TeachJava 2003

1
TeachJava! 2003

• Corky Cartwright
• Dung Nguyen
• Stephen Wong
• Charlie Reis, James Hsia, Peter Centgraf

2
From C To Java

• C and Java have similar syntax.Dont be
  misled!
• Profoundly different semantics high-level
  objects vs. bytes in memory
• Profoundly different programming modelsJava is
  object-oriented (OO)C is object-based (OB) in
  normal usage

3
Encouraging Note

• Writing OO programs in Java is remarkably easy --
  provided we recognize that it is fundamentally
  different than writing OB programs in C.
• There is little common conceptual ground, other
  than syntactic conventions, beween OO programing
  (OOP) in Java and OB programming in C

4
Guiding Vision

• Program design in Java is data-directed.Design
  the data abstractions first they will determine
  the structure of the code. In OOP circles, this
  data design process is often called
  object-modeling.
• Common data abstractions are codified as design
  patterns

5
Secondary Theme

• DrJava, our lightweight, reactive environment for
  Java, facilitates active learning with DrJava
  learning Java is a form of exploration.
• DrJava is not a toy DrJava is developed using
  DrJava. It includes everything that we believe
  is important and nothing more.

6
What Is an Object?

• Collection of fields representing the properties
  of a conceptual or physical object.
• Collection of operations called methods for
  observing and changing the fields of the object.
• These fields and methods often called the
  members of the object.

7
Example Phone Directory

• Task maintain a directory containing the office
  address and phone number for each person in the
  Rice Computer Science Dept.
• Each entry in such a directory has a natural
  representation as an object with three fields
  containing a persons
• name
• address
• phone number
• represented as character strings.

8
Summary of Entry format

• Fields
• String name
• String address
• String phone
• Methods
• String getName()
• String getAddress()
• String getPhone()

9
Java Method Invocation

• A Java method m is executed by sending a method
  call o.m()
• to an object o, called the receiver. The method
  m must be a member of o.
• The code defining the method m can refer to the
  receiver using the keyword this.

10
Finger Exercise

• In the DrJava programming environment, open the
  program file Entry.java, compile it, and type the
  following statements in the Interactions pane
• Entry e new Entry("Corky","DH 3104","x
  6042")e.getName()
• e.getPhone()

11
Java Expressions

• Java supports essentially the same expressions
  over primitive types (int, float, double,
  boolean) as C.
• Notable differences
• boolean is a distinct type from int
• no unsigned version of integer types
• explicit long type

12
Finger Exercise

• Evaluate the following
• -5 3
• -(5 3)
• 5 3
• 5./3.
• 5 / 0
• 5./0.
• 3 .1 .1 - 3.
• 5
• 5. 6.

13
Finger Exercise cont.

• 72. - 32. 1.8
• (72. - 32.) 1.8
• 72. - 30. - 12.
• 72. - (30. - 12.)

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Java Statements

• Essentially the same form as in C assignment,
  if, while, for, return,
• But well-written Java programs consist primarily
  of assignment, if, and return statements (with
  smattering of for).
• Focus on assignment and if for most of the week.

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Assignment

• Restricted form of assignment variable
  definition
• type var exp
• Example
• int x 5

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Finger Exercise

• int x 5
• xx
• double d .000001
• double dd dd
• dd
• dddd
• 1. dd
• 1. dddd

17
Finger Exercise cont.

• Evaluate
• int x 7
• if (x 5) y 0 else y 10
• y
• Did you get the behavior that you expected?
  Repeat the exercise with corrected syntax.
• Evaluate
• boolean switch1 (x 7)
• switch1
• Repeat the exercise with corrected syntax.

18
Classes Object Templates

• A Java program is a collection of classes.
• A class is an object template consisting of a
  collection of members--fields and methods-- plus
  a constructor, which is a special method used to
  initialize objects when created.

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Example Class Entry

• class Entry
• / fields /
• String name
• String address
• String phone
• / constructor /
• Entry(String n, String a, String p)
• this.name n
• this.address a
• this.phone p
• / accessors /
• String getName() return this.name
• String getAddress() return this.address
• String getPhone() return this.phone

20
Finger Exercise

• In the Definitions pane of DrJava, enter the
  Java program defining the Entry class. In the
  Interactions pane, evaluate the following program
  text
• Entry e new Entry("Corky", "DH 3104", "x
  6042")
• e.getName()
• e.getAddress()
• e.getPhone()
• Save your program for future use in a file named
  Entry.java.

21
Java Data Types

• Two fundamental categories
• Primitive types int, boolean, double, float,
  char, long, short, byte (first three are most
  common)
• Object types all class instances (objects)
  belong to object types, which are disjoint from
  the primitive types
• Value of primitive type (e.g., true, 0) are not
  objects

22
Object Types

• Organized in a strict hierarchy with the
  universal type Object at the top.
• Every class C except Object has an immediate
  superclass, which is the parent of C in the
  hierarchy. In a class definition (like our Entry
  example), the default superclass is Object.
• A descendant in the class hierarchy is called a
  subclass. B is a subclass of A iff A is a
  superclass of B. Entry is a subclass of Object
  Object is a superclass of Entry.

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Example the String class

• The String class is built-in to Java, just like
  Object.
• Finger Exercise evaluate
• String s "Corky"
• Object o s
• o
• s
• o s
• String t "Cork" "y
• t s
• hello hello
• Multiple copies of the same String may exist do
  not use to test String equality.

24
Object Types cont.

• Each subclass C inherits (includes) all of the
  members of its superclass.
• The declared members of C augment the inherited
  members with one exception if C declares a
  method m with exactly the same name and types as
  an inherited method, then the new definition of m
  overrides (replaces) the inherited definition.

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Inheritance from Object

• The Object class has several members that its
  children inherit. They include the methods
• public String toString() which gives a String
  representation for the object.
• public boolean equals(Object o) which compares
  this to o

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Example of Overriding

• class Entry
• / fields /
• String name
• String address
• String phone
• / constructor /
• Entry(String n, String a, String p)
• this.name n
• this.address a
• this.phone p
• / accessors /
• String getName() return this.name
• String getAddress() return this.address
• String getPhone() return this.phone

27
Finger Exercise

• Open your Entry class into the DrJava Definitions
  pane.
• Compile your program and evaluate
• Entry e new Entry("Corky", "DH 3104", "x
  6042")e
• Add the definition of toString() from the
  previous slide to your Entry class.
• Compile your program and evaluate
• Entry e new Entry("Corky", "DH 3104", "x
  6042")

28
The Wrapper Classes

• How do we treat primitive values as objects?
  Java includes a builtin wrapper class for each
  primitive type.
• Examples
• Integer is the wrapper class for int
• Boolean is the wrapper class for boolean
• Double is the wrapper class for double

29
Finger Exercise

• In the DrJava Interactions evaluate the
  following
• Integer i new Integer(5)
• i
• Integer j i
• i j
• Integer k new Integer(5)
• i k
• i.equals(k)

30
Constructors

• Given a class definition, Java provides a
  mechanism called new for creating new instances
  of the class.
• To exploit the new, the class must provide a
  special method called a constructor that
  specifies how the fields of the created object
  are initialized.
• A constructor method has the same name as the
  class and does not contain the return type in the
  heading.
• Example
• Entry(String n, String a, String p)
• this.name n
• this.address a
• this.phone p
• If a class does not include a constructor, Java
  provides a default constructor (of no arguments)
  that does nothing.

31
Finger Exercise

• Add a second constructor
• Entry(String n)
• to the Entry class where n specifies the name of
  the entry. Use any reasonable default value,
  e.g., "unknown" for the address and phone fields.
• Evaluate the expression
• new Entry("Matthias")