A better way to write testable code is not to create cooperating classes by new operator in class code, but to pass cooperating classes as constructor arguments.
class TestedClass{
private final Helper helper;
public TestedClass(Helper helper){
this.helper = helper;
}
public aMethodUsesHelper(){
//hello is weird name for method that returns int but it is link to your code
int aVar =this.helper.hello();
// ...
}
// ...
Then in test class:
Helper helper = mock(Helper.class);
when(helper.hello()).thenReturn(10);
TestedClass tested = new Tested(helper);
// ...