Download 03-ch2-java-basics

Java basics
Chapter 2
Slides still stolen (and in a very exciting format!)
Trey Kirk
1
DisplayForecast.java
// Authors: J. P. Cohoon and J. W. Davidson
// Purpose: display a quotation in a console window
public class DisplayForecast {
Three comments
// method main(): application entry point
public static void main(String[] args) {
System.out.print("I think there is a world market for");
System.out.println(" maybe five computers.");
System.out.println("
Thomas Watson, IBM, 1943.");
}
}
// class
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// Authors: J. P. Cohoon and J. W. Davidson
// Purpose: display a quotation in a console window
public class DisplayForecast {
Method main() is part of
DisplayForecast
// method main(): application entry point
public static void main(String[] args) {
System.out.print("I think there is a world market for");
System.out.println(" maybe five computers.");
System.out.println("
Thomas Watson, IBM, 1943.");
}
}
Indentation indicates subcomponents
Statements are
part of method
main()
3
Good whitespacing
// Authors: J. P. Cohoon and J. W. Davidson
// Purpose: display a quotation in a console window
public class DisplayForecast {
Whitespace
// method main(): application entry point
public static void main(String[] args) {
System.out.print("I think there is a world market for");
System.out.println(" maybe five computers.");
System.out.println("
Thomas Watson, IBM, 1943.");
}
}
Whitespace separates program elements
Whitespace between program elements is
ignored by Java
4
Bad whitespacing
 The same program without any whitespacing or comments:
public class DisplayForecast2 { public static void
(String[] args) { System.out.print("I think there
world market for"); System.out.println(" maybe
computers."); System.out.println("
Thomas Watson,
1943."); } }
main
is a
five
IBM,
5
Identifiers
 Identifiers are names for variables, classes, etc.
 Good ones are compact, but inidicate what they stand for
 radius, width, height, length
 Bad ones are either too long
 theRadiusOfTheCircle
 theWidthOfTheBoxThatIsBeingUsed
 the_width_of_the_box_that_is_being_used
 Or too short
 a, b, c, d, e
 Good identifiers will help the graders understand your
program!
6
Keywords
 Some words are reserved, and can’t be used as identifiers
// Authors: J. P. Cohoon and J. W. Davidson
// Purpose: display a quotation in a console window
public class DisplayForecast {
}
// method main(): application entry point
public static void main(String[] args) {
System.out.print("I think there is a world market for");
System.out.println(" maybe five computers.");
System.out.println(" Thomas Watson, IBM, 1943.");
}
7
Capitalization
 Case matters!
 public ≠ Public ≠ PUBLIC
 This is different than FORTRAN and BASIC
 This is the same as C/C++
 You can use Public as a identifier
 Not recommended, though!
8
Statements
 A statement in Java is (usually) a single line
 Example: System.out.println (“Hello world!”);
 All statements must end with a semi-colon
 That tells Java that the statement is finished
9
A bit of humor:
1989 Computer
Advertisement
Guess the price!
10
Variables
11
Defining variables
 We’ve seen variables before in math
 y = mx + b
 Here y, m, x, and b can hold any value
 To store things in a computer program, we also use variables
 Example:
 int x = 5;
 Visualization:
 This defines an integer variable with value 5
x
5
 The variable is x
 The type is int
12
More on variables
 An integer variable can only hold integers
 In other words, it can’t hold 4.3
 To hold floating point values, we use the double type
 double d = 4.3;
d
4.3
 The variable is d
 The type is double
13
Primitive variable assignment
 Assignment operator =
 Allows the variable
memory to
location
be updated
for a variable to be updated
target
=
Name of previously
defined object
 Consider
int j = 11;
j = 1985;
expression ;
Expression t o be
evaluat ed
j
1985
11
14
Primitive variable assignment
 Consider
int a = 1;
int aSquared = a * a;
a = 5;
aSquared = a * a;
 Consider
int i = 0;
i = i + 1;
 Consider
int asaRating;
asaRating = 400;
a
1
5
aSquared
25
1
i
1
0
asaRating
400
15
Primitive variable assignment
 Consider
double x = 5.12;
double y = 19.28;
double rememberX = x;
x = y;
y = rememberX;
x
19.28
5.12
y
19.28
5.12
rememberX
5.12
16
Printing variables
 To print a variable to the
System.out.println() statement:
screen,
put
it
in
a
 int x = 5;
 System.out.println (“The value of x is “ + x);
 Important points:
 Strings are enclosed in double quotes
 If there are multiple parts to be printed, they are
separated by a plus sign
17
public class SolvingABC {
From this
week’s
lab
public static void main(String[] args) {
// variable definitions and initializations
int a = 3;
int b = 12;
int c = 6;
int d = 1;
// calculate results
double result1 = d *
double result2 = c +
double result3 = d double result4 = c *
double result5 = b /
Note that I don’t
show a lot of
comments so that
the code will fit on
a single slide
a;
2 * a;
b / c;
b % c;
2;
// display the results
System.out.println();
System.out.println("result1
System.out.println("result2
System.out.println("result3
System.out.println("result4
System.out.println("result5
System.out.println();
Also note all the
semi-colons
}
}
:
:
:
:
:
"
"
"
"
"
+
+
+
+
+
result1);
result2);
result3);
result4);
result5);
Variable initialization
 Note that the following
 int x;
 x = 5;
 is (mostly) the same as the following:
 int x = 5;
19
You can only declare variables once
 The following code will not work:
 int x = 5;
 int x = 6;
 Java can have only one variable named x
 So you can’t declare multiple variables with the same
name
 (we’ll see ways around this later in the semester)
20
Today’s demotivators
21
Types
23
Primitive variable types
 Java has 8 (or so) primitive types:
 float
real numbers
 double
two values: true and false
 boolean
 char
a single character
 byte
 short
integer numbers
 int
 long
 Also the void “type”, which we will see later
 We’ll only be using half of the types in this course: int,
24
double, boolean, and char
Primitive real (floating-point) types
 A float takes up 4 bytes of space
 Has 6 decimal places of accuracy: 3.14159
 A double takes up 8 bytes of space
 Has 15 decimal places of accuracy: 3.14159265358979
 Always use doubles
 It will save you quite a headache!
25
Primitive integer types

Consider a byte:
0
1
0
0
0
1
0
1


1 byte = 8 bits
Each bit has two possibilities: 0 or 1


28 = 256
Thus, a byte can have any one of 256 values

A Java byte can have values from -128 to 127


From -27 to 27-1
C/C++ has unsigned versions; Java does not
26
Primitive integer types
Type
Bytes Minimum value
Maximum value
byte
1
-27=-128
27-1=127
short 2
-215=
-32,768
215-1=
32,767
int
4
-231=-2,147,483,648
231-1=2,147,483,647
long
8
-263=-9,223,372,036,
854,775,808
263-1=9,223,372,036,
854,775,807
27
Increment and decrement operators
 ++
 Increments a number variable by 1
 - Decrements a numeric variable by 1
 Consider
int i = 4;
++i;
System.out.println(i);
System.out.print(++i);
System.out.println(i++);
System.out.println(i);
//
//
//
//
//
//
i
4
5
6
7
define
increment
display
update then display
display then update
display
28
Why C++ was named C++

The increment operator adds one to the
integer value
– Or makes it ‘one better’

So when Bjarne Stroustrup was making
the successor to C, he was making a ‘one
better’ language
29
Primitive character type
 All




characters have a integer equivalent
‘0’ = 48
‘1’ = 49
‘A’ = 65
‘a’ = 97
 Thus, you can refer to ‘B’ as ‘A’+1
30
Primitive boolean type
 The boolean type has only two values:
 true
 false
 There are boolean-specific operators
 && is and
 || is or
 ! is not
 etc.
 We’ll see those operators in a few slides
31
Variables must be declared before use
 The following code will not work:
 x = 5;
 System.out.println (x);
 Java requires you to declare x before you use it
32
Variable initialization
 Consider the following code:
int x;
System.out.println(x);
 What happens?
 Error message:
 variable x might not have been initialized
 Java also requires you to give x a value before you use it
33
Constants
 Consider the following:
final int x = 5;
 The value of x can NEVER be changed!
 The value assigned to it is “final”
 This is how Java defines constants
 Constants have a specific naming scheme
 MILES_PER_KILOMETER
 All caps, with underscores for spaces
34
Expressions
 What is the value used to initialize expression
int expression = 4 + 2 * 5;
 What value is displayed
System.out.println(5 / 2.0);
 Java rules in a nutshell
 Each operator has a precedence level and an associativity
 Operators with higher precedence are done first
 * and / have higher precedence than + and  When floating-point is used the result is floating point
35
Question on expressions
 Does the following statement compute the average of double
variables a, b, and c? Why or why not?
double average = a + b + c / 3.0;
36
Java operators

The following are the common operators for ints:
 +-/*%
 Division is integer division
 6 / 2 yields 3
 7 / 2 yields 3, not 3.5
 Because everything is an int, the answer is an int
 Modulus is %
 Returns the remainder
 7 % 2 yields 1
 6 % 2 yields 0

Floats and doubles use the same first four operators
 +-/*
 7.0 / 2.0 yields 3.5
 7.0 / 2 yields 3.5
 7 / 2.0 yields 3.5
 7 / 2 yields 3
37
Java operators
 Booleans have their own operators
 && is AND
 Only true when both operands are true
 true && true yields true
 false && true yields false
 || is OR
 True when either of the operands (or both) are true
 true || false yields true
 false || false yields false
 ! is NOT
 Changes the value
 !true yields false
 !false yields true
38
New York Drivers
39
System.out.println
 Can print multiple things by using the + operator
 Let int i = 7;
 Example: System.out.println (“i = “ + i);
 Prints i = 7
 Can also have the statement on multiple lines
System.out.println (
“hello world!”
)
;
 But can’t have the String on multiple lines
System.out.println (
“hello
world!”
);
40
System.out.println
 System.out.println (“result:
 What does it print?
 result: 0
 System.out.println (“result:
 What does it print?
 result: 2
 System.out.println (“result:
 What does it print?
 result: 0.6
 System.out.println (“result:
 What does it print?
 result: 34.0
 System.out.println (“result:
 What does it print?
 result: 7.0
“ + 3/5);
“ + 5 % 3);
“ + 3/5.0);
“ + 3+4.0);
“ + (3+4.0));
41
Scanner usage
42
Interactive programs
 Programs that interact with their users through statements
performing input and output
43
Reading in a value from the keyboard
 We will see this in more detail later in this slide set
 For now (and for lab 2), this is what you need to know
 To read in values from the keyboard, you first have to create
a Scanner object
 Don’t worry about what an object is, what a Scanner is, or
about creation of these things
 We’ll get to them later
 To do this, use the following code:
Scanner stdin = new Scanner (System.in);
44
Reading in more values from the keyboard
 You should have this only once in your program.
 From then on, when you want to read in a value into a
variable, use the following:
int x = stdin.nextInt();
double d = stdin.nextDouble();
 Or
x = stdin.nextInt();
d = stdin.nextDouble();
45
Scanner usage example
import java.util.*;
public class ScannerUsage {
public static void main (String args[]) {
Scanner stdin = new Scanner (System.in);
System.out.println ("Enter first value");
int x = stdin.nextInt();
int y;
System.out.println ("Enter second value");
y = stdin.nextInt();
int z = x + y;
System.out.println ("The sum of " + x + " and " +
y + " is " + z);
}
}
46
Program demo…

ScannerUsage.java

Note that all this code is available on the
website!
47
How to make Java work with the Scanner
class
 In Java 1.5, do a:
import java.util.*;
 To create a new Scanner:
Scanner stdin = new Scanner (System.in);
48
Today’s demotivators
49
Program Examples
50
Example program: temperature
conversion
// Purpose: Convert a Celsius temperature to Fahrenheit
public class CelsiusToFahrenheit {
// main(): application entry point
public static void main(String[] args) {
// set Celsius temperature of interest
int celsius = 28;
// convert to Fahrenheit equivalent
int fahrenheit = 32 + ((9 * celsius) / 5);
// display result
System.out.println("Celsius temperature");
System.out.println("
" + celsius);
System.out.println("equals Fahrenheit temperature");
System.out.println("
" + fahrenheit);
}
}
51
Program demo…

CelsiusToFahrenheit.java
52
Computation
 Programmers frequently write small programs for computing
useful things
 Example – body mass index (BMI)
 Measure of fitness
 Ratio of person’s weight to the square of the person’s
height
 Weight in is kilograms, height is in meters
 Person of interest is 4.5 feet and weighs 75.5 pounds
 Metric conversions
 Kilograms per pound 0.454
 Meters per foot 0.3046
53
Program outline for BMI.java
// Purpose: Compute BMI for given weight and height
public class BMI {
// main(): application entry point
public static void main(String[] args) {
// define constants
// set up person's characteristics
// convert to metric equivalents
// perform bmi calculation
// display result
}
}
54
BMI.java: define constants
KILOGRAMS_PER_POUND
0.454
// define constants
final double KILOGRAMS_PER_POUND = 0.454;
final double METERS_PER_FOOT = 0.3046;
METERS_PER_FOOT
0.3046
55
BMI.java: personal characteristics
weightInPounds
75.5
// set up person's characteristics
double weightInPounds = 75.5; // our person’s weight
double heightInFeet = 4.5;
// our person’s height
heightInFeet
4.5
56
BMI.java: convert to metric equivalents
metricWeight
34.2770
// convert to metric equivalents
double metricWeight = weightInPounds *
KILOGRAMS_PER_POUND;
double metricHeight = heightInFeet *
METERS_PER_FOOT;
metricHeight
1.3706
57
BMI.java: perform BMI calculation
// perform bmi calculation
double bmi = metricWeight / (metricHeight *
metricHeight);
bmi
18.2439
58
BMI.java: display result
bmi
18.2439
// display result
System.out.println("A person with");
System.out.println(" weight " + weightInPounds + " lbs");
System.out.println(" height " + heightInFeet + " feet");
System.out.println("has a BMI of " + Math.round(bmi));
Math.round(bmi) is 18
Operator evaluation depend upon its operands
59
public static void main(String[] args) {
// define constants
final double KILOGRAMS_PER_POUND = 0.454;
final double METERS_PER_FOOT = 0.3046;
// set up person's characteristics
double weightInPounds = 75.5; // our person’s weight
double heightInFeet = 4.5;
// our person’s height
// convert to metric equivalents
double metricWeight = weightInPounds *
KILOGRAMS_PER_POUND;
double metricHeight = heightInFeet * METERS_PER_FOOT;
// perform bmi calculation
double bmi = metricWeight / (metricHeight * metricHeight);
// display result
System.out.println("A person with");
System.out.println(" weight " + weightInPounds + " lbs");
System.out.println(" height " + heightInFeet + " feet");
System.out.println("has a BMI of " + Math.round(bmi));
}
Program demo…

BMI.java
61
Common program elements
 Constant
 Symbolic name for memory location whose value does not
change
 KILOGRAMS_PER_POUND
 Variable
 Symbolic name for memory location whose value can
change
 weightInPounds
62
BMI Calculator
63
Interactive program for BMI
 Program outline
import java.util.*;
// Purpose: Compute BMI for user-specified
// weight and height
public class BMICalculator {
// main(): application entry point
public static void main(String[] args) {
//
//
//
//
//
//
//
}
}
defining constants
displaying legend
set up input stream
get person's characteristics
convert to metric equivalents
perform bmi calculation
display result
64
public static void main(String[] args) {
// define constants
//...
// displaying legend
System.out.println ("BMI Calculator\n");
// set up input stream
Scanner stdin = new Scanner (System.in);
// get person's characteristics
System.out.print("Enter weight (lbs): ");
double weight = stdin.nextDouble();
System.out.print("Enter height (feet): ");
double height = stdin.nextDouble();
// convert to metric equivalents
double metricWeight = weight * KILOGRAMS_PER_POUND;
double metricHeight = height * METERS_PER_FOOT;
// perform bmi calculation
double bmi = metricWeight / (metricHeight * metricHeight);
// display result
//...
}
import java.util.*;
class BMICalculator {
public static void main(String[] args) {
// define constants
final double KILOGRAMS_PER_POUND = 0.454;
final double METERS_PER_FOOT = 0.3046;
// displaying legend
System.out.println ("BMI Calculator\n");
// set up input stream
Scanner stdin = new Scanner (System.in);
// get person's characteristics
System.out.print("Enter weight (lbs): ");
double weight = stdin.nextDouble();
System.out.print("Enter height (feet): ");
double height = stdin.nextDouble();
// convert to metric equivalents
double metricWeight = weight * KILOGRAMS_PER_POUND;
double metricHeight = height * METERS_PER_FOOT;
// perform bmi calculation
double bmi = metricWeight / (metricHeight * metricHeight);
// display result
System.out.println("A person with");
System.out.println(" weight " + weight + " lbs");
System.out.println(" height " + height + " feet");
System.out.println("has a BMI of " + Math.round(bmi));
}
}
Program demo…

BMICalculator.java
67
Scanner API
public Scanner(InputStream in)
// Scanner(): convenience constructor for an
// InputStream
public Scanner(File s)
// Scanner(): convenience constructor for a filename
public int nextInt()
// nextInt(): next input value as an int
public short nextShort()
// nextShort(): next input value as a short
public long nextLong()
// nextLong(): next input value as a long
public double nextDouble()
// nextDouble(): next next input value as a double
public float nextFloat()
// nextFloat(): next next input value as a float
public String next()
// next(): get next whitespace-free string
public String nextLine()
// nextLine(): return contents of input line buffer
public boolean hasNext()
// hasNext(): is there a value to next
68
Casting
69
Casting
 Consider the following code
double d = 3.6;
int x = Math.round(d);
 Java complains (about loss of precision). Why?
 Math.round() returns a long, not an int
 So this is forcing a long value into an int variable
 How to fix this
double d = 3.6;
int x = (int) Math.round(d);
 You are telling Java that it is okay to do this
 This is called “casting”
 The type name is in parenthesis
70
More casting examples
 Consider
double d = 3.6;
int x = (int) d;
 At this point, x holds 3 (not 4!)
 This truncates the value!
 Consider
int x = 300;
byte b = (byte) x;
System.out.println (b);
 What gets printed?
 Recall that a byte can hold values -128 to 127
 44!
 This is the “loss of precision”
71
More on println()
72
System.out.println()
public static void main(String[] args) {
System.out.print("I think there is a world market for");
System.out.println(" maybe five computers.");
System.out.println("
Thomas Watson, IBM, 1943.");
}
 Class System supplies objects that can print and read values
 System variable out references the standard printing object
 Known as the standard output stream
 Variable out provides access to printing methods
 print(): displays a value
 println(): displays a value and moves cursor to the next
line
73
print() vs. println()
 What do these statements output?
System.out.print
System.out.println
System.out.println
System.out.println
System.out.println
(“foo”);
(“bar”);
();
(“foo”);
(“bar”);
 Output
foobar
foo
bar
74
Escape sequences
 Java provides escape sequences
characters
 \b
backspace
 \n
newline
 \t
tab
 \r
carriage return
 \\
backslash
 \"
double quote
 \'
single quote
for
printing
special
75
Escape sequences
 What do these statements output?
System.out.println("Person\tHeight\tShoe size");
System.out.println("=========================");
System.out.println("Hannah\t5‘1\"\t7");
System.out.println("Jenna\t5'10\"\t9");
System.out.println("JJ\t6'1\"\t14");
 Output
Person Height Shoe size
=========================
Hannah 5‘1"
7
Jenna
5'10"
9
JJ
6'1"
14
76
What we wish computers could do
77