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java/samples/introduction/A.java

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+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+
+package basic;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 07.10.2022
+ */
+public class A {
+    private A a;
+    private A b;
+    
+    public A () {
+       this(null);
+       System.out.println ("A::A()"); 
+    }
+    
+    public A (A a) {
+        System.out.println("A::A(A)");
+    }
+    
+    public void a () {
+        System.out.println("A::a()");
+    }
+    
+    public void a (A a) {
+        System.out.println("A::a(A)");
+    }
+    
+    public static void main (String[] args) {
+        System.out.println("Hello, Java!!!");
+        A a = new A (null);
+        a.a(a);
+    }
+}

java/samples/introduction/B.java

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+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package basic;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 07.10.2022
+ */
+public class B extends A {
+
+    public B() {
+        super(null);  // this();
+        System.out.println("B::B()");
+    }
+
+    public static void main(String[] args) {
+        B b = new B();
+        b.a();
+    }
+
+    @Override
+    public void a() {
+        System.out.println("B::a()");
+        super.a();
+    }
+}

java/samples/introduction/C.java

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+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package basic;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 07.10.2022
+ */
+public class C implements I {
+
+    public void a() {
+        System.out.println("C::a()");
+    }
+
+    public void a(A a) {
+        System.out.println("C::a(A)");
+    }
+
+    public static void main(String[] args) {
+        C c = new C();
+        c.a();
+        c.a(null);
+        c.b();
+    }
+}

java/samples/introduction/I.java

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+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package basic;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 07.10.2022
+ */
+public interface I {
+
+    public static final A a = new A(null);
+    A b = null;
+
+    public abstract void a();
+
+    void a(A a);
+
+    default void b() {
+        System.out.println("I:b() - default");
+    }
+}

java/samples/java-packages/source/DemoActionListener.java

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+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+
+package gui;
+
+import java.awt.Color;
+import java.awt.Container;
+import java.awt.event.ActionEvent;
+import java.awt.event.ActionListener;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 28.11.2022
+ */
+public class DemoActionListener implements ActionListener{
+    
+    private Container cp;
+    private Color defaultColor;
+    
+    public DemoActionListener ( Container cp) {
+        this.cp = cp;
+        defaultColor = cp.getBackground();
+    }
+
+    @Override
+    public void actionPerformed(ActionEvent e) {
+        System.out.println ("Action performed");
+        Color c = cp.getBackground();
+        cp.setBackground(c == defaultColor ? Color.BLUE : defaultColor)        ;
+    }
+
+}

java/samples/java-packages/source/DemoCollection.java

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+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package std;
+
+import java.io.Serializable;
+import java.util.Collection;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.Set;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 05.12.2022
+ */
+public class DemoCollection {
+
+    public class User implements Serializable {
+
+        private String login;
+        private String password;
+
+        public User(String login, String password) {
+            this.login = login;
+            this.password = password;
+        }
+
+        public String getLogin() {
+            return login;
+        }
+
+        public void setLogin(String login) {
+            this.login = login;
+        }
+
+        public String getPassword() {
+            return password;
+        }
+
+        public void setPassword(String password) {
+            this.password = password;
+        }
+
+        @Override
+        public String toString() {
+            return "User{" + "login=" + login + ", password=" + password + '}';
+        }
+    }
+
+    public class Info implements Serializable {
+
+        String info;
+
+        public Info(String info) {
+            this.info = info;
+        }
+
+        public String getInfo() {
+            return info;
+        }
+
+        public void setInfo(String info) {
+            this.info = info;
+        }
+
+        @Override
+        public String toString() {
+            return "Info{" + "info=" + info + '}';
+        }
+    }
+
+    public static void main(String[] args) {
+        DemoCollection demo = new DemoCollection();
+//        demo.print(demo.init(5));
+        Map<User, Info> m = demo.init();
+        demo.printMap(m);
+    }
+
+    private Set<Integer> init(int size) {
+        int[] ai = {123, 345, 4564, 345, 123};
+        HashSet<Integer> hs = new HashSet<>();
+        for (int i : ai) {
+            System.out.println("Add " + i + " is "
+                    + hs.add(new Integer(i)));
+        }
+        return hs;
+    }
+
+    private Map<User, Info> init() {
+        HashMap<User, Info> hm = new HashMap<>();
+        hm.put(new User("first", "12344"), new Info("first message"));
+        hm.put(new User("second", "283479"), new Info("secondt message"));
+        return hm;
+    }
+
+    private <T> void printIterator(Collection<T> c) {
+        if (c == null || c.isEmpty()) {
+            System.out.println("Collection is null or empty");
+            return;
+        }
+        Iterator<T> i = c.iterator();
+        while (i.hasNext()) {
+            T t = i.next();
+            System.out.println(t);
+        }
+    }
+
+    private <T> void print(Collection<T> c) {
+        if (c == null || c.isEmpty()) {
+            System.out.println("Collection is null or empty");
+            return;
+        }
+        for (T t : c) {
+            System.out.println(t);
+        }
+    }
+
+    private void printMap(Map<User, Info> m) {
+        // 1. Map.Entry
+        for (Map.Entry<User, Info> tm : m.entrySet()) {
+            System.out.println( tm.getKey() + " --> "
+                    + tm.getValue());
+        }
+        System.out.println("------------------------------");
+        // 2. keySet -> get(...)
+        for (User u : m.keySet()) {
+            Info i = m.get(u);
+            System.out.println(u + " --> " + i);
+        }
+    }
+}

java/samples/java-packages/source/DemoFunInterface.java

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+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package std;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 05.12.2022
+ */
+@FunctionalInterface
+public interface DemoFunInterface {
+
+    public String message(int x);
+
+    public static String info() {
+        return "Demo Functional interface";
+    }
+    
+    default public void demo () {
+        System.out.println("Demo method");
+    }
+}

java/samples/java-packages/source/DemoInteger.java

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+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+
+package lang;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 14.11.2022
+ */
+public class DemoInteger {
+    public static void main(String[] args) {
+        Integer i = new Integer ("1234");
+        Integer j = Integer.valueOf("12345", 7);
+        Integer k = ++i;
+        i = ++i;
+        String s = "asdf";
+        s += "gth";
+        String t = new String (s);
+        System.out.println("s: " + s);
+        System.out.println(k);
+    }
+}

java/samples/java-packages/source/DemoJFrame.java

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+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package gui;
+
+import java.awt.Container;
+import java.awt.FlowLayout;
+import java.awt.event.ActionEvent;
+import java.awt.event.ActionListener;
+import javax.swing.JButton;
+import javax.swing.JFrame;
+import javax.swing.JLabel;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 28.11.2022
+ */
+public class DemoJFrame extends JFrame {
+
+    private JLabel lbl;
+
+    public DemoJFrame(String title) {
+        super(title);
+//        setBounds (x, y, w, h);
+//        setTitle (title);
+        setLocation(120, 80);
+        setSize(640, 480);
+        setResizable(false);
+        setDefaultCloseOperation(DO_NOTHING_ON_CLOSE);
+        init();
+        setVisible(true);
+    }
+
+    public static void main(String[] args) {
+        DemoJFrame demo = new DemoJFrame("Demo JFrame");
+    }
+
+    private void init() {
+//        DemoWindowListener wl = new DemoWindowListener();
+//        addWindowListener(wl);
+        addWindowListener(new DemoWindowAdapter());
+        Container cp = getContentPane();
+        cp.setLayout(null);
+        //        cp.setLayout(new FlowLayout(FlowLayout.CENTER, 10, 5));
+        lbl = new JLabel("Demo label");
+        lbl.setBounds(120, 80, 160, 32);
+        JButton b = new JButton("Demo button");
+        b.setBounds(120, 140, 160, 32);
+        b.addActionListener(new DemoActionListener(cp));
+        // class ? implements ActionListener { .... }
+        b.addActionListener(new ActionListener() {
+            @Override
+            public void actionPerformed(ActionEvent e) {
+                String s = lbl.getText();
+                lbl.setText(s + "*");
+            }
+        });
+        cp.add(lbl);
+        cp.add(b);
+    }
+}

java/samples/java-packages/source/DemoLambda.java

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+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package std;
+
+import java.util.ArrayList;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 05.12.2022
+ */
+public class DemoLambda {
+
+    public static void main(String[] args) {
+        DemoLambda demo = new DemoLambda();
+        ArrayList<Integer> al = demo.init();
+        demo.print(al);
+        al.removeIf((Integer x) -> x % 2 != 0);
+        demo.print(al);
+        al.forEach((Integer x) -> {
+            System.out.print(x * 2 + ", ");
+        });
+        System.out.println("");
+    }
+
+    private ArrayList<Integer> init() {
+        int[] ai = {1, 2, 3, 4, 5, 6, 77, 78};
+        ArrayList<Integer> al = new ArrayList<>();
+        for (int i : ai) {
+            al.add(i);
+        }
+        return al;
+    }
+
+    private void a(int x) {
+
+        // new ? implements DemoFunInterface { ... }
+        DemoFunInterface d = x1 -> {
+            return String.valueOf(x1);
+        };
+    }
+
+    private void print(ArrayList<Integer> al) {
+        for (Integer i : al) {
+            System.out.print(i + ", ");
+        }
+        System.out.println();
+    }
+
+}

java/samples/java-packages/source/DemoObject.java

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+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package lang;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 14.11.2022
+ */
+public class DemoObject implements Cloneable {
+    private int id = 12344;
+
+    public static void main(String[] args) {
+        DemoObject obj = new DemoObject();
+        System.out.println("obj: " + obj.id);
+        try {
+            DemoObject clone = (DemoObject) obj.clone();
+            System.out.println("clone: " + clone.id );
+            DemoObject clone2 = (DemoObject) clone.clone();
+            System.out.println("clone2: " + clone2.id );
+        } catch (CloneNotSupportedException e) {
+            System.out.println("Error: " + e.getMessage());
+        }  
+    }
+    
+    @Override
+    public Object clone () throws CloneNotSupportedException {
+        DemoObject t = new DemoObject();  // id = 12344
+//        DemoObject t = (DemoObject) super.clone();
+        t.id = id + 1;     // id = 12345
+        if ( t.id > 1000000) {
+            throw new CloneNotSupportedException ("Id overflow");
+        }
+        return t;
+    }
+}

java/samples/java-packages/source/DemoRuntime.java

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+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package lang;
+
+import java.io.IOException;
+import java.util.concurrent.TimeUnit;
+import java.util.logging.Level;
+import java.util.logging.Logger;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 14.11.2022
+ */
+public class DemoRuntime {
+
+    private String cmd = "calc.exe";
+
+    public static void main(String[] args) {
+        new DemoRuntime().run();
+    }
+
+    public void run() {
+        Runtime r = Runtime.getRuntime();
+        try {
+            Process p = r.exec(cmd);
+            p.waitFor(5, TimeUnit.SECONDS);
+            if (p.isAlive()) {
+                System.out.println("Process is alive");
+                p.destroyForcibly();
+            } else {
+                System.out.println("Process is done");
+            }
+            System.out.println("Exit code: " + p.exitValue());
+        } catch (IOException | InterruptedException ex) {
+            System.out.println("Error: " + ex.getMessage());
+        }
+    }
+}

java/samples/java-packages/source/DemoString.java

@@ -0,0 +1,27 @@
+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package lang;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 14.11.2022
+ */
+public class DemoString {
+
+    public static void main(String[] args) {
+        String s = "askjfsj akfkdjfal    askdfjlasj asldkjflasj 1234";
+//        String[] as = s.split("\\s+");
+//        for (String t: as) {
+//            System.out.println(t);
+//        }
+        System.out.println(s.replaceAll("\\p{Digit}", "*"));
+        String n = "2202 2202 2222 2222";
+        String t = n.substring(5, 14);
+        String x = "**** ****";
+        n = n.replace(t, x);
+        System.out.println(n);
+    }
+}

java/samples/java-packages/source/DemoWindowAdapter.java

@@ -0,0 +1,22 @@
+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package gui;
+
+import java.awt.event.WindowAdapter;
+import java.awt.event.WindowEvent;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 28.11.2022
+ */
+public class DemoWindowAdapter extends WindowAdapter {
+
+    @Override
+    public void windowClosing(WindowEvent e) {
+        System.out.println("Window closing");
+        e.getWindow().dispose();
+    }
+}

java/samples/java-packages/source/DemoWindowListener.java

@@ -0,0 +1,53 @@
+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package gui;
+
+import java.awt.event.WindowEvent;
+import java.awt.event.WindowListener;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 28.11.2022
+ */
+public class DemoWindowListener implements WindowListener {
+
+    @Override
+    public void windowOpened(WindowEvent e) {
+        System.out.println("Window opened");
+    }
+
+    @Override
+    public void windowClosing(WindowEvent e) {
+        System.out.println("Window closing");
+        e.getWindow().dispose();
+    }
+
+    @Override
+    public void windowClosed(WindowEvent e) {
+        System.out.println("Window closed");
+    }
+
+    @Override
+    public void windowIconified(WindowEvent e) {
+        System.out.println("Window iconified");
+    }
+
+    @Override
+    public void windowDeiconified(WindowEvent e) {
+        System.out.println("Window deiconified");
+    }
+
+    @Override
+    public void windowActivated(WindowEvent e) {
+        System.out.println("Window activated");
+    }
+
+    @Override
+    public void windowDeactivated(WindowEvent e) {
+        System.out.println("Window deactivated");
+    }
+
+}

java/samples/java-packages/source/EmptyStackException.java

@@ -0,0 +1,15 @@
+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+
+package lang;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 07.11.2022
+ */
+class EmptyStackException extends Exception {
+
+}

java/samples/java-packages/source/FullStackException.java

@@ -0,0 +1,15 @@
+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+
+package lang;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 07.11.2022
+ */
+class FullStackException extends Exception {
+
+}

java/samples/java-packages/source/Stack.java

@@ -0,0 +1,38 @@
+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package lang;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 07.11.2022
+ */
+public class Stack <T> {
+
+    public final int DEFAULT_SIZE = 128;
+    private T[] stack;
+    private int top;
+
+    public Stack(T[] stack) {
+//        size = size < 1 ? DEFAULT_SIZE : size;
+        this.stack = stack;
+        top = 0;
+    }
+
+    public void push(T x) throws FullStackException {
+        if (top == DEFAULT_SIZE) {
+            throw new FullStackException();
+        }
+        stack[top] = x;
+        ++top;
+    }
+
+    public T pop() throws EmptyStackException {
+        if (top < 1) {
+            throw new EmptyStackException();
+        }
+        return stack[--top];
+    }
+}

java/samples/multithread/ping-pong/Ball.java

@@ -0,0 +1,27 @@
+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package mt;
+
+import java.util.logging.Level;
+import java.util.logging.Logger;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 09.01.2023
+ */
+public class Ball {
+
+    public synchronized void hit(String name, int delay) {
+        System.out.print(name + " -> ");
+        try {
+            notify();
+            wait();
+            Thread.currentThread().sleep(delay);
+        } catch (InterruptedException ex) {
+            System.out.println("Exception: " + ex.getMessage());
+        }
+    }
+}

java/samples/multithread/ping-pong/DemoThread.java

@@ -0,0 +1,31 @@
+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package mt;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 09.01.2023
+ */
+public class DemoThread {
+
+    public static void main(String[] args) {
+        SimpleThread t1 = new SimpleThread('*', 10);
+        SimpleThread t2 = new SimpleThread('a', 10);
+        SimpleThread t3 = new SimpleThread('$', 10);
+        Thread mt = new Thread(t1, "First");
+        Thread m1 = new Thread(t2, "Second");
+        Thread m2 = new Thread(t3, "Third");
+        m2.setPriority(2);
+        mt.setPriority(7);
+
+        mt.start();
+        m1.start();
+        m2.start();
+
+        System.out.println(m2.getId() + "." + m2.getName() + ", "
+                + m2.getPriority());
+    }
+}

java/samples/multithread/ping-pong/PingPong.java

@@ -0,0 +1,54 @@
+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package mt;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 09.01.2023
+ */
+public class PingPong implements Runnable {
+
+    public static final int ROUND = 10;
+    private String name;
+    private int delay;
+    private Ball ball;
+
+    public PingPong(String name, int delay, Ball ball) {
+        this.name = name;
+        this.delay = delay;
+        this.ball = ball;
+    }
+
+    public static void main(String[] args) throws InterruptedException {
+        Ball ball = new Ball();
+        PingPong gamer1 = new PingPong("ping", 300, ball);
+        PingPong gamer2 = new PingPong("pong", 500, ball);
+
+        Thread t1 = new Thread(gamer1);
+        Thread t2 = new Thread(gamer2);
+
+        System.out.println("Start game...");
+        t1.start();
+        t2.start();
+        try {
+            t1.join(8000);
+            t2.join(8000);
+            synchronized (ball) {
+                ball.notifyAll();
+            }
+        } catch (InterruptedException e) {
+            System.out.println("Error: " + e.getMessage());
+        }
+        System.out.println("End over.");
+    }
+
+    @Override
+    public void run() {
+        for (int i = 0; i < ROUND; ++i) {
+            ball.hit(name, delay);
+        }
+    }
+}

java/samples/multithread/ping-pong/SimpleThread.java

@@ -0,0 +1,40 @@
+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package mt;
+
+/**
+ *
+ * @author (C)Y.D.Zakovryashin, 09.01.2023
+ */
+public class SimpleThread implements Runnable {
+
+    private char c;
+    private int num;
+
+    public SimpleThread(char c, int num) {
+        this.c = c;
+        this.num = num;
+    }
+
+    @Override
+    public void run() {
+        for (int i = 0; i < num; ++i) {
+            System.out.print("-" + c);
+//            try {
+//                Thread.currentThread().sleep(500);
+//            } catch (InterruptedException e) {
+//                System.out.println("Thread \""
+//                        + Thread.currentThread().getName()
+//                        + "\" is interrupted");
+//            }
+        }
+        System.out.println();
+    }
+    
+    public Thread getThread () {
+        return Thread.currentThread();
+    }
+}

java/samples/multithread/ping-pong/UDPClient.java

@@ -0,0 +1,158 @@
+/*
+ * Демонстрационное приложение.
+ */
+package udp;
+
+import java.io.BufferedReader;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.net.DatagramPacket;
+import java.net.DatagramSocket;
+import java.net.InetAddress;
+import java.net.SocketException;
+import java.nio.ByteBuffer;
+
+/**
+ * Класс клиента. Реализует запрос и получение файла от сервера.
+ *
+ * @author (C)Y.D.Zakovryashin, 16.01.2023
+ */
+public class UDPClient {
+
+    private DatagramSocket socket;
+    private DatagramPacket packet;
+    private byte[] buffer;
+    private String fileName;
+
+    public static void main(String[] args) {
+        System.out.println("Client started...");
+        try (DatagramSocket s = new DatagramSocket()) {
+            UDPClient client = new UDPClient();
+            client.socket = s;
+            /**
+             * Запрос у пользователя имени файла для загрузки.
+             */
+            client.getFileName();
+            /**
+             * Запрос файла с сервера.
+             */
+            client.sendFileName();
+            /**
+             * Чтение ответа сервера, содержащего либо размер файла, либо код
+             * ERROR, который означает отсутствие запрошенного файла на сервере.
+             */
+            long size = client.getFileSize();
+            if (size > UDPServer.ERROR) {
+                try {
+                    /**
+                     * Загрузка файла с сервера.
+                     */
+                    client.getFile(size);
+                    System.out.println("File \"" + client.fileName
+                            + "\" is downloaded successfully.");
+                } catch (IOException e) {
+                    /**
+                     * Произошла ошибка чтения файла.
+                     */
+                    System.err.println("File \"" + client.fileName
+                            + "\" isn\'t downloaded.\n" + e.getMessage());
+                }
+            } else {
+                /**
+                 * Сервер вернул код ERROR, который означает, что на сервере нет
+                 * файла с запрошенным именем.
+                 */
+                System.out.println("File \"" + client.fileName + "\" not found");
+            }
+        } catch (SocketException ex) {
+            System.out.println("Error #1: " + ex.getMessage());
+        } catch (IOException ex) {
+            System.out.println("Error #2: " + ex.getMessage());
+        }
+        System.out.println("Client stoped.");
+    }
+
+    /**
+     * Чтение имени запрашиваемого файла с консоли.
+     *
+     * @return строку с именем файла.
+     */
+    private void getFileName() {
+        try (BufferedReader in = new BufferedReader(
+                new InputStreamReader(System.in));) {
+            /**
+             * Цикл запроса имени файла должен выполняться до тех пор, пока
+             * пользователь не введёт непустое имя файла.
+             */
+            do {
+                System.out.println(">>> Input file name for download:");
+                fileName = in.readLine();
+            } while (fileName == null || fileName.trim().isEmpty());
+        } catch (IOException e) {
+            System.out.println("Errror: " + e.getMessage());
+        }
+        System.out.println("Step 1");
+    }
+
+    /**
+     * Отправка запроса с именем файла.
+     *
+     * @param fileName строка с именем файла.
+     * @throws IOException в случае общей ошибки ввода/вывода.
+     */
+    private void sendFileName() throws IOException {
+        buffer = fileName.getBytes();
+        /**
+         * В пакет заносится адрес сервера, его порт и сериализованное имя
+         * файла.
+         */
+        packet = new DatagramPacket(buffer, buffer.length,
+                InetAddress.getByName(UDPServer.UDP_SERVER_ADDRESS),
+                UDPServer.UDP_SERVER_PORT);
+        socket.send(packet);
+        System.out.println("Step 2");
+    }
+
+    /**
+     * Получение первого пакета от сервера с размером файла и отправка пакета с
+     * подтверждением о готовности принять файл.
+     *
+     * @return размер файла.
+     * @throws IOException в случае общей ошибки ввода/вывода.
+     */
+    private long getFileSize() throws IOException {
+        buffer = new byte[8];
+        packet = new DatagramPacket(buffer, buffer.length);
+        /**
+         * Получение пакета с размером файла.
+         */
+        socket.receive(packet);
+        long size = ByteBuffer.wrap(packet.getData()).getLong();
+        if (size > UDPServer.ERROR && size < Integer.MAX_VALUE) {
+            buffer = UDPServer.OK.getBytes();
+        } else {
+            buffer = UDPServer.NO.getBytes();
+        }
+        /**
+         * Подготовка и отправка пакета с подтверждением о готовности принять
+         * файл.
+         */
+        packet.setData(buffer);
+        packet.setLength(buffer.length);
+        socket.send(packet);
+        System.out.println("Step 3");
+        return size;
+    }
+
+    private void getFile(long size) throws IOException {
+        int s = size > Integer.MAX_VALUE ? Integer.MAX_VALUE : (int) size;
+        buffer = new byte[s];
+        packet.setData(buffer);
+        socket.receive(packet);
+        try (FileOutputStream fos = new FileOutputStream("Copy-" + fileName)) {
+            fos.write(buffer);
+        }
+        System.out.println("Step 4");
+    }
+}

java/samples/multithread/ping-pong/UDPServer.java

@@ -0,0 +1,120 @@
+/*
+ * Демонстрационное приложение. Сервер приложения.
+ */
+package udp;
+
+import java.io.BufferedReader;
+import java.io.DataOutputStream;
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.net.DatagramPacket;
+import java.net.DatagramSocket;
+import java.net.InetAddress;
+import java.net.SocketException;
+import java.nio.ByteBuffer;
+
+/**
+ * Класс сервера. Обеспечивает отправку клиенту запрошенного файла.
+ *
+ * @author (C)Y.D.Zakovryashin, 16.01.2023
+ */
+public class UDPServer {
+
+    public static final int UDP_SERVER_PORT = 12345;
+    public static final String UDP_SERVER_ADDRESS = "localhost";
+    public static final String OK = "Ok";
+    public static final String NO = "No";
+    public static final int ERROR = -1;
+    public static final int BUFFER_SIZE = 255;
+    private DatagramSocket socket;
+    private DatagramPacket packet;
+    private byte[] buffer;
+    private String fileName;
+    private long fileLength;
+    private boolean exists;
+
+    public static void main(String[] args) {
+        System.out.println("Server started...");
+        try (DatagramSocket s = new DatagramSocket(UDP_SERVER_PORT)) {
+            UDPServer server = new UDPServer();
+            server.socket = s;
+            /**
+             * Ожидание запроса клиента с именем файла.
+             */
+            server.getFileName();
+            /**
+             * Отправка клиенту результата поиска файла: либо размера найденного
+             * файла, либо кода ERROR, который означает отсутствие запрошенного
+             * файла на сервере.
+             */
+            server.sendResult();
+            /**
+             * Отправка файла с сервера.
+             */
+            if (server.exists && server.getClientReply()) {
+                server.sendFile();
+                System.out.println("Server: File \"" + server.fileName
+                        + "\" is sent successfully.");
+            }
+        } catch (SocketException ex) {
+            System.out.println("Server error #1: " + ex.getMessage());
+        } catch (IOException e) {
+            /**
+             * Произошла ошибка ввода/вывода файла.
+             */
+            System.err.println("Server error #2: " + e.getMessage());
+        }
+        System.out.println("Server stoped.");
+    }
+
+    /**
+     * Чтение имени запрашиваемого файла с консоли.
+     *
+     * @return строку с именем файла.
+     */
+    private void getFileName() throws IOException {
+        buffer = new byte[BUFFER_SIZE];
+        packet = new DatagramPacket(buffer, BUFFER_SIZE);
+        socket.receive(packet);
+        fileName = new String(packet.getData(), 0, packet.getLength());
+        System.out.println("Step 1: file name: " + fileName);
+    }
+
+    private void sendResult() throws IOException {
+        File f = new File(fileName);
+        buffer = new byte[8];
+        if (exists = f.exists()) {
+            fileLength = f.length();
+            ByteBuffer.wrap(buffer).putLong(fileLength);
+        } else {
+            ByteBuffer.wrap(buffer).putLong(ERROR);
+        }
+        packet.setData(buffer);
+        packet.setLength(8);
+        socket.send(packet);
+        System.out.println("Step 2");
+    }
+
+    private boolean getClientReply() throws IOException {
+        buffer = new byte[4];
+        packet.setData(buffer);
+        socket.receive(packet);
+        String r = new String(packet.getData()).trim();
+        System.out.println("Step 3");
+        return r.equals("Ok");
+    }
+
+    private void sendFile() throws IOException {
+        buffer = new byte[(int) fileLength];
+        try (FileInputStream fis = new FileInputStream(fileName)) {
+            int s = fis.read(buffer);
+        }
+        packet.setData(buffer);
+        socket.send(packet);
+        System.out.println("Step 4");
+    }
+
+}