public class Util {
public static boolean compare(Pair p1, Pair p2) {
return p1.getKey().equals(p2.getKey()) &&
p1.getValue().equals(p2.getValue());
}
}
public class Pair {
private K key;
private V value;
public Pair(K key, V value) {
this.key = key;
this.value = value;
}
public void setKey(K key) { this.key = key; }
public void setValue(V value) { this.value = value; }
public K getKey() { return key; }
public V getValue() { return value; }
}
我们可以像下面这样去调用泛型方法：
Pair p1 = new Pair<>(1, "apple");
Pair p2 = new Pair<>(2, "pear");
boolean same = Util.compare(p1, p2);
或者在Java1.7/1.8利用type inference，让Java自动推导出相应的类型参数：
Pair p1 = new Pair<>(1, "apple");
Pair p2 = new Pair<>(2, "pear");
boolean same = Util.compare(p1, p2);
边界符
现在我们要实现这样一个功能，查找一个泛型数组中大于某个特定元素的个数，我们可以这样实现：
public static int countGreaterThan(T[] anArray, T elem) {
int count = 0;
for (T e : anArray)
if (e > elem) // compiler error
++count;
return count;
}
class Fruit {}
class Apple extends Fruit {}
class Orange extends Fruit {}
下面这个例子中，我们创建了一个泛型类Reader，然后在f1()中当我们尝试Fruit f = fruitReader.readExact(apples);编译器会报错，因为List与List之间并没有任何的关系。
public class GenericReading {
static List apples = Arrays.asList(new Apple());
static List fruit = Arrays.asList(new Fruit());
static class Reader {
T readExact(List list) {
return list.get(0);
}
}
static void f1() {
Reader fruitReader = new Reader();
// Errors: List cannot be applied to List.
// Fruit f = fruitReader.readExact(apples);
}
public static void main(String[] args) {
f1();
}
}
static class CovariantReader {
T readCovariant(List<? extends T> list) {
return list.get(0);
}
}
static void f2() {
CovariantReader fruitReader = new CovariantReader();
Fruit f = fruitReader.readCovariant(fruit);
Fruit a = fruitReader.readCovariant(apples);
}
public static void main(String[] args) {
f2();
}
public class GenericsAndCovariance {
public static void main(String[] args) {
// Wildcards allow covariance:
List<? extends Fruit> flist = new ArrayList();
// Compile Error: can't add any type of object:
// flist.add(new Apple())
// flist.add(new Orange())
// 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);
}
}
public class GenericWriting {
static List apples = new ArrayList();
static List fruit = new ArrayList();
static void writeExact(List list, T item) {
list.add(item);
}
static void f1() {
writeExact(apples, new Apple());
writeExact(fruit, new Apple());
}
static void writeWithWildcard(List<? super T> list, T item) {
list.add(item)
}
static void f2() {
writeWithWildcard(apples, new Apple());
writeWithWildcard(fruit, new Apple());
}
public static void main(String[] args) {
f1(); f2();
}
}
这样我们可以往容器里面添加元素了，但是使用super的坏处是以后不能get容器里面的元素了，原因很简单，我们继续从编译器的角度考虑这个问题，对于List<? super Apple> list，它可以有下面几种含义：
List<? super Apple> list = new ArrayList();
List<? super Apple> list = new ArrayList();
List<? super Apple> list = new ArrayList
Object[] stringLists = new List[]; // compiler error, but pretend it's allowed
stringLists[0] = new ArrayList(); // OK
// An ArrayStoreException should be thrown, but the runtime can't detect it.
stringLists[1] = new ArrayList();
public class ErasedTypeEquivalence {
public static void main(String[] args) {
Class c1 = new ArrayList().getClass();
Class c2 = new ArrayList().getClass();
System.out.println(c1 == c2); // true
}
}
public class Node {
public T data;
public Node(T data) { this.data = data; }
public void setData(T data) {
System.out.println("Node.setData");
this.data = data;
}
}
public class MyNode extends Node {
public MyNode(Integer data) { super(data); }
public void setData(Integer data) {
System.out.println("MyNode.setData");
super.setData(data);
}
}
看完上面的分析之后，你可能会认为在类型擦除后，编译器会将Node和MyNode变成下面这样：
public class Node {
public Object data;
public Node(Object data) { this.data = data; }
public void setData(Object data) {
System.out.println("Node.setData");
this.data = data;
}
}
public class MyNode extends Node {
public MyNode(Integer data) { super(data); }
public void setData(Integer data) {
System.out.println("MyNode.setData");
super.setData(data);
}
}
MyNode mn = new MyNode(5);
Node n = mn; // A raw type - compiler throws an unchecked warning
n.setData("Hello"); // Causes a ClassCastException to be thrown.
// Integer x = mn.data;
