Garbage collection

21 Sep 2014

Guy Steele is a smart man who gave a talk sixteen1 years ago about the design2 of the Java3 programming4 language.56 In Guy Steele’s talkGrowing a Language” he does not tell what garbage collection means, and says that is a task for you to try in your spare time. I have some spare time this day, and so will try to tell what garbage collection is.

A person is the thing that more than one of in a group can be called folks, but only one of cannot be called folks.

Garbage is trash - it is that for which there is no use. What is garbage to some folks may not be to other folks. What is garbage in one place may not be garbage in a place other than that first place. Then the thing which makes garbage is not what a thing is, but if any person has use for it. The finding of what is garbage is the task of finding if any person has use for that thing.

To refer to something is to point at it. I referred7 to the talk of Guy Steele in place of writing out the text of the talk. A reference is a thing which only refers to an other8 thing.

When no one has use of this talk of Guy Steele, and no web page like this one refers to it, it will be garbage. This will not be true for a long time.

In a programming language, things can make use of other things much like a person makes use of things, and like the way this page refers to the talk of Guy Steele. Things may refer to other things, in which case they have use for them.

Collection is the act of putting some things so they are close to each other.

Garbage collection is the act of collecting garbage, and also putting it out of the way to make room for new things. If this task is the job of a programmer9 in a programming language, we say the programming language does not have garbage collection. If the programming language does this and the programmer does not have to, we say it does have garbage collection.

A simple way to find things that are garbage might be to take a thing and check with each other thing to see if it has use of it - to see if it refers to it. If not, that thing is garbage. This is a method of garbage collection.


If a left shoe has use for a right shoe, and that right shoe has use for that left shoe, but no other things have use for one shoe nor the other, we say they are both garbage. But this means that asking every other thing if it refers to a first thing does not work for finding garbage for collection.

If we draw lines that go one way from each thing to all the things it refers to, we end up with a web of such links. If this mess were brought up in the air and shook with weak force such that groups of things with no links to other groups fell down, each such group of things we would call a cycle. Finding each cycle is a job for garbage collection, as only one of these cycles is not garbage.


As well as using primitives10 I hope you all know in this text, I have also used words Guy Steele defined11 in his talk, which I point out when I do so with small notes at the foot of the page. He may assume other primitives I don’t define to define these words; you should read his talk to learn those words. I change some of his words that tell what words mean to tell what other words mean as well, so that words he uses to define words are defined in place, but I have not done this with all the words he uses. When I do this I put a “(“ glyph to the left of these words and a “)” glyph to the right of them. When I define words in the body of the text, they are not words Guy Steele had need to define, or I want to define them differently.12

  1. Sixteen is twice eight

  2. A design is a plan for how to build a thing. To design is to build a thing in one’s mind but not yet in the real world — or, better (more good) yet, to plan how the real thing can be built.

  3. Java is a brand name for a computer programming language

  4. To program is to make up a list of things to do and choices to make, to be done by a computer. Such a list is called a program. A noun can be made from a verb in such a way that it means that which is done as meant by that verb; to make such a noun, we add “ing” to the verb stem. Thus we can speak of “programming.”

  5. A language is a vocabulary (a set of words) and rules for what a string of words might mean to a person or a machine that hears them.

  6. A programming language is a language that we can use to tell a computer a program to do.

  7. We can use a verb in the past tense if we add a “d” or “ed” sound at the end of the verb. In the same way we can form what we call a past participle, which is a form of the verb that says of a noun that what the verb means has been done to it.

  8. The word other means “not the same.” The phrase other than means “not the same as.”

  9. One more rule for making words: if we add the syllable (a bit of sound that a mouth and tongue can say all at one time, more or less, in a smooth way; each word is made up of one or more syllables) “er” to a verb stem, we make a noun that names a person or thing that does what the verb says to do. For example, a buyer is one who buys. A user is one who does use.

  10. A primitive is a word for which we can take it for granted that we all know what it means. As a rule, we can speak of two or more of a thing if we add an “s” or “z” sound to the end of a word that names it.)

  11. To define a word is to tell what it means. When you define a word, you add it to your vocabulary (a set of words) and to the vocabulary of each person who hears you. Then you can use the word.

  12. I took Steele’s challenge to be explaining garbage collection given all the words he had defined so far at the point in his talk when he makes the challenge. Therefore these footnotes are only for your convenience and amusement if you haven’t read the talk, and are all but this one taken verbatim from there.