Java Master - S11.GenericsClass

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Notes and Exersice for Generics Class of Completed Java Master Class.

Notes

Help to find out error at compile time.

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ArrayList myList = new ArrayList();
myList.add(1);
myList.add("Hi");

Here, different types of data will be added into myList. Error will occur at run time.

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ArrayList<Integer> myList = new ArrayList<>();
myList.add(1);
myList.add("Hi");	// will show error

So error can be handled at compile time.

Define type for class to avoid adding other type of data

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Team<FootballPlayer> myTeam = new Team<>();
myTeam.add(new BasketballPlayer("mike")) // X Error

Make sure T is correct type

  • Team will allow Team
  • But T here has to be Player
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    class Team<T extends Player> {}
  • so that Team shows error at compile time
  • at the same time, no need to cast player inside class

Make sure parameter is certain type

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class Team<T extends Player> {
	void playWith(Team<T> team){..};
}
  • T in Team must be one of Player class
  • when passing parameter, team will follow the same team type.

Demo: create own generics class

Player

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public class Player {

	private String name;

	public Player(String name) {
		super();
		this.name = name;
	}

	public String getName() {
		return name;
	}
	
}

FootballPlayer

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public class FootballPlayer extends Player{

	public FootballPlayer(String name) {
		super(name);
	}
}

BasketballPlayer

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public class BasketballPlayer extends Player {

	public BasketballPlayer(String name) {
		super(name);
	}
}

Team

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import java.util.List;

public class Team<T extends Player> {

	private String name;
	private List<T> members;
	
	public Team(String name) {
		super();
		this.name = name;
	}

	public String getName() {
		return name;
	}

	void addPlayer(T player) {
		members.add(player);
	}
}

Main

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public class GenericsMain {

	public static void main(String[] args) {

		BasketballPlayer michael = new BasketballPlayer("Michael");
		FootballPlayer beckham = new FootballPlayer("BeckHam");
		
		Team team1 = new Team("1st team"); // warning
		Team<String> team2 = new Team<>("2nd team"); // error 
		
		Team<Player> team3 = new Team<>("3rd team");
		team3.addPlayer(beckham);
		team3.addPlayer(michael);
		// able to add both types of player
		
		Team<FootballPlayer> team4 = new Team<>("4th team");
		team4.addPlayer(beckham);
		team4.addPlayer(michael);	// error
		// only accept football player
	}
}