repositories { jcenter() }
dependencies { testCompile "org.mockito:mockito-core:1.+" }

apply plugin: ‘java‘
apply plugin: ‘idea‘

sourceCompatibility = 1.5
version = ‘1.0‘

repositories {
    mavenCentral()
    jcenter()
}

dependencies {
    testCompile group: ‘junit‘, name: ‘junit‘, version: ‘4.11‘
    testCompile "org.mockito:mockito-core:1.8.5"
}

package com.mengdd.examples.mockito;

public class HelloWorld {
    private MyRobot mRobot;

    public MyRobot getRobot() {
        return mRobot;
    }

    public void setRobot(MyRobot robot) {
        this.mRobot = robot;
    }

    /**
     * This is the method we want to test.
     * When there is an robot, this method return the robot‘s information
     * otherwise, return some sorry text
     */
    public String sayHello() {
        MyRobot robot = getRobot();
        if (null != robot) {
            return robot.getSomeInfo();
        }

        return "No robot here, sorry!";
    }

    /**
     * MyRobot class
     */
    public static class MyRobot {

        /**
         * Get some information from somewhere, the implementation may varies
         */
        public String getSomeInfo() {
            return "Hello World -- From robot";
        }
    }
}

package com.mengdd.examples.mockito;

import org.junit.Test;

import static org.hamcrest.CoreMatchers.is;
import static org.junit.Assert.assertThat;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.when;


public class HelloWorldTest {
    @Test
    public void testSayHelloWhenThereIsRobot() throws Exception {
        // We want to test the sayHello() method in the class HelloWorld.
        // But we don‘t care about how MyRobot get its information.(We can treat it as external interface).

        // Maybe the robot rely on many complicated things.(Although it‘s simple in this case.)
        // And when the robot‘s actions change in the future, we don‘t need to change this test case.

        // We can mock the MyRobot‘s action to make it simple and just test whether sayHello() can do its own work.


        // Mock steps
        HelloWorld.MyRobot robot = mock(HelloWorld.MyRobot.class); // Mock MyRobot class
        String mockInfo = "some mock info";
        when(robot.getSomeInfo()).thenReturn(mockInfo); // Set behavior for mock object

        // real object
        HelloWorld helloWorld = new HelloWorld();//This is the real objec we want to test
        helloWorld.setRobot(robot);//set the mock robot to real object

        // execute the target method we want to test
        String result = helloWorld.sayHello();

        // assert the result is what we want to have
        assertThat(result, is(mockInfo));
    }
}

@Test
public void testSayHelloWhenThereIsNoRobot() throws Exception {
    HelloWorld helloWorld = new HelloWorld();
    helloWorld.setRobot(null);

    String result = helloWorld.sayHello();

    assertThat(result, is("No robot here, sorry!"));
}