Introduction to Java 15-121 Fall 2020 Margaret Reid-Miller - - PowerPoint PPT Presentation

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

• f 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

• f 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:

<variable> = <expression>;

• 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

null founder

• A primitive variable holds an actual value:

int count = 15121;

• An object variable holds a reference (address) to the
• bject.

String founder = new String("Carnegie");

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Object vs Primitive Data

15121 count

A String object

"Carnegie" a35f3cd

founder

a35f3cd reference: computer generated address length() substring()

…

• bject data
• bject 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();

• bject

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

• 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)

• r, 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

• 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

• rder 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 #