## Object-Oriented Design & Patterns

#### Cay S. Horstmann

### Chapter 1

### A Crash Course in Java

## Chapter Topics

- Hello, World in Java
- Documentation Comments
- Primitive Types
- Control Flow Statements
- Object References
- Parameter Passing
- Packages
- Basic Exception Handling
- Strings
- Reading Input
- Array Lists and Linked Lists
- Arrays
- Static Fields and Methods
- Programming Style

## "Hello, World" in Java

- Simple but typical class

  #### Ch1/helloworld/Greeter.java

- Features:
  - Constructor `Greeter(String aName)`
  - Method `sayHello()`
  - Instance field `name`
- Each feature is tagged `public` or `private`

## "Hello, World" in Java

- Construct new objects with `new` operator

      new Greeter("World")

- Can invoke method on newly constructed object

      new Greeter("World").sayHello()

- More common: store object reference in object variable

      Greeter worldGreeter = new Greeter("World");

- Then invoke method on variable:

      String greeting = worldGreeter.sayHello();

## "Hello, World" in Java

- Construct separate class to test your class

  #### Ch1/helloworld/GreeterTest.java

- `main` method is called when program starts
- `main` is `static`: it doesn't operate on any objects
- There are no objects yet when `main` starts
- In OO program, `main` constructs objects and invokes methods

## Using the SDK

- Create a new directory to hold your files
- Use a text editor to prepare files (Greeter.java, GreeterTest.java)
- Open a shell window
- `cd` to directory that holds your files
- Compile and run

      javac GreeterTest.java
      java GreeterTest

  Note that Greeter.java is automatically compiled.
- Output is shown in shell window

## Using the SDK

## Using BlueJ

- Download BlueJ from http://www.bluej.org
- No test program required
- Select Project->New and supply directory name
- Click on New Class... and type in Greeter class
- Compile the class
- Right-click the class to construct an object
- Right-click the object to invoke a method

## Using BlueJ

## Documentation Comments

- Delimited by `/** ... */`
- First sentence = summary
- `@param parameter` explanation
- `@return` explanation
- Javadoc utility extracts HTML file

## Documentation Comments - Summary

## Documentation Comments - Detail

## Documentation Comments

- Easy to keep documentation in sync with code
- You must document all classes and methods
- The pros do it--check out the API docs
- Install and bookmark the API docs now!

## Documentation Comments - API Docs

## Primitive Types

- 8 primitive types
- int, long, short, byte
- double, float
- char
- boolean
- suffixes L = long, F = float
- character constants 'a', '\n', '\x2122'
- Casts (int) x, (float) x
- Math class has methods that operate on numbers:

      y = Math.sqrt(x);

## Control Flow

- if
- while
- do/while
- for
- Variable can be declared in for loop:

      for (int i = 1; i <= n; i++)
      {
         . . .
      }
      // i no longer defined here

## Object References

- Object variable holds a reference

      Greeter worldGreeter = new Greeter("World");

- Can have multiple references to the same object

      Greeter anotherGreeter = worldGreeter;

- After applying mutator method, all references access modified object

      anotherGreeter.setName("Dave");
      // now worldGreeter.sayHello() returns "Hello, Dave!"

## The `null` Reference

- `null` refers to no object
- Can assign `null` to object variable:

      worldGreeter = null;

- Can test whether reference is `null`

      if (worldGreeter == null) . . .

- Dereferencing `null` causes `NullPointerException`

## The `this` Reference

- Refers to implicit parameter of method call
- Example: Equality testing

      public boolean equals(Greeter other)
      {
         if (this == other) return true;
         return name.equals(other.name);
      }

- Example: Constructor

      public Greeter(String name)
      {
         this.name = name;
      }

## Parameter Passing

- Java uses "call by value":
  Method receives copy of parameter value
- Copy of object reference lets method modify object

      public void copyNameTo(Greeter other)
      {
         other.name = this.name;
      }

-     Greeter worldGreeter = new Greeter("World");
      Greeter daveGreeter = new Greeter("Dave");
      worldGreeter.copyNameTo(daveGreeter);

## No Reference Parameters

- Java has no "call by reference"

-     public void copyLengthTo(int n)
      {
         n = name.length();
      }
      public void copyGreeterTo(Greeter other)
      {
         other = new Greeter(name);
      }

- Neither call has any effect after the method returns

      int length = 0;
      worldGreeter.copyLengthTo(length); // length still 0
      worldGreeter.copyGreeterTo(daveGreeter) // daveGreeter unchanged

## Packages

- Classes are grouped into packages
- Package names are dot-separated identifier sequences

      java.util
      javax.swing
      com.sun.misc
      edu.sjsu.cs.cs151.alice

- Unique package names: start with reverse domain name

## Packages

- Add `package` statement to top of file

      package edu.sjsu.cs.cs151.alice;
      public class Greeter { . . . }

- Class without package name is in "default package"
- Full name of class = package name + class name

      java.util.ArrayList
      javax.swing.JOptionPane

## Importing Packages

- Tedious to use full class names
- `import` allows you to use short class name

      import java.util.ArrayList;
      . . .
      ArrayList a; // i.e. java.util.ArrayList

- Can import all classes from a package

      import java.util.*;

## Importing Packages

- Cannot import from multiple packages

      import java.*.*; // NO

- If a class occurs in two imported packages, `import` is no help.

      import java.util.*;
      import java.sql.*;
      . . .
      java.util.Date d; // Date also occurs in java.sql

- Never need to import `java.lang`.

## Packages and Directories

- Package name must match subdirectory name.

      edu.sjsu.cs.sjsu.cs151.alice.Greeter

  must be in subdirectory

      basedirectory/edu/sjsu/cs/sjsu/cs151/alice

- Always compile from the base directory

      javac edu/sjsu/cs/sjsu/cs151/alice/Greeter.java

  or

      javac edu\sjsu\cs\sjsu\cs151\alice\Greeter.java

- Always run from the base directory

      java edu.sjsu.cs.cs151.alice.GreeterTest

## Exception Handling

- Example: `NullPointerException
  `

      String name = null;
      int n = name.length(); // ERROR

- Cannot apply a method to `null`
- Virtual machine throws exception
- Unless there is a handler, program exits with stack trace

      Exception in thread "main" java.lang.NullPointerException
         at Greeter.sayHello(Greeter.java:25)
         at GreeterTest.main(GreeterTest.java:6)

## Checked and Unchecked Exceptions

- Compiler tracks only checked exceptions
- `NullPointerException` is not checked
- `IOException` is checked
- Generally, checked exceptions are thrown for reasons beyond the programmer's control
- Two approaches for dealing with checked exceptions
  - Declare the exception in the method header (preferred)
  - Catch the exception

## Declaring Checked Exceptions

- Example: Opening a file may throw `FileNotFoundException`:

      public void read(String filename) throws FileNotFoundException
      {
         FileReader reader = new FileReader(filename);
         . . .
      }

- Can declare multiple exceptions

      public void read(String filename)
         throws IOException, ClassNotFoundException

      public static void main(String[] args)
         throws IOException, ClassNotFoundException

## Catching Exceptions

-     try
      {
         code that might throw an IOException
      }
      catch (IOException exception)
      {
         take corrective action
      }

- Corrective action can be:
  - Notify user of error and offer to read another file
  - Log error in error report file
  - In student programs: print stack trace and exit

        exception.printStackTrace();
        System.exit(1);

## The finally Clause

- Cleanup needs to occur during normal and exceptional processing
- Example: Close a file

      FileReader reader = null;
      try
      {
         reader = new FileReader(name);
         ...
      }
      finally
      {
         if (reader != null) reader.close();
      }

## Strings

- Sequence of Unicode characters
- `length` method yields number of characters
- `""` is the empty string of length 0, different from `null`
- `charAt` method yields characters:
  `char c = s.charAt(i);`

## Strings

- `substring` method yields substrings:
  `"Hello".substring(1, 3)` is `"el"
  `
- Use `equals` to compare strings
  `if (greeting.equals("Hello"))`
- `==` only tests whether the object references are identical:
  `if ("Hello".substring(1, 3) == "el") ... // NO!`

## `StringTokenizer`

- Use `StringTokenizer` to break string into substrings

      String countries = "Germany,France,Italy";
      StringTokenizer tokenizer = new StringTokenizer(countries, ",");
      while (tokenizer.hasMoreTokens())
      {
         String country = tokenizer.nextToken();
         ...
      }

## String concatenation

- `+` operator concatenates strings:

      "Hello, " + name

- If one argument of `+` is a string, the other is converted into a string:

      int n = 7;
      String greeting = "Hello, " + n;
         // yields "Hello, 7"

- `toString` method is applied to objects

      Date now = new Date();
      String greeting = "Hello, " + now;
         // concatenates now.toString()
         // yields "Hello, Wed Jan 17 16:57:18 PST 2001"

## Converting Strings to Numbers

- Use static methods
  `Integer.parseInt`
  `Double.parseDouble`
- Example:

      String input = "7";
      int n = Integer.parseInt(input);
         // yields integer 7

- If string doesn't contain a number, throws a `NumberFormatException`(unchecked)

## Reading Input

- Use input dialog:

      String input = JOptionPane.showInputDialog("How old are you?");

- If user cancels, result is `null`:

      if (input != null) age = Integer.parseInt(input);

## Reading Input

- Must call `System.exit(0)`

- #### Ch1/input1/InputTest.java

- Also have message dialog

      JOptionPane.showMessageDialog(null, "Hello, World");

## Reading Input

- Read console input from System.in
- System.in is an InputStream: reads bytes
- We want a Reader that reads characters
- Turn System.in into InputStreamReader
- Also want to read entire lines
- Use BufferedReader:

      BufferedReader console = new BufferedReader(
         new InputStreamReader(System.in));
      System.out.println("How old are you?");
      String input = console.readLine();
      int age = Integer.parseInt(input);

- #### Ch1/input2/InputTest.java

## The `ArrayList` class

- Collects objects of any class type
- `add` appends to the end

      ArrayList countries = new ArrayList();
      countries.add("Belgium");
      countries.add("Italy");
      countries.add("Thailand");

- `size` method yields number of elements
  `for (int i = 0; i < countries.size(); i++) . . .`
- `get` gets an element; must cast to correct type:
  `String country = (String)countries.get(i);`
- `set` sets an element
  `countries.set(1, "France");``
  `

## The `ArrayList` class

- Insert and remove elements in the middle`
  countries.add(1, "Germany");
  countries.remove(0);`
- Not efficient--use linked lists if needed frequently
  `
  `

## Linked Lists

- Efficient insertion and removal
- `add` appends to the end

      LinkedList countries = new LinkedList();
      countries.add("Belgium");
      countries.add("Italy");
      countries.add("Thailand");

- Use iterators to edit in the middle

## List Iterators

- Iterator points between list elements
- next retrieves element and advances iterator
  ListIterator iterator = countries.listIterator();
  while (iterator.hasNext())
  {
  String country = (String) iterator.next();
  . . .
  }
- add adds element before iterator position
- remove removes element returned by last call to next

## Arrays

- Drawback of array lists: can't store numbers
- Arrays can store objects of any type, but their length is fixed
  `int[] numbers = new int[10];`
- Array variable is a reference

## Arrays

- `length` member yields number of elements

      for (int i = 0; i < numbers.length; i++)

- Can have array of length 0; not the same as `null`:

      numbers = new int[0];

- Array access with `[]` operator:
  `int n = numbers[i];`
- Multidimensional array

      int[][] table = new int[10][20];
      int t = table[i][j];

## Command-Line Arguments

- `void main(String[] args)`
- `args` parameter of `main` is initialized with command-line arguments
- Example:
  `java GreeterTest Mars`
- `args.length` is 1
  `args[0]` is `"Mars"`

## Static Fields

- Shared among all instances of a class
- Example: shared random number generator

      public class Greeter
      {
         . . .
         private static Random generator;
      }

- Example: shared constants

      public class Math
      {
         . . .
         public static final double PI = 3.14159265358979323846;
      }

## Static Methods

- Don't operate on objects
- Example: `Math.sqrt`
- Example: factory method

      public static Greeter getRandomInstance()
      {
         if (generator.nextBoolean()) // note: generator is static field
            return new Greeter("Mars");
         else
            return new Greeter("Venus");
      }

- Invoke through class:

      Greeter g = Greeter.getRandomInstance();

- Static fields and methods should be rare in OO programs

## Programming Style: Case Convention

- variables, fields and methods:
  start with lowercase, use caps for new words:

      name
      sayHello

- Classes:
  start with uppercase, use caps for new words:

      Greeter
      ArrayList

- Constants:
  use all caps, underscores to separate words

      PI
      MAX_VALUE

## Programming Style: Property Access

- Common to use `get/set` prefixes:

      String getName()
      void setName(String newValue)

- Boolean property has `is/set` prefixes:

      public boolean isPolite()
      public void setPolite(boolean newValue)

## Programming Style: Braces

- "Allman" brace style: braces line up

      public String sayHello()
      {
         return "Hello, " + name + "!";
      }

- "Kernighan and Ritchie" brace style: saves a line

      public String sayHello() {
         return "Hello, " + name + "!";
      }

## Programming Style: Fields

- Some programmers put fields before methods:

      public class Greeter
      {
         private String name;
         public Greeter(String aName) { . . . }
         . . .
      }

- From OO perspective, it is better to list the `public` interface first
- All fields should be `private`
- Don't use default (package) visibility

## Programming Style: Miscellaneous

- Spaces around operators, after keywords, but not after method names

      Good: if (x > Math.sqrt(y))
      Bad: if(x>Math.sqrt (y))

- Don't use C-style arrays:

      Good: int[] numbers
      Bad: int numbers[]

- No magic numbers

      Good: h = HASH_MULTIPLIER * h + val[off];
      Bad: h = 31 * h + val[off];