Easy way to concatenate two byte arrays

Here's a nice solution using Guava's com.google.common.primitives.Bytes:

byte[] c = Bytes.concat(a, b);

The great thing about this method is that it has a varargs signature:

public static byte[] concat(byte[]... arrays)

which means that you can concatenate an arbitrary number of arrays in a single method call.

Most straightforward:

byte[] c = new byte[a.length + b.length];
System.arraycopy(a, 0, c, 0, a.length);
System.arraycopy(b, 0, c, a.length, b.length);

The most elegant way to do this is with a ByteArrayOutputStream.

byte a[];
byte b[];

ByteArrayOutputStream outputStream = new ByteArrayOutputStream( );
outputStream.write( a );
outputStream.write( b );

byte c[] = outputStream.toByteArray( );

Another possibility is using java.nio.ByteBuffer.

Something like

ByteBuffer bb = ByteBuffer.allocate(a.length + b.length + c.length);
bb.put(a);
bb.put(b);
bb.put(c);
byte[] result = bb.array();

// or using method chaining:

byte[] result = ByteBuffer
        .allocate(a.length + b.length + c.length)
        .put(a).put(b).put(c)
        .array();

Note that the array must be appropriately sized to start with, so the allocation line is required (as array() simply returns the backing array, without taking the offset, position or limit into account).