调用method.getReturnType（）。newInstance（）（静态内部类的静态方法）时出现InstantiationException

Question

I get

run: method: foo
Return type: class java.lang.Integer
Exception in thread "main" java.lang.InstantiationException: java.lang.Integer
  at java.lang.Class.newInstance0(Class.java:359)
  at java.lang.Class.newInstance(Class.java:327)
  at newinstancetest.NewInstanceTest.main(NewInstanceTest.java:10)
Java Result: 1 BUILD SUCCESSFUL (total time: 0 seconds)

When I run this: package newinstancetest; import java.lang.reflect.Method;

public class NewInstanceTest {

public static void main(String[] args) throws NoSuchMethodException, InstantiationException, IllegalAccessException {
    Method method = InnerClass.class.getDeclaredMethod("foo", null);
    System.out.println("method: " + method.getName());
    System.out.println("Return type: " + method.getReturnType());
    Object obj = method.getReturnType().newInstance();
    System.out.println("obj: " + obj);
}

public static class InnerClass {
    public static Integer foo() {
        return new Integer(1);
    }
}
}

Shouldn't the "obj" + obj print the reference to a new object? Any idea why i get an exception instead?

Answer 1

The return type for the method is Integer which does not have a no-arg constructor. To replicate the instance in the foo method, you could do

Object obj = method.getReturnType().getConstructor(int.class).newInstance(1);

Answer 2

Integer does not have a contructor with no argument. You can use the Integer(int) instead for example:

Object obj = method.getReturnType().getConstructor(int.class).newInstance(0);

If you meant to call the foo method, then you can use:

Object obj = method.invoke(null); //null for static

Answer 3

At runtime, the method getReturnType() in

Object obj = method.getReturnType().newInstance();

returns a Class<Integer> instance. The Integer class has two constructors, one with int and one with String .

When you call newInstance() , you're expecting the default no-arg constructor of the returned class object, which doesn't exist.

You need to get the constructors

Constructor[] constructors = d.getReturnType().getConstructors();

then iterate and use the one that matches best.