How can I write a byte array to a file in Java?

You can use IOUtils.write(byte[] data, OutputStream output) from Apache Commons IO.

KeyGenerator kgen = KeyGenerator.getInstance("AES");
kgen.init(128);
SecretKey key = kgen.generateKey();
byte[] encoded = key.getEncoded();
FileOutputStream output = new FileOutputStream(new File("target-file"));
IOUtils.write(encoded, output);

As Sebastian Redl points out the most straight forward now java.nio.file.Files.write. Details for this can be found in the Reading, Writing, and Creating Files tutorial.

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Old answer: FileOutputStream.write(byte[]) would be the most straight forward. What is the data you want to write?

The tutorials for Java IO system may be of some use to you.

As of Java 1.7, there's a new way: java.nio.file.Files.write

import java.nio.file.Files;
import java.nio.file.Paths;

KeyGenerator kgen = KeyGenerator.getInstance("AES");
kgen.init(128);
SecretKey key = kgen.generateKey();
byte[] encoded = key.getEncoded();
Files.write(Paths.get("target-file"), encoded);

Java 1.7 also resolves the embarrassment that Kevin describes: reading a file is now:

byte[] data = Files.readAllBytes(Paths.get("source-file"));