PowerMock calls real method - unit-testing

Question

I am trying to spy private method with PowerMock but on the line when I define the what should be returned when the private method is called, it calls the method and I am getting and Null Pointer Exception. What PowerMock is calling real method on this line ?
myService= PowerMockito.spy(new MyService(myParam));
.....
PowerMockito.when(myService, "getCLientBy", anyString(), anyString(), anyString()).thenRetur`n(Client.of(setName, new HashSet<>())); // here it calls real method

Answer 1

Ensure that you prepare your class to be used in spy by adding #PrepareForTest(MyService.class)
#RunWith(PowerMockRunner.class)
// We prepare MyService for test because it's final
// or we need to mock private or static methods
#PrepareForTest(MyService.class)
public class YourTestCase {
//...
#Test
public void spyingWithPowerMock() {
MyService classUnderTest = PowerMockito.spy(new MyService(myParam));
//.....
// use PowerMockito to set up your expectation
PowerMockito.doReturn(Client.of(setName, new HashSet<>()))
.when(classUnderTest, "getClientBy", anyString(), anyString(), anyString());
//...
Also make sure provide the correct method name to be invoked.

Answer 2

#user1474111 and #Nkosi
I've built a small simulation of your example.
Maybe you also need to add the Client class in the PrepareForTest annotation.
#RunWith(PowerMockRunner.class)
#PrepareForTest({ MyService.class, Client.class })
public class Example1Test {
#Test
public void testPowerMockito() throws Exception {
MyService myService = PowerMockito.spy(new MyService("myParam"));
PowerMockito.when(myService, "getClientBy", ArgumentMatchers.anyString(), ArgumentMatchers.anyString(),
ArgumentMatchers.anyString()).thenReturn(Client.of("setName", new HashSet<String>()));
myService.run();
Assert.assertEquals("setName", myService.getClient().getName());
}
}
public class MyService {
private Client client;
public MyService(String param) { }
private Client getClientBy(String a, String b, String c) {
return new Client(a + b + c);
}
public Client getClient() {
return this.client;
}
public void setClient(Client client) {
this.client = client;
}
public void run() {
setClient(getClientBy("A", "B", "C"));
}
}
public class Client {
private final String name;
public Client(String name) {
this.name = name;
}
public static Client of(String name, HashSet<String> hashSet) {
return new Client(name);
}
public String getName() {
return name;
}
}