public class SomeClass {

    public int someMethod(MutableInteger mutableInteger) {
        mutableInteger.setValue(mutableInteger.getValue()+1);
        return -1;
    }
}

public class SomeClassTest {

    @Test
    public void testSomeMethod() {
        MutableInteger mutableInteger = new MutableInteger(2);
        SomeClass instance = new SomeClass();
        int expResult = -1;
        int result = instance.someMethod(mutableInteger);
        assertAll(() -> assertEquals(expResult, result),
            () -> assertEquals(3, mutableInteger.getValue()));
    }
}

public class MutableInteger {
    private int value;

    public MutableInteger(int value) {
        this.value = value;
    }

    public int getValue() {
        return value;
    }

    public void setValue(int value) {
        this.value = value;
    }
}