an issue with primitive value in unit test

I am quite confused about writing the unit test to test myMethod, because it cannot cover the logics defined factory, because the default value of boolean is false, it means if I don't changed flag to true, it won't be executed at all. How can I define flag is true in my test in order factory method can be executed?

  @Service
    public class MyService{

    private boolean flag;
    private String result;

    public void myMethod(String input1, String input2) {
        factory(input1, input2);
    }

    private void factory(String input1, String input2) {
        if (flag) {
            result = input1 + input2;
        }
    }

Here is my test:

 @Test
    public void myMethod() {
        assertEquals(myMethod("Hello", "World"), "HelloWorld");
    }