泛型为方法返回类型 [英] Generic as method return type

问题描述

我环视了一下StackOverflow，找到了我面临的问题的答案。我遇到了很多很好的答案，但仍然没有回答我的问题。

如何找到返回类型的参数化类型通过检查？

Java泛型：获得泛型方法返回类型的类

http://qussay.com/2013/09/28/handling-java-generic-types-with-反射/

HTTP：// gafter .blogspot.com / search？q = super + type + token

所以这就是我想要做的。
使用反射，我想获得所有方法和它的返回类型（非泛型）。
我一直在使用 Introspector.getBeanInfo 来做到这一点。然而，当我遇到返回类型未知的方法时，我遇到了限制。

  public class Foo {
 
公共字符串名称; 
 
 public String getName（）{
 return name; 
} 
 
 public void setName（final String name）{
 this.name = name; 
} 
} 
 
 public class Bar< T> {
 
 T对象; 
 
 public T getObject（）{
 return object; 
} 
 
 public void setObject（final T object）{
 this.object = object; 
 
 
 $ b @Test 
 public void testFooBar（）抛出NoSuchMethodException，SecurityException，IllegalAccessException，
 IllegalArgumentException，InvocationTargetException {
 
 Foo foo = new Foo（）; 
 Bar< Foo> bar = new Bar< Foo>（）; 
 bar.setObject（foo）; 
方法mRead = bar.getClass（）。getMethod（getObject，null）; 
 
 System.out.println（Foo.class）; // Foo 
 System.out.println（foo.getClass（））; // Foo 
 System.out。 println（Bar.class）; // Bar 
 System.out.println（bar.getClass（））; // Bar 
 System.out.println（mRead.getReturnType（））; // java.lang.Object 
 System.out.println（mRead.getGenericReturnType（））; // T 
 System.out.println（mRead.getGenericReturnType（））; // T 
 System.out.println（mRead.invoke（bar，null）.getClass（））; // Foo 
} 
  

如何知道方法返回类型 T 是否为泛型？
我没有奢望在运行时拥有一个对象。
我正在试验Google TypeToken 或者使用抽象类来获取类型信息。
我想为 getObject 关联 T 和 Foo $ c $> $ Bar object。

有些人认为java不保留泛型信息。在这种情况下，为什么第一次投射和第二次投射不会。

bar.setObject（（Foo）fooObject）; //这个工程
Object object = 12;
bar.setObject（（Foo）object）; //引发铸造错误

任何帮助都是值得赞赏的。

解决方案

  Bar< Foo> bar = new Bar< Foo>（）; 
方法mRead = bar.getClass（）。getMethod（getObject，null）; 
 TypeToken< Bar< Foo>> tt = new TypeToken< Test.Bar< Foo>>（）{}; 
 Invokable< Bar< Foo>，Object> inv = tt.method（mRead）; 
 System.out.println（inv.getReturnType（））; // Test $ Foo 
  

也许这就是您要搜索的内容。
TypeToken和Invokable来自Google Guava。

€：修复了关于@PaulBellora的注释的代码

I have looked around on StackOverflow to find the answer for the problem I am facing. I came across many good answers but still it doesn't answer my question.

Get type of a generic parameter in Java with reflection

How to find the parameterized type of the return type through inspection?

Java generics: get class of generic method's return type

http://qussay.com/2013/09/28/handling-java-generic-types-with-reflection/

http://gafter.blogspot.com/search?q=super+type+token

So here is what I want to do. Using Reflection, I want to get all methods and its return type (non-generic). I have been using Introspector.getBeanInfo to do so. However I hit the limitation when I run into a method where return type is unknown.

public class Foo {

    public String name;

    public String getName() {
        return name;
    }

    public void setName(final String name) {
        this.name = name;
    }
}

public class Bar<T> {

    T object;

    public T getObject() {
        return object;
    }

    public void setObject(final T object) {
        this.object = object;
    }
}

@Test
    public void testFooBar() throws NoSuchMethodException, SecurityException, IllegalAccessException,
            IllegalArgumentException, InvocationTargetException {

        Foo foo = new Foo();
        Bar<Foo> bar = new Bar<Foo>();
        bar.setObject(foo);
        Method mRead = bar.getClass().getMethod("getObject", null);

        System.out.println(Foo.class);// Foo
        System.out.println(foo.getClass());// Foo
        System.out.println(Bar.class);// Bar
        System.out.println(bar.getClass());// Bar
        System.out.println(mRead.getReturnType()); // java.lang.Object
        System.out.println(mRead.getGenericReturnType());// T
        System.out.println(mRead.getGenericReturnType());// T
        System.out.println(mRead.invoke(bar, null).getClass());// Foo
    }

How do I know if the method return type T is generic or not? I don't have a luxury to have an object at runtime. I am experimenting with Google TypeToken or use an abstract class to get type information. I want to associate T to Foo for the getObject method for Bar<Foo> object.

Some have argued that java doesn't preserve the generic information. In that case why first casting works and second casting doesn't.

Object fooObject = new Foo();
bar.setObject((Foo) fooObject); //This works
Object object = 12;
bar.setObject((Foo) object); //This throws casting error

Any help is appreciated.

解决方案

Bar<Foo> bar = new Bar<Foo>();
Method mRead = bar.getClass().getMethod( "getObject", null );
TypeToken<Bar<Foo>> tt = new TypeToken<Test.Bar<Foo>>() {};
Invokable<Bar<Foo>, Object> inv = tt.method( mRead );
System.out.println( inv.getReturnType() ); // Test$Foo

Maybe this is what you are searching for. TypeToken and Invokable are from Google Guava.

€: Fixed the code with respect to the comment of @PaulBellora

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