Java程序辅导

Data and Variables
• Data Types
• Expressions
– Operators
– Precedence
• String Concatenation
• Variables
– Declaration
– Assignment
– Shorthand operators
Review
– class
• All code in a java file is written in a
class
• public class  { ….. }
– Static methods
• public static void  () {
….. }
• Made up of statements;
• Used in procedural programming
– Decompose large tasks into
smaller subtasks
– Create a method for each unique 
subtask to eliminate redundancy
– Errors
• Syntax (won’t compile)
• Runtime (won’t run)
• Logic (incorrect result)
– Identifiers
• Must begin with a letter,
underscore, or $
• Must contain only letters, 
digits, underscores, or $
• Can not be a Java
keyword/reserved word
Data Types
• Java is a “strongly typed” language.
– Each piece of data has a specific “type”
• Java supports two categories of data types
– Primitive data
– Objects (covered later)
Primitive Data Types
• Integer (Whole Number)
• Java keywords for Integer Types
• int (4 bytes)
• byte (1 byte)
• short (2 bytes)
• long (8 bytes)
• We will use the int type
– minimum value      -2,147,483,648
– maximum value       2,147,483,647
• Examples: 0,  35, +148, -250
Primitive Data Types (cont.)
• Real (Floating point, Decimal)
• Java keywords for Real Number Types
• double (8 bytes) (double precision)
• float  (4 bytes)
• We will use the double type
– minimum value 4.94065645841246544e-324
– maximum value 1.79769313486231570e+308
• Examples:   23.   .03   -5.5   10.0583   34e-20
Primitive Data Types (cont.)
• Character (Single Character)
• Java keyword for Character Type
– char (2 bytes)
• Enclosed in single quotes ‘ ’
• Examples: ‘a’ ‘&’ ‘X’ ‘8’
• Use escape sequence for
– Single Quote   ‘\’’
– Backslash   ‘\\’
Primitive Data Types (cont.)
• boolean (Logical Values)
• Java keyword
– boolean
• Only two different values (Java keywords)
– true
– false
• Covered Later
Practice
• Which of the following are legal int
literals?
• 5.
• -1
• 22
• ‘7’
• 1.5
• -6875309
• 10.0
• 2.3
Practice
• What primitive data type would you use 
to store…
• a person’s middle initial?
• number of people in class?
• cost of lunch?
• distance to class?
• number of siblings a person has?
• your grade in a class?
Expressions
• Expression – A simple value or a set of 
operations that produces a value.
• Evaluation – The process of obtaining the
value of an expression.
• Expressions have values, statements do not.
• Two types
– Arithmetic
– Boolean (covered later)
Expressions (cont.)
• A simple value or a set of operations that
produce a value
– Literal Value
• 42 -365  0  +9812
• 28.9  0.24  207.  0.0  -.98
• true false
• ‘a’ ‘m’   ‘X’   ‘!’   ‘\\’    ‘\’’
– Operations: combining values
• (5 * 6) + 32
• Use expressions in print statements
– System.out.println(4);
– System.out.println(2 + 2);
Arithmetic Operators and Operands
• Operators: indicate the operation to be
performed
– Addition (+) (5 + 2)
– Subtraction (-)              (5  - 2)
– Multiplication  (*)          (5 * 2)
– Division  ( / ) (5 / 2)    (5.0 / 2.0)
– Remainder (mod)  (%)   (5 % 2)
• Operands: values used in the expression
– Literal
– Expression  (3 + 2) * (6 / 2)
Division
• Integer Division - result is an integer, the
remainder is dropped (truncation).
– examples:
• 22 / 4   = 5
• 116 / 5 =  23
• Floating Point Division – “normal division”.
– examples:
• 22.0 / 4. = 5.5
• 116.0 / 5.0 = 23.2
Remainder (Mod)
• % - The remainder operator
– Returns the remainder of division
– examples:
• 22 % 4 = 2
• 22. % 4.0 = 2.0
• 5.2 % 2.4 = 0.4
• 1 % 5 = 1
• 0 % 5 = 0
• 5 % 0 = undefined (runtime error)
• Useful applications
– Testing for even/odd (number % 2 = 0 means even)
– Extract final digit (number % 10)
Precedence
• Precedence – The order of evaluating 
expressions
• Multiplication, Division, and Modulo take 
precedence over Addition and Subtraction.
• Unary operators (+, -) (pos, neg) take
precedence over all 5 operators.
• Within same level of precedence, evaluate
from left to right
• Override precedence with parentheses.
Precedence Examples
• 8 * 4 / 10 + (4 + 2) * 5  % 2
• 46 % 8 * 2 / 7 + 11 / 4 * 3
Mixing Types
Promotion: a copy of a value is converted to a “higher” type.
– Does not lose information about the value
– Integer to a double
– Result of operation between integer and double is a double 
(integer is promoted to a double to perform the operation) 
– 6.4 + 8 = 14.4
Casting: a copy of a value is converted to another type
– Double to integer
– Loses the fraction part
– Requires a cast – put the name of the type you want in 
parentheses in front of the value you want to cast
– (int) 5.62          (result is 5)
– (int) 5.0 / 2.0   (result is 5 / 2.0 = 2.5)
– (int) (5.0 / 2.0)  (result is 2)
More Casting
• Only casts value immediately following cast For example: 
– 23 / 2 = 11 
– (double) 23 / 2 = 11.5  (23. / 2, 23 / 2., 23. / 2.)
– (double) (23 / 2) = 11.0 
• Example of when we may want to use casting: 
– We have some books that are 0.15 feet wide and we 
want to know how many of them will fit in a bookshelf 
that is 2.5 feet wide. 
– Then, 2.5 / 0.15 = 16.666 books. 
• How are we going to put 2/3 of a book on the shelf?
– Instead, we need to see how many whole books can fit 
on the shelf: (int) (2.5 / 0.15) = 16 
String Concatenation
• Combining several strings into a single string, or 
combining a string with other data into a new, 
longer string
– Addition operator (+)
– Result of adding a String literal and a primitive data 
type is a String literal (primitive type is promoted to a
String literal to perform the operation)
– Examples:
• "hello" + 42 is "hello42"
• 1 + "abc" + 2 is "1abc2"
• "abc" + 1 + 2   is   "abc12" 
• 1 + 2 + "abc" is "3abc"
• "abc" + 9 * 3   is "abc27"
• "12" + 3        is "123"
Expression Practice
• Consider the data type in the answers. For example, 5 (an int) is
totally different from 5.0 (a double)
• 5.5 + 2 / 3
• 2 + 3.5 * -2
• 5 * 10 / 5.0 + 3 * (int) 2.5 / 10
• 8 + 4 + "3" + 4 + 10 / 2 + "6" + (7 + 8)
In-Class Exercise
1. 2 * 3 - 4 % 2 / 2 + 10 / (double)(10 / 4) 
2. 42 % 6 + 35 / 5 % 4 - 16. / (12 % 8)
3. (int) 5.0 / 2.0 + (4 + 6) / 5 * 2
4. 3 * (5 - 3) + 3 - 3 * 2 
5. 9 / 2.0 - 2 - 10 / (int) (5 * 0.5) 
6. "1" + 2 / 3 + "four" + 5 % 6 + 7 + (8 + 9)
Variables
• Variable – A memory location with a name and a type 
that stores a value.
• Declaration – A request to set aside a new variable 
with a given type and name.
–  ;
• Variable names are identifiers
– Style guidelines:  start with lowercase letter, capitalize each
subsequent word
• Examples
int age;
double weight;
char firstInitial; 
Variable Assignment
• Assignment – Giving a value to a variable.
• A value is assigned to a variable using an assignment statement:
 = ;
• Equal sign (=) is the operator for assignment
• The value of the expression on the right-hand side of the
assignment is stored in the variable on the left-hand side of the
assignment and is the result of the assignment operation
• Examples
double height = 70;
double weight = 195;
double bmi = weight / (height * weight) * 703;
char firstInitial = ‘M’;
Initialization
• Giving a variable an initial value is known
as the initialization of the variable
int age; //uninitialized
age = 35; //now initialized
Declaration/Assignment Variations
//declare and assign a value to
//a variable in one statement
  = ;
– int value = 10;
– int answer =  5 * 6;
//declare multiple variables of
//the same type in one statement
 , , …, ;
– int value, answer;  
– int number = 5, sum;
– int age1 = 5, age2 = 8;
Practice
• Which of the following choices is the 
correct syntax for declaring a real number 
variable named ‘grade’ and initializing its 
value to 4.0?
1. 4.0 = grade;
2. double grade = 4.0;
3. grade = double 4.0;
4. 4.0 = double grade;
5. int grade = 4.0;
Variables (cont.)
Changing the value of a variable using “assignment”:
What will the values of x, y, and z be after the following 
statements are executed?
int x = 3, y = 7, z;
z = x + y;
x = x + 2;
x = y – 5;
y = z – x;
Variables (cont.)
Changing the value of a variable using a “shorthand” 
method:
– Special Assignment Operators that increment or 
decrement a value by a set amount
Standard Assignment
x = x + 1;
y = y – 7;
z = z * 2;
a = a / 3;
Shorthand Assignment
x += 1;
y -= 7;
z *= 2;
a /= 3;
Variables (cont.)
Changing the value of a variable using
increment/decrement (++/--) operators
Increment a value by 1
i++;             (post increment)
++i;             (pre increment)
Decrement a value by 1
i--;               (post decrement)
--i;               (pre decrement)
Post versions evaluate to the older (original) value
– “evaluate using current value of i, then increment i”
Pre versions evaluate to the new (final) value
– “increment i, then evaluate using new i“
Pre/Post Increment/Decrement
Examples
What are the values of age1, age2, age3, and years after 
the following statements are executed?
int age1 = 21;
int age2 = 50; 
int years;
int age3 = age2 – age1++;
years = ++age1 + age2--;
years++;
Common Syntax errors
• A variable can't be used until it is assigned a 
value.
int x;
System.out.println(x); // ERROR: x has no value
• You may not declare the same variable twice.
int x;
int x;               // ERROR: x already exists
int x = 3;
int x = 5;           // ERROR: x already exists
Printing an expression or variable value
• Use + to print a String and an expression value on one line
System.out.println("Grade: " + (95.1 + 71.9) / 2);
• Output:  Grade: 83.5
System.out.println("Grade: " + (95.1 + 71.9) + 2);
• Output:  Grade: 167.02
• Use + to print a String and a variable's value on one line.
double grade = (95.1 + 71.9 + 82.6) / 3.0;
System.out.println("Your grade was " + grade);
• Output:   Your grade was 83.2
int students = 11 + 17 + 4 + 19 + 14; 
System.out.println("There are " + students + 
" students in the course.");
• Output:  There are 65 students in the course.
Practice
• What does the following program output?
public class Variables { 
public static void main(String[] args) {
int num1, num2, num3; 
num1 = 4; 
num2 = 12; 
num3 = num1; 
num2 /= 3; 
num1 = num2++ + 4; 
num3 += num1; 
int num4 = --num1; 
num4++; 
System.out.print("num1 = " + --num1 + "\nnum2 = " + 
++num2 + "\nnum3 = " + num3++);
System.out.println(" num4 = " + num4--); 
}
} 
In-Class Assignment
• Meadowdale Dairy Farm sells organic brown eggs 
to local customers. They charge $3.25 for a dozen 
eggs, or 45 cents for individual eggs that are not 
part of a dozen. 
• Write a class that includes a variable for the 
number of eggs in the order and assign the value 
27 to this variable. 
• Calculate and display the amount owed with a full 
explanation as follows: 
You ordered 27 eggs. That’s 2 dozen at $3.25
per dozen and 3 loose eggs at 45 cents each for 
a total of $7.85.
• Save the class as Eggs.java. 
In-Class Assignment
• Write a class that calculates and displays the 
conversion of an integer variable storing a 
number of dollars into currency 
denominations—20s, 10s, 5s, and 1s. Assign 
the value of 57 to the variable. Print the 
resulting conversion as shown below (the 
output values must be calculated, not 
printed out using literal values).
• Output: 
$57 converts to: 2 20s, 1 10s, 1 5s, and 2 1s.
• Save the class as Dollars.java.