numbers longer numbers longer - 5 months ago 42
Java Question

Can I override a private method in Java?

I know I can use reflection to invoke a private method, and to get or set the value of a private variable, but I want to override a method.

public class SuperClass {

public void printInt() {
System.out.println("I am " + getClass() + ". The int is " + getInt());
}

private int getInt() {
return 1;
}
}

public class SubClass extends SuperClass {

public static void main(String[] args) {
(new SubClass()).printInt();
}

public int getInt() {
return 2;
}
}


I want the
main
method in
SubClass
to print out
2
, but it prints out
1
.
I've heard this can be done through reflection, but I can't figure out how.
If not reflection, does anyone know of another way of doing it?
(Other than making
SuperClass.getInt()
protected, or copying and pasting the
printInt()
method into
SubClass
.)
If actually overriding the private method is not possible, is there a way of placing some sort of trigger on it that will invoke a method in my sub-class either before or after the private method executes?

Answer

Private methods are not inherited and cannot be overridden in any way. Whoever told you you can do it with reflection was either lying or talking about something else.

However, you can access the private method getInt of whatever subclass is invoking printInt like so:

public void printInt() throws Exception {
    Class<? extends SuperClass> clazz = getClass();
    System.out.println("I am " + clazz + ". The int is " +
                       clazz.getMethod("getInt").invoke(this) );
}

This will have the effect of the subclass' getInt method being called from the superclass' printInt. Of course, now this will fail if the subclass doesn't declare a getInt, so you have to add a check to be able to handle "normal" subclasses that don't try to "override" a private method:

public void printInt() throws Exception {
    Class<? extends SuperClass> clazz = getClass();

    // Use superclass method by default
    Method theGetInt = SuperClass.class.getDeclaredMethod("getInt");

    // Look for a subclass method
    Class<?> classWithGetInt = clazz;
    OUTER: while( classWithGetInt != SuperClass.class ){

        for( Method method : classWithGetInt.getDeclaredMethods() )
            if( method.getName().equals("getInt") && method.getParameterTypes().length == 0 ){
                theGetInt = method;
                break OUTER;
            }

        // Check superclass if not found
        classWithGetInt = classWithGetInt.getSuperclass();
    }

    System.out.println("I am " + classWithGetInt + ". The int is " + theGetInt.invoke(this) );
}

You still have to change superclass code to make this work, and since you have to change superclass code, you should just change the access modifier on getInt to protected instead of doing reflection hack-arounds.

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