Transcript Java Concepts Ch2

Java Concepts
Chapter 2 - Using Objects
Mr. Smith
AP Computer Science A
Basic Java
Syntax and Semantics
Data Types:
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Primitive data types
Objects
Numbers (integer and floating point)
Strings
Characters (“A”, “B”, etc.) (char)
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Objects
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Booleans (true and false) (boolean)
Numbers use operators (addition and multiplication)
Are sent messages
Must be instantiated before use (remember robots?)
Strings
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Are objects
Are sent messages
Do not need to be instantiated
Can be combined using the concatenation operator (+)
Basic Java
Syntax and Semantics
Numeric Data Types:
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6 numeric data types are used in Java:
 int (integer, no decimals)
 double (double precision floating-point
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numbers; i.e. numbers with decimals)
short
long
Not in AP CS subset.
byte
float
Basic Java
Syntax and Semantics
Some Java numeric data types:
I hope I don’t have to
memorize this
Basic Java
Syntax and Semantics
Literals:
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By literal, we mean any number, text,
or other information that represents a
specific value and does not change:
(PI)
 7.21
 “Hello World”
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In the statement:
 int month = 10;
 10 is the literal
Basic Java
Syntax and Semantics
Identifiers
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An identifier is the name of a variable, method, or
class
Java imposes the following rules on identifiers:
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Can be made up of letters, digits, underscore (_), and
dollar sign ($) characters.
Cannot start with a digit: score1 is legal but not 1score
Cannot use reserved symbols such as ! and %: money! is
not a legal identifier.
Spaces are not permitted: myScore is legal but not
my Score.
Cannot use reserved words such as new, public, while.
int newNum = 2; is legal but int new = 2; is not valid.
Identifiers are case sensitive: myScore and myscore are
different identifiers.
Basic Java
Syntax and Semantics
Questions/Practice:
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What is the data type of each of the following values?
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int
String
double
100Answers
void
my_cash
<myVar>
money$value
lucky number
illegal
illegal
legal
illegal
legal
illegal
Tell me whether each of these identifiers are legal or
illegal:
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56
“56”
56.0
Create a variable to store the school phone number
and use camel case.
Basic Java
Syntax and Semantics
Declarations
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Variables
A variable is a storage location in memory that
has a type, name (identifier), and value.
 Before using a variable for the first time, the
program must declare its type.
 Declare a variable in a variable declaration
statement (i.e. int year; )
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Several variables can be declared in a single
declaration.
 Remember to use camelCase for variable names
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Basic Java
Syntax and Semantics
Declarations
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Variables
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Syntax:
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<typeName> <variableName> = <value>; OR
<typeName> <variableName>;
Initial values can be assigned in the same
statement as its variable declaration:
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is the same as: int x;
int x, y, z = 7;
int y;
double p, q = 1.41, t;
int z = 7;
String name = “AP Computer Science”;
UrRobot karel = new UrRobot(1,1,East,0);
String name = 13;
Types do not match
Basic Java
Syntax and Semantics
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Objects
Constants
Declare the object variable, instantiate or create an
object, and assign the object to the variable.
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<className> <variableName> = new <className>()
Robot bot = new Robot(1, 1, North, 0);
Constants
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The value cannot change
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Literal
final double SALES_TAX_RATE = 7.00;
final indicates a variable is declared as a constant
The naming convention for constants is that they are
written in UPPERCASE with underlines between words.
Trying to change the value of a constant after it is
initialized will be flagged by the compiler as an error.
Basic Java
Syntax and Semantics
Assignment Statements
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The assignment operator is =
An assignment statement has the following form:
<variableName> = <expression>;
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The value of the expression on the right is
assigned to the variable on the left. It simply
replaces the value of the variable:
double celsius;
double fahrenheit = 82.5; //Assign 82.5 to variable fahrenheit
celsius = (fahrenheit – 32.0) * 5.0 / 9.0;
Basic Java
Syntax and Semantics
Assignment Statements
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It is an error to use a variable that has
never had anything assigned to it:
int myNumber;
System.out.println(myNumber);
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The solution to this problem is:
int myNumber;
myNumber = 20; // Assign a value to myNumber
System.out.println(myNumber);
Basic Java
Syntax and Semantics
Arithmetic Expressions
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An arithmetic expression consists of
operands and operators combined in a manner
used in algebra. The usual rules apply:
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Remember the mnemonic phrase:
“Please Excuse My Dear Aunt Sally”
Multiplication and division are evaluated before
addition and subtraction.
Operators of equal precedence are evaluated from left
to right.
Parentheses can be used to change the order of
evaluation.
Basic Java
Syntax and Semantics
Common operators and their precedence:
Basic Java
Syntax and Semantics
Terminal I/O for
Different Data types
Scanner class: reads from the input stream
**** new for Java 5.0, but not tested on AP exam
Here are the methods in the Scanner class:
METHOD
DESCRIPTION
double nextDouble()
Returns the first double in the input line.
Leading and trailing spaces are ignored.
int nextInt()
Returns the first integer in the input line.
Leading and trailing spaces are ignored.
String nextLine()
Returns the input line, including leading and
trailing spaces. Warning: A leading newline is
returned as an empty string.
Terminal I/O for
Different Data types
The following program illustrates the major features of terminal
I/O:
import java.util.Scanner;
public class TestTerminalIO {
public static void main (String [] args)
{ Scanner in = new Scanner(System.in);
System.out.print ("Enter your name (a string): ");
String name = in.nextLine();
System.out.print("Enter your age (an integer): ");
int age = in.nextInt();
System.out.print("Enter your weight (a double): ");
double weight = in.nextDouble();
Terminal I/O for
Different Data types
System.out.println ("Greetings " + name +
". You are " + age + " years old and you weigh " +
weight + " pounds.");
}
}
Terminal I/O for
Different Data types
Warning: A leading newline is returned as an empty string.
// now change the order of the input,
// putting the string last
System.out.print("Enter your age (an integer): ");
int age = in.nextInt();
System.out.print("Enter your weight (a double): ");
double weight = in.nextDouble();
System.out.print ("Enter your name (a string): ");
String name = in.nextLine();
Terminal I/O for
Different Data types
The string name is the empty string. The reason is
that the method nextDouble ignored but did not
consume the new line that the user entered following
the number. Therefore, this newline character was
waiting to be consumed by the next call of nextLine,
which was expecting more data.
To avoid this problem, you should either input all lines
of text before the numbers or, when that is not
feasible, run an extra nextLine after numeric input to
eliminate the trailing newline characters.
Terminal I/O for
Different Data types
Warning: A leading newline is returned as an empty string.
// now change the order of the input,
// putting the string last
System.out.print("Enter your age (an integer): ");
int age = in.nextInt();
System.out.print("Enter your weight (a double): ");
double weight = in.nextDouble();
in.nextLine();
// to consume the newline char
System.out.print ("Enter your name (a string): ");
String name = in.nextLine();
TemperatureConversion
Create a TemperatureConversion class as follows:
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Enter a prompt for the Fahrenheit temperature in the console (using
the Scanner class), and allow the user to enter the temperature there.
Convert the temperature from Fahrenheit to Celsius
Convert the temperature from Fahrenheit to Kelvin
Print a message to the console that shows the conversion, such as:
50.0 F = 10.0 C
50.0 F = 283.15 K
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Do the same thing to allow the user to enter the temperature in Celsius
and then print the conversion to Fahrenheit and Kelvin.
0.0 C = 32.0 F
0.0 C = 273.15 K
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Do the same thing to allow the user to enter the temperature in Kelvin
and then print the conversion to Fahrenheit and Celsius.
283.15 K = 50.0 F
283.15 K = 10.0 C
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The conversion formulas can easily be found on the internet
Please try these conversions where Fahrenheit = 50 and Celsius = 0.
Also try a couple of other examples of your choice to verify that the
program is working correctly.
Basic Java
Syntax and Semantics
Division:
The semantics of division are different for integer
and floating-point operands.
Thus:
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5.0 / 2.0
5/2
yields 2.5
yields 2 (a quotient in which the decimal portion
of the answer is simply dropped)
Modulus:
The operator % yields the remainder obtained
when one number is divided by another. Thus:
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9%5
9.3 % 5.1
yields 4
yields 4.2
Basic Java
Syntax and Semantics
Mixed-Mode Arithmetic
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Intermixing integers and floating-point numbers is called
mixed-mode arithmetic.
When arithmetic operations occur on operands of
different numeric types, the less inclusive type (int) is
temporarily and automatically converted to the more
inclusive type (double) before the operation is
performed.
Examples:
but
5.0 / 2.0
5/2
yields 2.5 (both double)
yields 2 (both int)
5.0 / 2
yields 2.5 (mixed)
Basic Java
Syntax and Semantics
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Mixed-mode assignments are also allowed,
provided the variable on the left is of a more
inclusive type than the expression on the right.
Otherwise, a syntax error occurs.
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double d;
int i;
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i = 45;
d = i;
-- OK, because i is an integer and 45 is an integer.
-- OK, because d is more inclusive than i.
The value 45.0 is stored in d.
-- Syntax error because i is less inclusive than d.
i = d;
 Can put an integer into a double but not a
double into an integer.
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Basic Java
Syntax and Semantics
Difficulties associated with mixed-mode arithmetic
can be circumvented using a technique called
“casting”. This allows one data type to be explicitly
converted to another.
The cast operator, either (int) or (double), appears
immediately before the expression it is supposed to
convert.
The (int) cast simply throws away the digits after the
decimal part.
The (double) cast inserts a .0 after the integer.
Basic Java
Syntax and Semantics
Examples:
double x, y;
int m, n;
a)
x = (double) 5 / 4;
x = 5.0 / 4;
x = 1.25
b)
y = (double) (5 / 4);
y = (double) (1);
y = 1.0
c)
m = (int) (x + 0.5);
if x = 1.1, then
m = (int) (1.6);
m=1
Basic Java
Syntax and Semantics
String Expressions and Methods
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Simple Concatention
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The concatenation operator uses the plus symbol (+)
String firstName,
lastName,
fullName,
lastThenFirst;
//declare four string
//variables
firstName = “Debbie”;
lastName = “Klipp”;
//initialize firstName
//initialize lastName
fullName = firstName +” “ + lastName; //yields “Debbie Klipp”
lastThenFirst = lastName +”, “+ firstName; //yields “Klipp, Debbie”
Basic Java
Syntax and Semantics
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Concatenating Strings and Numbers
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Strings also can be concatenated to numbers. (The
number is automatically converted to a string
before the concatenation operator is applied.)
String message;
int x = 20, y = 35;
message = “Bill sold ” + x + “ and Sylvia sold ” + y
+ “ subscriptions.”;
// yields “Bill sold 20 and Sylvia sold 35 subscriptions.”
Basic Java
Syntax and Semantics
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Precedence of Concatenation
 The concatenation operator has the same
precedence as addition, which can lead to
unexpected results:
a) “number ” + 3 + 4
number 34
b) “number ” + (3 + 4)
number 7
c) “number ” + 3 * 4
number 12
d) 3 + 4 + “ number”
7 number
Basic Java
Syntax and Semantics
The length Method
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Strings are objects and implement several
methods.
A String returns its length in response to a
length message:
String theString;
int theLength;
theString = “the cat sat on the mat.”;
theLength = theString.length(); // yields 23
What is the result of printing
the following to the console?
If you don’t know the answer, then create a client class and print it to the
console using a System.out.println() statement.
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Calculate the following and print the results to the console:
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15 / 6
15.0 / 6.0
15.0 / 6
15 / 6.0
Find the remainder of 8 / 3
Find the remainder of 8.95 / 1.1
(double) 15 / 6
(double) (15 / 6)
(double) 15 / 6 + 0.6
(int) ((double) 15 / 6 + 0.6)
______________________
______________________
______________________
______________________
Syntax: _____________ Answer: _______
Syntax: _____________ Answer: _______
______________________
______________________
______________________
______________________
Output the following string functions to the console:
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“Age “ + 2 + 8
“Age “ + (2 + 8)
“Age “ + 2 * 8
2 + 8 + “ Age”
If name = “John Doe”, then what
______________________
______________________
______________________
______________________
is name.length(); _______
What is the result of printing
the following to the console?
If you don’t know the answer, then create a client class and print it to the
console using a System.out.println() statement.
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Calculate the following and print the results to the console:
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15 / 6
15.0 / 6.0
15.0 / 6
15 / 6.0
Find the remainder of 8 / 3
Find the remainder of 8.95 / 1.1
(double) 15 / 6
(double) (15 / 6)
(double) 15 / 6 + 0.6
(int) ((double) 15 / 6 + 0.6)
2
2.5
2.5
2.5
Syntax: 8 % 3
Syntax: 8.95 % 1.1
2.5
2.0
3.1
3
Output the following string functions to the console:
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“Age “ + 2 + 8
“Age “ + (2 + 8)
“Age “ + 2 * 8
2 + 8 + “ Age”
If name = “John Doe”, then what
Age 28
Age 10
Age 16
10 Age
is name.length(); 8
Answer:
Answer:
2
.15
What is the result of printing
the following to the console?
If you don’t know the answer, then create a client class and print it to the
console using a System.out.println() statement.
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Calculate the following and print the results to the console:
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(3.0 + 6.0) / 2 * 3
3.0 + 6.0 / 2 * 3
3.0 + 6.0 / (2 * 3)
8 + 6 + 3.5 - 6.0
(int) 15.0 / 4 * 4
(double) (15 / 6 + 9)
(int) Math.PI * 10
(int) (4.0 / 2) * 3 + 1.5
3.0 / 6.0 + (3 * 3)
3 / 6 + 7 / 14
(((6 * 4) + (7 * 2)* 2) + 5)
13.5
12.0
4.0
11.5
12
11.0
30
7.5
9.5
0
57