Java 17 Recipes

7-5. Safely Enabling Types or Methods to Operate 
on Objects of Various Types
 Problem
Your application uses many different object types, and there are containers within your 
class that are available for holding each of these different types. You are interested in 
ensuring your application remains bug-free, yet you want to dynamically change the 
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type of object a particular container may hold. In other words, you want to define a 
generic container, but can specify its type each time a new instance of the container is 
instantiated.
 Solution
Use generic types to decouple the type from the container. Generics are a way to abstract 
over object types, not explicitly declaring the type of an object or container. All collection 
types are parameterized to allow you to specify, at the time of instantiation, the type of 
elements the collection can hold. The following example code demonstrates how to use 
generics in various scenarios. The comments in the code indicate where the generics are 
utilized.
public class MainClass {
static List
team;
private static void loadTeam() {
System.out.println("Loading team...");
// Use of the diamond operator
team = new ArrayList<>();
Player player1 = new Player("Josh", "Juneau", 5);
Player player2 = new Player("Duke", "Java", 15);
Player player3 = new Player("Jonathan", "Gennick", 1);
Player player4 = new Player("Bob", "Smith", 18);
Player player5 = new Player("Steve", "Adams", 7);
team.add(player1);
team.add(player2);
team.add(player3);
team.add(player4);
team.add(player5);
}
public static void main(String[] args) {
loadTeam();
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// Create a list without specifying a type
List objectList = new ArrayList();
Object obj1 = "none";
objectList.add(obj1);
// Create a List that can be of type that is any superclass of 
Player (or only Player type itself)
List myTeam = objectList;
for (Object p : myTeam) {
System.out.println("Printing the objects...");
System.out.println(p.toString());
}
// Create a Map of String keys and String values
Map strMap = new HashMap<>();
strMap.put("first", "Josh");
strMap.put("last", "Juneau");
System.out.println(strMap.values());
}
}
Note When we generally talk about a collection or a collection type, you can read 
this as those types that make up the Java collections framework. this includes all 
the classes and interfaces from the Collection and Map interfaces. Collection 
types generally refer to types that descend from the Collection interface.