A few simple test cases for the Observable from https://gist.github.com/dlhartveld/5057313. Uses JUnit and Mockito.

ObservableTest.java

package com.hartveld.examples;

import static org.mockito.Mockito.verify;
import static org.mockito.Mockito.verifyNoMoreInteractions;

import java.util.function.Consumer;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.Mock;
import org.mockito.runners.MockitoJUnitRunner;

@RunWith(MockitoJUnitRunner.class)
public class ObservableTest {

  @Mock
  private Consumer<String> strTarget;
  @Mock
  private Consumer<Integer> intTarget;

  @Test
  public void testSubscribe() {
    Observable<String> strings = onNext -> {
      onNext.accept("Hello");
      onNext.accept("World");
    };

    strings.subscribe(strTarget);

    verify(strTarget).accept("Hello");
    verify(strTarget).accept("World");
    verifyNoMoreInteractions(strTarget);
  }

  @Test
  public void testMap() {
    Observable<String> strings = onNext -> {
      onNext.accept("Hello");
      onNext.accept("World");
    };

    strings.map(s -> s.substring(0, 2)).subscribe(strTarget);

    verify(strTarget).accept("He");
    verify(strTarget).accept("Wo");
    verifyNoMoreInteractions(strTarget);
  }

  @Test
  public void testFilter() {
    Observable<String> strings = onNext -> {
      onNext.accept("Hello");
      onNext.accept("World");
    };

    strings.filter(s -> s.startsWith("W")).subscribe(strTarget);

    verify(strTarget).accept("World");
    verifyNoMoreInteractions(strTarget);
  }

  @Test
  public void testBind() {
    Observable<String> strings = onNext -> {
      onNext.accept("XYZ");
      onNext.accept("X");
    };

    Observable<Integer> ints = onNext -> {
      onNext.accept(1);
      onNext.accept(2);
      onNext.accept(3);
    };

    strings.bind(s -> ints.filter(i -> i == s.length())).subscribe(intTarget);

    verify(intTarget).accept(1);
    verify(intTarget).accept(3);
    verifyNoMoreInteractions(intTarget);
  }

  @Test
  public void testFilter2() {
    Observable<String> strings = onNext -> {
      onNext.accept("Hello");
      onNext.accept("World");
    };

    strings.filter2(s -> s.startsWith("W")).subscribe(strTarget);

    verify(strTarget).accept("World");
    verifyNoMoreInteractions(strTarget);
  }

}