package com.helloacm;
 
import java.util.concurrent.atomic.AtomicBoolean;
 
public class Example {
    // is the data available?
    private final AtomicBoolean pending = new AtomicBoolean(true);
 
    // data field
    private final Object data = new Object();
 
    public Object get() {
        // when it is true, return the data and set to false
        if (pending.compareAndSet(true, false)) {
            return data;
        }
        return null;
    }
}

package com.helloacm;
 
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
 
import static org.junit.jupiter.api.Assertions.assertNotNull;
import static org.junit.jupiter.api.Assertions.assertNull;
 
public class ExampleTest {
    // test instance
    private Example instance;
 
    @BeforeEach
    public void setup() {
        instance = new Example();
    }
 
    @Test
    public void test_get_twice_should_return_null() {
        assertNotNull(instance.get());
        assertNull(instance.get());
    }
}