getInstance methods of Mac class throw NullPointerException if algorithm parameter is null. This behavior is not specified either in the method or in the package/class description. Here is an example:
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import javax.crypto.Mac;
import java.security.NoSuchAlgorithmException;
import java.security.NoSuchProviderException;
import java.security.Provider;

public class A {
    public static void main(String[] args) {
        try {
            Mac.getInstance(null);
        } catch (NoSuchAlgorithmException e) {
            System.out.println("NoSuchAlgorithmException was thrown");
        } catch (NullPointerException e) {
            System.out.println("NPE was thrown");
        }
        try {
            Mac.getInstance(null, "SunJCE");
        } catch (NoSuchAlgorithmException e) {
            System.out.println("NoSuchAlgorithmException was thrown");
        } catch (NullPointerException e) {
            System.out.println("NPE was thrown");
        } catch (NoSuchProviderException e) {
            System.out.println("NoSuchProviderException was thrown");
        }
        try {
            Mac.getInstance(null, new Provider("A", 0.1, "C") {});
        } catch (NoSuchAlgorithmException e) {
            System.out.println("NoSuchAlgorithmException was thrown");
        } catch (NullPointerException e) {
            System.out.println("NPE was thrown");
        }
    }
}
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aag@gaff:~/work/bugs/doc> javac A.java;java A
NPE was thrown
NPE was thrown
NPE was thrown
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###@###.### 2004-12-07 15:05:20 GMT