Unit tests with Mocks on Mastermind and Random() Class

I need to programm Mastermind and ive done this with a random key. I now need to Unit test them. its easy for the control methods in my Helperclass but i dont know how to really mock or test the method with a Random Object from Class Random or a method using an Object from Scanner Class here the Code:

public int[] randomKey(){
        int[] key = new int[4];
        Random random1 = new Random();
        for(int i = 0;i < 4;i++) {
            key[i] = random1.nextInt(6);//Random Zahl for the Key
    }
        return key;
    }

Thats the Random method in HelperClass How you do there the Mock object and test

private String[] guess(){ //Input from guessing
        int k = 1;
        Scanner eingabewert = new Scanner(System.in);
        String[] guess = new String[4];
        for(int i = 0; i < 4;i++){
            k = 1;
            while(k==1) {
                System.out.println("Jetzt die " + (i + 1) + ".te Farbe eingeben ");
                guess[i] = eingabewert.nextLine();
                if(guess[i].equals("red")||guess[i].equals("blue")||guess[i].equals("yellow")||guess[i].equals("green")||guess[i].equals("purple")||guess[i].equals("brown")){
                    k = 0;
                }
            }
        }
        return guess;
    }

my helpless try was in the Test class for the method with the Random() Object

    @Mock
    private java.util.Random random;
    @InjectMocks
    private HelperClass helperClass;
    @Before
    public void setUp() throws Exception {
        MockitoAnnotations.initMocks(this);
    }
    @Test
            void testRandomKey(){
            Random mockedItem = Mockito.mock(Random.class);
            when(mockedItem.nextInt(6)).thenReturn(1);


    }


}

I dont know how to go on or basically understood how this actually works pls help me and try to find a short solution for the test. okay if its just to the randomKey method. Im using Intellij Idea as IDE btw.