How to create a unit test for this cae

I am new to java and Unit testing. I am facing a problem in writing a unit test to one of the test methods. Can you please help me with this.

In different .class file

Class   classA{ 
    /*......*/
    classB  method1_ClassA(parameters){
        /*.....
        .....
        .....
        .....
        .....*/
        String  some_Identifier =   method2_ClassA(parameters)

    }
    public  String  method2_ClassA(parameters){
        /*contains call to the database*/
    }
}

In different .class file

Class classB{
    /*.....*/
}

In different .class file

Class   classC{
    /*.......*/
    public  classB  method1_ClassC(parameters){
            classA  obj_ClassA  =   new classA();
            return  obj_ClassA.method1_classA(parameters);
    }
    /*.......*/

}

I am trying to do Unit Testing to the classC. One of the methods in ClassC calls a method of class A. I tried to mock this method, which is method1_classA. But this method has a call to another method which is present in the same class. Can somebody please help me on how to create a unit test for classC.