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#590 explanation for Trampoline

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Ilkka Seppälä 2020-07-25 15:48:21 +03:00
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---
## Intent
Trampoline pattern is used for implementing algorithms recursively in Java without blowing the stack
and to interleave the execution of functions without hard coding them together
It is possible by representing a computation in one of 2 states : done | more
(completed with result, or a reference to the reminder of the computation,
something like the way a java.util.Supplier does).
Trampoline pattern is used for implementing algorithms recursively in Java without blowing the stack and to interleave
the execution of functions without hard coding them together.
## Explanation
Trampoline pattern allows to define recursive algorithms by iterative loop.
Recursion is a frequently adopted technique for solving algorithmic problems in a divide and conquer
style. For example calculating fibonacci accumulating sum and factorials. In these kinds of problems recursion is
more straightforward than their loop counterpart. Furthermore recursion may need less code and looks more concise.
There is a saying that every recursion problem can be solved using a loop with the cost of writing code that is more
difficult to understand.
However recursion type solutions have one big caveat. For each recursive call it typically needs an intermediate value
stored and there is a limited amount of stack memory available. Running out of stack memory creates a stack overflow
error and halts the program execution.
Trampoline pattern is a trick that allows us define recursive algorithms in Java without blowing the stack.
Real world example
> A recursive Fibonacci calculation without the stack overflow problem using the Trampoline pattern.
In plain words
> Trampoline pattern allows recursion without running out of stack memory.
Wikipedia says
> In Java, trampoline refers to using reflection to avoid using inner classes, for example in event listeners. The time overhead of a reflection call is traded for the space overhead of an inner class. Trampolines in Java usually involve the creation of a GenericListener to pass events to an outer class.
**Programmatic Example**
Here's the `Trampoline` implementation in Java.
When `get` is called on the returned Trampoline, internally it will iterate calling `jump` on the returned `Trampoline`
as long as the concrete instance returned is `Trampoline`, stopping once the returned instance is `done`.
```java
public interface Trampoline<T> {
T get();
default Trampoline<T> jump() {
return this;
}
default T result() {
return get();
}
default boolean complete() {
return true;
}
static <T> Trampoline<T> done(final T result) {
return () -> result;
}
static <T> Trampoline<T> more(final Trampoline<Trampoline<T>> trampoline) {
return new Trampoline<T>() {
@Override
public boolean complete() {
return false;
}
@Override
public Trampoline<T> jump() {
return trampoline.result();
}
@Override
public T get() {
return trampoline(this);
}
T trampoline(final Trampoline<T> trampoline) {
return Stream.iterate(trampoline, Trampoline::jump)
.filter(Trampoline::complete)
.findFirst()
.map(Trampoline::result)
.orElseThrow();
}
};
}
}
```
Using the `Trampoline` to get Fibonacci values.
```java
public static Trampoline<Integer> loop(int times, int prod) {
if (times == 0) {
return Trampoline.done(prod);
} else {
return Trampoline.more(() -> loop(times - 1, prod * times));
}
}
log.info("start pattern");
var result = loop(10, 1).result();
log.info("result {}", result);
// start pattern
// result 3628800
```
## Class diagram
![alt text](./etc/trampoline.urm.png "Trampoline pattern class diagram")
@ -27,18 +124,16 @@ Use the Trampoline pattern when
* For implementing tail recursive function. This pattern allows to switch on a stackless operation.
* For interleaving the execution of two or more functions on the same thread.
## Known uses(real world examples)
## Known uses
* Trampoline refers to using reflection to avoid using inner classes, for example in event listeners.
The time overhead of a reflection call is traded for the space overhead of an inner class.
Trampolines in Java usually involve the creation of a GenericListener to pass events to an outer class.
## Tutorials
* [Trampolining: a practical guide for awesome Java Developers](https://medium.com/@johnmcclean/trampolining-a-practical-guide-for-awesome-java-developers-4b657d9c3076)
* [Trampoline in java ](http://mindprod.com/jgloss/trampoline.html)
* [cyclops-react](https://github.com/aol/cyclops-react)
## Credits
* [library 'cyclops-react' uses the pattern](https://github.com/aol/cyclops-react)
* [Trampolining: a practical guide for awesome Java Developers](https://medium.com/@johnmcclean/trampolining-a-practical-guide-for-awesome-java-developers-4b657d9c3076)
* [Trampoline in java ](http://mindprod.com/jgloss/trampoline.html)
* [Laziness, trampolines, monoids and other functional amenities: this is not your father's Java](https://www.slideshare.net/mariofusco/lazine)
* [Trampoline implementation](https://github.com/bodar/totallylazy/blob/master/src/com/googlecode/totallylazy/Trampoline.java)
* [What is a trampoline function?](https://stackoverflow.com/questions/189725/what-is-a-trampoline-function)
* [Modern Java in Action: Lambdas, streams, functional and reactive programming](https://www.amazon.com/gp/product/1617293563/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=1617293563&linkId=ad53ae6f9f7c0982e759c3527bd2595c)
* [Java 8 in Action: Lambdas, Streams, and functional-style programming](https://www.amazon.com/gp/product/1617291994/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=1617291994&linkId=e3e5665b0732c59c9d884896ffe54f4f)