BAEL-5908: Single.just vs fromCallable

rxjava-modules/rxjava-core-2/README.md

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+## RxJava
+
+This module contains articles about RxJava.
+
+### Relevant articles:

rxjava-modules/rxjava-core-2/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>rxjava-core-2</artifactId>
+    <version>1.0-SNAPSHOT</version>
+    <name>rxjava-core-2</name>
+
+    <parent>
+        <groupId>com.baeldung.rxjava-modules</groupId>
+        <artifactId>rxjava-modules</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+    </parent>
+
+</project>

rxjava-modules/rxjava-core-2/src/main/java/com/baeldung/rxjava/justvscallable/EmployeeRepository.java

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+package com.baeldung.rxjava.justvscallable;
+
+public interface EmployeeRepository {
+    String findById(Long id);
+}

rxjava-modules/rxjava-core-2/src/main/resources/logback.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<configuration>
+    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
+        <encoder>
+            <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n
+            </pattern>
+        </encoder>
+    </appender>
+
+    <root level="INFO">
+        <appender-ref ref="STDOUT" />
+    </root>
+</configuration>

rxjava-modules/rxjava-core-2/src/test/java/com/baeldung/rxjava/justvscallable/EmployeeRepositoryTest.java

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+package com.baeldung.rxjava.justvscallable;
+
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.reset;
+import static org.mockito.Mockito.times;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.mockito.Mockito;
+
+import rx.Single;
+import rx.observers.TestSubscriber;
+
+class EmployeeRepositoryTest {
+
+    public EmployeeRepository repository = mock(EmployeeRepository.class);
+
+    @BeforeEach
+    public void beforeEach() {
+        reset(repository);
+    }
+
+    @Test
+    void givenNoSubscriber_whenUsingJust_thenDataIsFetched() {
+        Mockito.when(repository.findById(123L))
+            .thenReturn("John Doe");
+
+        Single<String> employee = Single.just(repository.findById(123L));
+
+        Mockito.verify(repository, times(1))
+            .findById(123L);
+    }
+
+    @Test
+    void givenASubscriber_whenUsingJust_thenReturnTheCorrectValue() {
+        TestSubscriber<String> testSubscriber = new TestSubscriber<>();
+        Mockito.when(repository.findById(123L))
+            .thenReturn("John Doe");
+
+        Single<String> employee = Single.just(repository.findById(123L));
+        employee.subscribe(testSubscriber);
+
+        testSubscriber.assertValue("John Doe");
+        testSubscriber.assertCompleted();
+    }
+
+    @Test
+    void givenNoSubscriber_whenUsingFromCallable_thenNoDataIsFetched() {
+        Single<String> employee = Single.fromCallable(() -> repository.findById(123L));
+
+        Mockito.verify(repository, never())
+            .findById(123L);
+    }
+
+    @Test
+    void givenASubscriber_whenUsingFromCallable_thenReturnCorrectValue() {
+        TestSubscriber<String> testSubscriber = new TestSubscriber<>();
+        Mockito.when(repository.findById(123L))
+            .thenReturn("John Doe");
+
+        Single<String> employee = Single.fromCallable(() -> repository.findById(123L));
+        employee.subscribe(testSubscriber);
+
+        Mockito.verify(repository, times(1))
+            .findById(123L);
+        testSubscriber.assertCompleted();
+        testSubscriber.assertValue("John Doe");
+    }
+}