Java程序辅导

Introduction to Java
15-121 Fall 2020
Margaret Reid-Miller
Academic Integrity
• You must do your own work.
• Discussion with other students is limited to 
clarifying the assignments or at a high non-
code level (pictures).
• If you need help, ask the course staff or me 
(see the Staff page for office hours).
• You should never look at another person’s 
program nor allow another student to look 
at your program (except recitation labs).
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What if I cheat on a assignment?
• Record a negative score on the assignment.
• Reduce your final semester letter grade.
• Prevent you from dropping, withdrawing, or 
changing to pass/fail.
• Send letters to Student Affairs, your advisor, 
various deans, etc.
• May result in long-term suspension.
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Life happens
So I give you some late days.  How many late 
days in total?
How many late days can you use for a single 
assignment?
What is the penalty for turning an assignment 
late when you run out of late days?
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4
2
No credit, so always submit something on time
Quizzes
• When is our first quiz?
• How often do we have a quiz?
• How many quizzes will be counted?
• Can your arrange to make up a quiz?
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Tomorrow
Every Thursday
10 (1-2 will be dropped)
You can’t, but each is worth only 0.5%
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Java vs Python
• Program Translation:
• Java is compiled, Python is interpreted
• Java syntax:
• Uses braces {} to group statements (indentation 
is not semantic, but recommended)
• Most statements end in semi-colons.
• Java programs:
• Often have several classes, each in a file with 
same name.
• Java programs start with the method main.
There is no required textbook.
• I will post related readings, however, from a 
free on-line text book Introduction to 
Programming Using Java by David J. Eck that 
you may find helpful for your understanding.
• You are welcome to use other books on data 
structures using Java.
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Today
• Data types (primitive/reference)
• Basic Operators
• Variable declaration
• Strings
• Before next lecture, review and compare 
syntax of LetterCounter1.java and 
LetterCounter1.py
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Java requires that you declare the type 
of each variable before it is used.
• You also need to declare the type of the expression 
return from a method
Basic types:
boolean (true or false)
int (integer)
double (floating point decimal)
String (sequence of characters)
…
void return type of a method that returns no value
• Java checks that the types match when you compile. 
Compiler error messages often indicate a type mismatch.
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Primitive Data Types
• Java has two categories of data:
• primitive data (e.g., number, character)
• object data (programmer created types)
• There are 8 primitive data types:
byte, short, int, long, float, double,     
char, boolean
• Primitive data are only single values; they have no 
special capabilities
• You cannot define new primitive data types.
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Common Basic Types
Type Description Example of Literals
int integers (whole numbers) 42, 60634, -8, 0
double real numbers 0.039, -10.2, 4.2E+72
char single characters
boolean logical values
String List of characters "", "a", "ab6&"
'a', 'B', '&', '6'
true, false
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Mostly use int and double
Type Storage Range of Values
byte 8 bits -128 to 127
short 16 bits -32,768 to 32,727
int 32 bits -2,147,483,648 to 2,147,483,647 
long 64 bits -9x1018 to 9x1018
float 32 bits ±10-45 to ±1038, 7 significant digits
double 64 bits ±10-324 to ±10308, 15 significant digits
Recall
• What is 210 in base 10, approximately?
• 220 ?
• 230 ?
• 232 ?
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103
1 million
1 billion
4 billion
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Note the limited range of int
Type Storage Range of Values
byte 8 bits -128 to 127
short 16 bits -32,768 to 32,727
int 32 bits -2 billion to 2 billion, approximately
long 64 bits -9x1018 to 9x1018
float 32 bits ±10-45 to ±1038, only 7 significant digits! 
double 64 bits ±10-324 to ±10308, 15 significant digits
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Basic Operators
Type Description Operators
int integers +, -, *, /, %, ++, --
double real numbers +, -, *, /
char single characters +, - (int arithmetic)
boolean relational
logical 
==, !=, <, >, <=, >=
! , &&, ||
String Strings + (concatenation)
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Before you can use a variable, 
you must declare its type.
• You can declare a variable only once in a 
method. 
• Examples:
int numDimes;
double length;
char courseSection;
boolean done;
String lastName;
camelCase is the 
Java convention
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Declaring Variables
• Declaring a variable instructs the compiler to set 
aside a portion of memory large enough to hold data 
of that type.
int count;
double length;
count                length
• No value has be put in memory yet. That is, the 
variable is undefined.
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Assignment Statements
• An assignment statement stores a value into a 
variable's memory location:
 = ;
• The type of the variable and expression must match.
• The first assignment to a variable initializes it.
count = 3; count
length = 72.3 + 2.0;   length
3
74.3
19
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Assignment statements shortcuts
int n;
n = 0; int n = 0;
n = n + 1; n += 1; n++;
n = n - 1; n -= 1; n--;
Other assignment operators:
*=, /=, %=
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Strings
https://docs.oracle.com/javase/8/docs/api/java/lang/String.html
• String is not a primitive type, but a class/type
• String variables store references to memory 
addresses
• Strings are immutable! That is, you cannot change a 
string, but you can assign a new string to a variable.
• A string cannot span more than one line:
"Not a valid
String literal”
How do you include a newline or single quote in a string? 
WRONG!
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As with Python, Java has escape 
sequences
Escape sequence: a two-character sequence that 
represent a single special character.
Sequence Meaning
\t
\n
\"
\'
\\
tab character
newline character
double quote
single quote
backslash character
System.out.println(
"What \"character\" is this \\? ");
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An object of type String is a 
sequence of (unicode) characters.
• When we declare a variable of type String, it does 
not create an object. 
String founder; 
• To create an object we use the new operator:
founder = new String("Carnegie");
• Strings have a shortcut way of creating them:
String founder2 = "Mellon"; 
initializes its state
nullfounder
• A primitive variable holds an actual value:
int count = 15121;
• An object variable holds a reference (address) to the 
object.
String founder = new String("Carnegie");
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Object vs Primitive Data
15121count
A String object
"Carnegie"
a35f3cdfounder
a35f3cd
reference: computer 
generated address
length()
substring()…
object data
object methods
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String Length
int length()
• int indicates that the data type of the value the 
method returns.
• Returns the number of characters in this string.
• Example:
String founder = "Carnegie";
int numChar = founder.length();
object dot operator method
method header
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Getting a single character
char charAt(int index)
Returns the character at a specified index
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C a r n e g i e M e l l o n
Example:
String school = "Carnegie Mellon";
char firstChar = school.charAt(0);
WARNING: You cannot assign a char to an object of type String 
without first converting the char to a String object ! 
e.g., String initial = "" + firstChar;
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Substrings
String substring(int startIndex, int endIndex)
String substring(int startIndex)
Returns a new string consisting of the substring starting at 
startIndex (inclusive) and ending at endIndex (exclusive) 
or, if one parameter, to the last character.
Example:
String school = "Carnegie Mellon";
String founder = school.substring(0, 8);
String founder2 = school.substring(9);
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C a r n e g i e M e l l o n
An example of 
method overloading, 
where the number of 
parameters differs.
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Replacing Characters
String replace(char oldChar, char newChar)
Returns a new String object resulting from replacing 
every occurrence of oldChar with newChar.  
• The original String object is unchanged. 
(Strings are immutable!)
Example:
String founder = "Carnegie";
System.out.println(
founder.replace('e', 'E'));
System.out.println(founder);
OUTPUT:
CarnEgiE
Carnegie 
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Changing Case
String toUpperCase()
Returns a new String object with all letters 
converted to uppercase.
String toLowerCase()
Returns a new String object with all letters 
converted to lowercase.
Example:
String founder = "Carnegie";
String upper = founder.toUpperCase();
String lower = founder.toLowerCase();
Immutable: You 
need to print or 
assign the result to 
a variable!
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Equals
boolean equals(String str) 
Returns true if str represents the same sequence of 
characters as this string.
Example:
String founder = "Carnegie";
founder.equals(“Carnegie”);  // returns true
founder.equals(“carnegie”);  // returns false
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Testing For Equality
• For primitive values use == for equality testing.
• For objects, use the equals method for testing 
equal contents. 
• The argument must be the same type as the object on which 
equals() is called.
• For example:
if (day == 1 && month.equals(“APRIL”)) {
System.out.println(“It’s April Fool’s Day”);
}
WARNING: Never use == with strings!  The 
results are unpredictable!
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Compare the lexicographical 
order of two strings
int compareTo(String str)
Returns: 
• value < 0 if this string is “less than” str
• value = 0 if they are equal
• value > 0 if this string is “greater than” str
Example:
“mellon”.compareTo(“carnegie”); // returns positive #
“Mellon”.compareTo(“mellon”);   // returns negative #