Java 17 Recipes

7-7. Making Your Objects Iterable
 Problem
You have created a custom collection–based class that wraps (instead of extends) the 
underlying collection type. Without exposing the internal implementation details of 
your class, you want objects of your class to become iterable, especially with the use of a 
foreach statement.
 Solution
Have your class extend the Iterable interface, where T is the element type of the 
collection to be iterated. Implement the iterator() method to return the Iterator 
object from this collection. The example for this recipe is the StockPortfolio class. 
Internally, StockPortfolio manages a collection of Stock objects. We want users of our 
class to treat StockPortfolio objects as iterable objects using a foreach statement. The 
StockPortfolio class follows.
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public class StockPortfolio implements Iterable {
Map portfolio = new HashMap<>();
public void add(Stock stock) {
portfolio.put(stock.getSymbol(), stock);
}
public void add(List stocks) {
for (Stock s : stocks) {
portfolio.put(s.getSymbol(), s);
}
}
@Override
public Iterator iterator() {
return portfolio.values().iterator();
}
public static void main(String[] args) {
StockPortfolio myPortfolio = new StockPortfolio();
myPortfolio.add(new Stock("ORCL", "Oracle", 500.0));
myPortfolio.add(new Stock("AAPL", "Apple", 200.0));
myPortfolio.add(new Stock("GOOG", "Google", 100.0));
myPortfolio.add(new Stock("IBM", "IBM", 50.0));
myPortfolio.add(new Stock("MCD", "McDonalds", 300.0));
// foreach loop (uses Iterator returned from iterator() method)
System.out.println("====Print using legacy for-each loop====");
for (Stock stock : myPortfolio) {
System.out.println(stock);
}
System.out.println("====Print using Java 8 foreach 
implementation====");
myPortfolio.forEach(s->System.out.println(s));
}
}
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The following code is that of the Stock class.
public class Stock {
private String symbol;
private String name;
private double shares;
public Stock(String symbol, String name, double shares) {
this.symbol = symbol;
this.name = name;
this.shares = shares;
}
public String getSymbol() {
return symbol;
}
public String getName() {
return name;
}
public double getShares() {
return shares;
}
public String toString() {
return shares + " shares of " + symbol + " (" + name + ")";
}
}
The main() method creates a StockPortfolio and then calls the add() method 
to add several stocks to the portfolio. Both variations of the foreach loop (legacy and 
forEach implementation) then loop over and print all the stocks in the portfolio. 
Running the StockPortfolio class results in the following output.
100.0 shares of GOOG (Google)
200.0 shares of AAPL (Apple)
50.0 shares of IBM (IBM)
500.0 shares of ORCL (Oracle)
300.0 shares of MCD (McDonalds)
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Note the order of the lines in the output may differ when you run the 
StockPortfolio class in your environment because the underlying 
implementation uses a hashMap . a hashMap does not guarantee 
the order of the elements stored in the map, and this extends to its iterators. if you 
wanted the iterator to return elements sorted by the stock symbol, you could use 
one of the sorted collections, such as treeMap or treeSet , instead of 
hashMap.