?????????
????????????渴?????????д??????????????????t??
??????????????????????????????—— ???? ??
?????????????????????????????????????Java????????У???????????????????????δ?????????“???????”??о???
??????y???????????????????????????????????????????????????????????????д?????????????????“?????????????”???
???????????? ???? ??????
??????????
????????????????Java??????????????????????? ?????? ????????
?????????????????????????????????
???????????????н?????????????????????????е??????????????
????public class Holder1 {
????private Object a;
????public Holder1(Object a) {
????this.a = a;
????}
????Object get() {
????return a;
????}
????}
?????????????????ж??????????????????????????????????洢??????????????????????洢 ???????? ??????????????洢????????? ??????? ??
????????????????????????????????????????????????????顣
????class Automobile {}
????public class Holder2<T> {
????private T a;
????public Holder2(T a) {
????this.a = a;
????}
????public void set(T a) {
????this.a = a;
????}
????public T get() {
????return a;
????}
????public static void main(String[] args) {
????Holder2<Automobile> h2 =
????new Holder2<Automobile>(new Automobile());
????Automobile a = h2.get(); // No cast needed
????// h2.set("Not an Automobile"); // Error
????// h2.set(1); // Error
????}
????}
????????????????÷????????????????????????????д ??????? “T”?????????????????????滻??“T”??????????????????洢?? ??????? ??
???????????
??????????????????????????? ????????б? ?????????????????ɡ?
????????????????У? f() ??Ч??????????????????????
????//: generics/GenericMethods.java
????public class GenericMethods {
????public <T> void f(T x) {
????System.out.println(x.getClass().getName());
????}
????public static void main(String[] args) {
????GenericMethods gm = new GenericMethods();
????gm.f("");
????gm.f(1);
????gm.f(1.0);
????gm.f(1.0F);
????gm.f(‘c’);
????gm.f(gm);
????}
????} /* Output:
????java.lang.String
????java.lang.Integer
????java.lang.Double
????java.lang.Float
????java.lang.Character
????GenericMethods
????*///:~
????????????????????????????г?? ?????????? ??????????????????????????
?????????????? f() ??????????????????? ?????????? ???????????????????????????????????
????????
????Java????????ò??????????????ζ?? ??????????????????ù??????????????? ??
??????????????????????????????????????磬??????Object??????<T extends Bound>???????????Bound??????????????????????Bound?滻??T?????
????????????????????????
????//: generics/ErasedTypeEquivalence.java
????import java.util.*;
????public class ErasedTypeEquivalence {
????public static void main(String[] args) {
????Class c1 = new ArrayList<String>().getClass();
????Class c2 = new ArrayList<Integer>().getClass();
????System.out.println(c1 == c2);
????}
????} /* Output:
????true
????*///:~
????????????????????????????????”ArrayList<String>”??”ArrayList<Integer>”????????????????????????????“ArrayList”?????д???????????泣????????????????????? “c1”??”c2″????“ArrayList.class”????????“ArrayList<String>.class”??“ArrayList<Integer>.class” ??
??????????????????????2??????????Щ?????????????
??????ArrayList???????????????????????洢?????????????????????
????/**
????* The elements in this list?? followed by nulls.
????*/
????transient Object[] array;
????????get()??????????????
????@SuppressWarnings("unchecked") @Override public E get(int index) {
????if (index >= size) {
????throwIndexOutOfBoundsException(index?? size);
????}
????return (E) array[index];
????}
?????????E??????????Object???????????????????????н?????????Object??Object????
????????????????????????′??????????????????ие????
????//: generics/GenericHolder.java
????public class GenericHolder<T> {
????private T obj;
????public void set(T obj) { this.obj = obj; }
????public T get() { return obj; }
????public static void main(String[] args) {
????GenericHolder<String> holder =
????new GenericHolder<String>();
????holder.set("Item");
????String s = holder.get(); // Why it works?
????}
????} ///:~
??????? javap -c ????????????????????
????public void set(java.lang.Object);
????0: aload_0
????1: aload_1
????2: putfield #2; //Field obj:Object;
????5: return
????public java.lang.Object get();
????0: aload_0
????1: getfield #2; //Field obj:Object;
????4: areturn
????public static void main(java.lang.String[]);
????0: new #3; //class GenericHolder
????3: dup
????4: invokespecial #4; //Method "<init>":()V
????7: astore_1
????8: aload_1
????9: ldc #5; //String Item
????11: invokevirtual #6; //Method set:(Object;)V
????14: aload_1
????15: invokevirtual #7; //Method get:()Object;
????18: checkcast #8; //class java/lang/String --------Watch this line--------
????21: astore_2
????22: return
??????????? ??????????????????????????飬??????????????
???????????????????
????//: generics/ArrayMaker.java
????import java.lang.reflect.*;
????import java.util.*;
????public class ArrayMaker<T> {
????private Class<T> kind;
????public ArrayMaker(Class<T> kind) { this.kind = kind; }
????@SuppressWarnings("unchecked")
????T[] create(int size) {
????return (T[])Array.newInstance(kind?? size);
????}
????public static void main(String[] args) {
????ArrayMaker<String> stringMaker =
????new ArrayMaker<String>(String.class);
????String[] stringArray = stringMaker.create(9);
????System.out.println(Arrays.toString(stringArray));
????}
????} /* Output:
????[null?? null?? null?? null?? null?? null?? null?? null?? null]
????*///:~
?????????????????kind?????洢?Class?????“Array.newInstance();”?????????????????????????????????????????????????????????? ????????????????????????????? ????????????????????????????“@SuppressWarnings(“unchecked”)”??н????????
????????
??????Щ????????????????????????????????????????
????????????????????
????//: generics/GenericsAndCovariance.java
????import java.util.*;
????public class GenericsAndCovariance {
????public static void main(String[] args) {
????// Wildcards allow covariance:
????List<? extends Fruit> flist = new ArrayList<Apple>();
????// Compile Error: can’t add any type of object:
????// flist.add(new Apple());
????// flist.add(new Fruit());
????// flist.add(new Object());
????flist.add(null); // Legal but uninteresting
????// We know that it returns at least Fruit:
????Fruit f = flist.get(0);
????}
????} ///:~
????flist??????? List<? extends Fruit> ??????“?κδ?Fruit??ж??????????????б?”??????????ζ?????List???????κ??????Fruit???????????????? ????????? ???????????????ζ??“????????????Fruit?????????”??