.0 to radius;
a = 'A';
// Assign 'A' to a;
variable = variable * (expression);
is equivalent to
variable *= expression;
Similarly,
variable = variable + (expression);
is equivalent to
variable += expression;
Java Programming: From Problem Analysis to Program Design, 3e
33
One-way if Statements
if (radius >= 0) {
area = radius * radius * PI;
System.out.println("The area"
+ " for the circle of radius "
+ radius + " is " + area);
}
if (boolean-expression) {
statement(s);
}
Boolean
Expression
false
false
(radius >= 0)
true
true
Statement(s)
area = radius * radius * PI;
System.out.println("The area for the circle of " +
"radius " + radius + " is " + area);
(A)
Java Programming: From Problem
Analysis to Program Design, 3e
(B)
34
The Two-way if Statement
if (boolean-expression) {
statement(s)-for-the-true-case;
}
else {
statement(s)-for-the-false-case;
}
true
Boolean
Expression
Statement(s) for the true case
false
Statement(s) for the false case
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
Java Programming: From Problem Analysis to Program Design, 3e
12
35
if...else Example
if (radius >= 0) {
area = radius * radius * 3.14159;
System.out.println("The area for the “
+ “circle of radius " + radius +
" is " + area);
}
else {
System.out.println("Negative input");
}
Java Programming: From Problem Analysis to Program Design, 3e
36
Switch-case
switch ( N ) {
// (Assume N is an integer variable.)
case 1:
System.out.println("The number is 1.");
break;
case 2:
case 4:
case 8:
System.out.println("The number is 2, 4, or 8.");
System.out.println("(That's a power of 2!)");
break;
case 3:
case 6:
case 9:
System.out.println("The number is 3, 6, or 9.");
System.out.println("(That's a multiple of 3!)");
break;
case 5:
System.out.println("The number is 5.");
break;
default:
System.out.println("The number is 7 or is outside range 1 to 9.");
}
Java Programming: From Problem Analysis to Program Design, 3e
37
switch Statement Flow Chart
status is 0
Compute tax for single filers
break
Compute tax for married file jointly
break
Compute tax for married file separatly
break
Compute tax for head of household
break
status is 1
status is 2
status is 3
default
Default actions
Next Statement
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
Java Programming: From Problem Analysis to Program Design, 3e
44
38
Introducing while Loops
int count = 0;
while (count < 100) {
System.out.println("Welcome to Java");
count++;
}
while Loop Flow Chart
while (loop-continuation-condition) {
int count = 0;
while (count < 100) {
// loop-body;
System.out.println("Welcome to Java!");
Statement(s);
count++;
}
}
count = 0;
Loop
Continuation
Condition?
true
Statement(s)
(loop body)
false
(count < 100)?
false
true
System.out.println("Welcome to Java!");
count++;
Java Programming: From Problem Analysis
to Program Design, 3e (B)
(A)
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
39
6
do-while Loop
Statement(s)
(loop body)
true
do {
// Loop body;
Loop
Continuation
Condition?
false
Statement(s);
} while (loop-continuation-condition);
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
Java Programming: From Problem Analysis to Program Design, 3e
1
40
for Loops
for (initial-action; loopcontinuation-condition;
action-after-each-iteration) {
// loop body;
Statement(s);
}
int i;
for (i = 0; i < 100; i++) {
System.out.println(
"Welcome to Java!");
}
Initial-Action
Loop
Continuation
Condition?
i=0
false
(i < 100)?
true
Statement(s)
(loop body)
true
System.out.println(
"Welcome to Java");
Action-After-Each-Iteration
i++
(A)
(B)
false
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
Java Programming: From Problem Analysis to Program Design, 3e
21
41
For-loop
For (int i=1, j=10; i < j; i++, j-=2)
System.out.println("i="+i+" j="+j);
i=1 j=10
i=2 j=8
i=3 j=6
for (int i = 0; i < 4; i++){
for( char letter = 'A'; letter <= 'A' + i; letter++)
System.out.print(letter);
System.out.println();
}
A
AB
ABC
ABCD
Java Programming: From Problem Analysis to Program Design, 3e
42
Enhanced for Loop (for-each loop)
JDK 1.5 introduced a new for loop that enables you to traverse the complete array
sequentially without using an index variable. For example, the following code
displays all elements in the array myList:
for (double value: myList)
System.out.println(value);
In general, the syntax is
for (elementType value: arrayRefVar) {
// Process the value
}
You still have to use an index variable if you wish to traverse the array in a
different order or change the elements in the array.
Java Programming: From Problem Analysis to Program Design, 3e
43
Console Input (Read) statement
•
Standard input stream object: System.in
•
Input numeric data to program
–
•
Separate by blanks, lines, or tabs
To read data:
1. Create an input stream object of the class
Scanner (import java.util.*; or
import java.util.Scanner;)
2. Use the methods such as next(), nextLine(),
nextInt(), nextFloat(), nextDouble()
Java Programming: From Problem Analysis to Program Design, 4e
3e
44
Input (Read) statement
static Scanner console = new Scanner(System.in);
•
Java program:
static Scanner cin = new Scanner(System.in);
int feet;
int inches;
Suppose the input is: 23 7
Feet
= cin.nextInt();
Inches = cin.nextInt();
•
//Line 1
//Line 2
How to read a single character:
–
char ch; ch = console.next().charAt(0);
Java Programming: From Problem Analysis to Program Design, 4e
3e
45
Reading Input from the Console
1. Create a Scanner object
Scanner cin = new Scanner(System.in);
2. Use the methods next(), nextByte(), nextShort(), nextInt(),
nextLong(), nextFloat(), nextDouble(), or nextBoolean() to
obtain to a string, byte, short, int, long, float, double, or boolean
value. For example,
System.out.print("Enter a double value: ");
Scanner cin = new Scanner(System.in);
double d = cin.nextDouble();
Java Programming: From Problem Analysis to Program Design, 3e
46
public class CalculatingArea {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.println("This program calculates " +
"the area of a rectangle or a triangle");
System.out.print("Enter a and b (for rectangle) " +
"or a and h (for triangle): ");
int a = input.nextInt();
int b = input.nextInt();
System.out.print("Enter 1 for a rectangle or " +
"2 for a triangle: ");
int choice = input.nextInt();
double area = (double) (a * b) / choice;
System.out.println("The area of your figure is " + area);
}
}
Java Programming: From Problem Analysis to Program Design, 3e
47
Output (Write/Print/Display) statement
• Standard output object: System.out
• Methods
print
println
• Syntax
System.out.print(stringExp);
System.out.println(stringExp);
System.out.println();
Java Programming: From Problem Analysis to Program Design, 4e
3e
48
Input/Output using Dialog Boxes
Java Programming: From Problem Analysis to Program Design, 4e
3e
49
JOptionPane Input
Two ways of obtaining input.
1. Using the Scanner class (console input) needs
import java.util.Scanner;
2. Using JOptionPane input dialogs needs
import javax.swing.JOptionPane;
Java Programming: From Problem Analysis to Program Design, 3e
50
Getting Input from Input Dialog Boxes
String input = JOptionPane.showInputDialog(
"Enter an input");
51
Converting Strings to Integers
The input returned from the input dialog box is a string. If you
enter a numeric value such as 123, it returns “123”. To obtain
the input as a number, you have to convert a string into a
number.
To convert a string into an int value, you can use the static
parseInt() method in the Integer class as follows:
int intValue = Integer.parseInt(intString);
where intString is a numeric string such as “123”.
52
Converting Strings to Doubles
To convert a string into a double value, you can use the static
parseDouble method in the Double class as follows:
double doubleValue =Double.parseDouble(doubleString);
where doubleString is a numeric string such as “123.45”.
53
Displaying Text in a Message
Dialog Box
Instead of using System.out.println() for
console output
you can use the showMessageDialog method in the
JOptionPane class. JOptionPane is one of the
many predefined classes in the Java system, which can be
reused rather than “reinventing the wheel.”
54
The showMessageDialog Method
JOptionPane.showMessageDialog(null,
"Welcome to Java!",
"Display Message",
JOptionPane.INFORMATION_MESSAGE);
DEMO program: Open file ProgDialogBoxes.java
55
Exercise on statements
Write a Java program as a single class:
• single method main() to include:
• program segment to calculate the greatest
common divisor of two positive integer
values
• program segment to calculate the factorial of
a nonnegative integer value
• program segment to calculate the sum of
integer numbers in range from n1…n2, such
that n1<n2
56
Components of a Java program
Methods & Classes
• To be discussed in lesson 3.
Java Programming: From Problem Analysis to Program Design, 3e
57
Data Collections
Typical popular Data collections:
• Arrays
Detailed discussion on arrays follows
Java Programming: From Problem Analysis to Program Design, 3e
58
Declaring and creating arrays
Similar to C#
• Example for array objects:
int[] myArray;
// declaration
• This declares myArray to be an array reference variable
• It does not create an array — it only provides a place to put a
reference of an array, i.e. address of an array.
• Notice that the size is not part of the type
myArray = new int[10]; // creation
• The new int[10] creates the array of size 10
• Write as
int[] myArray = new int[10];
5/3/2017
// combined
Assoc. Prof. Stoyan 59
Bonev
An array’s size is not part of its type
• When you declare an array, you declare its type; you must not
specify its size
– Example:String[] names;
– orString names[];//not recommended
• When you create the array, you allocate space; you must
specify its size
– Example: names = new String[50];
• This is true even when the two are combined. Example:
String[] names = new String[50];
5/3/2017
Assoc. Prof. Stoyan 60
Bonev
Arrays of objects
• Suppose you declare and create an array of objects:
– Person[] people = new Person[20];
• You have created an array named people, but you have not
yet given values to each element in that array
• There is nothing wrong with this array, but
– it has 20 references to Persons in it
– all of these references are initially null
– you have not yet defined 20 Persons
– For example, people[12].name will give you a
nullPointerException if you do not define the
Persons.
5/3/2017
Assoc. Prof. Stoyan 61
Bonev
Here’s one way to initialize an array of objects:
Person[] people = new Person[20];
for (int i = 0; i < people.length; i++) {
people[i] = new Person(“John");
}
This approach has a slight drawback: all the array elements have
similar values!
Remember: index of array elements starts at 0 and ends at
(length – 1)
5/3/2017
Assoc. Prof. Stoyan 62
Bonev
Resizing an array
The following is legal:
int[] myArray = new int[10];
...and later in the program,
myArray = new int[500];
// legal!
This is legal because the array’s size is not part of its type,
but part of its value
5/3/2017
Assoc. Prof. Stoyan 63
Bonev
Length of an array
• Arrays are objects.
• Every array has an instance constant, length, that tells how
large the array is
• Example:
for (int i = 0; i < scores.length; i++)
System.out.println(scores[i]);
• Use of length is always preferred over using a literal
constant such as 10
• Arrays have a length variable,
• Strings have a length() method
5/3/2017
Assoc. Prof. Stoyan 64
Bonev
• There is a special syntax for giving initial values to the elements
of arrays
– This syntax can be used in place of new type[size]
– It can only be used in an array declaration
– The syntax is: { value, value, ..., value }
• Examples:
int[] primes = { 2,3,5,7,11,13,19};
String[] languages = { "Java","C","C++" };
5/3/2017
Assoc. Prof. Stoyan 65
Bonev
Initializing arrays
• To initialize an array of Person:
Person[] people =
{ new Person(“Elena"),
new Person(“Ivan"),
new Person(“Katya"),
new Person(“Dimitar") };
• Notice that you do not specify the size of the array
5/3/2017
Assoc. Prof. Stoyan 66
Bonev
Arrays - Recap
• Declare an array ref variable:
– int[] counts;
– String[] names;
– int[][] matrix; //this is an array of arrays
• Create an array and assign its reference to array ref
variable:
– counts = new int[5];
– names = new String[100];
– matrix = new int[5][10];
• Use:
– String months[] = {"Jan", "Feb", "Mar", "Apr",
"May", "Jun", "July", "Aug", "Sep", "Oct", "Nov",
"Dec"};
5/3/2017
– for(int i = 0; i < months.length; i++)
System.out.println("month: " + months[i]);
Assoc. Prof. Stoyan 67
Bonev
Array assignment
• Array assignment is object assignment:
• Object assignment does not copy values
Person p1; Person p2;
p1 = new Person(“Ivan");
p2 = p1;
// p1 and p2 refer to same person (“point to”)
• Array assignment does not copy values
int[] a1; int[] a2;
a1 = new int[10];
a2 = a1; // a1 and a2 refer to the same array
5/3/2017
Assoc. Prof. Stoyan 68
Bonev
Copying Arrays
Often, in a program, you need to duplicate an array or a part of an
array. In such cases you could attempt to use the assignment statement
(=), as follows:
list2 = list1;
Before the assignment
list2 = list1;
list1
After the assignment
list2 = list1;
Contents
of list1
list2
list1
list2
Contents
of list2
Garbage
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
5/3/2017
Contents
of list1
Contents
of list2
45
Assoc. Prof. Stoyan 69
Bonev
Copying Arrays
Using a loop:
int[] srcArray = {2, 3, 1, 5, 10};
int[] targetArray = new int[srcArray.length];
for (int i = 0; i < srcArrays.length; i++)
targetArray[i] = srcArray[i];
5/3/2017
Assoc. Prof. Stoyan 70
Bonev
Arrays of arrays
Arrays of arrays
Or
Multi – Dimensional arrays
5/3/2017
Assoc. Prof. Stoyan 71
Bonev
Arrays of arrays = Multidimensional arrays
• One-dimensional arrays are used to store linear
collections of elements, i.e. vector.
• Two-dimensional arrays are used to store a matrix or a
table.
• A two-dimensional array is one-dimensional array in
which each element is another one-dimensional array.
(look fwd after 7 slides)
• A multidimensional array is an array in which each
element is another array
5/3/2017
Assoc. Prof. Stoyan 72
Bonev
Arrays of arrays = Multidimensional arrays
• A three-dimensional array consists of an array of twodimensional arrays, each of which is an array of onedimensional arrays.
int[][][] x = new int[2][2][5];
• x[0] and x[1] are two-dimensional arrays.
• x[0][0], x[0][1], x[1][0], x[1][1] are
one-dimensional arrays and each contains 5 elements
• x.length is 2
• x[0].length and x[1].length are 2
• x[0][0].length, x[0][1].length,
x[1][0].length, x[1][1].length are 5
5/3/2017
Assoc. Prof. Stoyan 73
Bonev
Arrays of arrays
• The elements of an array can be arrays.
• Declaration: int[][] table;
• Definition: table = new int[10][15];
• Combined:
int[][] table = new int[10][15];
• The first index (10) is called the row index;
• The second index (15) is the column index
5/3/2017
Assoc. Prof. Stoyan 74
Bonev
Example array of arrays
• int[][] table = new int[3][2];
or,
• int[][] table = { {1, 2}, {3, 6}, {7, 8} };
An array of three rows and two columns
• For example, table[1][1] contains 6
• table[2][1] contains 8, and
0
1
1 3
2 7
0
5/3/2017
1 • table[1][2] is “array out of bounds”
2
6 • To “zero out this table”:
8
for (int i = 0; i < 3; i++)
for (int j = 0; j < 2; j++)
table[i][j] = 0;
Assoc. Prof. Stoyan 75
Bonev
Declaring Variables of 2-D Arrays and Creating 2D Arrays
// Declare array ref var
dataType[][] refVar;
// Create array & assign its reference to variable
refVar = new dataType[10][10];
// Combine declaration and creation in one stmt
dataType[][] refVar = new dataType[10][10];
// Alternative syntax, not recommended
dataType refVar[][] = new dataType[10][10];
5/3/2017
Assoc. Prof. Stoyan 76
Bonev
Declaring Variables of 2-D Arrays and
Creating 2-D Arrays
// declare array ref var, create 2D array and assign its reference to array ref var
// all three above activities in one step - statement
int[][] matrix = new int[10][10];
matrix[0][0] = 3;
for (int i = 0; i < matrix.length; i++)
for (int j = 0; j < matrix[i].length; j++)
matrix[i][j] = (int)(Math.random() * 1000);
// matrix.length – numr of rows or matrix capacity
// matrix[i].length – capacity of row nmr i
5/3/2017
Assoc. Prof. Stoyan 77
Bonev
Two-dimensional Array Illustration
0 1
2
3
4
0 1
2
3
4
0
2
0
0
0
1
1
1
4
2
2
2
7
8
9
3
3
10
11
12
4
4
matrix = new int[5][5];
7
matrix[2][1] = 7;
3
1
2
3
5
6
int[][] array = {
{1, 2, 3},
{4, 5, 6},
{7, 8, 9},
{10, 11, 12}
};
matrix.length? 5
array.length? 4
matrix[0].length? 5
array[0].length? 3
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
5/3/2017
1
6
Assoc. Prof. Stoyan 78
Bonev
Lengths of Two-dimensional
Arrays
int[][] x = new int[3][4];
x
x[0][0] x[0][1] x[0][2] x[0][3]
x[0].length is 4
x[1][0] x[1][1] x[1][2] x[1][3]
x[1].length is 4
x[2][0] x[2][1] x[2][2] x[2][3]
x[2].length is 4
x[0]
x[1]
x[2]
x.length is 3
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
5/3/2017
8
Assoc. Prof. Stoyan 79
Bonev
Ragged Arrays
Each row in a two-dimensional array is itself an array. So,
the rows can have different lengths. Such an array is
known as a ragged array. For example,
int[][] matrix = {
{1, 2, 3, 4, 5},
matrix.length is 5
{2, 3, 4, 5},
matrix[0].length is 5
{3, 4, 5},
matrix[1].length is 4
{4, 5},
matrix[2].length is 3
matrix[3].length is 2
{5}
matrix[4].length is 1
};
5/3/2017
Assoc. Prof. Stoyan 80
Bonev
Associative Arrays
import java.util.Hashtable;
…
// associative array - special case – hashtable
Hashtable<String, String> ht = new
Hashtable<String, String>();
ht.put("Sofia", "BG");
ht.put("Berlin", "DE");
System.out.print(" " + ht.get("Sofia"));
5/3/2017
Assoc. Prof. Stoyan 81
Bonev
The ArrayList and Vector Classes
You can create an array to store objects. But the array’s size is fixed once
the array is created. Java provides the ArrayList class that can be used to
store an unlimited number of objects.
java.util.ArrayList
+ArrayList()
Creates an empty list.
+add(o: Object) : void
Appends a new element o at the end of this list.
+add(index: int, o: Object) : void
Adds a new element o at the specified index in this list.
+clear(): void
Removes all the elements from this list.
+contains(o: Object): boolean
Returns true if this list contains the element o.
+get(index: int) : Object
Returns the element from this list at the specified index.
+indexOf(o: Object) : int
Returns the index of the first matching element in this list.
+isEmpty(): boolean
Returns true if this list contains no elements.
+lastIndexOf(o: Object) : int
Returns the index of the last matching element in this list.
+remove(o: Object): boolean
Removes the element o from this list.
+size(): int
Returns the number of elements in this list.
+remove(index: int) : Object
Removes the element at the specified index.
+set(index: int, o: Object) : Object
5/3/2017
Sets the element at the specified index.
Assoc. Prof. Stoyan 82
Bonev
ArrayList demo example
package sbarraylist;
import java.util.ArrayList;
public class SBArrayList {
public static void main(String[] args) {
ArrayList ar = new ArrayList();
ar.add("Sofia");
ar.add("Varna");
System.out.println(ar.toString());
for (int i=ar.size()-1; i>=0; i--) System.out.print(ar.get(i) +" ");
}
}
5/3/2017
Assoc. Prof. Stoyan 83
Bonev
Practical tasks
.
5/3/2017
Assoc. Prof. Stoyan 84
Bonev
Initializing arrays with input values
java.util.Scanner input = new Scanner(System.in);
System.out.println("Enter " + matrix.length + " rows and " +
matrix[0].length + " columns: ");
for (int row = 0; row < matrix.length; row++) {
for (int column = 0; column < matrix[row].length; column++) {
matrix[row][column] = input.nextInt();
}
}
5/3/2017
Assoc. Prof. Stoyan 85
Bonev
Initializing arrays with random values
for (int row = 0; row < matrix.length; row++) {
for (int column = 0; column < matrix[row].length; column++) {
matrix[row][column] = (int)(Math.random() * 100);
}
}
5/3/2017
Assoc. Prof. Stoyan 86
Bonev
Printing arrays
for (int row = 0; row < matrix.length; row++) {
for (int column = 0; column < matrix[row].length; column++) {
System.out.print(matrix[row][column] + " ");
}
System.out.println();
}
5/3/2017
Assoc. Prof. Stoyan 87
Bonev
Summing all elements
int total = 0;
for (int row = 0; row < matrix.length; row++) {
for (int column = 0; column < matrix[row].length; column++) {
total += matrix[row][column];
}
}
5/3/2017
Assoc. Prof. Stoyan 88
Bonev
Summing elements by column
for (int column = 0; column < matrix[0].length; column++) {
int total = 0;
for (int row = 0; row < matrix.length; row++)
total += matrix[row][column];
System.out.println("Sum for column " + column + " is "
+ total);
}
5/3/2017
Assoc. Prof. Stoyan 89
Bonev
Demo programs - Example
// Package Declaration
import java.lang.*; // not required
// Program start class
public class HelloWorld
{
// Main begins program execution
public static void main(string[] args)
{
// This is a single line comment
/* This is a
multiple
line comment */
System.out.println("Hello World!");
}
}
90
import java.util.*;
public class test11 {
public static void main(String[] args) {
System.out.println("Hello, it’s: ");
System.out.println(new Date());
}
}
91
Demo Programs
Demonstrate end-of-file controlled loop using
standard input from the keyboard and standard
output to the screen
Solution to implement in a version:
as console application
Problem:
To accumulate sum of stream of input integers
terminated by end-of-data (CTRL/Z) indicator 92
Demo Program – Source Code
// Java demo program, standard Input (keyboard) / Output (screen)
// Class Scanner (Import java.util.Scanner;)
// Methods next(), nextInt(), nextFloat(), nextDouble(), nextLine(), hasNext()
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
int sum=0, arg;
Scanner cin = new Scanner(System.in);
while (cin.hasNext())
{
sum += cin.nextInt();
}
System.out.println("Final sum = " + sum);
}
}
93
Thank You
For
Your Attention!
94