The argument of type null should explicitly be cas

问题:

Please have a look at the following example, the first call to getMethod() produces a Warning in Eclipse. The second one doesn't work and fails with a NoSuchMethodException.

The argument of type null should explicitly be cast to Class<?>[] for the invocation of the varargs method getMethod(String, Class<?>...) from type Class<Example>. It could alternatively be cast to Class for a varargs invocation.

I followed the warning and nothing worked anymore.

import java.lang.reflect.Method;


public class Example
{
  public void exampleMethod() { }

  public static void main(String[] args) throws Throwable
  {
   Method defaultNull = Example.class.getMethod("exampleMethod", null);         
   Method castedNull = Example.class.getMethod("exampleMethod", (Class<?>) null);
 }
}

The second call produces this error:

Exception in thread "main" java.lang.NoSuchMethodException: 
    Example.exampleMethod(null)
        at java.lang.Class.getMethod(Class.java:1605)
        at Example.main(Example.java:12)

Can someone explain this behaviour to me? What's the correct way to avoid the warning?

回答1:

The second parameter to the getMethod method is a VarArg argument. The correct use is : If reflected method has no parameter, then no second parameter should be specified. If the reflected method has parameter, so each parameter should be specified in the next way:

import java.lang.reflect.Method;


public class Example {

    public void exampleMethodNoParam() {
        System.out.println("No params");
    }

    public void exampleMethodWithParam(String arg) {
        System.out.println(arg);
    }

    public static void main(String[] args) throws Throwable {
        Example example = new Example();
        Method noParam = Example.class.getMethod("exampleMethodNoParam");
        Method stringParam = Example.class.getMethod("exampleMethodWithParam", String.class);
        noParam.invoke(example);
        stringParam.invoke(example, "test");
        //output 
        //No params
        //test
    }
}

UPDATE

So, in your case, when you specify null the compiler doesn't know what type do you specify. When you try to cast the null to a Class which is unknown but anyway is a class, you get an exception because there is no

public void exampleMethod(Class<?> object) { }

signature of exampleMethod.

回答2:

The getMethod defined in java API is:

Method java.lang.Class.getMethod(String name, Class<?>... parameterTypes) 

If the method you called has no arguments,then you should supply a empty array or a array with 0 length for the second argument.like this:

Method defaultNull = Example.class.getMethod("exampleMethod", new Class<?>[0]);

Invoke the method you should also set a empty array.

defaultNull.invoke(example , new Object[0]);

回答3:

You didn't cast null to Class<?>[], you cast it to Class<?>. Since no method matches that signature, it throws the exception. Correct your cast to properly identify the array.