Use the @Before method to instantiate a new Calculator and use that instance in each method.

package com.example;

import org.junit.Before;
import org.junit.Test;

import static org.junit.Assert.*;

public class CalculatorTest {

    private Calculator calculator;

    @Before
    public void setUp() throws Exception {
      calculator = new Calculator();
    }

    @Test
    public void addingMultipleNumbersProducesResult() throws Exception {
        //Calculator calculator = new Calculator();

        int answer = calculator.addNumbers(1 ,2, 3);

        assertEquals(6, answer);
    }

    @Test
    public void addingSingleNumberTotalsAppropriately() throws Exception {
        //Calculator calculator = new Calculator();

        int answer = calculator.addNumbers(1);

        assertEquals(1, answer);
    }
}

package com.example;

public class Calculator {

    public int addNumbers(int... numbers) {
        int total = 0;
        for (int number : numbers) {
            total += number;
        }
        return total;
    }
}