You can use java.util.Base64 package to decode the String to byte[]. Below code which I have used for encode and decode.

For Java 8 :

import java.io.UnsupportedEncodingException;
import java.util.Base64;

public class Example {

    public static void main(String[] args) {
        try {
            byte[] name = Base64.getEncoder().encode("hello World".getBytes());
            byte[] decodedString = Base64.getDecoder().decode(new String(name).getBytes("UTF-8"));
            System.out.println(new String(decodedString));
        } catch (UnsupportedEncodingException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

For Java 6 :

import java.io.UnsupportedEncodingException;
import org.apache.commons.codec.binary.Base64;

public class Main {

    public static void main(String[] args) {
        try {
            byte[] name = Base64.encodeBase64("hello World".getBytes());
            byte[] decodedString = Base64.decodeBase64(new String(name).getBytes("UTF-8"));
            System.out.println(new String(decodedString));
        } catch (UnsupportedEncodingException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

You can also use Apache Commons Codec (http://commons.apache.org/codec/)

String example = "SGVsbG8gV29ybGQ="
byte[] decoded = org.apache.commons.codec.binary.Base64.decodeBase64(example .getBytes());

As of Java 8, there is an officially supported API for Base64 encoding and decoding. In time this will probably become the default choice.

The API includes the class java.util.Base64 and its nested classes. It supports three different flavors: basic, URL safe, and MIME.

Sample code using the "basic" encoding:

import java.util.Base64;

byte[] bytes = "Hello, World!".getBytes("UTF-8");
String encoded = Base64.getEncoder().encodeToString(bytes);
byte[] decoded = Base64.getDecoder().decode(encoded);
String decodedStr = new String(decoded, StandardCharsets.UTF_8);

The documentation for java.util.Base64 includes several more methods for configuring encoders and decoders, and for using different classes as inputs and outputs (byte arrays, strings, ByteBuffers, java.io streams).