Java generics. h"p:// h"p://

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1 Java generics h"p:// h"p:// CSCI 136: Fundamentals of Computer Science II Keith Vertanen Copyright 2014

2 Overview Abstract Data Types (ADTs) A colleckon of data and operakons on that data Implemented by some underlying data structure Thus far: Class depends on data type we wanted to store Today: StackOfStrings, GemList, StackOfIntegers, StackOfDoubles, StackOfEnemies, StackOfStackOfStrings, Generics: roll your own mulk- purpose colleckons 2

3 Stack that holds Strings public class StackOfStrings private class Node private String item; private Node next; private Node first = null; public boolean isempty() return (first == null); Implemented as a linked list, but clients of StackOfStrings don't need to know the details Clients may care about performance guarantees: ü push() takes constant Kme ü pop() takes constant Kme... public void push(string s) Node node = new Node(); node.item = s; node.next = first; first = node; public String pop() if (first == null) throw new RuntimeException("Stack is empty!"); String result = first.item; first = first.next; return result; 3

4 public class StackOfDoubles private class Node private double item; private Node next; private Node first = null; public boolean isempty() return (first == null); public void push(double s) Node node = new Node(); node.item = s; node.next = first; first = node; Stack that holds doubles... public double pop() if (first == null) throw new RuntimeException("Stack is empty!"); double result = first.item; first = first.next; return result; 4

5 public class StackOfGems private class Node private Gem item; private Node next; private Node first = null; public boolean isempty() return (first == null); Stack that holds Gems... public void push(gem s) Node node = new Node(); node.item = s; node.next = first; first = node; public Gem pop() if (first == null) throw new RuntimeException("Stack is empty!"); Gem result = first.item; first = first.next; return result; Repeated code is evil! (also prevents development of widely applicable container classes like ArrayList) 5

6 Java generics Added in Java 5.0 Java generics Create class that can hold any (reference) type But primikve types not allowed Must use wrappers like Integer and Double We've seen this before: ArrayList Thing inside <>'s is the type used by generic colleckon ArrayList<String> lines = new ArrayList<String>(); while (!StdIn.isEmpty()) lines.add(stdin.readline()); for (int i = lines.size() - 1; i >= 0; i- - ) System.out.println(lines.get(i)); Reverses the lines in a file. 6

7 Reversing numbers in a file: failure This will not work! Java generics like ArrayList only take reference data types, not primikve types like double. import java.util.arraylist; public class ReverseNums public static void main(string[] args) ArrayList<double> nums = new ArrayList<double>(); while (!StdIn.isEmpty()) nums.add(stdin.readdouble()); for (int i = nums.size() - 1; i >= 0; i- - ) System.out.println(nums.get(i)); 7

8 Using primikve wrapper classes: success Double class wraps a primikve double data type into an object so we can put it into the ArrayList. import java.util.arraylist; public class ReverseNums public static void main(string[] args) ArrayList<Double> nums = new ArrayList<Double>(); while (!StdIn.isEmpty()) nums.add(stdin.readdouble()); for (int i = nums.size() - 1; i >= 0; i- - ) System.out.println(nums.get(i)); 8

9 Java primikve wrapper classes Wrapper classes Provide a way to use primikves types with generics like ArrayList Usually primikve type capitalized But: SKck to primikves unless a wrapper is required Less overhead Primi5ve type byte short int long float double char boolean Wrapper class Byte Short Integer Long Float Double Character Boolean 9

10 Wrapper automagic Autoboxing / Unboxing Java 5.0 automagically converts to/ from wrapper as needed Example: storing and then summing doubles in a file int Integer ArrayList<Double> nums = new ArrayList<Double>(); Autoboxing: This works even though StdIn.readDouble() returns a primikve double but the ArrayList requires a Double object. while (!StdIn.isEmpty()) nums.add(stdin.readdouble()); double sum = 0.0; for (int i = 0; i < nums.size(); i++) sum += nums.get(i); System.out.println("sum = " + sum); Unboxing: This works even though the right side needs a primikve double. Java converts from Double to a primikve double. 10

11 Generic Stack class public class Stack<E> private class Node private E item; private Node next; private Node first = null; E is for Element, the type of the elements stored by the stack. Replaces all occurrences of String in StackOfStrings. We now have a class that can store any (reference) data type. public void push(e s) Node node = new Node(); node.item = s; node.next = first; first = node;... h"p:// h"p:// 11

12 Generic Stack class public class Stack<Item> private class Node private Item item; private Node next; E is for Element, but nothing special about E. Any Java idenkfier can be used such as Item. It just needs to be used consistently in the declarakon of the generic class. private Node first = null; public void push(item s) Node node = new Node(); node.item = s; node.next = first; first = node;... h"p:// h"p:// 12

13 Example of using the generic Stack Stack<String> s1 = new Stack<String>(); s1.push("it"); s1.push("was"); s1.push("the"); s1.push("best"); s1.push("of"); s1.push("times"); while (!s1.isempty()) System.out.println("S1 popped: " + s1.pop() + ", stack = " + s1); The type String gets subsktuted for the "wildcard" type in the generic Stack class. The object s1 can hold anything of type String (or any subclass, but since the String class is declared final there are no subclasses) 13

14 Programming ackvity Goal: Finish the generic Stack class Team 1: pop() Team 2: elementat() Team 3: indexof() Team 4: size() Send a representakve to enter your code Used in my Towers of Hanoi game: Replace all TBD with some code Stack<Disk> peg1 = new Stack<Disk>(); Stack<Disk> peg2 = new Stack<Disk>(); Stack<Disk> peg3 = new Stack<Disk>(); peg1 peg2 peg3 14

15 Java generics Summary Create one class that can store any object type Replace real type with "wildcard" in class blueprint PrimiKves need to use wrapper class Integer instead of int Double instead of double 15