简述
上篇我说到了类型擦除的缺点，也提供了一些解决方法。本篇我将说一下解决方法中的细节部分，也就是通配符。
通配符
子类型通配符
子类通配符 在语法中写为: List<? extends SomeThing> 它的意义可以理解为:一个具有任何从SomeThing继承的类型的列表。
超类型通配符
超类型通配符 在语法中写为: List<? super SomeThing> 它的意义可以理解为:一个继承自SomeThing的类型的列表。
概念比较晦涩难懂，接下来我用一段代码来解释其中的差异。起初我觉得很别扭，但是仔细考虑了很久才理清楚他们之间相关的关系。
一个基类Holder
public class Holder<T> {
private T value;
public Holder() {}
public Holder(T val) { value = val; }
public void set(T val) { value = val; }
public T get() { return value; }
public boolean equals(Object obj) {
return value.equals(obj);
}
public static void main(String[] args) {
Holder<Apple> Apple = new Holder<Apple>(new Apple());
Apple d = Apple.get();
Apple.set(d);
// Holder<Fruit> Fruit = Apple; // Cannot upcast
Holder<? extends Fruit> fruit = Apple; // OK
Fruit p = fruit.get();
d = (Apple)fruit.get(); // Returns 'Object'
try {
Orange c = (Orange)fruit.get(); // No warning
} catch(Exception e) { System.out.println(e); }
// fruit.set(new Apple()); // Cannot call set()
// fruit.set(new Fruit()); // Cannot call set()
System.out.println(fruit.equals(d)); // OK
}
}public class Wildcards {
// Raw argument:
static void rawArgs(Holder holder, Object arg) {
// holder.set(arg); // Warning:
// Unchecked call to set(T) as a
// member of the raw type Holder
// holder.set(new Wildcards()); // Same warning
// Can't do this; don't have any 'T':
// T t = holder.get();
// OK, but type information has been lost:
Object obj = holder.get();
}
// Similar to rawArgs(), but errors instead of warnings:
static void unboundedArg(Holder<?> holder, Object arg) {
// holder.set(arg); // Error:
// set(capture of ?) in Holder<capture of ?>
// cannot be applied to (Object)
// holder.set(new Wildcards()); // Same error
// Can't do this; don't have any 'T':
// T t = holder.get();
// OK, but type information has been lost:
Object obj = holder.get();
}
static <T> T exact1(Holder<T> holder) {
T t = holder.get();
return t;
}
static <T> T exact2(Holder<T> holder, T arg) {
holder.set(arg);
T t = holder.get();
return t;
}
static <T>
T wildSubtype(Holder<? extends T> holder, T arg) {
// holder.set(arg); // Error:
// set(capture of ? extends T) in
// Holder<capture of ? extends T>
// cannot be applied to (T)
T t = holder.get();
return t;
}
static <T>
void wildSupertype(Holder<? super T> holder, T arg) {
holder.set(arg);
// T t = holder.get(); // Error:
// Incompatible types: found Object, required T
// OK, but type information has been lost:
Object obj = holder.get();
}
public static void main(String[] args) {
Holder raw = new Holder<Long>();
// Or:
raw = new Holder();
Holder<Long> qualified = new Holder<Long>();
Holder<?> unbounded = new Holder<Long>();
Holder<? extends Long> bounded = new Holder<Long>();
Long lng = 1L;
rawArgs(raw, lng);
rawArgs(qualified, lng);
rawArgs(unbounded, lng);
rawArgs(bounded, lng);
unboundedArg(raw, lng);
unboundedArg(qualified, lng);
unboundedArg(unbounded, lng);
unboundedArg(bounded, lng);
// Object r1 = exact1(raw); // Warnings:
// Unchecked conversion from Holder to Holder<T>
// Unchecked method invocation: exact1(Holder<T>)
// is applied to (Holder)
Long r2 = exact1(qualified);
Object r3 = exact1(unbounded); // Must return Object
Long r4 = exact1(bounded);
// Long r5 = exact2(raw, lng); // Warnings:
// Unchecked conversion from Holder to Holder<Long>
// Unchecked method invocation: exact2(Holder<T>,T)
// is applied to (Holder,Long)
Long r6 = exact2(qualified, lng);
// Long r7 = exact2(unbounded, lng); // Error:
// exact2(Holder<T>,T) cannot be applied to
// (Holder<capture of ?>,Long)
// Long r8 = exact2(bounded, lng); // Error:
// exact2(Holder<T>,T) cannot be applied
// to (Holder<capture of ? extends Long>,Long)
// Long r9 = wildSubtype(raw, lng); // Warnings:
// Unchecked conversion from Holder
// to Holder<? extends Long>
// Unchecked method invocation:
// wildSubtype(Holder<? extends T>,T) is
// applied to (Holder,Long)
Long r10 = wildSubtype(qualified, lng);
// OK, but can only return Object:
Object r11 = wildSubtype(unbounded, lng);
Long r12 = wildSubtype(bounded, lng);
// wildSupertype(raw, lng); // Warnings:
// Unchecked conversion from Holder
// to Holder<? super Long>
// Unchecked method invocation:
// wildSupertype(Holder<? super T>,T)
// is applied to (Holder,Long)
wildSupertype(qualified, lng);
// wildSupertype(unbounded, lng); // Error:
// wildSupertype(Holder<? super T>,T) cannot be
// applied to (Holder<capture of ?>,Long)
// wildSupertype(bounded, lng); // Error:
// wildSupertype(Holder<? super T>,T) cannot be
// applied to (Holder<capture of ? extends Long>,Long)
}
} 在上述代码中包括以下几种通配符: 没有通配符、无界通配符、子类通配符、超类型通配符
无界通配符
无界通配符 在语法中写为: List<?>
无界通配符中会有一个地方叫捕获转换。
public class CaptureConversion {
static <T> void f1(Holder<T> holder) {
T t = holder.get();
System.out.println(t.getClass().getSimpleName());
}
static void f2(Holder<?> holder) {
f1(holder); // Call with captured type
}
@SuppressWarnings("unchecked")
public static void main(String[] args) {
Holder raw = new Holder<Integer>(1);
// f1(raw); // Produces warnings
f2(raw); // No warnings
Holder rawBasic = new Holder();
rawBasic.set(new Object()); // Warning
f2(rawBasic); // No warnings
// Upcast to Holder<?>, still figures it out:
Holder<?> wildcarded = new Holder<Double>(1.0);
f2(wildcarded);
}
}