通配符
上界通配符<? extends T>
我们先来看一个例子：
class Fruit {}
class Apple extends Fruit {}
现在我们定义一个盘子类:
class Plate<T>{
T item;
public Plate(T t){
item=t;
}
public void set(T t) {
item=t;
}
public T get() {
return item;
}
}
下面，我们定义一个水果盘子，理论上水果盘子里，当然可以存在苹果:
Plate<Fruit> p=new Plate<Apple>(new Apple());
装苹果的盘子”无法转换成“装水果的盘子：
cannot convert from Plate<Apple> to Plate<Fruit>
从上面代码我们知道，就算容器中的类型之间存在继承关系，但是Plate和Plate两个容器之间是不存在继承关系的。 在这种情况下，Java就设计成Plate<? extend Fruit>来让两个容器之间存在继承关系。我们上面的代码就可以进行赋值了。
Plate<? extends Fruit> p=new Plate<Apple>(new Apple());
我们通过一个更加详细的例子来看一下上界的界限：
class Food{}
class Fruit extends Food {}
class Meat extends Food {}
class Apple extends Fruit {}
class Banana extends Fruit {}
class Pork extends Meat{}
class Beef extends Meat{}
class RedApple extends Apple {}
class GreenApple extends Apple {}
在上面这个类层次中，Plate<? extend Fruit>，覆盖下面的蓝色部分
如果往盘子里面添加数据，会有什么问题没？
p.set(new Fruit());
p.set(new Apple());
按道理说我们将泛型类型设置为? extend Fruit。按理说我们往里面添加Fruit的子类应该是可以的。但是Java编译器不允许这样操作。
一个Plate<? extends Fruit>的引用，指向的可能是一个Plate类型的盘子，要往这个盘子里放Banana当然是不被允许的。Plate<? extends Fruit>代表某个只能放某种类型水果的盘子，而不是什么水果都能往里放的盘子。
但是上界通配符是允许读取操作的。例如代码：
Fruit fruit=p.get();
Object object=p.get();
上界通配符设定容器中只能存放Fruit及其派生类，那么获取出来的我们都可以隐式的转为其基类（或者Object基类）。
上界描述符Extends适合频繁读取的场景。
下界通配符<? super T>
容器中只能存放T及其T的基类类型的数据，如下图：
下界通配符<? super T>不影响往里面存储，但是读取出来的数据只能是Object类型。
下界通配符规定了元素最小的粒度，必须是T及其基类，那么我往里面存储T及其派生类都是可以的，因为它都可以隐式的转化为T类型。
PECS原则
最后简单介绍下Effective Java这本书里面介绍的PECS原则。
- 上界<? extends T>不能往里存，只能往外取，适合频繁往外面读取内容的场景
- 下界<? super T>不影响往里存，但往外取只能放在Object对象里，适合经常往里面插入数据的场景
<?>无限通配符
无界通配符 意味着可以使用任何对象，因此使用它类似于使用原生类型。
List<Object> 和 List<?> 是否等同？
泛型擦除
# 查看编译后的class文件
javap -verbose ***.class
首先看非泛型的版本：
public class SimpleHolder {
private Object obj;
public Object getObj() {
return obj;
}
public void setObj(Object obj) {
this.obj = obj;
}
public static void main(String[] args) {
SimpleHolder holder = new SimpleHolder();
holder.setObj("Item");
String s = (String) holder.getObj();
System.out.printf(s);
}
}
查看编译后文件可以得到:
警告: 二进制文件SimpleHolder包含genericity.SimpleHolder
Classfile /Users/remer/Documents/works/server/Crypt/out/production/Crypt/genericity/SimpleHolder.class
Last modified 2019-7-22; size 978 bytes
MD5 checksum 87b59b95e4e29d6502d17618928803fa
Compiled from "SimpleHolder.java"
public class genericity.SimpleHolder
{
public genericity.SimpleHolder();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
LineNumberTable:
line 7: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lgenericity/SimpleHolder;
public java.lang.Object getObj();
descriptor: ()Ljava/lang/Object;
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: getfield #2 // Field obj:Ljava/lang/Object;
4: areturn
LineNumberTable:
line 11: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lgenericity/SimpleHolder;
public void setObj(java.lang.Object);
descriptor: (Ljava/lang/Object;)V
flags: ACC_PUBLIC
Code:
stack=2, locals=2, args_size=2
0: aload_0
1: aload_1
2: putfield #2 // Field obj:Ljava/lang/Object;
5: return
LineNumberTable:
line 15: 0
line 16: 5
LocalVariableTable:
Start Length Slot Name Signature
0 6 0 this Lgenericity/SimpleHolder;
0 6 1 obj Ljava/lang/Object;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=3, locals=3, args_size=1
0: new #3 // class genericity/SimpleHolder
3: dup
4: invokespecial #4 // Method "<init>":()V
7: astore_1
8: aload_1
9: ldc #5 // String Item
11: invokevirtual #6 // Method setObj:(Ljava/lang/Object;)V
14: aload_1
15: invokevirtual #7 // Method getObj:()Ljava/lang/Object;
18: checkcast #8 // class java/lang/String
21: astore_2
22: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream;
25: aload_2
26: iconst_0
27: anewarray #10 // class java/lang/Object
30: invokevirtual #11 // Method java/io/PrintStream.printf:(Ljava/lang/String;[Ljava/lang/Object;)Ljava/io/PrintStream;
33: pop
34: return
LineNumberTable:
line 19: 0
line 20: 8
line 21: 14
line 22: 22
line 23: 34
LocalVariableTable:
Start Length Slot Name Signature
0 35 0 args [Ljava/lang/String;
8 27 1 holder Lgenericity/SimpleHolder;
22 13 2 s Ljava/lang/String;
}
SourceFile: "SimpleHolder.java"
泛型的版本，从字节码的角度来看看:
public class GenericHolder<T> {
T obj;
public T getObj() {
return obj;
}
public void setObj(T obj) {
this.obj = obj;
}
public static void main(String[] args) {
GenericHolder<String> holder = new GenericHolder<>();
holder.setObj("Item");
String s = holder.getObj();
}
}
查看class ：
警告: 二进制文件GenericHolder包含genericity.GenericHolder
Classfile /Users/remer/Documents/works/server/Crypt/out/production/Crypt/genericity/GenericHolder.class
Last modified 2019-7-22; size 1096 bytes
MD5 checksum 65b7cd303117c5bddbea6ecec53e4377
Compiled from "GenericHolder.java"
public class genericity.GenericHolder<T extends java.lang.Object> extends java.lang.Object
{
T obj;
descriptor: Ljava/lang/Object;
flags:
Signature: #13 // TT;
public genericity.GenericHolder();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
LineNumberTable:
line 7: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lgenericity/GenericHolder;
LocalVariableTypeTable:
Start Length Slot Name Signature
0 5 0 this Lgenericity/GenericHolder<TT;>;
public T getObj();
descriptor: ()Ljava/lang/Object;
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: getfield #2 // Field obj:Ljava/lang/Object;
4: areturn
LineNumberTable:
line 11: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lgenericity/GenericHolder;
LocalVariableTypeTable:
Start Length Slot Name Signature
0 5 0 this Lgenericity/GenericHolder<TT;>;
Signature: #25 // ()TT;
public void setObj(T);
descriptor: (Ljava/lang/Object;)V
flags: ACC_PUBLIC
Code:
stack=2, locals=2, args_size=2
0: aload_0
1: aload_1
2: putfield #2 // Field obj:Ljava/lang/Object;
5: return
LineNumberTable:
line 15: 0
line 16: 5
LocalVariableTable:
Start Length Slot Name Signature
0 6 0 this Lgenericity/GenericHolder;
0 6 1 obj Ljava/lang/Object;
LocalVariableTypeTable:
Start Length Slot Name Signature
0 6 0 this Lgenericity/GenericHolder<TT;>;
0 6 1 obj TT;
Signature: #28 // (TT;)V
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=3, args_size=1
0: new #3 // class genericity/GenericHolder
3: dup
4: invokespecial #4 // Method "<init>":()V
7: astore_1
8: aload_1
9: ldc #5 // String Item
11: invokevirtual #6 // Method setObj:(Ljava/lang/Object;)V
14: aload_1
15: invokevirtual #7 // Method getObj:()Ljava/lang/Object;
18: checkcast #8 // class java/lang/String
21: astore_2
22: return
LineNumberTable:
line 19: 0
line 20: 8
line 21: 14
line 22: 22
LocalVariableTable:
Start Length Slot Name Signature
0 23 0 args [Ljava/lang/String;
8 15 1 holder Lgenericity/GenericHolder;
22 1 2 s Ljava/lang/String;
LocalVariableTypeTable:
Start Length Slot Name Signature
8 15 1 holder Lgenericity/GenericHolder<Ljava/lang/String;>;
}
Signature: #37 // <T:Ljava/lang/Object;>Ljava/lang/Object;
SourceFile: "GenericHolder.java"
