Homework 4:

The Super Savitch Study Guide

courtesy of Adam Fleming

The material presented in HW4 WILL be on the midterm

Midterm will cover Chapters 6-9 and JoptionPane

Sample Midterm is a sample of form not content.

12 multiple choice
5 Short Answer
In many cases the amount of space allowed for answer is much larger than you need. Do not take that as a guide to how long the answer is.
Style will count.

Midterm Review:

Reference Types
classes and arrays are reference types.
Arrays: get length using <array_name>.length
it’s not a method, it’s an instance variable.
example:

double[] list = {4, 1, 9, 3, 2};

System.out.println(list.length); // output? 5 largest index of the array? 4

example:

char[] greet = {‘h’,’e’,’l’,’l’,’o’}; // greet points to an array

char[] cuss = greet;

cuss[4] = ‘!’;

// what is now stored in greet? "hell!"

// cuss and greet are the same object

What does this mean when we think of…
using the = and == operator?
- = makes two pointers point to the same object in memory. This is usually an undesirable effect
- == check to see if two pointers are equals, which means it checks to see if the two objects you test are actually the same object, which is very uncommon.
returning private reference types?
- When we return a reference type, we don’t return a copy of the object, like when we use int’s and char’s. We return the actual object!

Inheritance
extending
- parent and child classes
privacy:
- private inherited methods and variables are not accessible
using this and super
- this() calls constructor in class this refers to
- super() calls constructor in parent class
- cannot use super.super()
overriding vs. overloading
- make sure to use the same method headers
- call the overridden method using super.overridden_method_name()
every java class extends from Object class
Dynamic Binding:
- objects just "do the right thing"
- example:

public class Mammal{
public void speak(){
System.out.println(getWords());
}

public String getWords(){
return "nothing yet";
}
}

public class Person extends Mammal{
public String getWords(){
return "hello world.";
}

public void shout(){
System.out.println("HEY!");
}
}

public class Dog extends Mammal{
public String getWords(){
return "bark";
}

public void chase(){
System.out.println("chasing….");
}
}

Mammal M = new Mammal();
Person P = new Person();< BR > Dog D = new Dog();< BR >
M.speak(); // "nothing yet"
P.speak(); // "hello world"
P.shout(); // "HEY!"
D.speak(); // "bark"
D.chase(); // "chasing…."

M = new Dog(); // This is okay – a Dog IS A Mammal
M.speak(); // output? "bark"
M.chase(); // output? ** compile time error **
M = P;
M.speak(); // output? "hello world"
M.shout(); // output? ** compile time error **

// Why do we get the compile time errors? because chase and shout are not methods in the Mammal Class. Why does it print out the persons words and dogs words? That is the dynamic binding happening.

Important:
the variable type controls what method names can be used, but the object type controls which method to use.
- type checking happens at compile-time
- dynamic binding happens at run time
So, what is polymorphism?
- it means using the process of dynamic binding to allow different objects to use different method actions for the same method name.

Exception Handling
use try-throw-catch blocks
example:

try{

// normal stuff

// sketchy-exception-generating-code

// throw new Exception_Type("mabye a message");

// other stuff

}

catch(Exception_Type some_name)

{

// do stuff

// perhaps use some_name.getMessage(), which

// returns the message from the throw, or a default message

}

Important:
- execution stops when an exception is thrown and resumes at the point where the exception is caught.
- This means order matters in catch blocks, catch the more specific exception first
when you define your own class:
- make sure to have a default constructor that creates a default message and calls the superclass constructor, ie: super("this is an exception message");
- make sure to allow the user to specify a message, so have a constructor that takes a string and calls something like this: super(message);
all exceptions must be caught or declared in throws clause except
- descendents of the RuntimeException class OR
- descendents of the Error class

File IO

Text File IO
- Creating Streams
- Input Stream: new FileReader("file_name.xxx")
- Output Stream: newFileOutputStream("file_name.xxx")
- Manipulating Streams
- Input Manipulator: BufferedReader class
  - buffered_reader_name.readLine()
- Output Manipulator: PrintWriter class
  - print_writer_name.println("put stuff here");
- Detecting EOF
  - buffered_reader_name.readln() == null
Binary IO
- Creating Streams
- Input Stream: new FileInputStream("file_name.xxx")
- Output Stream: new FileOutputStream("file_name.xxx")
- Manipulating Streams
- Input Manipulator: DataInputStream class
  - data_input_name.readInt() or Long or Double, etc
- Output Manipulator: DataOutputStream class
  - data_output_name.writeInt(n) or Long or Double, etc
- Detecting EOF
  - data_input_name.readInt() throws EOFException
All Files
- close: stream_manipulator_name.close();

Remember these steps when you use files:

First, you must create a stream. You can think of the stream as representing the data in the file
- possible streams: FileReader, FileInputStream, FileOutputStream
Second, you should use some type of class that will give you methods to act on the data.
- possible stream manipulators: BufferedReader, PrintWriter, DataInputStream, DataOutputStream

So, why do we have this complicated two part process for working with files?
- Remember that the stream represents the data. We cannot work directly with the data, we need to use the manipulators.
- The manipulators provide us with methods for working with the data (streams) that greatly simplify file IO.

Relevant examples from the practice midterm:

Consider the following which might appear in a Java program:
char[ ] b = new char[10];
The expression b[0] can be used

a. to hold one value of type char .
b. as an argument to plug in for a parameter of type char .
c. as an argument to plug in for a parameter of type char[ ] .
d. only a. and b.
e. only a and c
f. none of the above.

The number of possible elements in an array a is equal to.
a. a.length()
b. a.length
c. a.size
d. a.size()
e. none of the above

Which of the following are reference types?

a. char
b. String
c. double[]
d. exactly two of the above. (You do not have to say which two.)
e. all of the above.
f. none of the above.

Consider the following which might appear in a Java program:
double[ ] a = new double[12];

a. the array a has thirteen indexed variables numbered 0 through 12.
b. the array a has twelve indexed variables numbered 0 through 11.
c. the array a has twelve indexed variables numbered 1 through 12.
d. the array a has eleven indexed variables numbered 1 through 11.
e. none of the above.

If a and b are array variables (that name some arrays) then the effect of
a = b;

a. is to copy the elements in the array b to the indexed variables of the array a .
b. is to copy the elements in the array a to the indexed variables of the array b .
c. is to make a and b two names for the same array,
d. nothing. It is illegal to use = with arrays.
e. none of the above.

Every exception class

a. has a method named getMessage .
b. has a method named getText .
c. has a method named getActionCommand .
d. all of the above.
e. none of the above.