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CSC205::Lecture Note::Week 04
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Overview Assignment(s) Code Rot13.java | Import.java | PolyNumber.java | XYZ.java | Robot.java | Robot2.java
A.java (arrays) | Exceptions.java | TestExceptions.java | Xception.java


Arrays

An array is a aggregate (collection, set, group, list) of individual data values with two characteristics: it is ordered and homogeneous.

The following are some array-related terms:

An array is homogeneous because each element must be of the same type. Example: an array of int values, an array of float values, an array of char values, and array of Object values (handles, references, pointers).

An array is ordered because it has a 1st element, a 2nd element, a 3rd element, and so on. Array elements are probably stored in contiguous memory locations [this is definitely true in C and C++; in Java it is left up to the JVM implementors].

Here is a list of items you should understand when using arrays:

Defining an Array of Primitive Types

The following creates an array of long values:

   long[] ssNbrs;      // an array of social security numbers

      // ssNbrs is an object variable that is going to point to
      // an array of  long  values; at this point and time, it 
      // doesn't point to anything (i.e. it is equal to  null)

   ...

   int n = fooA();    // get the array length somehow

   ...

   ssNbrs = new long[n];

      // array length is decided at run-time (i.e. we don't know
      // the array length at compile-time; the information is
      // obtained from a user, or from a database file, etc.)
      // two things occur when an array is defined:  memory is
      // allocated from the heap and the memory is initialized
      // to the appropriate default values  [0 for integral and
      // floating-point types,  false  for  boolean  arrays, and
      // null  for arrays of object references]

   ...

   for (int i = 0; i < ssNbrs.length; i++)
      ssNbrs[i] = -1L;  // initialize each element to -1

      // use the  <  relational operator is important; a common
      // programming defect is to use the  <=  operator -- what
      // would happen if we did?

   ...

   ssNbrs = new long[2 * n];

      // after an array has been created, its length if fixed; if you
      // need more elements, then you need to create a new array;
      // now  ssNbrs  references (i.e. points to) an entirely
      // different array of long values; the original array still
      // exists, but the reference to it has been lost and it are
      // not able to copy the original values into the new array

   long[] orig = ssNbrs;
   ssNbrs = new long[n * 3];

   for (int i = 0; i < orig.length; i++) ssNbrs[i] = orig[i];

      // the handle to the original array is stored in the array variable
      // named  orig.  The new array is allocated and the contents of
      // the original arrray is copied to the new array

   System.arraycopy(orig, 0, ssNbrs, 0, orig.length);

      // arraycopy(Object src, int srcOffset, 
      //           Object destdst, int destOffset, int count);
      //
      // the  arraycopy()  method copies a region of one array, src,
      // beginning at element  srcOffset, to another array, dst,
      // beginning at element  destOffset;  count  elements are copied.
      // note:  the arrays must already be allocated

   Tip:  In the control-EXPR of the loop, I'm better off using
         ssNbrs.length  rather than the variable  'n'  because 
         since 'n' is a variable, it could be modified such that 
         the EXPR  n == ssNbrs.length  is not true.
Defining an Array of Objects

The following code snippet an array of object variables.

   Integer[] iArray;

      // iArray  is a reference (handle, pointer) to an array of 
      // Integer references -- at this point and time, it doesn't
      // "point" to any array

   ...

   iArray = new Integer[10];

      // array length is known compile-time; an array to store 
      // 10 Integer references is created; each array element is 
      // initialized to   null   (recall,  null  is a Java keyword)

   ...

   for (int i = 0; i < iArray.length; i++)
      iArray[i] = new Integer(i);

         // an  Integer  object is created and a reference to it is
         // stored in  iArray; when the Integer object is created,
         // it is initialized to the value of 'i'...

Arrays are always stored on the heap. The reference to the array will be stored either on the stack or in the data segment depending on where it is defined.

The following are some comments on the efficiency of arrays:

Arrays.java | A.java

{TopOfPage} {Oracle.com::API Specification | Tutorial} {Derek Banas Videos} {Data Structure Visualizations}


Exceptions

Exceptions are "exceptional" conditions that can occur while a program is executing (i.e. running). Some exceptions are caused due to software defects (e.g. using a null object handle, or indexing beyond the bounds of an array).

   String s = null;
   String s1 = "hello";
   ...
   if (s.equals(s1)) { ... }     //this will cause an exception
   ...
   int[] array = new int[10];
   ...
   array[array.length] = 200;    //this will cause an exception

Exceptions are also generated by errors that are beyond the control of the program (e.g. the program needs to access a file that doesn't exist, or the program needs to fetch a webpage given an invalid [mal-formed] URL).

Exceptions can be "caught" by programs, but if an exception does occur that isn't caught, then the program (assuming it is non-graphical) terminates. (A GUI program returns to the user interface).

Exceptions are objects that are created from the Exception and RuntimeException classes.

          Throwable
             |
   Error               Exception
   .....               /  ...  \
                  IOException   RuntimeException 
                    (checked)     (unchecked)

There are two categories of exceptions: checked and unchecked. Checked exceptions must be caught (otherwise, your program will not compile). Unchecked exceptions typically are not caught (i.e. they cause your program to terminate when they occur).

A method can throw an exception when it encounters a situation it cannot handle. There are numerous methods that come with the JCL (java class library) that throw exceptions.

A method declaration must contain a list of the the checked exceptions it may throw.

   public int someMethod() throws IOException, MalformedURLException

A method does not need to declare RuntimeException inherited exceptions that it may throw.

If a method overrides a method from a parent class, then the child method cannot throw more checked exceptions that the parent method does.

A method can say it throws exceptions that it really doesn't throw.

Catching an Exception

Whenever a method is called that throws a checked exception, then that method call must be called from within a try block and an exception handler (catch block) must be implemented.

   try {
      ...
      someMethodThatThrowsIOException();
      ...
   } catch (IOException e) {
      ...
   }

If an exception occurs, then the flow control of the program jumps to a catch block. All remaining statements, if any, within the try block are skipped. If no exception is thrown, then the catch blocks are skipped.

A try block must be a compound statement. Ditto for the catchblocks.

The catch blocks are passed an argument: a reference to the exception (which may or may not be used).

Throwing an Exception

An exception is thrown as follows.

   throw new EOFException();

         or

   throw new EOFException("file's empty");
      //supply additional information about the exception
      //this information can help the client "fix" the problem

A method that throws an exception does not return to the caller.

An exception can be re-thrown. You may want to do this in the event an exception occurs: you catch it locally to clean up any locally allocated objects and/or system resources, and then "pass" the exception on to those who know about handling it.

Exception A can be caught, and exception B can be thrown.

Creating an Exception Class

You can create your own exceptions by extending Exception or some child class of Exception.

   class MyException extends Exception {
      public MyException() {}
      public MyException(String msg) { super(msg); }
   }
Terminology

The following exception terminology was obtained from Just Java by Peter van der Linden:

Note Java Other Languages
An error condition that happens at run time Exception Exception
Causing an exception to occur Throwing Raising
Capturing an exception that has just occurred, and executing statements to resolve it in some way Catching Handling
The block that does this Catch clause Handler
The sequence of call statements that brought control to the method where the exception happened Stack trace Call chain
Miscellaneous Comments

Detect errors at a low level, handle them at a high level.

Use exceptions only for exceptional situations.

YouTube.com::CS 61B Lecture 14: Exceptions

{TopOfPage} {Oracle.com::API Specification | Tutorial} {Derek Banas Videos} {Data Structure Visualizations}


Multi-dimensional Arrays

Multi-dimensional arrays are arrays-of-arrays. Example: a two-dimensional array is an array of one-dimensional arrays.

   int someArray[][];  // someArray  will reference an array whose elements
                       // are of type:   array-of-int

   /*
    * create a 4 element array of 4 element int arrays
    */
   someArray = new int[4][];        // create a 4 element array
   for (int i = 0; i < 4; i++)
      someArray[i] = new int[4];

   int[0][0]  int[0][1]   int[0][2]   int[0][3]
   int[1][0]  int[1][1]   int[1][2]   int[1][3]
   int[2][0]  int[2][1]   int[2][2]   int[2][3]
   int[3][0]  int[3][1]   int[3][2]   int[3][3]

   /*
    * create a 4 element array of variable length int arrays
    */
   a = new int[4][];
   for (int i = 0; i < 4; i++)
      a[i] = new int[i+1];

   int[0][0]
   int[1][0]  int[1][1]
   int[2][0]  int[2][1]  int[2][2]
   int[3][0]  int[3][1]  int[3][2]  int[3][3]

   The EXPR  a[i]  accesses the  i'th   one-dimensional array.

   The following code snippet creates a 2x3 table of Foo objects.

      Foo[][] foo = new Foo[2][3];
      for (int i = 0; i < foo.length; i++) 
         for (int j = 0; j < foo[i].length; j++)
            foo[i][j] = new Foo();

   or

      Foo[][] foo = new Foo[2][];
      for (int i = 0; i < foo.length; i++)  {
         foo[i] = new Foo[3];
         for (int j = 0; j < foo[i].length; j++)
            foo[i][j] = new Foo();
      }

Excluding some mathematical and scientific computing, multi-dimensional arrays rarely exceed two dimensions.

MultiArrays.java

{TopOfPage} {Oracle.com::API Specification | Tutorial} {Derek Banas Videos} {Data Structure Visualizations}


Interfaces

From the The Java Programming Language by Arnold, Gosling, Holmes.

The fundamental unit of programming in Java is the class, but the fundamental unit of object-oriented design is the type. Interfaces define types in an abstract form as a collection of methods or other types that form the contract for that type.

An interface is defined using the keyword interface rather than class.

An interface contains no instance variables, nor does it contain any method implementations. You cannot create instances of an interface.

   interface Foo { ... }

   //! Foo f = new Foo();   // not legal

An interface can declare three kinds of members:

All interface members are implicitly public.

A class guarantees to implement the methods of an interface by using the implements keyword when the class is defined.

   class X implements Y

Any class that implements an interface must define (i.e. implement) the methods, if any, declared in the interface.

A class can implement more that one interface. In addition, a class can extend another class and still implement one or more interfaces.

   class X implements Y, Z { ... }
   class X extends A implements Y, Z { ... }

To an extent, interfaces support the concept of multiple inheritance.

Common interfaces from the java.lang package:

An interface can be used to "tie" together classes that are in separate inheritance trees.

                  Object
                    |
                Assignment               Interface:  Submitable
                    |
            Exam   Program   Homework
            ...    ...       ...



             Object
               |
              Game                       Interface:  Submitable
               |
  Guessing  Yahtzee   Blackjack
  ...       ...       ....

Another example...

An interface can be used to encapsulate a collected of related manifest constants (i.e. simulate an enum in C).

   interface ExitStatus {
      int EXIT_SUCCESS = 0;
      int EXIT_FAILURE = 1;
   }

When an interface declares named constants, then they are implicitly defined to be public, static, and final. [blank finals are not allowed]

Methods declared in an interface are implicitly abstract.

An interface can extend (i.e. inherit) other interfaces.

   interface Sortable { ... }
   interface Searchable { ... }
   interface Container extends Sortable, Searchable { ... }
Any class that implements interface Container must also implement the Sortable and Searchable interfaces.

Although you cannot instantiate an object using an interface, you can use an interface as the type for variable and assign to that variable any object whose class implements the interface. This allows for polymorphic run-time binding.

   interface Container { ... }

   class List implements Container { ... }

   List list = new List();
   ...
   Container c = list;  // list is a container

From a naming convention perspective, many interfaces have a -able suffix applied to them.

{TopOfPage} {Oracle.com::API Specification | Tutorial} {Derek Banas Videos} {Data Structure Visualizations}


Recursion

Recursion is any solution technique in which large problems are solved by reducing them to smaller problems of the same form.

According to multiple sources, the text Thinking Recursively by Eric Roberts (John Wiley 1986) is an excellent resource on recursion.

Internally, recursive functions are supported using a stack.

Almost every recursive function has the same basic structure:

   if (test for simple case)
      return (simple solution computed w/o using recursion)
   else
      return (recursive solution involving a call to the same function)

A recursive function is a function that calls itself in its function body. The call uses different arguments that ultimately read values that allow the function to return a result.

A recursive algorithm must have a well-defined stopping state, or termination condition.

Tail recursion is when the only recursive call appears in the function and that recursive call is the last operation performed at that procedural level. Typically, tail recursive functions can be easily rewritten as non-recursive functions. The traverse linked-list example provides an example of tail recursion.

Some final comments:

Interesting Recursion Related Tidbit

Headline: "Microsoft's Java JIT compiler bungles tail recursion."

A programmer from England has discovered a flaw in Microsoft's Java environment running in Internet Explorer under Windows. On 10/20/1998 Andrew Kennedy posted a note to Java programming newsgroups showing that jview, Microsoft's command-line interface to its just-in-time compiler, calculates the factorial of 5 (i.e., 5 * 4 * 3 * 2 * 1 * 0!) to be 16. Kennedy writes:
> Microsoft seem to have confused addition with multiplication. > Such an easy mistake to make.
Followup postings pin down the likely source of the bug as incorrectly handled tail recursion in Microsoft's JIT compiler. Unfortunately there seems to be no way to turn off JIT. Kennedy speculates that this error may be responsible for the failure of a large number of Java applets to run in the IE 4 environment.

The Factorial Example

Computing the factorial of a number is popular example of using recursion.

   Mathematically:  n! = { 1  if n = 0; otherwise,  n * (n-1)! }

To compute n! you must:

  1. remember what  n  is
  2. go compute  (n-1)! 
  3. once  (n-1)!  is computed, multiply by  n  to get the final answer
   To compute (n-1)!, you 1st compute (n-2)!.  Computing (n-2)!
   requires finding (n-3)! and so on.  Eventually we end up at 0!
   and 0! is defined to be the value 1.
   
   In C:
      printf("%i\n", factorial(4));
   
      int factorial(int n) {
         if (n == 0) return 1;
         return n * factorial(n -1);
      }
   

   4 is pushed onto the stack
      3 is pushed onto the stack
         2 is pushed onto the stack
            1 is pushed onto the stack
               0 is pushed onto the stack
               returns 1
            1 * 1 returns 1
         2 * 1 returns 2
      3 * 2 returns 6
   4 * 6 returns 24
See factorial.cpp and Factorial.java.

Recall: When a function is called, EXPRs are evaluated and are passed by value. An implicit argument that is passed to a function is a return address. Function return values, if any, are placed in a secure location from which the calling program may retrieve it.

The Power Example

Raising a number to a power can also be solved recursively.
   Power function:   n**k = { 1  if k is 0; otherwise, n * (n**(k-1)) }
Computing  xn , where  x  is a real number and  n  is a non-negative integer, is called the power function and its calculation is usually accomplished with repeated multiplication:
   xn = x * x * x * ... *x * x

      where  x  multiplied with itself  n  times

   20 = 1
   21 = 2 * 1 = 2
   22 = 2 * 2 = 4
   23 = 2 * 2 * 2 = 8
   24 = 2 * 2 * 2 * 2 = 16
The power function can be implemented as follows:

   power(x, n) = xn = { 1, n = 0 | x * xn-1, n > 0 }

   double power(double x, int n) {
      if (n == 0) return 1;
      return x * power(x, n-1);
   }

See power.cpp and Power.java.

Coverting an Integer to a String

   printd(150);
   
   printd(int n) {
      if (n / 10)
         printd(n / 10);
      putchar(n % 10 + '0');
   }
   
   150 / 10 -- 15 pushed onto the stack
      15 / 10 -- 1 pushed onto the stack
         1 / 10  
         1 is printed
       5 is printed
   0 is printed
See printld.cpp and PrintLD.java.

{TopOfPage} {Oracle.com::API Specification | Tutorial} {Derek Banas Videos} {Data Structure Visualizations}


The instanceof Operator

The instanceof binary operator (i.e. has two operands) is used to test if an object variable is an instance of a specific class. The operator evaluates to true or false.

   void foo(Object o) {
      if (o instanceof String)
         System.out.println("the Object is type String");
      else if (o instanceof StringBuffer)
         System.out.println("the Object is type StringBuffer");
   }

The left operand is either an object variable or an array element, while the right operand must be a class name, interface name, or array type.

{TopOfPage} {Oracle.com::API Specification | Tutorial} {Derek Banas Videos} {Data Structure Visualizations}


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