• Software development
• Literals
• Variables
• Types
  • int
  • String
  • float
• Declarations
• Initialization vs Assignment
• Constant
• Expressions
• Comments

What You're Going to Learn in this Class

• Problem Solving
• Critical Thinking
• Elementary Java

Software Development: Some Basic Terminology and Concepts

Problem Statements

• The above is called a problem statement.
  • Problem statements are typically presented in informal language (i.e., English, not code)
  • However, the statement must be precise in order for the programmer to be able understand what is expected of them
    • Notice I specified the first name (as opposed to their last name, or full name)
    • The level of precision may depend on the actual needs of the entity making the problem statement: for example if you were to submit to CodeLab, you would need the exact text of the welcoming message; if it was just an informal exercise, you could get away with anything that seems welcoming and contains the name.

Developing an Algorithm

• The programmer must then create a step-by-step solution to the problem
  • This is known as an algorithm
  • It is this step where the problem-solving and critical thinking come into play
    • The problem solving is to be able to take the problem an produce the algorithm
    • The critical thinking is to have a sense of the issues one will face as well as deciding whether one's solution is indeed a solution
  • The eventual goal is to have a computer carry out (execute) the steps of the algorithm
  • The above example has a trivial algorithm:
    • Prompt for and read the name
    • Display the name with a welcome message

Implementing the Algorithm: Programming Languages

• Once an algorithm is constructed, the programmer implements the algorithm in a programming language, i.e., transforms it into a formal written specification in a language suitable (directly or indirectly) for being run on a computer.
  • A programming language is a language that is conducive to being read by humans, but is also formal enough to provide precise specification of algorithms
  • In other words, it is a way of writing precise instructions that is at the same time meant to be read by people.

Source Files and Editing

• The file containing the text of the algorithm written in the programming language is called a source file, and we speak of the (program) text as source code
  • Java source files usually have a single class within them, and have the same name as the class with a suffix of .java; e.g. if we name the class of our Hello World program HelloWorld, it would reside in a file named HelloWorld.java
• The source file is created and subsequently modified (edited using a program known as an editor)

Compilation

• While the source file is human-readable, it is not conducive (nor efficient) for execution; rather the source file is translated into another target language suitable for execution by the machine.
  • The process of translating the source language to the target language is known as compilation, and is carried out by a program (piece of software) known as a compiler
  • In many cases, the target language is the actual language of the computer's hardware (known as the hardware's instruction set), and thus is called machine language
    • The resulting file containing machine language is called an executable file
  • For Java, the source code is compiled into a language known as byte code and is stored in a file known as a class file (because our Java source files contains classes and each class is compiled to one of these files).
    • Class file names typically match the source file name but end with the suffix .class rather than .java; e.g. HelloWorld.class

Execution and Interpretation

• Once one had produced the executable or class file, one can execute the program
  • An executable file can be run directly on the hardware (this is essentially what happens when you run Word, a video editor, and many other programs; i.e., you double click the icon on the desktop, or select the program from a menu or directory.
  • For Java, the class file does not run directly on the hardware; rather it is run by yet another piece of software, know as the Java interpreter. This piece of software creates a virtual machine, a simulation of a machine in software. For Java, this machine is known as the Java virtual machine — JVM, and this software-based 'machine' views the byte code in the class file as its machine code.
    • The process of running on a virtual machine is strictly called interpretation; i.e., the interpreter program is interpreting the byte code instructions as if they were hardware instructions; we often loosely refer to this as execution as well.

Testing

• testing is performed at many levels and in many ways:
  • individual components (methods), classes and full applications all require testing, and the testing at each level is different in nature
  • testing is also done by the programmer, their immediate team, and typically a QA team
  • testing can be performed by eyballing the code; i.e., reading it over and looking for problem, running the code on various test cases, and using special software created specifically for the purpose of code-testing

Debugging

• diagnostic prints: the programmer places print statements ate strategic points in the program in an attempt to determine what is going wrong.
• the programmer uses a debugger that permits them to step through the program and examine the contents of memory. If the debugger is able to display the names (i.e., symbols) of the variables (to make it easier for the programmer to examine them), it is called a symbolic debugger.
  • Debuggers vary widely in capability and the support they provide to the programmer

Integrated Development Environments (IDE)

• IntelliJ
• Eclipse
• NetBeans
• Dr Java
• Code::Blocks

Basic Imperative Java

Basic Programming Elements

• Actions or imperatives
• Decisions or conditionals
• Repetition or iteration

Organizational Elements

• methods
• classes

Methods

• As the representation of a subtask, a method can be called or invoked, i.e., one can initiate the performance of the subtask by name.
• As an example, nextInt is a method of the Scanner class. Once calls nextInt by writing: scanner.nextInt()
  • The exact details of the rest of that code fragments, i.e., the use of scanner as well as the ., and ()'s will be explained below.

Classes

• For example, the Scanner class is a set of logically-related methods who purpose is to provide the functionality of reading input.
• Classes are used to create object or instances of the class
  • We create such object using the new keyword followed by the name of the class, followed by optional additional information necessary for the creation.
    • For example, to create a Scanner object capable of reading input from the keyboard, we write:

      new Scanner(System.in)
      						

      the System.in is additional information supplied to the creation process informing it that the input is to come from the keyboard.

Basic Program Structure & Boilerplate

• The basic structure of a class is:

  public class ClassName {
  	method
  	method
  	…
  	data
  	…
  }
  		

  The ClassName typically matches the name of the containing source file (without the java suffix
• A class that contains a method named main can be executed as an application, i.e., it can be submitted to the Java interpreter to be run as a program.
  • When a class is submitted to the Java interpreter to be executed, executed begins from the first statement of the main method, and proceeds until the last statement has been reached, at which point the program terminates
• We will be restricting ourselves for the moment to single-file (and thus single class) applications
  • By single class, we mean that we will be creating/defining/coding one class only
    • We will, however, in general be using other classes — we've already seen use of Scanner and we will shortly investigate the System and other classes provided by Java.
      • Classes provided by the Java system are called predefined
  • We can thus now explain our basic boilerplate in a bit more detail:

    1. public class ClassName {
    2.    public static void main(String [] args) {
    3.       …
    4.    }
    5. }
    				

    Notes

    • Line 1 is known as a class header and introduces a class (in particular provides the class name); and line 5 is the closing brace for the class.
      • The code between the class header and terminating } (lines 2-4 above) is called the class body
      • Line 2 is known as a method header and introduces a method (in this case a method named main; and line 4 is the closing brace for the class.
        • The code between the method header and corresponding terminating } is called the method body
        • As this will be the only class we write, and we wish to execute it, we have a main method.
        • For the moment, all our code will be at line 3, i.e., the body of the main method.

Indentation

• In our boilerplates above, we see that a class is composed of methods (and data). Each method is indented relative to the class header.
• In turn, the method body is indented relative to the method header
• We will see further indentation in the context of conditionals and loops.

public class ClassName {
	public static void main(String [] args) {
		…	
		…
		if …
			…
		else
			…
		…
		…
		for … {
			…
			if …
				…
			else
				…
		}
		…
	}
}

Imperative Java

• calculate this expression
• test this condition and then do this if the condition is true
• repeat this body of code if this condition is true
• declare this variable

Literals

• An entity whose value is literally what it is written as
  • 1, 15
• Integer, string, floating point
  • 0, 15, "Hello", "", 1.5, 3.14

Variables

• Location in the memory of the computer that can contain a value
• Associated with a variable is it's name, known as an identifier
• Identifiers:
  • Must start with a letter or _, followed by sequence of letters, digits, and/or _'s
  • By convention: when used for a variable, must begin with lowerCase letter, multi-words have subsequent words capitalized
    • Known as camel case

    x
    str
    socialSecurityNumber
    studentId
    		

Types

• Specifies the set of values a variable can contain
• int, String

Arithmetic Expressions

• The basic arithmetic operators are

  +	addition
  -	subtraction
  *	multiplication
  /	division

The String + (concatenation) Operator

• Multiple values to be sent to System.out may be concatenated using the (String) + operator
• If one of the operands of a + operator is a String (a) the operator is a concatenation operator (and not an addition operator), (b) the other operator is turned into a String (if it isn't one already)

Expressions

• 3 + 5 evaluates to 8
• x * y evaluates to the product of the values contained in the variables x and y
• z == x + 1 evaluates to true if z is equal to the value contained in x plus 1

• Operators, operands, evaluation (result/value of an expression), subexpressions
  • operands are the values manipulated (by operators)
    • literals, constants, variables, results of method calls, subexpressions
  • operators are typically symbols that act upon their operands
    • binary operator takes two operands

      a + b
      						

    • unary operator takes one operand

      -b
      						

    • tertiary operator takes three operands

      hasFever ? "sick" : "healthy"
      						

  • In the above terms, an expression is a sequence of one or more operands with their operators

    x + 1 * y
    						

    • expressions are evaluated and the result is called the value of the expression
  • subexpression is an expression within a larger (surrounding) expression

    a * b + c
    				

    • a subexpression is usually an operand of an operator of the larger expression
      • In the above example a * b is a subexpression, and is the left operand of the + operator

Integer Division

• This is why (88+91)/2 evaluates to 89
• This is not an error, rather it is the semantics of integer division

P02.2 — Integer Division

1. public class IntegerDivision {
2.	public static void main(String [] args) {
3.		System.out.println("30/3: " + 30/3);
4.		System.out.println("10/3 + 10/3 + 10/3: " + (10/3 + 10/ 3 + 10/ 3));
5.		System.out.println("1/3 + 1/3 + 1/3 + 1/3 + 1/3 + 1/3 + 1/3 + 1/3 + 1/3 + 1/3: " +
6.			(1/3 + 1/3 + 1/3 + 1/3 + 1/3 + 1/3 + 1/3 + 1/3 + 1/3 + 1/3));
7.	}
8. }

30/3: 10
10/3 + 10/3 + 10/3: 9
1/3 + 1/3 + 1/3 + 1/3 + 1/3 + 1/3 + 1/3 + 1/3 + 1/3 + 1/3: 0

Floating point

• double, float
• When is floating point appropriate?
• Integer, floating point, (fixed point), and mixed mode arithmetic
• floating point literals
  • 1.5, 3.14
• Back to semester average problem
• counting (integers/exact number) vs measuring (floating point/approximation)

Declarations

• Declares a variable and its type for subsequent use in the program
• Variables must be declared before first use
• General form:

  type variable-name;
  		

• Examples:

  int x;
  
  String s;
  int grossPay, taxOwed;
  String word, lastName;
  		

• Multiple variables may be declared in the same statement

Getting Values into Variables: Initialization & Assignment

Initialization

• A variable may be given an initial value when it is declared:

  int x = 12;
  String word1 = "Hello", word2 = "Goodbye";
  		

• Initializations may also be expressions (of already declared and initialized variables):

  int num1 = 13, num2 = 15;
  int average = (num1 + num2) / 2;
  
  System.out.println("The average of " + num1 + " and " + num2 + " is " + average);
  		

Assignment

• Following its declaration, the value of a variable may be modified using the assignment operator

  int x = 12;
  int y;
  
  y = x;
  
  System.out.println(y);	// prints 12
  		

• The value on the right-hand side of the = is assigned to the variable on the left-hand side.
• The value in the left hand variable prior to the assignment is overwritten as a result of the assignment
• The right-hand-side (RHS of the assignment can be any expression; the left-hand=side (LHS) must be a variable.
• An equivalent version of the above num1, num2, and average example using assignment:

  int num1, num2;
  num1 = 13;
  num2 = 15;
  
  int average;
  average = (num1 + num2) / 2;
  
  System.out.println("The average of " + num1 + " and " + num2 + " is " + average);
  		

• Initialization may be used if the value of the variable is known at declaration time
• After the declaration, assignment must be used.

Statements

• Statements are the basic units of execution
• Unlike expressions, statements do not result in a value
  • The general rule is only one statement is allowed as the true-part/false-part of the if, and the loop body of the for
  • If more than one statement is needed, surrounding it with braces ({}s) has the effect of making it a single statement for this purpose

    if (average >= 60) {
    	System.out.println("You passed!");
    	System.out.println("Good job!");
    }
    				

• Some statement contain other statement, e.g. the if statement, and the for statement
• As a general rule statements that do not contain other statements end with a semicolon (;); those that DO contain other statements do not

Method Calls

The line System.out.println("Hello world!"); consists of a statement that displays/prints the text Hello world (without quotes) on the screen.
• System.out is a pre-defined object representing the screen
  • By pre-defined, we mean something that has been provided for you by the system
• The code println("Hello world!") is a message we are sending to the System.out object, requesting it to display the text Hello world
  • A message consists of a name (println), followed by the contents of the message ("Hello world!"), known as the arguments.
    • In this message there is only one argument
    • The println message requests that the argument be displayed on the screen
      • The screen is properly known as the standard output
      • (The keyboard is known as the standard input; we'll introduce it later)
    • We would thus say: The println message with the argument "Hello world!" is being sent to the object System.out
  • The text is enclosed within quotes ("…") to allow us to determine where it begins and ends ("Hello world!" vs " Hello world!" vs " Hello world! ")
• Statements in general are terminated with a semicolon (;)

Screen (Console) Output

• pre-defined screen object System.out

  System.out.println(first_name);
  		

• messages that can be sent to System.out:
  • print(argument) — displays argument on the screen
  • println(argument) — same as print but adds a newline to the end
    • cursor is thus positioned to the beginning of the next line
  • println() — newline only is sent to the screen
    • This is an example of overloading — allowing two different messages with the same name as long as they differ in argument structure (in our case 1 vs 0 arguments).
  • There are other messages that can be sent as well; we'll encounter them as we proceed through the course

Keyboard (Console) Input

• pre-defined keyboard object System.in, analogous to System.out
• requires a bit more work to use than System.out:
  • A Scanner object must first be created, and then THAT is what we send messages to:

    import java.util.*;				// At top of source file -- more boilerplate required when using Scanner
    …
    …
    Scanner scanner = new Scanner(System.in);
    				

    • Note this is our first example of an object that is NOT pref-defined — rather we are creating it
  • Messages that can be sent to Scanner objects:
    • nextInt — // retrieves the next value typed in at the keyboard (must be an int)
    • next — // retrieves the next value typed in at the keyboard

    int x;
    x = scanner.nextInt(); 	// takes the next value typed in at the keyboard (must be an int) and places it into x
    
    String s;
    s = scanner.next();		// takes the next value typed in at the keyboard (a string) and places it into s
    				

import java.util.Scanner;		// at top of source file … before class header
…
public class … {
	public static void main(String [] args) {
		…
		Scanner scanner = new Scanner(System.in);
		…
		… = scanner.next();	// or nextInt
		…
	}
}

A Comment About Comments

• Comments: why, where, and how
• 'Useless' comments

Adding Some Additional Features to our Grading Program

• Want to be able to assign a letter grade based on average
  • Need to be able to make decisions (if the average is greater than 90, the grade letter is an 'A') (conditionals)
• How about a weighted average: project 20%, midterm 30%, final 50%
  • More elaborate arithmetic (no longer simply add and divide by 3) (precedence, additional arithmetic operators)
• How about class average, highest grade in class, lowest grade in class
  • More decisions (is this grade the largest so far?) (cascaded/nested conditionals)
  • Need some sort of memory to keep running totals, or highest grade so far (variables)
    • Enough of these things to remember, it would be better to recall them by a name than a memory number
• 20 projects-- project average is the average of the highest 10 that were submitted on time
  • More complicated decision making (switch/logical operators)
  • Group of projects (arrays)
  • Wife wants to help-- give her this subtask to do while I do something else (methods)
• Add student id and email address to roster
  • Notion of the set of data associated with a particular student-- student data now in individual folders (classes)

The Essentials

• Manipulating values (calculations)
  • Numbers: arithmetic operations and expressions, various mathematical functions (e.g. absolute value)
  • Strings: string operations and expressions: extracting specific characters or sequences of characters; combining strings
• Making decisions (conditionals)
  • Are two values equal? Are we finished reading data? Is a number even? Does a string contain a vowel? Is a number a prime?
• Repeating actions (iteration)
  • Calculate grades for 10 students; calculating grades for 100 … 1,000 … 1,000,000 students; read in addresses until an address outside of New York is encounter.
• Dividing up a problem into simpler, individual task (methods)
  • Grade calculations as:
    • read in grades
    • calculate average
    • calculate letter grade
    • print result

Files used in Lecture 2

Lab for Lecture 2