8.12

#### Lecture 7: Accumulator methods, continuedðŸ”—

Methods on trees, and accumulator-style methods on lists

##### OverviewðŸ”—

We continue with our ancestry trees example from Lecture 6, working through two subtle examples with accumulators, and then ending with a brief discussion of how accumulators can behave in potentially unexpected ways.

##### 7.1Finding the younger of two IATsðŸ”—

In the section below, we’re going to need to compare two IATs and find the younger of them. This is similar to the bornInOrBefore method, except we’re going to return an IAT instead of a boolean. We want a signature
 // In IAT: // To return the younger of this ancestor tree and the given ancestor tree IAT youngerIAT(IAT other);

Let’s work through the cases:
• If this IAT is Unknown, and the given IAT is Unknown, return Unknown.

• If this IAT is Unknown, and the given IAT is a Person, return the Person.

• If this IAT is a Person, and the given IAT is Unknown, return the Person.

• If this IAT is a Person, and the given IAT is Person, return the younger of the two Persons.

The first two cases are nice: if this IAT is Unknown, we just return the given IAT, regardless of what it is.
 // In Unknown: // To return the younger of this Unknown and the given ancestor tree public IAT youngerIAT(IAT other) { return other; }
But the other two Person cases are harder, since the behavior depends on what the given IAT is.
 // In Person: // To return the younger of this Person and the given ancestor tree public IAT youngerIAT(IAT other) { /* Template * Fields: * this.yob -- int * ... others as before * Methods: * this.youngerIAT(IAT) -- IAT * ... others as before * Methods on fields * ... others as before * Parameters: * other -- IAT * Methods on parameters * other.youngerIAT(IAT) -- IAT */ }
Again, inside this method we can’t determine the age of the given IAT, since it might be Unknown. So we need a helper method that we invoke on the given IAT (like we invoked on this.mom above) where we pass along this Person’s age:
 // In IAT: // To return the younger of this ancestor tree and the given ancestor tree IAT youngerIAT(IAT other); // To return either this ancestor tree (if this ancestor tree is younger // than the given yob) or the given ancestry tree IAT youngerIATHelp(IAT other, int otherYob);
 // In Unknown: public IAT youngerIAT(IAT other) { return other; } // To return either this Unknown (if this Unknown is younger than the // given yob) or the given ancestry tree public IAT youngerIATHelp(IAT other, int otherYob) { return other; }
 // In Person: // To return the younger of this Person and the given ancestor tree public IAT youngerIAT(IAT other) { /* Template * Fields: * this.yob -- int * ... others as before * Methods: * this.youngerIAT(IAT) -- IAT * this.youngerIATHelp(IAT, int) -- IAT * ... others as before * Methods on fields * ... others as before * Parameters * other -- IAT * Methods on parameters * other.youngerIAT(IAT) -- IAT * other.youngerIATHelp(IAT, int) -- IAT */ return other.youngerIATHelp(this, this.yob); } // To return either this Person (if this Person is younger than the // given yob) or the given ancestry tree public IAT youngerIATHelp(IAT other, int otherYob) { /* same template as above */ if (this.yob > otherYob) { return this; } else { return other; } }
This implementation of youngerIAT for Person is quite strange at first sight.

Do Now!

Why should we pass both this and this.yob? And why should invoking a method on other be of any help here?

By invoking a method on other, we allow Java’s dynamic dispatch to determine whether other is a Unknown or a Person precisely the remaining question we had to figure out! We pass this along because it might be the desired result for the method. We pass this.yob because (like with bornInOrBefore) without it, youngerIATHelp would not be able to access the year of birth of the given IAT.

(Look at the purpose statement for other.youngerIATHelp very carefully, substituting “this” and “other” appropriately: If other is an Unknown, then other.youngerIATHelp(this, this.yob) will return this Person. On the other hand, if other is another Person, then other.youngerIATHelp(this. this.yob) will return other if other is younger than this Person, or else this Person.)

##### 7.2Finding the youngest grandparentðŸ”—

To determine the youngest grandparent of a given IAT, let’s try a simpler method first: determine the youngest parent of a given IAT.

 // In IAT: // To compute the youngest parent of this ancestry tree IAT youngestParent();
 // In Unknown: // To compute the youngest parent of this Unknown public IAT youngestParent() { return new Unknown(); }
 // In Person: // To compute the youngest parent of this Person public IAT youngestParent() { /* Template: * Fields: * this.mom -- IAT * this.dad -- IAT * ... others as before * Methods: * this.youngestParent() -- IAT * this.youngerIAT(IAT other) --- IAT * this.youngerIATHelp(IAT other, int otherYob) --- IAT * Methods of fields: * this.mom.youngestParent() -- IAT * this.mom.youngerIAT(IAT other) --- IAT * this.mom.youngerIATHelp(IAT other, int otherYob) --- IAT * this.dad.youngestParent() -- IAT * this.dad.youngerIAT(IAT other) --- IAT * this.dad.youngerIATHelp(IAT other, int otherYob) --- IAT */ }
This is fairly straightforward: we have this.mom and this.dad, and we have youngerIAT to return the younger of them:
 // In Person: // To compute the youngest parent of this Person public IAT youngestParent() { return this.mom.youngerIAT(this.dad); }

Now to implement youngestGrandparent, we can just return the youngerIAT of a Person’s parents’ youngestParents:
 // In IAT: // To compute the youngest grandparent of this ancestry tree IAT youngestGrandparent();
 // In Unknown: // To compute the youngest grandparent of this Unknown public IAT youngestGrandparent() { return new Unknown(); }
 // In Person: // To compute the youngest grandparent of this Person public IAT youngestGrandparent() { /* Template: * Fields: * this.mom -- IAT * this.dad -- IAT * ... others as before * Methods: * this.youngestParent() -- IAT * this.youngestGrandparent() -- IAT * this.youngerIAT(IAT other) --- IAT * this.youngerIATHelp(IAT other, int otherYob) --- IAT * Methods of fields: * this.mom.youngestParent() -- IAT * this.mom.youngestGrandparent() -- IAT * this.mom.youngerIAT(IAT other) --- IAT * this.mom.youngerIATHelp(IAT other, int otherYob) --- IAT * this.dad.youngestParent() -- IAT * this.dad.youngestGrandparent() -- IAT * this.dad.youngerIAT(IAT other) --- IAT * this.dad.youngerIATHelp(IAT other, int otherYob) --- IAT */ return this.mom.youngestParent().youngerIAT(this.dad.youngestParent()); }

How can we generalize this to greatgrandparents and beyond? Notice that the implementations of youngestParent and youngestGrandparent are pretty similar: they both invoke youngerIAT on the youngest appropriate relative on this.mom’s side and the youngest appropriate relative on this.dad’s side.

We can simplify these methods with an accumulator, which keeps track of what number generation we’re examining:
 // this.andrew.youngestAncInGen(1) ==> this.andrew.youngestParent() // this.andrew.youngestAncInGen(2) ==> this.andrew.youngestGrandparent() // this.andrew.youngestAncInGen(3) ==> this.andrew.youngestGreatGrandparent() // this.andrew.youngestAncInGen(0) ==> ?
The youngest IAT 1 generation away from this.andrew is this.andrew’s youngest parent. The youngest IAT 2 generations away from this.andrew is this.andrew’s youngest grandparent. The youngest IAT 0 generations away from this.andrew is...this.andrew!

Our code can then simplify to the following:
 // In IAT: // To compute the youngest ancestor in the given generation of this ancestry tree IAT youngestAncInGen(int gen);
 // In Unknown: // To compute the youngest ancestor in the given generation of this Unknown public IAT youngestAncInGen(int gen) { if (gen == 0) { return this; } else { return new Unknown(); } }
 // In Person: // To compute the youngest ancestor in the given generation of this Person public IAT youngestAncInGen(int gen) { /* Template: * Fields: * this.mom -- IAT * this.dad -- IAT * ... others as before * Methods: * this.youngestAncInGen(int gen) -- IAT * this.youngerIAT(IAT other) --- IAT * this.youngerIATHelp(IAT other, int otherYob) --- IAT * Methods of fields: * this.mom.youngestAncInGen(int gen) -- IAT * this.mom.youngerIAT(IAT other) --- IAT * this.mom.youngerIATHelp(IAT other, int otherYob) --- IAT * this.dad.youngestAncInGen(int gen) -- IAT * this.dad.youngerIAT(IAT other) --- IAT * this.dad.youngerIATHelp(IAT other, int otherYob) --- IAT * Parameters: * gen -- int */ if (gen == 0) { return this; } else { return this.mom.youngestAncInGen(gen - 1).youngerIAT(this.dad.youngestAncInGen(gen - 1)); } }

Do Now!

Complete the definition of youngestGrandparent, now that youngestAncInGen is defined.

The gen parameter is our accumulator, where this time it is “counting down” to decide at what depth to stop the recursion.

##### 7.3Potential hazards of accumulator-style methodsðŸ”—

Exercise

Design a method append for lists of Strings twice: first in direct style, and then again using an accumulator parameter.

Do you notice any differences in the output? (Did you write enough tests?)