A Problem Solved: Representing Coins

Problem statement

Imagine that we need to represent coins in various programs. A coin has two sides, heads and tails. Additionally, a coin has a monetary value and shows the year in which it was minted. Let’s decide that these characteristics are sufficient for our purposes and create a class of coins.

Designing the class

We’ll name the class Coin and give it four data fields, as follows:

• sideUp—a designation for heads or tails
• name—the coin’s name as a string
• value—a representation of the coin’s value
• year—a positive integer denoting the year the coin was minted

Let’s also give the class the following methods:

• Accessor methods that return the coin’s visible side—heads or tails—its name, its value in cents, and its mint year
• Boolean-valued methods that test whether the coin’s visible side is heads or tails
• A method that simulates a coin toss by assigning a designation for heads or tails to sideUp at random
• The method toString

After naming these methods and refining their specifications, we can write the following comments and method headers:

/** Returns the string "HEADS" if the coin is heads side up;
    otherwise, returns "TAILS". */
public String getSideUp()

/** Returns the coin's name as a string. */
public String getName()

/** Returns the value of the coin in cents. */
public int getValue()

/** Returns the coin’s mint year as an integer. */
public int getYear()

/** Returns true if the coin is heads side up. */
public boolean isHeads()

/** Returns true if the coin is tails side up. */
public boolean isTails()

/** Tosses the coin (sets sideUp randomly to one of two values). */
public void toss()

/** Returns the coin as a string in the form coin-name/year/side-up. */
public String toString()

Notice that we have not specified any set methods. Thus, once we create a Coin object, its value and mint date cannot be altered, just like a real coin. The method getSideUp could have returned the value of the third field sideUp, but instead, we chose to have it return a string. We did this so that the data type we eventually choose for sideUp will not be a part of the method’s specification and, in fact, will be hidden from the client.