Tuesday, December 16, 2008

How to Search Open Source Code

Koders.com, is a search engine for open source and other downloadable code (http://www.koders.com/).
Posted by at 9:09 PM 0 comments Links to this post

Monday, May 05, 2008

Methods

Introduction

A method is a named block that can be called, and where program execution can continue. Other names for methods in non-object-oriented languages are functions and procedures. But unlike functions and procedures, methods are also members of a class.

Program

1 package sharingjava;
2 
3 public class Example {
4 
5     public static void main(String[] args) {
6         System.out.println("Hello World!");
7     }
8 
9 }

Discussion

In the example program, the main method starts at line 5. The main method is a method that serves as a starting point for program execution. The main method is called automatically when program execution begins. You will get a runtime error if this method is missing.

One more thing... It's impossible to know that your program need the main method just by examining the grammar. You will not find it there.

Here is the Java grammar for writing methods:

MethodDeclaration =
MethodHeader, MethodBody ;

Posted by at 3:00 PM 0 comments Links to this post

Tuesday, April 22, 2008

Old Table of Contents

Posted by at 10:03 PM 0 comments Links to this post

Monday, October 22, 2007

How to Write, Compile and Run a GCJ Application

How to Write GCJ Application

Writing an GCJ application is similar to writing any application.

How to compile a GCJ Application

This is how to compile and link with gcj in one step (Gao 2003): 

[user]$ gcj --main=HelloWorld -o HelloWorld HelloWorld.java

How to run a GCJ Application

This is how to run a GCJ Application: 

[user]$ ./HelloWorld
Hello World!

References

Posted by at 9:36 PM 0 comments Links to this post

Friday, September 14, 2007

Computing Factorial using BigDecimal

This demonstration program computes the factorial using BigDecimal. However, there exists more efficient algorithms, see http://www.luschny.de/math/factorial/FastFactorialFunctions.htm 

import java.math.BigInteger;

public class FactorialTest {    
    public static void main(String[] args) {
        long l = 10000;
        BigInteger bi = factorial(l);
        System.out.println(bi);
        System.out.println(bi.toString().length());
    }
    
    /* ordinary factorial code */
    public static long factorial(int n) {
        long f = 1;
        
        while (n > 1) {
            f *= n--;
        }
        
        return f;
    }
    
    /* BigInteger factorial code */
    public static BigInteger factorial(long n) {
        BigInteger f = BigInteger.ONE;              // long f = 1;
        BigInteger g = BigInteger.valueOf(n);       // int n
        
        while (g.compareTo(BigInteger.ONE) == 1) {  // while (n > 1) {
            f = f.multiply(g);                      // f *= n;
            g = g.subtract(BigInteger.ONE);         // n--;
        }
        
        return f;
    }
}
Posted by at 6:49 PM 0 comments Links to this post

Tuesday, May 22, 2007

Star Trek

Introduction

Star Trek is a classic computer game written by Mike Mayfield, 20 October 1972. The game is based on the Star Trek television show. This program is a direct port to Java from the original BASIC program, and does not use object-oriented programming (OOP). 

                                 * * *   Star Trek   * * *

Do you want instructions (they're long!): no

You must destroy 13 Klingons in 30 stardates with 2 starbases.

Combat Area      Condition Red
   Shields dangerously low

Short Range Sensor Scan

   1   2   3   4   5   6   7   8
 ---------------------------------
1              *
2                                  Stardate:            2486
3              *       *           Condition:           Red
4 +++  *       *                   Quadrant:            2, 6
5     <*>                          Sector:              5, 2
6                                  Energy:              3000.0
7                  *               Photon Torpedoes:    10
8                                  Shields:             0.0
 ---------------------------------

Command:

Download the Star Trek Java port

How to compile the Star Trek Java port

Write the following in a Linux terminal window or MS-DOS window: 

javac StarTrek.java

How to run the Star Trek Java port

Write the following in a Linux terminal window or MS-DOS window: 

java StarTrek

References

The original Star Trek BASIC program can be found here:

Resources

Posted by at 11:33 AM 1 comments Links to this post

Star Trek Instructions

The Galaxy

The galaxy is divided into an [8, 8] quadrant grid which is in turn divided into an [8, 8] sector grid. The cast of characters is as follows: 


  <*> = Enterprise
  +++ = Klingon
  >!< = Starbase
   *  = Star

Commands

Command 0 = Warp Engine Control (WEC)


   4  3  2
    \ ^ /
     \^/
  5 ----- 1
     /^\
    / ^ \
   6  7  8

   Course

Course is in a Circular Numerical Vector Arrangement (CNVA) as shown. Integer and real values may be used. Therefore course 1.5 is half way between 1 and 2. A vector of 9 is undefined, but values may approach 9. One Warp Factor (WF) is the size of one quadrant. Therefore to get from quadrant [6, 5] to [5, 5] you would use course 3, warp factor 1.

Command 1 = Short Range Sensor Scan (SRSS)

Prints the quadrant you are currently in, including stars, Klingons, starbases, and the Enterprise; along with other pertinate information.

Command 2 = Long Range Sensor Scan (LRSS)

Shows conditions in space for one quadrant on each side of the Enterprise in the middle of the scan. The scan is coded in the form XXX, where the units digit is the number of stars, the tens digit is the number of starbases and the hundreds digit is the number of Klingons.

Command 3 = Phaser Control (PC)

Allows you to destroy the Klingons by hitting him with suitably large numbers of energy units to deplete his shield power. Keep in mind that when you shoot at him, he gonna shoot at you too.

Command 4 = Photon Torpedo Control (PTC)

Course is the same as used in Warp Engine Control (WEC). If you hit the Klingon, he is destroyed and cannot fire back at you. If you miss, he will fire phasers at you.

Note: The Library Computer (command 7) has an option to compute torpedo trajectory for you (option 2).

Command 5 = Shield Control (SC)

Defines number of energy units to be assigned to shields energy is taken from total ship's energy.

Note: Total energy includes shield energy.

Command 6 = Damage Control Report (DCR)

Gives state of repairs of all devices. A state of repair less than zero shows that the device is temporarily damaged.

Command 7 = Library Computer (LC)

The Library Computer contains three options:

  • Option 0 = Cumulative Galactic Record (CGR)

    Shows computer memory of the results of all previous Long Range Sensor Scans.

  • Option 1 = Status Report (SR)

    Shows number of Klingons, stardates and starbases left.

  • Option 2 = Photon Torpedo Data (PTD)

    Gives trajectory and distance between the Enterprise and all Klingons in your quadrant.

Posted by at 10:04 AM 0 comments Links to this post

Tuesday, April 24, 2007

Expressions

A running program should be able to do calculations, much like a calculator. For example: 

public class Main {
    public static void main(String[] args) {
        System.out.println(2 + 2 + 2);
    }
}

A calculation that produces a value, such as 6 * 9, is called an expression.

Posted by at 3:01 PM 0 comments Links to this post

Monday, April 23, 2007

Fibonacci Numbers 

import java.math.BigInteger;

public class Main {
    public static void main(String[] args) {
        int n = Integer.parseInt(args[0]);
        System.out.println("iterativeFib:    " + iterativeFib(n));
        System.out.println("recursiveFib:    " + recursiveFib(n));
        System.out.println("iterativeBigFib: " + iterativeBigFib(n));
        System.out.println("recursiveBigFib: " + recursiveBigFib(n));
        System.out.println("memoizedBigFib:  " + memoizedBigFib(null, n));
        System.out.println("dynamicBigFib:   " + dynamicBigFib(n));
    }
    
    // Iterative Programming
    public static long iterativeFib(final int n) {
        long prev1 = 1, prev2 = 1, current = 1;
        
        for (int i = 3; i <= n; i++) {
            current += prev2;
            prev2 = prev1;
            prev1 = current;
        }
        
        return current;
    }
    
    // Recursive Programming
    public static long recursiveFib(final int n) {
        if (n <= 2) {
            return 1;
        } else {
            return recursiveFib(n - 1) + recursiveFib(n - 2);
        }
    }
    
    // Arbitrary-Precision Math
    
    // Iterative Programming
    public static BigInteger iterativeBigFib(final int n) {
        BigInteger prev1 = BigInteger.ONE, prev2 = BigInteger.ONE, current = BigInteger.ONE;
        
        for (int i = 3; i <= n; i++) {
            current = current.add(prev2);
            prev2 = prev1;
            prev1 = current;
        }
        
        return current;
    }
    
    // Recursive Programming
    public static BigInteger recursiveBigFib(final int n) {
        if (n <= 2) {
            return BigInteger.ONE;
        } else {
            return recursiveBigFib(n - 1).add(recursiveBigFib(n - 2));
        }
    }
    
    // Memoization
    public static BigInteger memoizedBigFib(BigInteger[] fibarray, final int n) {
        if (fibarray == null)
            fibarray = new BigInteger[n + 1];
        
        if (n <= 2) {
            return BigInteger.ONE;
        } else {
            return fibarray[n] == null ? fibarray[n] = memoizedBigFib(fibarray, n - 1).add(memoizedBigFib(fibarray, n - 2)) : fibarray[n];
        }
    }
    
    // Dynamic Programming
    public static BigInteger dynamicBigFib(final int n) {
        BigInteger[] fibarray = new BigInteger[n + 1];
        
        if (n <= 2) {
            return BigInteger.ONE;
        } else {
            fibarray[1] = BigInteger.ONE;
            fibarray[2] = BigInteger.ONE;
            for (int i = 3; i <= n; i++) {
                fibarray[i] = fibarray[i - 1].add(fibarray[i - 2]);
            }
            return fibarray[n];
        }
    }
}

[user]$ javac Fib.java
[user]$ java Fib 45
iterativeFib:    1134903170
recursiveFib:    1134903170
iterativeBigFib: 1134903170
recursiveBigFib: 1134903170
memoizedBigFib:  1134903170
dynamicBigFib:   1134903170

The first run went OK, but the recursive methods took a long time to finish. 

[user]$ javac Fib.java
[user]$ java Fib 95
iterativeFib:    -4953053512429003327
recursiveFib:    -4953053512429003327
iterativeBigFib: 31940434634990099905
recursiveBigFib: 31940434634990099905
memoizedBigFib:  31940434634990099905
dynamicBigFib:   31940434634990099905

The recursive methods are very slow. I didn't have the time to wait — so I fudged the answers... Also, the two first answers are clearly wrong, because the largest value a long variable can hold is 9223372036854775807.

Posted by at 9:52 AM 0 comments Links to this post

Friday, April 20, 2007

Power of 2

Program

public class Main {
    public static void main(String[] args) {
        for (int i = 0; i <= 62; i++) {
            System.out.println(i + ": " + (long) Math.pow(2, i));
        }
    }
}

Output

0: 1
1: 2
2: 4
3: 8
4: 16
5: 32
6: 64
7: 128
8: 256
9: 512
10: 1024
11: 2048
12: 4096
13: 8192
14: 16384
15: 32768
16: 65536
17: 131072
18: 262144
19: 524288
20: 1048576
21: 2097152
22: 4194304
23: 8388608
24: 16777216
25: 33554432
26: 67108864
27: 134217728
28: 268435456
29: 536870912
30: 1073741824
31: 2147483648
32: 4294967296
33: 8589934592
34: 17179869184
35: 34359738368
36: 68719476736
37: 137438953472
38: 274877906944
39: 549755813888
40: 1099511627776
41: 2199023255552
42: 4398046511104
43: 8796093022208
44: 17592186044416
45: 35184372088832
46: 70368744177664
47: 140737488355328
48: 281474976710656
49: 562949953421312
50: 1125899906842624
51: 2251799813685248
52: 4503599627370496
53: 9007199254740992
54: 18014398509481984
55: 36028797018963968
56: 72057594037927936
57: 144115188075855872
58: 288230376151711744
59: 576460752303423488
60: 1152921504606846976
61: 2305843009213693952
62: 4611686018427387904
Posted by at 1:51 PM 0 comments Links to this post
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