Adding a method to Java class in different file without extending the class

Question

package com.companyxyz.api.person;

public class Man {
    public var1;
    public var2;
    ...
    private static void createAuth() {
    ...
    }
    // public methods go here
    ...
}

I want to create a new public method that accesses the private method, createAuth, but in a different file. Is there a way to create this new method without writing it or accessing it via an extended class?

Thank you.

See the Java tutorial page [Controlling Access to Members of a Class](http://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html). — Bobulous, May 21 '13 at 19:50
see http://stackoverflow.com/questions/6680674/can-a-java-class-add-a-method-to-itself-at-runtime — IvanNik, May 21 '13 at 19:53

score 5 · Answer 1 · answered May 21 '13 at 19:48

5

No. You can't access a private method from an other class. Because it's ... private.

answered May 21 '13 at 19:48

Thierry

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I understand that. I want to "continue" the Man class by adding new methods to it without touching the original file. – user717236 May 21 '13 at 19:50
1

As Duncan Jones says, you can use reflection, but it's another story. – Thierry May 21 '13 at 19:50

Duncan Jones · Answer 2 · 2013-05-21T20:00:30.147

4

A private method is not accessible to any external classes, (this includes subclasses).

A workaround might be to use reflection, however this isn't a generally recommended approach for a number of reasons (brittleness, performance problems, breaking encapsulation, etc).

edited May 21 '13 at 20:00

answered May 21 '13 at 19:48

Duncan Jones

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Notwithstanding the fact you're breaking the encapsulation of the member variable, reflection isn't very safe either - most of the methods you're going to use throw a multitude of exceptions (which are difficult to handle correctly) and the security manager, if present, can veto these action too. – Faelkle May 21 '13 at 19:55

A.H. · Answer 3 · 2013-05-21T20:33:01.417

There is no clean and recommended general way to do this in Java. private is private.

But you did not state why you want to do this and what the specific constraints are. Therefore I throw two options into the mix:

You can decompile the class file for Man, set everything you want to protected or public and recompile (and repackage into a jar file). Perhaps there is no need to decompile, perhaps some bit manipulation on the class file can do the job, too.
You can write a custom ClassLoader with a bytecode manipulation library to modify the bytecode of the class at runtime. Then you can also add additional access paths to the stuff you want. Note however that this is extremely advanced/complicated stuff.

Both ways are nothing you can use / should use for normal applications or the usual framework.

Adding a method to Java class in different file without extending the class

3 Answers3