Day3 Summary

CSC 224 -- Summary for Day3, June 25, 2001.

Practice Problems

True or False: a static method can call only static methods.
Answer: This is generally true, but here is a trick. You create a new object of the class containing the static method, then call dynamic methods from this object. For example:
public class Test
{
    public static void main(String[] args)
    {
       Test t = new Test();
       System.out.println(t.addOne(5));
    }     public int addOne(int x)
    {
       return x + 1;
    }
}
True or False: a static method can only be called by static methods. Answer: False, static methods can be called by any method.
What is wrong?
1 private void main(String args)
2 {
3     RepeatSentence("I will eat my spinach.", 20);
4 private void RepeatSentence(String sentenceToRepeat; int n)
5     for(i = 1; i <= 20; i =+ 1);
6     {
7        System.println(sentenceToRepeat);
8     }
9 }
Answer: Line 1 should be public and static. Also, it should have [ ] after String.
In Line 3, RepeatSentence should start with a lower case r.
There should be a closing brace } between lines 3 and 4.
In Line 4, repeatSentence should be static. There should also be a comma, not a semicolon between String sentenceToRepeat and int n.
An opening brace should be between lines 4 and 5.
The variable i must be declared, either in the for statement or between lines 4 and 5.
The for loop should go from 1 to n, not 1 to 20.
In Line 5, i =+ 1 should be i += 1, or better yet, i++. There should be no ; at the end of Line 5.
In Line 7, System.println should be System.out.println.
Write a Java command line application that reads hours, minutes, seconds using the Keyboard class, then prints this information out formatted as a 24 hour time.
Answer:
import cs1.Keyboard;
import java.text.DecimalFormat;
public class Time
{
    public static void main(String[] args)
    {
       int h, m, s;
       String h2, m2, s2;
       DecimalFormat f;
       f = new DecimalFormat("00");
       h = Keyboard.readInt();
       m = Keyboard.readInt();
       s = Keyboard.readInt();
       h2 = f.format(h);
       m2 = f.format(m);
       s2 = f.format(s);
       System.out.println(h2 + ":" + m2 + ":" + s2);
    }
}
Which method do you use to draw a circle in an applet? The center is at (100, 150) and the radius is 50.
Answer:
import java.applet.Applet;
import fava.awt.*;
public class Circle extends Applet
{
    public void init()
    {
       setSize(300, 300);
    }
    public void paint(Graphics g)
    {
       g.drawOval(50, 100, 100, 100);
    }
}

New Topics

Every object is derived eventually from the Object class. By default, a.equals(b), has the same meaning as a == b, that is do a and b refer to the same object in physical memory. However, often equals is overridden. For example, for String objects, a.equals(b) means that a and b consist of identical characters.

The toString method is also a method of the Object class that can be overridden. By default, toString prints the name of the object and its location in memory. Usually a more useful action for toString to take is to print some information about the contents of the object.

Conclusion: use a.equals(b) for String objects, not a == b.

Go over examples from Day3.txt or Day3.zip.
- Convert.
  Use valueOf to convert from int or double to String.
  Use Integer.parseInt and Double.parseDouble to convert from int and double to String.
- Kid1.
  To create a Kid object, either use
  Kid kid1;
  kid1 = new Kid("Brittany", 36594, 'F', 9);
  or
  Kid kid1 = new Kid("Brittany", 36594, 'F', 9);
  Here is a diagram showing this situation:
  
  The constructors for Kid are
  public Kid()
  and
  public Kid(String aName, int anId, char aGender, int anAge)
  The signature for the first constructor is ( ).
  The signature for the second constructor is (String, int, char, int).
  They are different constructors because their signatures are different.
  Methods with the same name and different signatures are called overloaded methods.
  The accessor methods for Kid are getName, getId, getGender, getAge. There are no mutator methods, so the private instance variables name, id, gender, and age are read only.
  The other methods for Kid are hasBirthday (adds 1 to age) and print.
- Clock.
  The private instance variables are hours, minutes, and seconds. The constructors are the default constructor and the parameterized constructor specifying hours, minutes, and seconds. The methods are tick (add one second to time) and display (display the time in 24 hour format).
- Finalize.
  Although constructors are very similar in C++ and Java, there is no exact equivalent to a destructor. Destructors are not necessary because the garbage collector reclaims memory automatically. However, there is a finalize method that runs just before the garbage collector reclaims the memory of its object. In a small project, finalize is not likely to run unless the garbage collector is specifically invoked with a call to System.gc. Even when gc is called, the behavior of finalize is unpredictable.
- PigLatin
  Translate an English phrase into Pig Latin. "This is a test" becomes "isthay isyay ayay estay."
  The application uses the Java utility program StringTokenizer to break up a phrase into words. It then checks whether a word begins with a vowel, a single consonant, or a two letter consonant combination before translating the word.
- IntArray
  In C++, space foran array can be either automatic, or dynamically created using new with a pointer. In Java, arrays are always dynamically created, as shown in this diagram.
  
  Here are three syntaxes for declaring, allocating space, and initializing an array:
  Syntax 1:
  int[] a;
  a = new int[3];
  a[0] = 32; a[1] = 18; a[2] = 70;
  Syntax 2:
  int[] a = new int[3];
  a[0] = 32; a[1] = 18; a[2] = 70;
  Syntax 3:
  int[] a = {32, 18, 70};
  We will cover two-dimensional arrays later.
- KidArray.
  An array of objects is actually an array of reference variables.
  
  Each Kid object in the array is initialized with its own constructor call.
- CharStack.
  A stack is an array of elements for which only the top element is accessible. The private instance variables are the array, which actually stores the elements, top (the index of the current top element) and size (the number of elements in the array).
  The methods are top (returns the value of the top elements), push (push a new element onto the top of the stack), pop (remove the top element of the stack), isEmpty (returns true is the stack is empty), and isFull (returns true if the stack is full).
- Derived.
  Illustrates how a derived class works. The class Derived inherits the instance variable a from the class Base. First the constructor for the base class fires, then the constructor for the derived class fires.
- Messages.
  This example shows how overriding methods in Java works. If the methods names and signatures in the base class and derived class are identical, then the method in the derived class overrides the method in the base class.
  These methods behave like virtual methods in C++: the method called does not depend on the type of the reference variable, it depends on call to new that created the object.
- Temperature1.
  Creating a class that displays an applet with controls consists of these steps:
  1. Extend (inherit) the Applet class and implement the ActionListener class. Here is more information on what implements means.
  2. Specify the height and width of the applet.
  3. Declare private instance reference variables for the controls. Here is a table of Common Controls. It also gives a brief explanation of the difference between AWT and Swing controls. We will use AWT for applets and Swing for frame based user interfaces.
  4. Define an init method that does the following: (a) creates the objects for the controls with calls to new, (b) add an action listener to controls that need one, (c) add the controls to the applet, (d) set the background color of the applet, (e) set the size of the applet.
  5. Implement an actionPerformed method for the ActionListener interface. This satisfies the "implements ActionListener" requirement. When a class implements an interface like ActionListener, it agrees to implement (supply method bodies) for all the methods of the interface. The ActionListener only has the actionPerformed method, but other interfaces have more. Here is a link to Common Listener Interfaces and the methods that must be implemented for each.
- Temperature2.
  The steps to designing a user interface in a frame window is similar to designing one in an applet. We use Swing instead of AWT. We also use an inner class for SubmitActionListener that implements the ActionListener interface. Note: ActionListener is not a class that can be used directly. It must be implemented by a user defined class. The descriptions of Controls and Listeners also applies to frame based user interfaces.