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public FilteredList (List
@Override
public Iterator
}
CS 61B Iterators and Iterables Spring 2021 Exam Prep Discussion 5: February 16, 2021
1 Filtered List
We want to make a FilteredList class that selects only certain elements of a List during iteration. To do so, we¡¯re going to use the Predicate interface defined below. Note that it has a method, test that takes in an argument and returns True if we want to keep this argument or False otherwise.
public interface Predicate
}
For example, if L is any kind of object that implements List
standard java.util.List), then writing
FilteredList
gives an iterable containing all items, x, in L for which filter.test(x) is True. Here, filter is of type Predicate. Fill in the FilteredList class below.
import java.util.*;
public class FilteredList
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8 9}
}
2 Iterator of Iterators
Implement an IteratorOfIterators which will accept as an argument a List of Iterator objects containing Integers. The first call to next() should return the first item from the first iterator in the list. The second call to next() should return the first item from the second iterator in the list. If the list contained n iterators, the n+1th time that we call next(), we would return the second item of the first iterator in the list.
Note that if an iterator is empty in this process, we continue to the next iterator. Then, once all the iterators are empty, hasNext should return false. For example, if we had 3 Iterators A, B, and C such that A contained the values [1, 3, 4, 5], B was empty, and C contained the values [2], calls to next() for our IteratorOfIterators would return [1, 2, 3, 4, 5].
1 import java.util.*;
2 public class IteratorOfIterators ______________________________ {
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5 public IteratorOfIterators(List
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13 } 14
15 @Override
16 public boolean hasNext() {
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21 } 22
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25 @Override
26 public Integer next() {
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31 }
32 }
Iterators and Iterables 3
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public class DMSComparator implements __________________________ {
@Override
public int compare(Animal o1, Animal o2) {
int first = o1.speak(new Animal()); int second = o2.speak(new Animal()); int third = o1.speak(new Dog());
int fourth = o2.speak(new Dog());
if (________________________________________________________) { return 0;
} else if (_________________________________________________) { return 1;
} else { return -1;
} }
}
4 Iterators and Iterables
3 DMS Comparator
Implement the Comparator DMSComparator, which compares Animal instances. An Animal instance is greater than another Animal instance if its dynamic type is more specific. See the examples to the right below.
In the second and third blanks in the compare method, you may only use the integer variables predefined (first, second, etc), relational/equality oper- ators (==, >, etc), boolean operators (&& and ||), integers, and parentheses.
As a challenge, use equality operators (== or !=) and no relational operators (>, <=, etc). There may be more than one solution.
class Animal {
int speak(Dog a) { return 1; } int speak(Animal a) { return 2; }
}
class Dog extends Animal {
int speak(Animal a) { return 3; } }
class Poodle extends Dog {
int speak(Dog a) { return 4; }
}
Examples:
Animal animal = new Animal(); Animal dog = new Dog(); Animal poodle = new Poodle();
compare(animal, dog) // negative number
compare(dog, dog) // zero
compare(poodle, dog) // positive number