Category: Java

Java: Download File

Frédéric Hannes

The following code will allow you to download a file in java from a source URL to a target File.

public static void download(URL orig, File dest) throws IOException {
	OutputStream os = null;
	InputStream is = null;
	try {
		byte[] buf;
		int count, length = 0;
		os = new BufferedOutputStream(new FileOutputStream(dest));
		is = orig.openConnection().getInputStream();
		buf = new byte[4096];
		while ((count = is.read(buf)) != -1) {
			os.write(buf, 0, count);
			length += count;
		}
	} finally {
		is.close();
		os.close();
	}
}

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JNI For Delphi Preview

Frédéric Hannes

I’m currently developing a new modernized JNI wrapper for Delphi 2010 and up.  The old wrappers found here and here do work, but my wrapper requires less work to set up and uses a lot of new Delphi language features to ensure it works smoothly. The wrapper has been rewritten from scratch to include all features of JNI in JDK 1.6. Note that this wrapper is NOT backwards compatible with the old JNI wrappers, but switching to this one shouldn’t be too hard. At the moment the wrapper itself is far from complete, but it’s a preview including a small sample application. Afterwards I might also be wrapping JVMTI and create a bigger more user-friendly framework to encapsulate JNI. You can download the preview here: http://ibeblog.com/files/JNIForDelphi%200.1.rar

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Delphi <=> Java: Datatypes

Frédéric Hannes

So you know Delphi or Java and want to learn the other language? A very important thing to know is what datatypes you are using, as these differ in each language.

As you may know, Java does not have any unsigned datatypes, Delphi however has both signed and unsigned datatypes. Because of this the unsigned types are not listed in the table below, but in order for the integer types these are Byte/UInt8, Word/UInt16, Cardinal/LongWord/UInt32, UInt64. Delphi also has ansi strings/chars, these can be stores in string and Char but those can also hold unicode versions, the actual ansi types are AnsiString and AnsiChar. And of course Delphi also comes with a Pointer type as well as it has the ability to create types that hold pointers to specific types.

JavaDelphiTypeData
byteShortInt, Int88-bit signed integer-128 to 127
shortSmallInt, Int1616-bit signed integer-32768 to 32767
intInteger, LongInt, Int3232-bit signed integer-2147483648 to 2147483647
longInt6464-bit signed integer-9223372036854775808 to 9223372036854775807
floatSingle32-bit floating point value
doubleDouble64-bit floating point value
booleanBooleanboolean valuetrue or false
charChar (stores both ansi and unicode chars), WideChar, UnicodeChar16-bit unicode charactera single unicode character
Stringstring (stores both ansi and unicode strings), Widestring, UnicodeStringunicode stringa string consisting out of unicode characters
ObjectTObjectan objectobject pointer

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Java: ShellSort

Frédéric Hannes

ShellSort is not a comparison-based sorting algorithm like those previously shown on this blog, it uses the property of InsertionSort that nearly sorted arrays of values are sorted very quickly. It has a performance of O(n log² n) which makes it a lot faster than a lot of other algorithms, certainly the O(n²) ones, but not entirely the fastest.

public static void shellSort(int[] ia) {
	int l = ia.length;
	if (l == 1)
		return;
	int inc = l / 2;
	while (inc != 0) {
		for (int i = inc; i != l; i++) {
			int t = ia[i];
			int j = i;
			while (j >= inc && ia[j - inc] > t) {
				ia[j] = ia[j - inc];
				j = j - inc;
			}
			ia[j] = t;
		}
		inc = (int) (inc / 2.2);
	}
}

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Java: SelectionSort

Frédéric Hannes

SelectionSort is another sorting algorithm that has a performance of O(n²) like BubbleSort. It sorts an array of numbers by finding the smallest element in the unsorted part of the array and switching it with the current item. This is repeated until the entire array is sorted.

public static void selectionSort(int[] ia) {
	int l = ia.length;
	if (l == 1)
		return;
	for (int i = 0; i != l; i++) {
		int s = i;
		for (int j = i; j != l; j++)
			if (ia[j] < ia[s])
				s = j;
		int t = ia[i];
		ia[i] = ia[s];
		ia[s] = t;
	}
}

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Java: BubbleSort

Frédéric Hannes

The bubblesort algorithm is one of the slowest sorting algorithms around as it’s performance is O(n²). As a result of this the algorithm is barely ever used in any real life applications, however, even though the algorithm will always stay rather slow, it can be tweaked to improve the code’s performance.

The implementation of the algorithm you see below has been optimized in various ways and it also makes use of the property that all items in an array of numbers that come after the last index that was sorted in a run through the array by the algorithm are already sorted.

public static void bubbleSort(int[] ia) {
	int l = ia.length, i = l;
	if (l == 1)
		return;
	while (i != 0) {
		int last = 0;
		for (int j = 1; j != i; j++)
			if (ia[j - 1] > ia[j]) {
				int t = ia[j];
				ia[j] = ia[j - 1];
				ia[j - 1] = t;
				last = j;
			}
		i = last;
	}
}

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Java: Reading and writing text files

Frédéric Hannes

A lot of people starting off with Java will often want to read or write text files. Here’s some code to do just that:

public static String readFile(String fileName) throws IOException {
	FileInputStream fis = new FileInputStream(fileName);
	BufferedInputStream bis = new BufferedInputStream(fis, 1024);
	byte[] buffer = new byte[1024];
	String result = "";
	int len;
	while ((len = bis.read(buffer)) != -1) {
		byte[] tmp = new byte[len];
		System.arraycopy(buffer, 0, tmp, 0, len);
		result += new String(tmp);
	}
	bis.close();
	return result;
}

public static void writeFile(String fileName, String content)
		throws IOException {
	FileOutputStream fos = new FileOutputStream(fileName);
	BufferedOutputStream bos = new BufferedOutputStream(fos, 1024);
	bos.write(content.getBytes());
	bos.close();
}

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