Generalized Containers CSIS 3701: Advanced Object Oriented Programming

Generalizing Containers Example: Stack class that stores strings public class Stack { private String[] contents; private int top; public Stack(int s) { contents = new String[s]; top = 0; } public void push(String x) {contents[top++] = x;} public String pop() {return contents[--top];}

Generalizing Containers Problem: Need different stack class for different types of contents public class IntStack { private Integer[] contents; … public class EmployeeStack { private Employee[] contents; …

Generalizing Containers Goal: one stack class that can store any type of contents public class Stack { private anything[] contents; Tools: –Polymorphism: Can assign subclass object to superclass variable –Java hierarchy: All Java objects have Object class as ultimate superclass Key: Can assign any Java object to a variable of type Object

Containers of Objects Use Object as type stored by container public class Stack { private Object[] contents; private int top; public Stack(int s) { contents = new Object[s]; top = 0; } public void push(Object x) {contents[top++] = x;} public Object pop() {return contents[--top];}

Containers of Objects Example: Stack stooges = new Stack(4); Stooges.push(“Larry”); Stooges.push(“Curley”); Stooges.push(“Moe”); public class Stack { public void push(Object x) {contents[top++] = x;} private Object[] contents; Storing String objects in Object array

Limits of Containers of Objects Inspector methods return Object type instead of original type stored Will cause error if assigned to variable of original type stooges.push(“Moe”); String name = stooges.pop(); Must explicitly cast back to original type: String name = (String)stooges.pop(); Object Type mismatch error!

Limits of Containers of Objects Cannot store simple types in containers –Store in type wrappers stooges.push(new Integer(3)); –Is done automatically by later versions of Java Can store multiple types of object in single container stooges.push(“Larry”); stooges.push(3); stooges.push(new Clock()); –Problem if must cast back to original type – what was it?

Java Container Classes Java provides many built-in generalized containers –Stack, Queue, Hashtable, … –In java.util.* All implemented by storing Object elements Most used container type: ArrayList –Can access elements by index (like array) –Can insert/remove elements (like linked list) –Can grow dynamically (like linked list) –Has built-in search

ArrayList Class “Array” type methods: –public void set(int index, Object x); store x in index th element –public Object get(int index); return object at index th element Will need to cast back to original type Throws ArrayIndexOutOfBoundsException if index illegal –public int size();

ArrayList Class “Linked list” type methods: –public void add(Object x); add x to end of list –public void add(int index, Object x); insert x at index th element (shifting others to right) –public Object remove(int index); remove object at index th element (shifting others to left) and return it

ArrayList Class “Search” type methods: –public boolean contains(Object x); returns true if x in list –public void indexOf(Object x); return index of first occurrence of x (or -1 if x not in list) –public void indexOf(Object x, int index); return index of first occurrence of x starting at index Based on equals method for object x

ArrayList Example Rewrite of NameList to use ArrayList –no maximum since list grows dynamically –no current, since can add dynamically to end public class NameList { private ArrayList names; // List of names stored public NameList() { names = new ArrayList(); }

ArrayList Example Use “linked list” method to add new name to end Use search to make sure not already in list public void add(String name) throws InListException { if (names.contains(name)) throw new InListException(); names.add(name); } Use “array” method to iterate through list (casting back to original type) public String toString() { String result = ""; for (int i = 0; i < names.size(); i++) result += (String)names.get(i)+","; return result; }

Object Equality Java containers use equals method for search public boolean contains(Object x) { for (int i = 0; i < size(); i++) if x.equals(get(i)) return true; return false; May need to override equals in your classes if –Store in container –Search for equivalent objects (perhaps to avoid duplication) Will need to define what it means for two objects to be “equivalent”

Object Equality Built-in equals method takes an Object as parameter –Must check type of parameter first using instanceof operator –Then cast to appropriate type before comparing relevant fields Basic form of a equals method for some class C: public boolean equals(Object x) { if (this == x) return true; if (x instanceof C) { C y = (C)x; compare relevant fields, returning either true or false } return false; } Do they refer to the same object? Is it the same type as C? If so, cast to class C and compare relevant fields

Object Equality Example: equals for Employee class –Define as equivalent if have same name public boolean equals(Object x) { if (this == x) return true; if (x instanceof Employee) { Employee e = (Employee)x; return name.equals(e.name); } return false; }

Generics (Templates) Containers with type checking –Define generically: stores object of “type ” public class classname {… –Define the actual type when specific object created classname variable = new classname(); –Originated by C++ –Recently implemented (in limited form) in Java

Defining Generics Example: Pair class –Stores two different values of same type –Type determined when object constructed public class Pair { private T thing1, thing2; public Pair() { thing1 = thing2 = null; } Stores two variables, both of type T

Defining Generics public class Pair { … public void set(T value, int which) { if (which == 1) thing1 = value; if (which == 2) thing2 = value; } public T get(int which) { if (which == 1) return thing1; if (which == 2) return thing2; return null; } Methods take/return values of generic type T

Using Generics Define actual type stored when object declared Pair p1 = new Pair(); p1.set("Fred", 1); p1.set("Barney", 2); String name = p1.get(1); Note that do not need to cast back to original type – compiler now knows original type

Using Generics Can create generics of different types Pair p2 = new Pair(); p2.set(12, 1); p2.set(13, 2); int x = p2.get(1); p2.set(“Fred”, 1); Unlike polymorphism, Java does type checking

Limits of Generics Java does not allow arrays to be used with generics –This is legal in C++! public class StackTemplate { private T[] contents; private int top; public StackTemplate(int s) { contents = new T[s]; top = 0; Compiler error

Generics and Java Containers Many Java containers upgraded to use generics Example: Vector class ArrayList a = new ArrayList(); a.add(“Larry”); a.add(3); ArrayList that only contains Strings This is legal This is not

Generics and Java Containers Can use generic containers to define your own containers –Example: Generic Stack class that uses generic ArrayList class public class StackTemplate { private ArrayList contents; public StackTemplate() {contents = new ArrayList();} public void push(T x) {contents.add(x);} public T pop() { return contents.remove(contents.size()-1); } }