Can a Test Class have no member variables?

Question

I am testing some HTTP callouts in Apex and instead of passing a newly created MyHttpCalloutMockImpl object in the test method every time, thought of declaring the same at class level and reusing it in each test method that tests a callout.

But this fails to compile stating "Variable does not exist". What is the reason for this? Here goes my code:

@isTest(seeAllData = false)

public class MyControllerTest {

HttpCalloutMockImpl mockImpl = new HttpCalloutMockImpl();

@testSetup static void setup() {
    //insert some setup data

}

public static testMethod void testMyMethod1() {
    Test.startTest();
    MyController ctrlObj = new MyController();
    Test.setMock(HttpCalloutMock.class, mockImpl);
    ctrlObj.sync1();
    Test.stopTest();
}

public static testMethod void testMyMethod2() {
    Test.startTest();
    MyController ctrlObj = new MyController();
    Test.setMock(HttpCalloutMock.class, mockImpl);
    ctrlObj.sync2();
    Test.stopTest();
}
}

Answer 1

You cannot see instance variables in static methods (though the converse is not true). As stated in the comments, add the static keyword:

static MyMock mock = new MyMock();

Answer 2

Each test method in a test class is executed in a separate transaction. Although you'd be able to "see" a static variable in each test method in the sense that it would be defined, it'd always be null (unless you write a static initializer, which case you'd just get a new instance) because static variables are reset at transaction boundaries. You can't store and reuse data from one test method to another.