Computer Science 111 Fundamentals of Computer Programming I

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Computer Science 111Fundamentals of Computer
Programming I

• Basic Java Syntax
• and Semantics

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Terminology

• Syntax the formal rules for combining words and
  symbols to form legal statements within the
  language.
• Semantics the meaning of the statements i.e.,
  what the statement does when executed.

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Literals

• Literals are items whose values do not change.
• "Hello World!" is a string literal234 is an
  integer literal-3145 is also an integer
  literal3.14 is a floating point literal5.25E7
  is also a floating point literal

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Numeric Data Types and Variables

• We use the type int for integers (4 bytes)
• We use the type double for floating point numbers
  (8 bytes)
• A variable is an item whose value can change
  during the execution of the program. A variable
  has
• A name
• A type
• A location in memory
• A value stored in this location

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Declaring Variables

• Before a variable can be used in a program, it
  must be declared. This consists of giving first
  the type of the variable and then its name.
• double fahrenheitint age, countKeyboardReader
  readerint num20

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Object Instantiation.

• When we declare a variable, memory space is
  created for the value of the variable. For a
  variable which is a reference to an object, the
  object is not created simply by declaring the
  variable, only space for the pointer to the
  object.
• Creating the object itself is called
  instantiating the object. This is done by
  asking for a new object of the given type.

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Object Instantiation (cont.)

• KeyboardReader reader

• reader new KeyboardReader()

• OrKeyboardReader reader new KeyboardReader()

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Comments

• We often need to put comments into the source
  code of a program. A comment has no effect on the
  execution it is there for the benefit of
  someone who reads the program.
• Examples would be programmers name, purpose of
  the program, explanations to clarify parts of the
  program, etc.
• Anything following a // on a single line is a
  comment.
• Anything between / and / , even many lines,
  is a comment.

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Identifiers in Java

• Identifiers are used for naming variables,
  methods, objects, etc.
• Identifiers are strings of
• Letters
• Digits
• Underscores
• Dollar signs
• Can not start with digit
• No spaces
• Can not be one of Javas Reserved Words

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Naming Conventions

• Names should be meaningful
• Begin names of variables and methods with lower
  case letter.
• If name is combination of words, run the words
  together and capitalize all but the first.
• Class names usually begin with capital.

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Assignment Statements

• ltvariablegt ltexpressiongt
• Note single variable on left hand side
• Expression should yield result which is of type
  appropriate for the variable.
• Expression created with literals, variables,
  operators, parentheses to form well-formed
  expression
• Semantics
• Expression is evaluated using current values of
  variables
• Resulting value is stored as new value of the
  variable.

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Common Operators
 
 
 
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Examples with integer operations

• Integer division yields the whole number part of
  the result
• 6/3 yields 2
• 6/4 yields 1
• 5/9 yields 0 (hmm)
• Modulus gives the remainder upon division
• 6 3 yields 0
• 6 4 yields 2
• 5 9 yields 5

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Examples with precedence

• 6 4 5 10 5 5
• 6 4 5 6 20 26
• (6 4) 5 10 5 50
• 10 5 6 1 5 6 1 -1 1 -2
• 10 (5 6 1) 10 (-1 1) 10 (-2) 12
• 4 11 6 / 2 4 5 / 2 4 2 6

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Types of Errors

• Syntax Errors caught by compiler
• Run-time occur while program is executing
  (maybe divide by 0)
• Logic errors runs but gives incorrect results.
  celsius (fahrenheit 32.0) (5/9)

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Testing as you go

• As you develop more complex programs, a piece of
  advice is to develop small pieces of the program
  at a time and test each before moving on.
• This avoids having a large piece of code with an
  errror (bug) and trying to locate it within a big
  program.
• To do this, you can put in extra output
  statements (writer.print) to see values of
  intermediate results, etc.
• This technique can also be used to find errors.
  Put in statements that print messages and
  variables at various points in the program to
  trace changes.

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If a bullfrog had wings, he wouldn't ...