使用泛型的Class.cast使用ClassCastException

Question

I am writing a generic method which will validate a property by trying a class.cast on it but I keep getting a ClassCastException

... Class to Test

public <T> T get(Properties p, String propKey, Class<T> clazz) throws Exception {

    T val = null;

    Object propValue = p.get(propKey);

    if(propValue== null) {
        throw new Exception("Property (" + propKey + ") is null");
    }

    try {
        val = clazz.cast(propValue); // MARKER

    } catch(Exception e) {
        throw new Exception("Property (" + propKey + ") value is invalid value of type (" + clazz + ")", e);
    }



    return val;
}

... Test Class

@Before
public void setUp() {
    propUtil = new PropUtil();
    properties = new Properties();
    properties.setProperty("test.int.prop", "3");
}

@Test
public void testGet() {

    try {

        assertEquals(new Integer(3), propUtil.get(properties, "test.int.prop", Integer.class));
    } catch (Exception e) {
        System.out.println(e);
    }
}

The code at commented at MARKER is causing the ClassCastException.

Any ideas much appreciated.

Answer 1

The Properties class is a Hashtable stores String objects, especially when you call setProperty . You have added the String "3", not the integer 3 . You are effectively attempting to cast "3" as an Integer , so that correctly throws a ClassCastException . Try

assertEquals("3", propUtil.get(properties, "test.int.prop", String.class));

Or if you want to have get return an Integer , then just use a Hashtable<String, Integer> , or even better, use a HashMap<String, Integer> .

Answer 2

Assuming that Properties here is java.util.Properties , the values are always String s.

You should use the getProperty() method, rather than the get() method that happens to be visible from HashTable because this class was written back when the Java folks were less careful about composition versus inheritance.

Answer 3

This Line

properties.setProperty("test.int.prop", "3");

puts a java.lang.String in properties

and You pass Integer.class to you generic method. So the ClassCastException is expected!

If you want to Test for Integer.class you have to put an Integer

properties.put("test.int.prop", 3);

Note in the above line the use of put since the Properties class is extending Hashtable

If your intention is to put a String and test for an Integer then you have to somehow parse that String to an Integer value

Answer 4

Thanks for your replies. I realise the basic act of casting from String to Integer wasn't possible. I was just trying to make the method slicker and do the conversion check for me. I've just worked out the solution I was lookin using Reflection which is :

    Object propValue = p.get(propKey);
    Constructor<T> constructor = clazz.getConstructor(String.class);
    val = constructor.newInstance(propValue);

ie Using the public constructor that takes the String.class (ie the String property value)

Works a treat.