Java has great support for Zip format and writing code in Java to zip/unzip file is really simple. Java 8 makes it even simpler. The following example shows how to zip and unzip file.

In the following Java code we use java.util.zip package to zip content into a file and then unzip the file and read the content.

package com.farenda.java.io;

import java.io.BufferedOutputStream;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.Writer;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.stream.Collectors;
import java.util.zip.ZipEntry;
import java.util.zip.ZipInputStream;
import java.util.zip.ZipOutputStream;

public class ZipUnzipExample {

    public static void main(String[] args) throws IOException {
        String filename = "/tmp/sample-file.zip";

        createZipFile(filename, "This data will be zipped.");

        System.out.println("Unzipped content: " + readZipFile(filename));
    }

    private static void createZipFile(String filename, String content) throws IOException {
        // try-with-resources will close everything automatically
        try (Writer writer = createWriter(filename)) {
            System.out.println("Zipping: " + content);
            writer.write(content);
        }
    }

    private static Writer createWriter(String filename) throws IOException {
        OutputStream os = createOutputStream(filename);
        return new OutputStreamWriter(os);
    }

    private static OutputStream createOutputStream(String filename) throws IOException {
        OutputStream fos = Files.newOutputStream(Paths.get(filename));
        ZipOutputStream zos = new ZipOutputStream(fos);
        // Sample zip entry, here: a filename
        ZipEntry entry = new ZipEntry("sample-file.txt");
        zos.putNextEntry(entry);
        return new BufferedOutputStream(zos);
    }

    private static String readZipFile(String filename) throws IOException {
        // try-with-resources will close everything automatically
        try (BufferedReader reader = createReader(filename)) {
            // use Java 8 Streams to load all lines:
            return reader.lines().collect(Collectors.joining());
        }
    }

    private static BufferedReader createReader(String filename) throws IOException {
        InputStream zis = createInputStream(filename);
        return new BufferedReader(new InputStreamReader(zis));
    }

    private static InputStream createInputStream(String filename) throws IOException {
        InputStream is = Files.newInputStream(Paths.get(filename));
        ZipInputStream zis = new ZipInputStream(is);
        // for many entries read one by one
        ZipEntry entry = zis.getNextEntry();
        System.out.println("Zip entry: " + entry.getName());
        return zis;
    }
}

Notice how we use try-with-resources from Java 7 to close all streams and prevent resource leakage. Also we use Java 8 Streams to read all lines (1 in this case) from unzipped input stream.

Also note how code has been split to small, highly cohesive, methods that do only one thing.

Running the example shows that zipping and unzipping works as expected:

Zipping: This data will be zipped.
Zip entry: sample-file.txt
Unzipped content: This data will be zipped.