Java 17 Recipes

7-8. Iterating Collections
 Problem
Your application contains Collection types, and you want to iterate the elements 
within them.
 Solution
Generate a stream on any type that extends or implements java.util.Collection, 
and then perform the desired task(s) on each element of the collection. In the following 
code, an ArrayList loaded with Stock objects demonstrates the concept of streams.
public class StreamExample {
static List myStocks = new ArrayList();
private static void createStocks(){
myStocks.add(new Stock("ORCL", "Oracle", 500.0));
myStocks.add(new Stock("AAPL", "Apple", 200.0));
myStocks.add(new Stock("GOOG", "Google", 100.0));
myStocks.add(new Stock("IBM", "IBM", 50.0));
myStocks.add(new Stock("MCD", "McDonalds", 300.0));
}
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public static void main(String[] args){
createStocks();
// Iterate over each element and print the stock names
myStocks.stream()
.forEach(s->System.out.println(s.getName()));
boolean allGt = myStocks.stream()
.allMatch(s->s.getShares() > 100.0);
System.out.println("All Stocks Greater Than 100.0 Shares? " + allGt);
// Print out all stocks that have more than 100 shares
System.out.println("== We have more than 100 shares of the 
following:");
myStocks.stream()
.filter(s -> s.getShares() > 100.0)
.forEach(s->System.out.println(s.getName()));
System.out.println("== The following stocks are sorted by 
shares:");
Comparator byShares = Comparator.
comparing(Stock::getShares);
Stream sortedByShares = myStocks.stream()
.sorted(byShares);
sortedByShares.forEach(s -> System.out.println("Stock: " + 
s.getName() + " - Shares: " + s.getShares()));
// May or may not return a value
Optional maybe = myStocks.stream()
.findFirst();
System.out.println("First Stock: " + maybe.get().getName());
List newStocks = new ArrayList();
Optional maybeNot = newStocks.stream()
.findFirst();
Consumer myConsumer = (s) ->
{
System.out.println("First Stock (Optional): " + s.getName());
};
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maybeNot.ifPresent(myConsumer);
if(maybeNot.isPresent()){
System.out.println(maybeNot.get().getName());
}
newStocks.add(new Stock("MCD", "McDonalds", 300.0));
Optional maybeNow = newStocks.stream()
.findFirst();
maybeNow.ifPresent(myConsumer);
}
}
The output is:
Oracle
Apple
Google
IBM
McDonalds
All Stocks Greater Than 100.0 Shares? false
== We have more than 100 shares of the following:
Oracle
Apple
McDonalds
== The following stocks are sorted by shares:
Stock: IBM - Shares: 50.0
Stock: Google - Shares: 100.0
Stock: Apple - Shares: 200.0
Stock: McDonalds - Shares: 300.0
Stock: Oracle - Shares: 500.0
First Stock: Oracle
First Stock (Optional): McDonalds
The results of executing this code demonstrate the concept of using streams. External 
iteration (for loops) is no longer a requirement for iterating over a data collection.
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