Transcript Console Output, Variables, Literals, and Introduction to Type

Console Output, Variables, Literals,
and Introduction to Type
CS0007: Introduction to Computer Programming
Review
 A General-Purpose Programming Language is…
 a programming language designed to be used for writing software in a wide
variety of application domains.
 A programming language is platform-specific if…
 once compiled, it can only be guaranteed to be able to run on the machine it
was compiled for.
 A High-Level Programming Language is…
 a language that is easily read and written by humans, and is needed to be
translated before a machine can use it. It provided a high level of abstraction
from the details of the workings of the computer’s hardware
 Compilation is…
 the process of translating high-level language into machine language for
platform-specific languages or some other intermediate code representation
for other languages. A program called a compiler does this.
Review
 The Java Virtual Machine (JVM) is…
 a program that reads instructions from Java Byte Code and
executes them for the platform that the JVM is installed on.
 Steps to create and run Java Program:
1.
2.
3.
4.
Write the source code in the Java programming language.
Run the source code through the Java Compiler (Interpreter)
Run the resulting Java Byte Code through the JVM
The programs executes!
 Procedural Programming is…
 a programming methodology in which a program was made up
of one of more procedures.
Review
 Object-Oriented Programming (OOP) is…
 a programming methodology in which a programs parts are
abstracted into the interaction between Objects.
 An object is…
 an abstraction of a real-world (or sometimes not) entity that
consists of data (attributes) and the procedures that act upon the
objects data (methods).
 Encapsulation refers to…
 the combining of data and code into a single object.
 Data Hiding refers to…
 the object’s ability to hide its data from outside the object.
The Java Console
 The console or console screen is a screen where the input
and output is simple text.
 This is a term from the days when computer terminals were
only text input and output.
 Now when using the console for output, it is said that you are
using the standard output device.
 Thus when you print to the console, you are using standard output.
 In Java, you can write to the console, using the methods
included in the Java API.
Java API
 The Java Application Programmer Interface (API) is a standard
library of prewritten classes for performing specific operations.
 These classes are available to all Java programs.
 There are TONS of classes in the Java API. That do MANY
different tasks.
 If you want to perform an operation, try either Googling it before
implementing it from scratch.
 Often you will get a link to the Java API that will reference the class
and method/data attribute you need.
 Example: “Java Round”
 Other times, you will get an example
 Example: “Java Ceiling”
Print and Println
 Like stated before, you can perform output to the Java console using the Java
API.
 Two methods to do this are print, and println.
 print – print some text to the screen
 println – print some text to the screen and add a new line character at the
end (go to the next line).
 These two methods are methods in the out class, which itself is contained in
the System class.
 This creates a hierarchy of objects.
 System has member objects and methods for performing system level operations (such as
sending output to the console)
 out is a member of the System class and provides methods for sending output to the
screen.
 System, out, print, and println are all in the Java API.
 Because of this relationship of objects, something as simple as printing to the
screen is a relatively long line of code.
 We’ve seen this before…
Dissecting println
System.out.println(“Hello World!”);
 The dot operator here tells the compiler that we are going to use
something inside of the class/object preceding it.
 So System.out says that we are using the out object in the
System class.
 The text displayed is inside of the parentheses.
 This is called an argument
 We will talk more about arguments when we go over functions, but for now,
just know that whatever is in the parentheses will be printed to the console
for the print and println methods.
 The argument is inside of double quotes, this makes it a string
literal.
 A String is a type in Java that refers to a series of characters
 We will go over literals, types, and strings later
Console Output Examples 1 & 2
 New Topics:
 print
 println
Escape Sequences
 What would be the problem with this?:
System.out.println("He said "Hello" to me");
 The compiler does not know that the "s aren’t ending and starting a
new string literal.
 But, we want double quotes in our string…How do we fix this?
 Answer: Escape Sequences
 Escape Sequences allow a programmer to embed control characters or
reserved characters into a string. In Java they begin with a backslash and
then are followed by a character.
 Control Characters are characters that either cannot be typed with a
keyboard, but are used to control how the string is output on the screen.
 Reserved Characters are characters that have special meaning in a
programming language and can only be used for that purpose.
Escape Sequences
 Common escape Sequences:
 \n – Newline – Advances the cursor to the next line
 \t – Horizontal Tab – Advances the cursor to the next tab
stop
 \\ – Backslash – The backslash character
 \’ – Single Quote – The single quote Character
 \” – Double Quote – The double quote Character
 Even though they are written as two characters, the character
represented by an escape sequence is only one character in
memory.
Escape Sequence Examples
 New Topics:
 Escape Sequences
Variables
 A variable is a named storage location in the computer’s
memory.
 If we want to hold some value in memory for later use, we need
to use as variable.
 Variables have 3 characteristics:
 Name – the name we use in our code to refer to the memory
location.
 Type – characteristic that defines what legal values the variable
can hold.
 Value – what is actually held inside of the memory location.
Variable - Naming
 Variables and all other identifiers follow the same naming
rules:
1.
2.
3.
4.
5.
The first character must begin with an alphabetic character
(A-Z or a-z), an underscore (_), or a dollar sign ($).
After the first character you must use an alphabetic character
(A-Z or a-z), a numeric character (0-9), an underscore (_), or
a dollar sign ($).
Identifiers are case-sensitive.
Identifiers cannot include spaces.
Identifiers cannot be reserved words.
Variables – Naming: Legal or Illegal?
 miles
 Legal
 3dGraph
 Illegal – cannot start with a numeric character
 firstName3
 Legal
 _miles
 Legal
 firstName#3
 Illegal – cannot have a pound (#) symbol
 first name
 Illegal – cannot have a space in an identifier
 class
 Illegal – cannot be a reserved word
Variables - Naming
 The name of the variable should describe what it holds, this
produces self-documenting programs.
 For example: If you have a variable that holds the number of miles a
car traveled you could call it miles.
 But what if the variable holds something that is normally two
words, like “first name”? I can’t use a space…
 Answer: Camel Casing
 Camel Casing is combining names that normally would be two words into
one, and capitalizing the start of a new word.
 Example: Instead of first name, use firstName.
 Variables by convention start with a lowercase letter.
 Identifiers for class names by convention start with an uppercase
letter.
Data Type
 Again, Data Type or just (Type) refers to the values that a
variable can hold.
 We have type for multiple reasons, namely:
 So the compiler knows how much memory to free up for the variable
 So the compiler knows the legal operations that can be done to/with the
variable.
 For example the int data type holds positive and negative
integers.
 We will be introducing new types both shortly and probably
throughout the semester
Variable Example
 New Topics:
 Variable Declaration
 Variable Assignment
 Variable Output
Variable Declaration
 Line 4 looks like: int
number;
 This is called a variable declaration.
 A variable declaration is a statement that tells the compiler the variables
name, and what data type it is.
 You MUST declare a variable before you can use it.
 General form of a variable declaration that is of a primitive
data type
dataType variableName;
 dataType – the data type of the variable.
 variableName – the identifier for the variable.
Assignment
 Line 9 looks like:
number = 5;
 This is called an assignment statement.
 An assignment statement stores a value on the right side of the = to the
variable on the left.
 For this reason the = is called the assignment operator in Java.
 This stores the value 5 into the memory location named number.
 5 here is called an integer literal.
 A Literal is a value that is explicitly written in the code of the program.
 Literals have type just like variables, that is why 5 is an integer literal.
 Question…
 If the types on either side of an assignment statement are not the
same, the result is a type-mismatch error. This happens because
Java is a strongly-typed language.
 A programming language is strongly-typed if it only allows operations
to be executed on operands of the same type.
Variable Usage
 Line 9 looks like:
System.out.println(number);
 This prints out the value stored in the variable number (5 in
this case)
 When a variable is used NOT on the left hand side of a
assignment statement, the value stored in the variable is
used.
 Question...
Primitive Data Types
 There are many types of data, you can even define your own
type.
 int is known as a primitive data type.
 A primitive data type is a built-in data type that is a basic
building block for all other types and DO NOT create objects.
 byte, short, int, long are all integer data types.
 float and double are floating-point (decimal) data
types.
Primitive Numeric Data Types
Data Type
Size
Range
byte
1 byte
Integers in the range of -128 to +128
short
2 bytes
Integers in the range of -32,768 to +32,767
int
4 bytes
Integers in the range of -2,147,483,648 to +2,147,483,647
long
8 bytes
Integers in the range of -9,223,372,036,854,775,808 to
+9,223,372,036,854,775,807
float
4 bytes
Floating-point numbers in the range of ±3.4x10-38 to
±3.4x1038 with 7 digits of accuracy
double
8 bytes
Floating-point numbers in the range of ±1.7x10-308 to
±1.7x10308 with 15 digits of accuracy
Notes on Type
 You want to choose a type based on what the variable should
do
 Why would you not use double for a variable that counts the
number of times the user clicks on something?
 Why would you not use int for a variable that calculates the
exact weight a bridge can hold?
 One good thing about types in Java is that the sizes of these
variables are the same on EVERY computer.
 Not the case in other languages.