import java.util.Random;

public class CodeWithPrivateMethod {

    public void meaningfulPublicApi() {

        if (doTheGamble("Whatever", 1 << 3)) {

            throw new RuntimeException("boom");

        }

    }

    private boolean doTheGamble(String whatever, int binary) {

        Random random = new Random(System.nanoTime());

        boolean gamble = random.nextBoolean();

        return gamble;

    }

}

import org.junit.Test;

import org.junit.runner.RunWith;

import org.powermock.api.mockito.PowerMockito;

import org.powermock.core.classloader.annotations.PrepareForTest;

import org.powermock.modules.junit4.PowerMockRunner;

import static org.mockito.Matchers.anyInt;

import static org.mockito.Matchers.anyString;

import static org.powermock.api.mockito.PowerMockito.when;

import static org.powermock.api.support.membermodification.MemberMatcher.method;

@RunWith(PowerMockRunner.class)

@PrepareForTest(CodeWithPrivateMethod.class)

public class CodeWithPrivateMethodTest {

    @Test(expected = RuntimeException.class)

    public void when_gambling_is_true_then_always_explode() throws Exception {

        CodeWithPrivateMethod spy = PowerMockito.spy(new CodeWithPrivateMethod());

        when(

                spy,

                method(CodeWithPrivateMethod.class, "doTheGamble",

                        String.class, int.class)).withArguments(anyString(),anyInt()

            ).thenReturn(true);

        spy.meaningfulPublicApi();

    }

}