Gist describing a potential usecase for VisibileForTesting annotation https://developer.android.com/reference/android/support/annotation/VisibleForTesting.html

Calculator.java

public class Calculator {
	private int a;
	private int b;
	
	public Calculator(int a, int b) {
		this.a = a;
		this.b = b;
	}
	
	public void sum() {
		doSum(a,b);
	}
	
	@VisibleForTesting(otherwise = VisibleForTesting.PRIVATE)
	protected int doSum(int a, int b) {
		return a + b;	
	}
}


public class CalculatorTest {

	@Test
	public void testSum() {
		// given a, b & calculator instance
		int a = 1;
		int b = 3;
		Calculator calc = new Calculator(a,b);
		
		// when we do sum
		int sum = calc.doSum(a,b);
		
		// then verify that sum is 4
		assertEquals(sum, 4);
	}
	
}