[CC150v4] 14.5 Java Reflection

Question

Explain what object reflection is in Java and why it is useful.

Solution

Java Reflection makes it possible to inspect classes, interfaces, fields and methods at runtime, without knowing the names of the classes, methods etc. at compile time. It is also possible to instantiate new objects, invoke methods and get/set field values using reflection.

For example, say you have an object of an unknown type in Java, and you would like to call a ‘doSomething’ method on it if one exists. Java’s static typing system isn’t really designed to support this unless the object conforms to a known interface, but using reflection, your code can look at the object and find out if it has a method called ‘doSomething’ and then call it if you want to. Like this:

Method method = foo.getClass().getMethod("doSomething", null);
method.invoke(foo, null);

Usage in Junit

One very common use case in Java is the usage with annotations. JUnit 4, for example, will use reflection to look through your classes for methods tagged with the @Test annotation, and will then call them when running the unit test.

Code

The following example code is covered in this post:

public class JavaReflection {

    public static void main(String[] args) {
        Method[] methods;

        methods = ListNode.class.getMethods();

        for (Method method : methods) {
            System.out.println(formatMethodName(method.getName() + "()")
                    + method.getDeclaringClass());
        }
    }

    private static String formatMethodName(String methodName) {
        for (int i = methodName.length(); i < 30; i++) {
            methodName += ".";
        }
        return methodName;
    }
}