代写代考 CS61B Lecture #25: Java Generics

CS61B Lecture #25: Java Generics
Last modified: Fri Oct 25 14:33:03 2019 CS61B: Lecture #25 1

The Old Days

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• Java library types such as List didn’t used to be parameterized. All Lists were lists of Objects.
• So you’d write things like this:
for (int i = 0; i < L.size(); i += 1) { String s = (String) L.get(i); ... } • That is, must explicitly cast result of L.get(i) to let the compiler know what it is. • Also, when calling L.add(x), was no check that you put only Strings into it. • So, starting with 1.5, the designers tried to alleviate these per- ceived problems by introducing parameterized types, like List.
• Unfortunately, it is not as simple as one might think.
Last modified: Fri Oct 25 14:33:03 2019 CS61B: Lecture #25 2

Basic Parameterization
• From the definitions of ArrayList and Map in java.util:
public class ArrayList implements List { public Item get(int i) { … }
public boolean add(Item x) { … }
public interface Map {
Value get(Key x);
• First (blue) occurrences of Item, Key, and Value introduce formal type parameters, whose “values” (which are reference types) get substituted for all the other occurrences of Item, Key, or Value when ArrayList or Map is “called” (as in ArrayList, or ArrayList, or Map>).
• Other occurrences of Item, Key, and Value are uses of the formal types, just like uses of a formal parameter in the body of a function.
Last modified: Fri Oct 25 14:33:03 2019 CS61B: Lecture #25 3

Type Instantiation
• Instantiating a generic type is analogous to calling a function.
• Consider again
public class ArrayList implements List { public Item get(int i) { … }
public boolean add(Item x) { … }
• When we write ArrayList, we get, in effect, a new type,
somewhat like
public String ArrayList implements List { public String get(int i) { … }
public boolean add(String x) { … }
• And then, likewise, List refers to a new interface type as well.
Last modified: Fri Oct 25 14:33:03 2019 CS61B: Lecture #25 4

Parameters on Methods
• Functions (methods) may also be parameterized by type. Example of use from java.util.Collections:
/** A read-only list containing just ITEM. */
static List singleton(T item) { … } /** An unmodifiable empty list. */
static List emptyList() { … }
The compiler figures out T in the expression singleton(x) by look- ing at the type of x. This is a simple example of type inference.
• In the call
List empty = Collections.emptyList();
the parameters obviously don’t suffice, but the compiler deduces the parameter T from context: it must be assignable to String.
Last modified: Fri Oct 25 14:33:03 2019 CS61B: Lecture #25 5

• Consider the definition of something that counts the number of times something occurs in a collection of items. Could write this as
/** Number of items in C that are equal to X. */
static int frequency(Collection c, Object x) { int n; n = 0;
for(Ty:c) {
if (x.equals(y))
return n; }
• But we don’t really care what T is; we don’t need to declare anything of type T in the body, because we could write instead
for (Object y : c) {
• Wildcard type parameters say that you don’t care what a type pa- rameter is (i.e., it’s any subtype of Object):
static int frequency(Collection c, Object x) {…}
Last modified: Fri Oct 25 14:33:03 2019 CS61B: Lecture #25 6

Subtyping (I)
• What are the relationships between the types
List, List