More from Chp 3: I/O, Java Statements

Input/Ouput

• Programs are useless unless there is interaction with the outside world. You must be able to
  • read input from the user
  • write output where the user can see it
• There are two ways of doing this:
  • console input/output:
  • GUI input/output
• Java is a language designed primarily for developing programs with a graphical interface. As a result, performing console I/O (particularly reading input) is rather awkward. Note: Many older languages were designed for console I/O only.
• To help ease you into the details of I/O (which can be a mess) the author of your text has written several classes that provide a clean I/O graphical interface.

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Console Output Briefly

To write output to a console window:

String name = "Dr. Seuss";
int age = 65;
System.out.print("My name is " + name + 
	         " and my age is " + age);

Here the symbol + is used as a concatenation operator and not numerical addition. We say + is an overloaded operator. Java knows how to interpret the + symbol by the context.

In Visual Cafe, this program will cause the output to appear in the console window.

Console output is easy but console input is a mess!

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GUI I/O using the javabook Package

Class for reading input: InputBox

Class for writing output: OutputBox (see page 112 for list of methods)

For example, click here.

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Making Change Example

Suppose you are asked to convert some number of cents into the correct change. Assume you have dollar bills, quarters, dimes, nickels, and pennies. How would you do it?

• Right now, let's just think about what variables the user needs to enter. Implement this.
• Always let the user know what is going on!
  • Always use a prompt to explain what the user is supposed to enter.
  • And always give FEEDBACK! Did the program properly read what the user entered?
• what algorithm to you use to compute change?

Java Statements

• A Java statement is the smallest unit that is a complete instruction.
• Statements must end with a semi-colon.
• Statements generally contain expressions (expressions have a value)
• One of the simplest is the Assignment Statement

<variable> = <expression>;

For Example:

int height;
height = 34;

The Java Assignment Statement

• The left-hand side must name a single memory location.
• The RHS must have a value;
• The value of the expression on RHS is placed in the memory location named on the LHS.

CORRECT: <variable> = <expression>;

int width, height;
width = 476;
height = 23;

INCORRECT: <expression> = <variable>;

2 = width;

width + 2 = height;

width + height = 43

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Expressions

• An expression is something that has a value.

• Literals:

  • integer: 23 67
  • real number: 12.5 -34.455
  • character literal: any single character on keyboard

    - use single quotes 'a' '8' 'H'

  • string literal:

    - use double quotes "oh boy!"

  • boolean literal: true false

• (named) variables:

  int width1, width2 = 23;
  width1 = width2;
  

  Here width2 can be treated as an expression with value 23.

• Objects

  • Any object is an expression

  • The value is the location of the Object. Recall:

    Player p1, p2;
    p2 = new Player();
    p1 = p2;

    Here p1 is set to the value of p2.

• Return value of a message

  • Value is the return value:
  • the value of dice.roll(2) is the value rolled.
  • E.g.

    int myRoll, myCheat;
    myRoll = dice.roll(2);

• Any mathematical expression (more on syntax later).

  width = 34 * 12;
  myRoll = 3 + dice.roll(2);

Initial Values

You must always set the value of a variable before using it. This is referred to as initializing the variable. This can be done at declaration time for primitives or objects:

  Player p = new Player();
  int width = 23;

Or later

  int width;
  width = 23;

What happens if you forget to initialize?

  Player p;
  p.takeTurn(); // p not initialized

Here, p doesn't point to anything so the compiler will give a Null pointer exception.

  int area, height, width=4;
  area = width * height;

Here, height is not initialized. Most compilers (but don't count on it!) will probably initialize height to 0. Some compilers will complain and not even let you compile. Bottom line: you can't predict what will happen - this is bad programming!

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Arithmetic Expressions

• Integer Arithmetic: +, -, * , e.g. 7 * 5

• Integer Division: / , %

7 / 2 - takes only integer part
7 % 2 - remainder after dividing

• Real Number Arithmetic: +, -, *, /

27.5 + -27.2

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Precedence

Which Operator Goes First? see page 94 in text

Examples:

int number = 5 * 7 - 15 % 6 + 3/4*5 + 3/(4*5) ;

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Some More Arithmetic Expressions

• Short-Cuts

  • Assignment Operators:
    +=, -=, *=, /=, %=<

  • Increment, Decrement, ++, -- , Pre and Post

• Relational Expressions

  • Comparision Operators <, >, ==, !=, <=, >=

  • Have Boolean Value

  • E.g.
    boolean isSmaller = (5 < 8 );

• Logical Expressions: AND (&&), OR (||), NOT (!)

  • These operate on boolean values

  • AND

    x	y	x && y
    T	T	T
    T	F	F
    F	T	F
    F	F	F

  • OR

    x	y	x || y
    T	T	T
    T	F	T
    F	T	T
    F	F	F

  • NOT

    x	!x
    T	F
    F	T

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Java is (sort-of) Strongly Typed

This means that if you create variables of different types, you can't always use them together.

Often numbers can be combined but be careful - the results may not always be what you expect!

Example 1:

  int age = 45;
  char letter = 'k';
  letter = age; // WON’T WORK 

Example 2: Mixing numerical types

int age = 45;
double fraction = 0.4;
fraction = age; // WILL WORK
age = fraction; // WON’T WORK (info can be lost)

Can force type change by type casting.

age = (int) fraction; // WILL WORK

Example 3: Mixing apples and oranges

int person = new Player(); // WON’T WORK
Dice mydice = new Player(); // WON’T WORK 

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Constants

• Value must be set at declaration and can't be changed.

• Convention: constants are all upper case
  

    final int ID = 1234;

• Why use constants?? Suppose you have two unrelated quantities with the same value:

    final int SIDES = 6;
    final int RESOLUTION = 6;

• Why not just use the number 6?

  Ans: Makes for code that is easier to modify.

• Why not just have one named constant instead? E.g.

    final int SIX = 6;

  Ans: Makes for more understandable code and easier to modify.

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The Math class

(see table on page 100-101)

Contains methods for standard math functions.

Note the format for message passing is a little different than what we have seen before because these methods are class methods and not instance methods.

Instance methods:

<object>.<method name>(<arguments>)

Class methods:

<class name>.<method name>(<arguments>)

Why make them class methods?? It is more convenient because we need not create a Math object in order to use these methods.

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