Why do we need getters?

Question

I have read the stackoverflow page which discusses "Why use getters and setters?", I have been convinced by some of the reasons using a setter, for example: later validation, data encapsulation, etc. But what is the reason of using getters anyway? I don't see any harm of getting a value of a private field, or reasons to validation before you get the a field's value. Is it OK to never use a getter and always get a field's value using dot notation?

Answer 1

I want to focus on practicalities, since I think you're at a point where you haven't seen the conceptual benefits line up just yet with the actual practice.

The obvious conceptual benefit is that setters and getters can be changed without impacting the outside world using those functions. Another Java-specific benefit is that all methods not marked as final are capable of being overriden, so you get the ability for subclasses to override the behavior as a bonus.

Overkill?

Yet you're probably at a point where you've heard these conceptual benefits before and it still sounds like overkill for your more daily scenarios. A difficult part of understanding software engineering practices is that they are generally designed to deal with very real world, large-scale codebases being managed by teams of developers. A lot of things are going to seem like overkill initially when you're just working on a small project of your own.

So let's get into some practical, real-world scenarios. I formerly worked in a very large-scale codebase. It a was low-level C codebase with a long legacy and sometimes barely a step above assembly, but many of the lessons I learned there translate to all kinds of languages.

Real-World Grief

In this codebase, we had a lot of bugs, and the majority of them related to state management and side effects. For example, we had cases where two fields of a structure were supposed to stay in sync with each other. The range of valid values for one field depended on the value of the other. Yet we ran into bugs where those two fields were out of sync. Unfortunately since they were just public variables with a very global scope ('global' should really be considered a degree with respect to the amount of code that can access a variable rather than an absolute), there were potentially tens of thousands of lines of code that could be the culprit.

As a simpler example, we had cases where the value of a field was never supposed to be negative, yet in our debugging sessions, we found negative values. Let's call this value that's never supposed to be negative, x. When we discovered the bugs resulting from x being negative, it was long after x was touched by anything. So we spent hours placing memory breakpoints and trying to find needles in a haystack by looking at all possible places that modified x in some way. Eventually we found and fixed the bug, but it was a bug that should have been discovered years earlier and should have been much less painful to fix.

Such would have been the case if large portions of the codebase weren't just directly accessing x and used functions like set_x instead. If that were the case, we could have done something as simple as this:

void set_x(int new_value)
{
    assert(new_value >= 0);
    x = new_value;
}

... and we would have discovered the culprit immediately and fixed it in a matter of minutes. Instead, we discovered it years after the bug was introduced and it took us meticulous hours of headaches to trace it down and fix.

Such is the price we can pay for ignoring engineering wisdom, and after dealing with the 10,000th issue which could have been avoided with a practice as simple as depending on functions rather than raw data throughout a codebase, if your hairs haven't all turned grey at that point, you're still generally not going to have a cheerful disposition.

The biggest value of getters and setters comes from the setters. It's the state manipulation that you generally want to control the most to prevent/detect bugs. The getter becomes a necessity simply as a result of requiring a setter to modify the data. Yet getters can also be useful sometimes when you want to exchange a raw state for a computation non-intrusively (by just changing one function's implementation), e.g.

Interface Stability

One of the most difficult things to appreciate earlier in your career is going to be interface stability (to prevent public interfaces from changing constantly). This is something that can only be appreciated with projects of scale and possibly compatibility issues with third parties.

When you're working on a small project on your own, you might be able to change the public definition of a class to your heart's content and rewrite all the code using it to update it with your changes. It won't seem like a big deal to constantly rewrite the code this way, as the amount of code using an interface might be quite small (ex: a few hundred lines of code using your class, and all code that you personally wrote).

When you work on a large-scale project and look down at millions of lines of code, changing the public definition of a widely-used class might mean that 100,000 lines of code need to be rewritten using that class in response. And a lot of that code won't even be your own code, so you have to intrusively analyze and fix other people's code and possibly collaborate with them closely to coordinate these changes. Some of these people may not even be on your team: they may be third parties writing plugins for your software or former developers who have moved on to other projects.

You really don't want to run into this scenario repeatedly, so designing public interfaces well enough to keep them stable (unchanging) becomes a key skill for your most central interfaces. If those interfaces are leaking implementation details like raw data, then the temptation to change them over and over is going to be a scenario you can face all the time.

So you generally want to design interfaces to focus on "what" they should do, not "how" they should do it, since the "how" might change a lot more often than the "what". For example, perhaps a function should append a new element to a list. However, you may want to swap out the list data structure it's using for another, or introduce a lock to make that function thread safe ("how" concerns). If these "how" concerns are not leaked to the public interface, then you can change the implementation of that class (how it's doing things) locally without affecting any of the existing code that is requesting it to do things.

You also don't want classes to do too much and become monolithic, since then your class variables will become "more global" (become visible to a lot more code even within the class's implementation) and it'll also be hard to settle on a stable design when it's already doing so much (the more classes do, the more they'll want to do).

Getters and setters aren't the best examples of such interface design, but they do avoid exposing those "how" details at least slightly better than a publicly exposed variable, and thus have fewer reasons to change (break).

Practical Avoidance of Getters/Setters

Is it OK to never use a getter and always get a field's value using dot notation?

This could sometimes be okay. For example, if you are implementing a tree structure and it utilizes a node class as a private implementation detail that clients never use directly, then trying too hard to focus on the engineering of this node class is probably going to start becoming counter-productive.

There your node class isn't a public interface. It's a private implementation detail for your tree. You can guarantee that it won't be used by anything more than the tree implementation, so there it might be overkill to apply these kinds of practices.

Where you don't want to ignore such practices is in the real public interface, the tree interface. You don't want to allow the tree to be misused and left in an invalid state, and you don't want an unstable interface which you're constantly tempted to change long after the tree is being widely used.

Another case where it might be okay is if you're just working on a scrap project/experiment as a kind of learning exercise, and you know for sure that the code you write is rather disposable and is never going to be used in any project of scale or grow into anything of scale.

Nevertheless, if you're very new to these concepts, I think it's a useful exercise even for your small scale projects to err on the side of using getters/setters. It's similar to how Mr. Miyagi got Daniel-San to paint the fence, wash the car, etc. Daniel-San finds it all pointless with his arms exhausted on top of that. Then Mr. Miyagi goes "hyah hyah hyoh hyah" throwing big punches and kicks, and using that indirect training, Daniel-San blocks all of them without realizing how he's even doing it.

Answer 2

Fields are not polymorphic.

The alternative to a getter would be a public field; however, fields are not polymorphic.

This means that you cannot extend the class and "override" the field without introducing weird behaviour. Basically, the value you get will depend on how you refer to the field.

Furthermore, you can't include the field in an interface and you can't perform validation (that applies more to a setter).

Answer 3

If a given field in a Java class be visible for reading (on the RHS of an expression), then it must also be possible to assign that field (on the LHS of an expression). For example:

class A {
    int someValue;
}

A a = new A();
int value = a.someValue; // if you can do this (potentially harmless)
a.someValue = 10;        // then you can also do this (bad)

Besides the above problem, a major reason for having a getter in a class is to shield the consumer of that class from implementation details. A getter does not necessarily have to simply return a value. It could return a value distilled from a Collection or something else entirely. By using a getter (and a setter), we free the consumer of the class from having to worry about the implementation changing over time.

Answer 4

In java you can't tell the compiler to allow read-only access to a public field from outside.

So exposing public fields opens the door to uncontroled modifications.