初始化变量.我不知道他们的类型[java]

Question

class pair<U,V>{
U first;
V second;
public pair() {
    first = new U(); //error
    second = new V(); //error
}   
public pair(U f,V s){
    first = f;
    second = s;
}
}

required:
找到的类:类型参数

是否有可能以其他方式初始化first/ secondwith(with-arguments)U/V类型的构造函数？

Answer 1

由于类型擦除, Java通常不允许这样做.您可以指定类型的构造函数参数,Class<U>并Class<V>为其传递给定类型参数的具体类类型(即Integer.class和String.classfor <Integer>和<String>).

也可以使用字节码级反射来提取类型,但是非常复杂,并且它并不总是适用于所有情况.如果您向下滚动本文,您可以找到使这成为可能的示例.为方便起见,我在下面贴了它.

static public Type getType(final Class<?> klass, final int pos) {
    // obtain anonymous, if any, class for 'this' instance
    final Type superclass = klass.getGenericSuperclass();

    // test if an anonymous class was employed during the call
    if ( !(superclass instanceof Class) ) {
        throw new RuntimeException("This instance should belong to an anonymous class");
    }

    // obtain RTTI of all generic parameters
    final Type[] types = ((ParameterizedType) superclass).getActualTypeArguments();

    // test if enough generic parameters were passed
    if ( pos < types.length ) {
        throw RuntimeException(String.format("Could not find generic parameter #%d because only %d parameters were passed", pos, types.length));
    }

    // return the type descriptor of the requested generic parameter
    return types[pos];
}

编辑:回复评论:

class pair<U,V>{
    U first;
    V second;
    public pair(Class<U> cu, Class<V> cv) {
        try {
            first = cu.newInstance();
            second = cv.newInstance();
        } catch (Exception e) {
            throw new IllegalArgumentException(e);
        }
    }   
    public pair(U f,V s){
        first = f;
        second = s;
    }
}