Java中此类/模板的用法是什么意思？

Question

package MyTest;

import java.lang.reflect.*;

public class CallingMethod {
    public static void main(String[] args) {
        String firstName = new String("Deepak");
        String lastName = new String("Kumar");
        String result = new String("");

        Class<String> cls = String.class;
        Class[] parametertype = new Class[] { String.class };
        Object[] arguments = new Object[] { lastName };
        try {
            Method concatMethod = cls.getMethod("concat", parametertype);
            result = (String) concatMethod.invoke(firstName, arguments);
            System.out.println(" Concatenated String is => " + result);
        } catch (Exception e) {
            System.out.println(e);
        }
    }
}

For the above code, I don't understand below parts.

Class<String> cls = String.class;
Class[] parametertype = new Class[] { String.class };
Object[] arguments = new Object[] { lastName };

So what does String.class mean? And String.class in new Class[] { String.class } means what? It means the first item in the array is a String.class type? Or all the items are String.class type? And new Object[] { lastName } ? It means the first item/instance of the array is lastName? Hope to get your expertise! Thanks!

Answer 1

The code is using reflection and method invocation to call the concat method in the String class, on the object firstName , using the arguments parameters. It is similar to calling:

result = firstName.concat(lastName);

Answer 2

• Class<String> cls = String.class; is a Class object which refers to the String class.
• Class[] parametertype = new Class[] { String.class }; is an array of Class objects, containing one element: String.class
• Object[] arguments = new Object[] { lastName }; is an array of Objects containing one element too

Answer 3

So what does String.class mean?

A reference to the String Class

And String.class in new Class[] { String.class } means what?

An array of Classes which has one reference to the String Class.

means the first item in the array is a String.class type?

The type of the first element is Class or Class<String>

And new Object[] { lastName }? It means the first item/instance of the array is lastName?

Yes.

I don't understand below parts.

It appears you are correct in all your questions so it appears you were right all along.

Do you have any other questions?

Answer 4

Every object in Java is associated with a type , eg, String or MyClass . The type is itself implemented by another object. So String.class and MyClass.class are referring to the class objects for those types.