Inheritance and Interfaces - Building Java Programs: A Back to Basics Approach - page 566

The compiler produces four errors: one error for each line that tries to access an
inherited private field, and a message about a missing Stock() constructor:
DividendStock.java:5: cannot find symbol
symbol : constructor Stock()
location: class Stock
public DividendStock(String symbol) {
∧
DividendStock.java:6: symbol has private access in Stock
DividendStock.java:7: totalShares has private access in Stock
DividendStock.java:8: totalCost has private access in Stock
The first problem is that even though a DividendStock does contain the symbol ,
totalShares , and totalCost fields by inheritance, it cannot refer to them directly
because they were declared private in the Stock class.
The second problem—the missing Stock() constructor—is a subtle and confus-
ing detail of inheritance. A subclass's constructor must always begin by calling a
constructor from the superclass. The reason is that a DividendStock object partially
consists of a Stock object, and you must initialize the state of that Stock object first
by calling a constructor for it. If you don't do so explicitly, the compiler assumes that
Stock has a parameterless Stock() constructor and tries to initialize the Stock data
by calling this constructor. Since the Stock class doesn't actually have a parameter-
less constructor, the compiler prints a bizarre error message about a missing Stock()
constructor. (It's a shame that the error message isn't more informative.)
The solution to this problem is to explicitly call the Stock constructor that accepts
a String symbol as its parameter. Java uses the keyword super for a subclass to
refer to behavior from its superclass. To call a constructor of a superclass, write the
keyword super , followed by the constructor's parameter values in parentheses:
super(<expression>, <expression>, ..., <expression>);
In the case of the DividendStock constructor, the following code does the trick.
Use the super keyword to call the superclass constructor, passing it the same
symbol value that was passed to the DividendStock constructor. This action will
initialize the symbol , totalShares , and totalCost fields. Then set the initial
dividends to 0.0 :
// constructs a new dividend stock with the given symbol
// and no shares purchased
public DividendStock(String symbol) {
super(symbol); // call Stock constructor
dividends = 0.0;
}