Java 17 Recipes

7-9. Iterating Over a Map
 Problem
You use one of the Map classes, such as HashMap or TreeMap, and you need to iterate the 
keys, values, or both. You also want to remove elements from the map while iterating 
over it.
 Solution
There are multiple ways to iterate a Map class. The method you choose should depend on 
which portions of the map you need to access and whether you need to remove elements 
from the map while iterating. The StockPortfolio1 class is a continuation of the 
StockPorfolio class shown in the previous recipe. It adds three methods—summary(), 
alertList(), and remove(List)—that demonstrate alternative methods for 
iterating over the portfolio map: Map portfolio = new HashMap<>();.
The following is the class.
import org.java17recipes.chapter07.recipe07_07.Stock;
import org.java17recipes.chapter07.recipe07_06.StockScreener;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
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public class StockPortfolio1 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);
}
}
public void remove(String stock) {
portfolio.remove(stock);
}
public void remove(List sellList) {
Iterator keyIter = portfolio.keySet().iterator();
while (keyIter.hasNext()) {
if (sellList.contains(keyIter.next())) {
keyIter.remove();
}
}
}
/**
* Utilize for-loop to traverse Map keys and apply filter to obtain desired
* stocks
* @return
*/
public List alertListLegacy() {
System.out.println("==Legacy technique for filtering and 
collecting==");
List alertList = new ArrayList<>();
for (Stock stock : portfolio.values()) {
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if (!StockScreener.screen(stock.getSymbol(), StockScreener.
Screen.PE, 20)) {
alertList.add(stock);
}
}
return alertList;
}
/**
* Utilize stream and filters to obtain desired stocks
* @return
*/
public List alertList(){
return
portfolio.values().stream()
.filter(s->!StockScreener.screen(s.getSymbol(), 
StockScreener.Screen.PE, 20))
.collect(Collectors.toList());
}
public void summary() {
System.out.println("==Legacy technique for traversing
Map.Entry==");
for (Map.Entry entry : portfolio.entrySet()) {
System.out.println("Stock = " + entry.getKey() + ", Shares = " 
+ entry.getValue().getShares());
}
System.out.println("==Utilization of new foreach and lambda 
combination==");
portfolio.forEach((k,v)->System.out.println("Stock = " + k + ", 
Shares = " + v.getShares()));
}
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@Override
public Iterator iterator() {
return portfolio.values().iterator();
}
public static void main(String[] args) {
StockPortfolio1 myPortfolio = new StockPortfolio1();
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)
for (Stock stock : myPortfolio) {
System.out.println(stock);
}
myPortfolio.forEach((stock)->System.out.println(stock));
List sellList = new ArrayList<>();
sellList.add("IBM");
sellList.add("GOOG");
myPortfolio.remove(sellList);
System.out.println("Portfolio Summary:");
myPortfolio.summary();
System.out.println("Alerts:");
for (Stock stock : myPortfolio.alertList()) {
System.out.println("Alert: " + stock.getSymbol());
}
}
}
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The output is:
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)
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)
Portfolio Summary:
==Legacy technique for traversing Map.Entry==
Stock = AAPL, Shares = 200.0
Stock = ORCL, Shares = 500.0
Stock = MCD, Shares = 300.0
==Utilization of new foreach and lambda combination==
Stock = AAPL, Shares = 200.0
Stock = ORCL, Shares = 500.0
Stock = MCD, Shares = 300.0
Alerts:
AAPL: PE = 6.152926256441124
ORCL: PE = 13.539727307058891
MCD: PE = 20.073565345827422
Alert: MCD