How do I create array of unique values from another array?

By Wayan in Core API, Lang Package 2 Comments

This code snippet show you how to create an array of unique numbers from another array of numbers.

package org.kodejava.lang;

import java.util.Arrays;

public class UniqueArray {
    /**
     * Return true if num appeared only once in the array.
     */
    public static boolean isUnique(int[] numbers, int num) {
        for (int number : numbers) {
            if (number == num) {
                return false;
            }
        }
        return true;
    }

    /**
     * Convert the given array to an array with unique values and
     * returns it.
     */
    public static int[] toUniqueArray(int[] numbers) {
        int[] temp = new int[numbers.length];
        // in case you have value of 0 in the array
        Arrays.fill(temp, -1);

        int counter = 0;
        for (int number : numbers) {
            if (isUnique(temp, number))
                temp[counter++] = number;
        }

        int[] uniqueArray = new int[counter];
        System.arraycopy(temp, 0, uniqueArray, 0, uniqueArray.length);
        return uniqueArray;
    }

    /**
     * Print given array
     */
    public static void printArray(int[] numbers) {
        for (int number : numbers) {
            System.out.print(number + " ");
        }
        System.out.println();
    }

    public static void main(String[] args) {
        int[] numbers = {1, 1, 2, 3, 4, 1, 4, 7, 9, 7};
        printArray(numbers);
        printArray(toUniqueArray(numbers));
    }
}

For other example to create unique array check the following example How do I remove duplicate element from array?.

How do I check whether a thread group has been destroyed?

By Wayan in Core API, Lang Package 0 Comment

You can use ThreadGroup.isDestroyed() method to check whether a thread group and its subgroups has been destroyed.

package org.kodejava.lang;

public class CheckGroupDestroy {
    public static void main(String[] args) {
        ThreadGroup grandParent = new ThreadGroup("GrandParent");
        ThreadGroup uncle = new ThreadGroup(grandParent, "Uncle");
        ThreadGroup parent = new ThreadGroup(grandParent, "Parent");
        ThreadGroup son = new ThreadGroup(parent, "Son");
        ThreadGroup daughter = new ThreadGroup(parent, "Daughter");
        ThreadGroup neighbour = new ThreadGroup("Neighbour");

        ThreadGroup[] groupArray = {
                grandParent, uncle, parent, son, daughter, neighbour
        };

        // Destroy 'parent' group and all its subgroups
        parent.destroy();

        // Check whether the group is destroyed. The result is,
        // GrandParent, Uncle, and Neighbour did not destroyed
        // because they are not Parent's subgroups
        for (ThreadGroup tg : groupArray) {
            if (tg.isDestroyed()) {
                System.out.println(tg.getName() + " is destroyed");
            } else {
                System.out.println(tg.getName() + " is not destroyed");
            }
        }
    }
}

The result is:

GrandParent is not destroyed
Uncle is not destroyed
Parent is destroyed
Son is destroyed
Daughter is destroyed
Neighbour is not destroyed

How do I destroy a thread group?

By Wayan in Core API, Lang Package 0 Comment

You can destroy a thread group by using destroy() method of ThreadGroup class. It will cleans up the thread group and removes it from the thread group hierarchy. It’s not only destroy the thread group, but also all its subgroups.

The destroy() method is of limited use: it can only be called if there are no threads presently in the thread group.

package org.kodejava.lang;

public class ThreadGroupDestroy {
    public static void main(String[] args) {
        ThreadGroup root = new ThreadGroup("Root");
        ThreadGroup server = new ThreadGroup(root, "ServerGroup");
        ThreadGroup client = new ThreadGroup(root, "ClientGroup");

        // Destroy 'root' thread groups and all its subgroup
        // ('server' & 'client')
        root.destroy();

        // Check if 'root' group and its subgroups already destroyed
        if (root.isDestroyed()) {
            System.out.println("Root group is destroyed");
        }

        if (server.isDestroyed()) {
            System.out.println("Server group is destroyed");
        }

        if (client.isDestroyed()) {
            System.out.println("Client group is destroyed");
        }
    }
}

How do I use try-with-resources statement?

By Wayan in Basic, Core API 0 Comment

The try-with-resources statement is introduced in the Java 7. With this new statement we can simplify resource management in our program, also known as ARM (Automatic Resource Management).

This statement is a try statement that declares one or more resources. After the program finish with the resource, it must be closed. The try-with-resources ensures that each resource is closed and the end of the statement.

Any object that implements java.lang.AutoCloseable, which includes all objects which implement java.io.Closeable, can be used as a resource.

package org.kodejava.basic;

import java.io.*;
import java.nio.charset.StandardCharsets;

public class TryWithResourceExample {
    public static void main(String[] args) {
        try {
            TryWithResourceExample demo = new TryWithResourceExample();
            demo.printStream("F:/tmp/data.txt");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private void printStream(String fileName) throws IOException {
        char[] buffer = new char[1024];

        try (InputStream is = new FileInputStream(fileName);
             Reader reader = new BufferedReader(
                     new InputStreamReader(is, StandardCharsets.UTF_8))) {

            while (reader.read(buffer) != -1) {
                System.out.println(buffer);
            }
        }
    }
}

How do I use string in switch statement?

By Wayan in Basic, Core API 0 Comment

Starting from Java 7 release you can now use a String in the switch statement. On the previous version we can only use constants type of byte, char, short, int (and their corresponding reference / wrapper type) or enum constants in the switch statement.

The code below give you a simple example on how the Java 7 extended to allow the use of String in switch statement.

package org.kodejava.basic;

public class StringInSwitchExample {
    public static void main(String[] args) {
        String day = "Sunday";
        switch (day) {
            case "Sunday":
                System.out.println("doSomething");
                break;
            case "Monday":
                System.out.println("doSomethingElse");
                break;
            case "Tuesday":
            case "Wednesday":
                System.out.println("doSomeOtherThings");
                break;
            default:
                System.out.println("doDefault");
                break;
        }
    }
}

How do I use multi-catch statement?

By Wayan in Core API, Lang Package 1 Comment

The multi-catch is a language enhancement feature introduces in the Java 7. This allows us to use a single catch block to handle multiple exceptions. Each exception is separated by the pipe symbol (|).

Using the multi-catch simplify our exception handling and also reduce code duplicates in the catch block. Let’s see an example below:

package org.kodejava.lang;

import java.io.IOException;
import java.sql.SQLException;

public class MultiCatchDemo {
    public static void main(String[] args) {
        MultiCatchDemo demo = new MultiCatchDemo();
        try {
            demo.callA();
            demo.callB();
            demo.callC();
        } catch (IOException | SQLException | ClassNotFoundException e) {
            e.printStackTrace();
        }
    }

    private void callA() throws IOException {
        throw new IOException("IOException");
    }

    private void callB() throws SQLException {
        throw new SQLException("SQLException");
    }

    private void callC() throws ClassNotFoundException {
        throw new ClassNotFoundException("ClassNotFoundException");
    }
}

How do I read all lines from a file?

By Wayan in Core API, IO 0 Comment

The java.nio.file.Files.readAllLines() method read all lines from a file. This method ensures that the file is closed when all bytes have been read or an I/O error, or other runtime exception, is thrown. Bytes from the file are decoded into characters using the specified charset.

Note that this method is intended for simple cases where it is convenient to read all lines in a single operation. It is not intended for reading in large files. This method is available since Java 7.

package org.kodejava.io;

import java.net.URI;
import java.nio.charset.Charset;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.List;
import java.util.Objects;

public class ReadFileAsListDemo {
    public static void main(String[] args) {
        ReadFileAsListDemo demo = new ReadFileAsListDemo();
        demo.readFileAsList();
    }

    private void readFileAsList() {
        String fileName = "/data.txt";

        try {
            URI uri = Objects.requireNonNull(this.getClass().getResource(fileName)).toURI();
            List<String> lines = Files.readAllLines(Paths.get(uri),
                    Charset.defaultCharset());

            for (String line : lines) {
                System.out.println(line);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

How do I get number of active thread in current thread?

By Wayan in Core API, Lang Package 0 Comment

In this example you’ll see how to obtain the number of active thread in the current thread. You can use the Thread.activeCount() method to get this information.

package org.kodejava.lang;

public class CountActiveThread {
    public static void main(String[] args) {
        Thread t = new Thread(() -> System.out.println("Hello..."));
        t.start();

        // Get the number of active threads in the current thread's
        // thread group.
        int activeThread = Thread.activeCount();
        System.out.format("Number of active threads of %s is %d %n",
                Thread.currentThread().getName(), activeThread);
    }
}

How do I get number of active thread group?

By Wayan in Core API, Lang Package 0 Comment

Use method activeGroupCount() of ThreadGroup class to get estimate number of active groups in the thread group and use activeCount() to get estimate number of active threads in a thread group.

package org.kodejava.lang;

public class ActiveGroupCount {
    public static void main(String[] args) {
        ThreadGroup root = new ThreadGroup("RootGroup");
        ThreadGroup server = new ThreadGroup(root, "ServerGroup");
        ThreadGroup client = new ThreadGroup(root, "ClientGroup");

        Thread t1 = new Thread(server, new ServerThread(), "ServerThread");
        Thread t2 = new Thread(client, new ClientThread(), "ClientThread");

        t1.start();
        t2.start();

        // Get estimate active groups in 'root' thread group
        int activeGroup = root.activeGroupCount();
        System.out.format("Estimated active group in %s is %d%n",
                root.getName(), activeGroup);

        // Get estimate active threads in 'root' thread group
        int activeThread = root.activeCount();
        System.out.format("Estimated active thread in %s is %d%n",
                root.getName(), activeThread);
    }
}

class ServerThread implements Runnable {
    public void run() {
        System.out.println("Running - Server Thread..");
    }
}


class ClientThread implements Runnable {
    public void run() {
        System.out.println("Running - Client Thread..");
    }
}

The example above print the following example output:

Estimated active group in RootGroup is 2
Estimated active thread in RootGroup is 2
Running - Client Thread..
Running - Server Thread..

How do I get thread group of a thread?

By Wayan in Core API, Lang Package 0 Comment

Use the getThreadGroup() method of Thread class to get the thread group to which the thread belongs.

package org.kodejava.lang;

public class GetThreadGroup {
    public static void main(String[] args) {
        // Create thread groups
        ThreadGroup group = new ThreadGroup("ThreadGroup");
        ThreadGroup anotherGroup = new ThreadGroup(group, "AnotherGroup");

        // Create threads and placed into thread group
        Thread t1 = new Thread(group, new FirstThread(), "Thread1");
        Thread t2 = new Thread(anotherGroup, new FirstThread(), "Thread2");

        // Start the threads
        t1.start();
        t2.start();

        // Use getThreadGroup() method of Thread class to get the object
        // of ThreadGroup then use the getName() method to get the name
        // of thread group.
        System.out.format("%s is a member of %s%n", t1.getName(),
                t1.getThreadGroup().getName());
        System.out.format("%s is a member of %s%n", t2.getName(),
                t2.getThreadGroup().getName());
    }
}

class FirstThread implements Runnable {
    public void run() {
        System.out.println("Start..");
    }
}