Capture Conversion解读

Let G name a generic type declaration with n type parameters A1,...,An with corresponding bounds U1,...,Un.

 

There exists a capture conversion from a parameterized type G<T1,...,Tn>  to a parameterized type G<S1,...,Sn>, where, for 1 ≤ i ≤ n :

 

（1）If Ti is a wildcard type argument of the form ?, then Si is a fresh type variable whose upper bound is Ui[A1:=S1,...,An:=Sn] and whose lower bound is the null type.

import java.io.BufferedInputStream;
import java.io.IOException;
import java.io.InputStream;
 
public class CaptureExampleUnboundedWildcard<E extends InputStream> {
    public void readFromIt(E readFrom) throws IOException{
        readFrom.read();
    }
 
    public E getSomething(){
        return null;
    }
 
    public static void main(String [] args) throws Exception{
        CaptureExampleUnboundedWildcard<?> ce=new CaptureExampleUnboundedWildcard<BufferedInputStream>();
 
        BufferedInputStream obj=new BufferedInputStream(null);
        InputStream i = ce.getSomething();   // ok
        ce.readFromIt(obj);  // compiler-time error
    }
}

这里的CaptureExampleUnboundedWildcard就是G，A1,...An就是<E extends InputStream>，相对应的上/下界声明(U1,..,Un）就是InputStream。

G<T1,...,Tn>是这里的CaptureExampleUnboundedWildcard<?>（在英文中叫a parameterized type，中文译为一个参数化的类型），在调用readFromIt()方法时就需要capture conversion了，也就是为方法中的E推断出G<S1,...,Sn>。

调用readFromIt()方法会报错，Eclipse的提示如下：

The method readFromIt(capture#1-of ?) in the type CaptureExampleUnboundedWildcard<capture#1-of ?> is not applicable for the arguments (BufferedInputStream)

其中有对Ui[A1:=S1,...,An:=Sn]这个表达式的理解。参考阅读：

（1）http://docs.oracle.com/javase/specs/jls/se8/html/jls-1.html#jls-1.3

（2）http://stackoverflow.com/questions/31206947/understanding-captured-conversion

也就是将声明的A1,...An用S1,...Sn来代替，并且新的这个S1,..,Sn的上界为Ui[A1:=S1,...,An:=Sn]。那么如上的例子中ce.readFromIt()方法调用中，实际的类型在new CaptureExampleUnboundedWildcard 时就已经确定为BufferedInputStream，可惜编译器并不保持这个类型变量，而是通过参数化类型声明CaptureExampleUnboundedWildcard<?>和类定义时的类型参数声明CaptureExampleUnboundedWildcard<E extends InputStream>来推断。推断后编译器只能知道新变量的上界为InputStream，具体是InputStream中什么具体的子类型，编译器并不知道，也就不允许放入BufferedInputStream了。

假如在如上类中添加一个方法，如下：

public <X extends InputStream> void readFromIt2(X x) throws IOException{
    	x.read();
}

调用如上方法然后传入一个BufferedInputStream的实例后，能够正常调用，因为编译器马上推断出了X类型变量的类型为BufferedInputStream，并且满足上界约束条件。

（2）If Ti is a wildcard type argument of the form ? extends Bi, then Si is a fresh type variable whose upper bound is glb(Bi, Ui[A1:=S1,...,An:=Sn]) and whose lower bound is the null type.

glb(V1,...,Vm) is defined as V1 & ... & Vm. 

public class CaptureExampleUpperBoundedWildcard<E extends InputStream> {
    public void readFromIt(E readFrom) throws IOException{
        readFrom.read();
    }
 
    public static void main(String [] args) throws Exception{
        //The ce is passed only ? extends Serializable as a type argument. Hence the capture converted type has an upper
        //bound same as the original bound during the declaration, which is InputStream
        //Note that it is not possible to create an Object of wildcard type. That is a compile
        //time error
 
        CaptureExampleUpperBoundedWildcard<? extends BufferedInputStream> ce=new CaptureExampleUpperBoundedWildcard<BufferedInputStream>();
 
    }
}

It is a compile-time error if, for any two classes (not interfaces) Vi and Vj, Vi is not a subclass of Vj or vice versa.

这里BufferedInputStream是InputStream类的子类，否则会出现编译错误。

（3）If Ti is a wildcard type argument of the form ? super Bi, then Si is a fresh type variable whose upper bound is Ui[A1:=S1,...,An:=Sn] and whose lower bound is Bi.

import java.io.BufferedInputStream;
import java.io.FileInputStream;
import java.io.FilterInputStream;
import java.io.IOException;
import java.io.InputStream;
 
public class CaptureExampleLowerBoundedWildcard<E extends InputStream> {
	public void readFromIt(E readFrom) throws IOException {
		readFrom.read();
	}
 
	public E getSomething(){
        return null;
    }
 
	public static void main(String[] args) throws Exception {
		CaptureExampleLowerBoundedWildcard<? super FilterInputStream> ce = new CaptureExampleLowerBoundedWildcard<InputStream>();
 
		BufferedInputStream obj = new BufferedInputStream(new FileInputStream("somefile"));
 
		// Note that subclasses of the lower bound are allowed in method
		// invocation due normal method invocation conversion
		ce.readFromIt(obj); // ok
 
		InputStream b = ce.getSomething(); // ok
 
	}
}

通过a parameterized type，也就是CaptureExampleLowerBoundedWildcard<? super FilterInputStream>与类定义时的类型参数声明CaptureExampleLowerBoundedWildcard<E extends InputStream>可知，新的类型变量下界为FilterInputStream，而上界为InputStream。那么满足这个条件的都可以，在new CaptureExampleLowerBoundedWildcard时传递了InputStream和FilterInputStream，而不能使用BufferedInputStream。

值得注意的是，ce.readFromIt(obj)中，这个obj是bufferedInputStream，也就是FilterInputStream的直接子类，按理说不满足如上的上下界，这是由于发生了method invocation conversion，具体就是a widening reference conversion转换，导致obj也满足了限定条件。　

　

当Ti为? super Bi，那么Bi应该为Ai的子类，举例如下：

 

interface IA {}
 
class B implements IA {}
 
class Test6<T extends IA> {
	public void test() {
		Test6<? super B> x = new Test6<B>(); // B不实现IA会报错
	}
}

（4）Otherwise, Si = Ti

public class CaptureExample<E, F extends E> {
    public void copy(List<E> toGet, List<F> copyFrom){
        for(F x:copyFrom){
            toGet.add(x);
        }
    }
 
    public static void main(String [] args){
        List<Object> aList=new ArrayList<Object>();
        List<String> copyFrom=new ArrayList<String>();
        copyFrom.add("Hi");
        CaptureExample<Object, String> ce=new CaptureExample<Object, String>();//Here is where the capture conversion takes place
        ce.copy(aList, copyFrom);
 
        System.out.println(aList);
 
    }
}

　

重新解读通配符：

e.g 1

public class CaptureExampleUnboundedWildcard<E extends InputStream> {
 
    // 方法形式参数表示设置值
    public void set(E readFrom) throws IOException{
        readFrom.read();
    }
 
    // 返回值为类型参数，表示取值
    public E get(){
        return null;
    }
 
    public static void main(String [] args) throws Exception{
 
        CaptureExampleUnboundedWildcard<?> ce = new CaptureExampleUnboundedWildcard<InputStream>();
 
        //The method set(capture#1-of ?) in the type CaptureExampleUnboundedWildcard<capture#1-of ?>
        //is not applicable for the arguments (BufferedInputStream)
        BufferedInputStream bis = null;
        ce.set(bis);
 
        //The method set(capture#2-of ?) in the type CaptureExampleUnboundedWildcard<capture#2-of ?>
        //is not applicable for the arguments (InputStream)
        InputStream is = null;
        ce.set(is);
 
        ce.set(null);
 
        Object o1 = ce.get();
        InputStream o2 = ce.get();
        // Type mismatch: cannot convert from capture#6-of ? to FilterInputStream
        FilterInputStream o3 = ce.get();
    }
}

从上面的实例可以看出，通配符为?时，只能设置null值，但是可以取值。

e.g 2

interface IA{}
class A{}
class B{}
 
public class CaptureExampleUpperBoundedWildcard<E extends A> {
 
	 // 方法形式参数表示设置值
    public void set(E readFrom) throws IOException{
 
    }
 
    // 返回值为类型参数，表示取值
    public E get(){
        return null;
    }
 
    public static void main(String [] args) throws Exception{
 
    	//Bound mismatch: The type ? extends B is not a valid substitute for the bounded parameter <E extends A>
    	//of the type CaptureExampleUpperBoundedWildcard<E>
        //CaptureExampleUpperBoundedWildcard<? extends IA> ce = new CaptureExampleUpperBoundedWildcard();
 
        CaptureExampleUpperBoundedWildcard<? extends IA> ce = new CaptureExampleUpperBoundedWildcard();
 
        // 当为<? extends Serializable>时不能存入任何元素，除了null值
        ce.set(null);  // 在方法的参数中就相当于给设置值，而在方法的返回中就相当于获取值
 
        Object o1 = ce.get();
        A o2 = ce.get();
        IA o3 = ce.get();
    }
 
}

从上面的实例可以看出，通配符为? extends X时，只能设置null值，但是可以取值。

声明中的E extends Y 与 通配符声明 ? extends X之间的关系如下：

（1）X与Y有一个为接口时，则相互之间没有限制。

（2）当X与Y都为类时，两个类之间必须有父子关系。抽象类也看作类

e.g 3

class SubBufferedInputStream extends BufferedInputStream{
    public SubBufferedInputStream(InputStream in) {
        super(in);
    }
}
public class CaptureExampleLowerBoundedWildcard<E extends FilterInputStream> {
 
    // 方法形式参数表示设置值
    public void set(E readFrom) throws IOException{
        readFrom.read();
    }
 
    // 返回值为类型参数，表示取值
    public E get(){
        return null;
    }
 
    public static void main(String [] args) throws Exception{
 
    	CaptureExampleLowerBoundedWildcard<? super BufferedInputStream> ce=new CaptureExampleLowerBoundedWildcard<FilterInputStream>();
 
        //Bound mismatch: The type ? super InputStream is not a valid substitute for
        //the bounded parameter <E extends FilterInputStream> of the type CaptureExampleLowerBoundedWildcard<E>
        // CaptureExampleLowerBoundedWildcard<? super InputStream> ce = new CaptureExampleLowerBoundedWildcard<FilterInputStream>();
 
        //Now we can call this without an error, because the capture converted type does have a lower bound.
        //Note that subclasses of the lower bound are allowed in method invocation due normal method invocation conversion
        BufferedInputStream obj= null;
        ce.set(obj);
 
        SubBufferedInputStream sis = null;
        ce.set(sis);
 
        //The method set(capture#3-of ? super BufferedInputStream) in the type
        //CaptureExampleLowerBoundedWildcard<capture#3-of ? super BufferedInputStream> is
        //not applicable for the arguments (FilterInputStream)
        FilterInputStream fis = null;
        ce.set(fis); // error
 
        FilterInputStream o1 = ce.get();
        // Type mismatch: cannot convert from capture#5-of ? super BufferedInputStream to SubBufferedInputStream
        SubBufferedInputStream o2 = ce.get(); // error
        // Type mismatch: cannot convert from capture#6-of ? super BufferedInputStream to BufferedInputStream
        BufferedInputStream o3  =  ce.get(); // error
 
    }
 
}

从上面的实例可以看出，通配符为? super X时，可以设置X及其子类，取值时能接收的最精确的类型为声明时E extends Y的Y类型。

X与Y不管是接口还是类必须要形成父子关系。

当声明为E，而通配符为?，? extends X或者? super X时，会是什么情况呢？也就是? extends X或者 ? super X时的类型X无限制。取值时只能以Object来接收。

　

参考文献：

（1）http://docs.oracle.com/javase/specs/jls/se7/html/jls-5.html#jls-5.1

（2）http://www.geekyarticles.com/2011/11/java-generics-capture-conversion.html

（3）https://www.ibm.com/developerworks/java/library/j-jtp04298/index.html