Java Compress/Decompress String/Data

Java includes built‑in classes for ZLIB compression — no extra libraries needed. The `Deflater` class does the heavy lifting, while `DeflaterOutputStream` wraps an output stream to compress data as it’s written. For decompression, there’s the matching `Inflater` and `InflaterOutputStream`. In this post, I’ll walk you through compressing a byte array, decompressing it back, and converting the result to Base64 for easy transport.

Compression

Let’s start with a static method that compresses a byte array using `DeflaterOutputStream`.

static byte[]compressBArray(byte[]bArray) throws IOException{
        ByteArrayOutputStream os=new ByteArrayOutputStream();
        try(DeflaterOutputStream dos=new DeflaterOutputStream(os)){
            dos.write(bArray);
        }
        return os.toByteArray();
}

Let's test:

byte[] input = "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"        .getBytes();
byte[] op = CompressionUtil.compressBArray(input);
System.out.println("original data length " + input.length +
        ",  compressed data length " + op.length);

Run it and you’ll see: **original data length 71, compressed data length 12**. That’s the power of ZLIB — repetitive data shrinks dramatically.

Decompression

Let's test:

public static byte[] decompress(byte[] compressedTxt) throws IOException {
        ByteArrayOutputStream os = new ByteArrayOutputStream();    
        try (OutputStream ios = new InflaterOutputStream(os)) {
            ios.write(compressedTxt);    
        }
        return os.toByteArray();
}

When you feed the compressed bytes into this method, you’ll get back the original `'input'` string — just as expected.

Let's convert the byte[] to Base64 to make it portable

The compressed output is a raw `byte[]`, which isn’t friendly for JSON, databases, or simple file storage. Base64 encoding turns it into a safe string that can travel anywhere.

byte[] bytes = {}; //the byte array    
String b64Compressed = new String(Base64.getEncoder().encode(bytes));
byte[] decompressedBArray = Base64.getDecoder().decode(b64Compressed);
//convert to original string if input was string
new String(decompressedBArray, StandardCharsets.UTF_8);

One thing to keep in mind: Base64 encoding adds about 33% in size to the already compressed data. But for storage and transport it’s well worth the trade‑off.

Here's the complete code and the test cases

**Full utility class:**

package compress;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.nio.charset.StandardCharsets;
import java.util.Base64;
import java.util.zip.DeflaterOutputStream;
import java.util.zip.InflaterOutputStream;

public class CompressionUtil {

    public static String compressAndReturnB64(String text) throws IOException {
        return new String(Base64.getEncoder().encode(compress(text)));
    }

    public static String decompressB64(String b64Compressed) throws IOException {
        byte[] decompressedBArray = decompress(Base64.getDecoder().decode(b64Compressed));
        return new String(decompressedBArray, StandardCharsets.UTF_8);
    }

    public static byte[] compress(String text) throws IOException {
        return compress(text.getBytes());
    }

    public static byte[] compress(byte[] bArray) throws IOException {
        ByteArrayOutputStream os = new ByteArrayOutputStream();
        try (DeflaterOutputStream dos = new DeflaterOutputStream(os)) {
            dos.write(bArray);
        }
        return os.toByteArray();
    }

    public static byte[] decompress(byte[] compressedTxt) throws IOException {
        ByteArrayOutputStream os = new ByteArrayOutputStream();
        try (OutputStream ios = new InflaterOutputStream(os)) {
            ios.write(compressedTxt);
        }
        return os.toByteArray();
    }

}

**Test cases to verify everything works:**

package compress;

import org.junit.jupiter.api.Test;

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

public class CompressionTest {

    String testStr = "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA";

    @Test
    void compressByte() throws IOException {
        byte[] input = testStr.getBytes();
        byte[] op = CompressionUtil.compress(input);
        System.out.println("original data length " + input.length + ",  compressed data length " + op.length);
        byte[] org = CompressionUtil.decompress(op);
        System.out.println(org.length);
        System.out.println(new String(org, StandardCharsets.UTF_8));
    }

    @Test
    void compress() throws IOException {

        String op = CompressionUtil.compressAndReturnB64(testStr);
        System.out.println("Compressed data b64" + op);
        String org = CompressionUtil.decompressB64(op);
        System.out.println("Original text" + org);
    }

}

 Note: Because `compress` and `decompress` work on `byte[]`, you can apply them to any binary data — images, serialized objects, you name it. Just wrap the logic into a small utility and you’re good to go.