How do I get char value of a string at a specified position?

By Wayan in Core API, Lang Package 0 Comment

The chartAt() method of the String class returns the char value at the specified index. An index ranges from 0 to length() - 1. If we specified an index beyond of this range a StringIndexOutOfBoundsException exception will be thrown.

These method use zero based index which means the first char value of the sequence is at index 0, the next at index 1, and so on, as for array indexing.

package org.kodejava.lang;

public class CharAtExample {
    public static void main(String[] args) {
        String[] colors = {"black", "white", "brown", "green", "yellow", "blue"};

        for (String color : colors) {
            // Get char value of a string at index number 3. We'll get the 
            // fourth character of each color in the array because the 
            // index is zero based.
            char value = color.charAt(3);
            System.out.printf("The fourth char of %s is '%s'.%n", color, value);
        }

    }
}

Here is the program output:

The fourth char of black is 'c'
The fourth char of white is 't'
The fourth char of brown is 'w'
The fourth char of green is 'e'
The fourth char of yellow is 'l'
The fourth char of blue is 'e'

How do I determine if a string match a pattern exactly?

By Wayan in Regex 0 Comment

If you want the entire string to match your regular expression pattern you can use the Matcher.matches() method. This method will return true if and only if entire input string matches with the matcher’s pattern.

If the pattern only needs to match the beginning of the string you can use the Matcher.lookingAt() method. You can find its example on the following address How do I check if a string starts with a pattern?.

package org.kodejava.regex;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class MatcherMatchesExample {

    public static void main(String[] args) {
        String[] inputs = {
                "blue sky",
                "blue sea",
                "blue",
                "blue lagoon"
        };

        // Creates an instance of Pattern using the compile method.
        Pattern pattern = Pattern.compile("blue");

        int match = 0;
        for (String s : inputs) {
            // Creates a matcher that will match the given input
            // against this pattern.
            Matcher matcher = pattern.matcher(s);

            // Check if the input match the pattern exactly and
            // increment the match counter.
            if (matcher.matches()) {
                match++;
            }

        }

        System.out.println("Number of input matched: " + match);
    }
}

The code above will only match one input that match exactly with the pattern (“blue”), because the other three elements of the array has another word beside blue.

How do I get the extension of a file?

By Wayan in Core API, IO 0 Comment

Below is an example that can be used to get the extension of a file. The code below assume that the extension is the last part of the file name after the last dot symbol. For instance if you have a file named data.txt the extension will be txt, but if you have a file named data.tar.gz the extension will be gz.

package org.kodejava.io;

import java.io.File;

public class FileExtension {
    private static final String EXT_SEPARATOR = ".";

    public static void main(String[] args) {
        File file = new File("data.txt");
        String ext = FileExtension.getFileExtension(file);
        System.out.println("Ext = " + ext);

        file = new File("F:/Temp/Data/data.tar.gz");
        ext = FileExtension.getFileExtension(file);
        System.out.println("Ext = " + ext);

        file = new File("F:/Temp/Data/HelloWorld.java");
        ext = FileExtension.getFileExtension(file);
        System.out.println("Ext = " + ext);
    }

    /**
     * Get the extension of the specified file.
     *
     * @param file a file.
     * @return the extension of the file.
     */
    private static String getFileExtension(File file) {
        if (file == null) {
            return null;
        }

        String name = file.getName();
        int extIndex = name.lastIndexOf(FileExtension.EXT_SEPARATOR);

        if (extIndex == -1) {
            return "";
        } else {
            return name.substring(extIndex + 1);
        }
    }
}

How do I check if a string starts with a pattern?

By Wayan in Regex 0 Comment

The example below demonstrate the Matcher.lookingAt() method to check if a string starts with a pattern represented by the Pattern class.

package org.kodejava.regex;

import java.util.Locale;
import java.util.Set;
import java.util.TreeSet;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class MatcherLookingAtExample {

    public static void main(String[] args) {
        // Get the available countries
        Set<String> countries = new TreeSet<>();
        Locale[] locales = Locale.getAvailableLocales();
        for (Locale locale : locales) {
            countries.add(locale.getDisplayCountry());
        }

        // Create a Pattern instance. Look for a country that start with
        // "I" with an arbitrary second letter and have either "a" or "e"
        // letter in the next sequence.
        Pattern pattern = Pattern.compile("^I.[ae]");
        System.out.println("Country name which have the pattern of " +
                pattern.pattern() + ": ");

        // Find country name which prefix matches the matcher's pattern
        for (String country : countries) {
            // Create matcher object
            Matcher matcher = pattern.matcher(country);

            // Check if the matcher's prefix match with the matcher's
            // pattern
            if (matcher.lookingAt()) {
                System.out.println("Found: " + country);
            }
        }
    }
}

The following country names is printed as the result of the program above:

Country name which have the pattern of ^I.[ae]: 
Found: Iceland
Found: Iran
Found: Iraq
Found: Ireland
Found: Italy

How do I trim the capacity of an ArrayList?

By Wayan in Core API, Util Package 0 Comment

For minimizing the storage used by an ArrayList to the number of its elements we can use the ArrayList.trimToSize() method call. In the code below we create an instance of ArrayList with the initial capacity set to 100. Later we only add five data into the list. To make the capacity of the ArrayList minimized we call the trimToSize() method.

package org.kodejava.util;

import java.util.ArrayList;

public class ListTrimToSize {
    public static void main(String[] args) {
        // Create an ArrayList with the initial capacity of 100.
        ArrayList<String> list = new ArrayList<>(100);
        list.add("A");
        list.add("B");
        list.add("C");
        list.add("D");
        list.add("E");
        list.add("F");

        System.out.println("Size: " + list.size());

        // Trim the capacity of the ArrayList to the size
        // of the ArrayList. We can do this operation to reduce
        // the memory used to store data by the ArrayList.
        list.trimToSize();
    }
}