Object-Oriented Design & Patterns

Object-Oriented Design & Patterns (3rd edition) by Cay Horstmann

Chapter 1 sections 1.1, 1.3, 1.5, 1.6

A Crash Course in Java

Chapter Topics

• Hello, World  in Java
  
• Documentation Comments
• Primitive Types
• Control Flow Statements
• Object References
• Parameter Passing

"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
  

  ch01/helloworld/GreeterTester.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
  

Another Code Example:Circle

• ch01/circle/Circle.java

• Create a csci2300 directory: mkdir csci2300
• Create a circle sub-director: mkdir csci2300/circle
• Download Circle.java code
• Fill in the code for constructors

• ch01/circle/CircleTest.java

• Download CircleTest.java code to csci2300/circle directory
• Create Circle objects using the two constructors you wrote. Call print method on those objects.
• Compile and run your code:

  javac CircleTest.java
  java CircleTest
            

Documentation Comments

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

Using javadoc

• javadoc -help

• javadoc -d docs src/*.java

• Circle Docs
• Easy to keep documentation in sync with code
• You must document all classes and methods
• run javadoc in your csci2300/circle directory (redirect the output to docs director)
• Look at the documentation
• Add documentation comments to the 2nd constructor
• Add documentation comments to getArea() method
• Re-run javadoc and look at the documentation: firefox docs/index.html

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
  

• Try it yourself: download, compile, and run CircleCopy.java