Version 3.2¹

Two class days

Handouts:

• Day 1: Lossy.java
• Day 2: Einstein.html (goes with Einstein.java example)

(First Day)

(Sec. 2.1) Character string literals

• String literals are enclosed in ""
• System.out.print and System.out.println—what's the difference?
  • println begins a new line after what it prints
  • println can have 0 or 1 parameters
• Listing 2.1: Countdown.java
  • String concatenation with +
  • String literals may occupy just one line of code.
  • Concatenation can be used to continue long strings on additional lines.
• Listing 2.2: Facts.java
  • When a number is concatenated with a string, the result is a string.
• Listing 2.3: Addition.java
• Escape sequences (figure 2.1)
  • Listing 2.4: Roses.java

(Sec. 2.2) Variables and assignment

- p. 66: Syntax of variable declaration - What does the syntax diagram mean? - Examples? - Reserved words: int, double, char, final - Listing 2.5: PianoKeys.java - Assignment (=): - p. 69: syntax - Listing 2.6: Geometry.java - Java is a strongly typed language: each variable must be declared with a type, and can only store values of compatible types. - Declaring constants with final

(Sec. 2.3) Primitive types

• Integer types
  • byte = 8 bits signed
  • short = 16 bits signed
  • int = 32 bits signed
  • long = 64 bits signed
• Floating point types
  • float = 32 bits
  • double = 64 bits
• char = character (Unicode?), 16 bits
• boolean = logical, 1 bit (at least conceptually)
  • Boolean values: true, false

Page 72: Syntax of a decimal integer literal

• Default literal types are int and double.
• Suffixes: F or f (float), L or l (long).

Besides primitive types, Java also has reference types (objects).

• String
• More reference types later

(2.4) Expressions

• Arithmetic operations: +, -, *, /, %
• Operator precedence:
  1. ()
  2. + and - (unary, as signs)
  3. *, /, and % left to right
  4. + and - (binary, meaning add and subtract) left to right
• Integer division: integer / integer = integer, the result is truncated (omit any fractional part or remainder)
• Listing 2.7: TempConverter.java
  • Why 9.0 / 5.0 instead of 9 / 5?
• Increment and decrement operators: ++, --
• Assignment operators: +=, -=, *=, etc.

(2.5) Data conversion

• Three ways to convert data to another type:
  1. By assignment to a variable of different type
  2. By [implicit] promotion (in a mixed type expression)

  3. By casting:

     (long) 21
     (double) 21
     (float) 2.1
     

• Narrowing conversions (to a smaller data type, may lose information) and widening conversions (to a larger data type, usually do not lose information—when might they ever?)

• Example: Lossy.java

Transition

In the second half of this chapter, the content changes from core Java language features to some of the important Java libraries. In addition, we will begin using classes that somebody has already defined. We will look in greater depth at using classes and objects in Chapter 3, and at writing our own classes in Chapter 4. For the time being, we will write only simple classes with a main method and nothing else.

(Second Day)

(2.6) Interactive programs: the Scanner class

• The Scanner class parses text data from the standard input stream or a file into tokens and converts them to typed values.
• Example:
  • Input stream: abc 123 45.67
  • Tokens: "abc", "123", "45.67"
  • Typed values: String "abc", int 123, double 45.67
• New in Java 1.5 ("5")

• Setting up:

  Scanner in0 = new Scanner(System.in); // stdin
  Scanner in1 = new Scanner("1 2 3 4"); // "reads" from the string
  Scanner in2 = new Scanner("friends.data"); // ditto
  Scanner in3 = new Scanner(new File("friends.data")); // file input
  

• Scanner methods include:
  • nextInt() ⇒ int
  • nextDouble() ⇒ double
  • … and similarly for the rest of the primitive data types.
  • next()
    • Returns the next token as a String
    • What is a token?
  • nextLine() ⇒ String
  • hasNext() ⇒ true or false
• Listings:
  • 2.8 Echo.java
  • 2.9 GasMileage.java

(2.7) Graphics

• Pixels, picture and monitor resolution
• Coordinate systems:
  • y goes down from 0 at the top;
  • x goes right from 0 at the left.

  

• Colors as RGB values
  • Each color component can be from 0 to 255.
  • How many colors are possible? 256³ = ?

• Some colors have names: Color.blue, etc.

(2.8-2.9) Applets and Drawing Shapes

• An Applet is a Java program that runs in a web browser.
  • Security considerations
  • How applets made Java "hot"
• An application is a stand-alone program.
  • That is, it runs without a web browser.
  • It still needs the Java interpreter, if it has been compiled only to byte code.
• Applet classes extend (inherit from) javax.swing.JApplet.
  • Explain the long name
• An applet does not have a main method
  • Its paint method is run automatically whenever the applet needs to be displayed.
  • Applets also have a run method; but at this stage, we are just using the method inherited from JApplet.
• The paint method uses methods of the class java.awt.Graphics (also called a graphics context). We may think of a Graphics object as something that can be drawn or painted on; a "page" for drawing; a graphical output device.
• Some methods of java.awt.Graphics (see Figure 2.12 on p. 99):
  • draw/fill-Rect/Oval/Line
  • drawString
  • getColor
  • setColor
• Listings:
  • 2.10 Einstein.java (black and white shapes)
    • See also the required HTML file (handout: Einstein.html)
    • Attributes like width and height should be in quotation marks.
    • A minimal HTML file contains these elements (draw as enclosing boxes): html, head, title, body.
    • For applets we also need to add an applet element. [Is that still correct?]
    • How to run an applet: compile first, create HTML page, then —
      1. in a web browser
      2. appletviewer Einstein.html
  • 2.11 Snowman.java (color picture)

Discussion

• Programming Projects, pp. 108–110 (time permitting). Begin with a header comment (!) and pseudocode, which becomes comments.
  • 2.2 average of three integers
  • 2.10 coins to value
    • Solution: CoinCounter.java (12:30 class) (restricted access; see Oncourse Forums for the login)
  • 2.11 making change
    • Solution: Money.java (5:30 class) (restricted access; see Oncourse Forums for the login). We didn't finish this, but sketched out the solution in pseudocode and coded most of it.
      
  • (?) 2.15–2.16 Applet(s) with name and Big Dipper (together in one applet?)
• Lab assignment: (p. 109, PP #2.8) time durations
  • Details