Java - JUnit - testing methods - MultipleMarkersError

I have just started learning JUnit testing. I have to write test for methoods below. I started from testting zero(). Can you tell me what i do wrongly there? And leave some helpful examples how should i test rest of this methods?

private Accumulator a;
int acc;

public void test() 
    {
        Kalkulator test = new Kalkulator();
        int result = test.zero(); // my error: MULTIPLE MARKERS
        assertEquals(0,result);
    }

// Methoods

public void setAccumulator( Accumulator a ){
    this.a = a;
    acc = a.get();
}

public Accumulator getAccumulator(){    
    return a;
}

public int get(){
        return acc;
}

public void zero(){
    acc = 0;
}

//***get***
public int get() 
{
         return acc;      
}