FTP

FTP/build.gradle

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+plugins {
+    id 'java-library'
+}
+
+dependencies {
+    api 'org.apache.commons:commons-math3:3.6.1'
+
+    implementation 'com.google.guava:guava:23.0'
+
+    compile group: 'org.jetbrains', name: 'annotations', version: '15.0'
+    compile group: 'commons-io', name: 'commons-io', version: '2.6'
+
+    testCompile group: 'org.hamcrest', name: 'hamcrest-all', version: '1.3'
+    testCompile group: 'org.junit.jupiter', name: 'junit-jupiter-api', version: '5.1.0'
+    testCompile group: 'org.junit.jupiter', name: 'junit-jupiter-params', version: '5.0.0-M4'
+
+    testImplementation 'junit:junit:4.12'
+}
+
+repositories {
+    jcenter()
+}
+task wrapper(type: Wrapper) {
+	description = 'Generates gradlew[.bat] scripts'
+	gradleVersion = '4.6'
+}

FTP/gradle/wrapper/gradle-wrapper.properties

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+#Sun Apr 22 23:21:38 MSK 2018
+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-4.6-all.zip

FTP/settings.gradle

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+rootProject.name = 'FTP'
+

FTP/src/main/java/FTPClient.java

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+import org.apache.commons.io.IOUtils;
+import org.jetbrains.annotations.NotNull;
+
+import java.io.*;
+import java.net.Socket;
+import java.nio.file.FileAlreadyExistsException;
+
+/**
+ * This class provides methods to work with FTPServer. These are:
+ *
+ * {@link FTPClient#list(String)} -- prints all files in the target directory.
+ *
+ * {@link FTPClient#get(String, String)} -- downloads and saves file from the
+ * server to the target file.
+ */
+@SuppressWarnings({"unused", "WeakerAccess"})
+public class FTPClient {
+
+    private final static int BUF_SIZE = 2048;
+
+    @NotNull
+    final private DataOutputStream os;
+    @NotNull
+    final private DataInputStream is;
+
+    /**
+     * Creates new client, that connected to the FTPServer.
+     *
+     * @param url servers url.
+     * @param port port to connect.
+     * @throws IOException if any IOException occurred.
+     */
+    public FTPClient(final String url, final int port) throws IOException {
+        final Socket socket = new Socket(url, port);
+        os = new DataOutputStream(socket.getOutputStream());
+        is = new DataInputStream(socket.getInputStream());
+    }
+
+    /**
+     * Creates new client, that connected to the FTPServer.
+     *
+     * @param url servers url.
+     * @throws IOException if any IOException occurred.
+     */
+    public FTPClient(final String url) throws IOException {
+        this(url, FTPServer.DEFAULT_PORT);
+    }
+
+    /**
+     * Prints list of all files (including folders), inside the source folder.
+     *
+     * Also for every file prints whether the file is a directory or not.
+     *
+     * @param source target folder.
+     * @throws IOException if any IOException occurred.
+     */
+    public void list(@NotNull final String source) throws IOException {
+        os.writeInt(1);
+        os.writeUTF(source);
+        os.flush();
+
+        final int number = is.readInt();
+        System.out.println("Total number of files: " + number);
+        for (int i = 0; i < number; i++) {
+            final String name = is.readUTF();
+            final boolean isFolder = is.readBoolean();
+            final String isFolderString = isFolder ? "is a folder." : "is not a folder.";
+            System.out.println("\"" + name + "\" " + isFolderString);
+        }
+    }
+
+    /**
+     * Downloads file from the server to the file with specified name. If file with
+     * the given name already exists, FileAlreadyExistsException will be thrown.
+     *
+     * @param source target file.
+     * @param destination path, to save file.
+     * @throws IOException if any IOException occurred.
+     */
+    public void get(@NotNull final String source, @NotNull final String destination) throws IOException {
+        os.writeInt(2);
+        os.writeUTF(source);
+        os.flush();
+
+        final File file = new File(destination);
+        if (!file.createNewFile()) {
+            throw new FileAlreadyExistsException();
+        }
+        final byte[] buf = new byte[BUF_SIZE];
+        try (final FileOutputStream os = new FileOutputStream(file)) {
+            final long size = is.readLong();
+            IOUtils.copyLarge(is, os, 0, size);
+        }
+    }
+
+    private class FileAlreadyExistsException extends RuntimeException {}
+}