Lab: Java Generics

Summary

We explore some basic use of generic values in Java.

Prerequisites

Classes and interfaces.

Repository

https://github.com/Grinnell-CSC207/lab-generics-2019

Preparation

a. Fork and clone the repository.

b. Import the repository into Eclipse.

Exercises

Exercise 1: Simple expandable arrays

The reading on generics shows how we build a generic “expandable array” class. You’ll find that generic class in the repository for this lab.

a. Read through SEAExpt.java and predict what the output will be.

b. Compile and run SEAExpt.java to see what the output is.

c. SimpleExpandableArray is supposed to expand the array when you set a value larger than the size. Do you expect it to do so for this example?

d. Within the portion of the set method of SimpleExandableArray that expands the array, add a call to System.err.println so that you can tell when the array expands.

e. Create an expandable array of strings, assign some values to it, and print them out. Here’s a start.

  ExpandableArray<String> strings = new SimpleExpandableArray<String>();
  ...
  for (int i = 0; i < 10; i++) {
    pen.println("strings[" + i + "] = " + strings.get(i));
  } // for

f. What do you expect to happen if you assign a string to an element of numbers or a number to an element of strings??

g. Check your answer experimentally.

h. What do you expect to happen if we leave out the type when we construct numbers, as in the following?

  ExpandableArray<BigInteger> numbers = new SimpleExpandableArray();

i. Check your answer experimentally.

j. What do you expect to happen if we leave out the type when we declare strings, as in the following?

  ExpandableArray strings = new SimpleExpandableArray<String>();

k. Check your answer experimentally.

l. What do you expect to happen if we leave out the type on both sides of the declaration, as in the following?

  ExpandableArray strings = new SimpleExpandableArray();

m. Check your answer experimentally.

n. Summarize what you’ve learned in these exercises.

Exercise 2: Searching

The reading on generics shows how we build a generic search method. You’ll find that code in the repository.

a. Read through SearchExpt.java and predict what the output will be.

b. Compile and run SearchExpt.java to see what the output is.

d. What do you expect to happen if you try to search strings with OddInteger.PRED or numbers with SmallString.PRED?

e. Check your answer experimentally.

f. What do you expect to happen if we try to generalize the declaration of strings, as in the following?

    Object[] strings = new Object[] { ... };

g. Check your answer experimentally.

h. Create a new SmallObject predicate whose hold method takes an object as a parameter, converts it to a string, and sees if that string has fewer than five characters. Do you expect that new predicate to work with the updated strings?

i. Check your answer experimentally.

j. Summarize what you’ve learned in these exercises.

Exercise 3: Predicates

a. What do you expect to happen if we restore the original declaration of strings and use the new comparator we just developed?

    String[] strings = new String[] { ... };
    pen.println("A small string: " + SearchUtils.search(strings, SmallObject.PRED));

b. Check your answer experimentally.

c. What do you expect to happen if we use the new SmallObject.PRED predicate to search numbers?

    pen.println("A small integer: " + SearchUtils.search(numbers, SmallObject.PRED));

d. Check your answer experimentally.

e. Summarize what you’ve learned in this exercise.

Exercise 4: Boxes

As you’ve noted, when two variables refer to the same mutable object, we can change the object through one variable and see the effect through the other variable. At times, that effect is undesirable, but at others it is desirable.

Unfortunately, if the two variables refer to the same immutable object, such as a string, we can’t propagate the change to one variable to the other variable. The typical solution to this problem is to have what is typically called a “box”. You can set the value in a box or get the value in a box.

  Box<String> s1 = new Box<String>("Hello");
  Box<String> s2 = s1;
  pen.println(s1.get());        // Prints "Hello"
  pen.println(s2.get());        // Prints "Hello"
  s1.set("Goodbye");
  pen.println(s1.get());        // Prints "Goodbye"
  pen.println(s2.get());        // Prints "Goodbye"
  s2.set("Whatever");
  pen.println(s1.get());        // Prints "Whatever"
  pen.println(s2.get());        // Prints "Whatever"

  Box<Integer> i1 = new Box<Integer>(42);
  Box<Integer> i2 = i1;
  pen.println(i2.get() + 3);          // Prints 45
  i2.set(21);
  pen.println(i1.get() * 2);          // Prints 42

Implement Box using generics. You should include a constructor that sets the initial value; a mutator, set, that changes the value; and an observer, get, that extracts the value.

For those with extra time

Finish the following alternate implementation of ExpandableArray

public class VectorBasedExpandableArray {
  Vector<T> values;
  ...
} // class VectorBasedExpandableArray