how to pass by a reference in java

Question

public class SecureSystem {
    final int low = 0;
    final int high = 1;
    HashMap<String, int[]> subject;
    HashMap<String, int[]> object;

    public SecureSystem() {
    subject = new HashMap<String, int[]>();
    object = new HashMap<String, int[]>();
    }
    ReferenceMonitor rm = new ReferenceMonitor();
    rm.ReferenceMonitor(subject,object);

    System.out.println(this.subject.get("a")); // how to make it print [1,2]?
    System.out.println(this.object.get("b")); // how to make it print [3,4]?
}
class ReferenceMonitor{
    ReferenceMonior(HashMap<String, int[]> subject, HashMap<String, int[]> object) {
        SecureSystem ss = new SecureSystem();
        ss.subject.put("a", new int{1,2});
        ss.object.put("a", new int{3,4})
    }
}

how do I make it do that? If I pass HashMaps to ReferenceMonitor class, and try to read contents, I get NullPointerError.

Thank you so much.

you never created objects for subject and "object". They are not initialized. — OldProgrammer, Sep 13 '13 at 21:31
You can't pass by reference in Java. The language doesn't support this mechanism. Everything in Java is passed by value. — NullUserException, Sep 13 '13 at 21:31
And even if you could pass something by reference, this code wouldn't work. — Thomas Junk, Sep 13 '13 at 21:33
I actaully have them initialized, I posted only a chunk of my code. I am sorry. — Nayana, Sep 13 '13 at 21:55
@NullUserException: true, but that said, the value that is passed is a reference to the object. Which is why you can re-assign the variable, i.e. var = new Object, but after exiting the method and returning to the parent method your parameter will not be changed because basically all you did was change the value of the reference. — Sebastiaan van den Broek, Sep 13 '13 at 22:04
Please don't think about other syntax errors, I've successfully compiled it. If anyone knows how to mimic the reference mechanism, please let me know. — Nayana, Sep 13 '13 at 22:21
You can create a container object to mimic c style pointers but in all my time with java I have never found it nessissary — Richard Tingle, Sep 13 '13 at 22:39

score 0 · Answer 1 · answered Sep 13 '13 at 21:31

0

You have not initialized the HashMaps. They are null because you have never created them.

HashMap<String, int[]> map = new HashMap<String, int[]>();

answered Sep 13 '13 at 21:31

leigero

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I actually do... it's just a simplified version of my code. The point of this post was to ask how to pass by a reference. – Nayana Sep 13 '13 at 21:51

score 0 · Answer 2 · edited May 23 '17 at 11:50

The problem is here:

class ReferenceMonitor{
    ReferenceMonior(HashMap<String, int[]> subject, HashMap<String, int[]> object) {
        //these three lines are the culprit
        SecureSystem ss = new SecureSystem();
        ss.subject.put("a", new int{1,2});
        ss.object.put("a", new int{3,4})
    }
}

You're putting the data in the subject and object maps inside the new SecureSystem ss local variable which will be unusable after the completion of the constructor call. You should put the data in the subject and object parameters so they're contents will be modified as well:

class ReferenceMonitor{
    ReferenceMonior(HashMap<String, int[]> subject, HashMap<String, int[]> object) {
        //code fixed
        //also, there's no need to create a new SecureSystem instance
        subject.put("a", new int[] {1,2});
        object.put("b", new int[] {3,4});
    }
}

This code can even be enhanced to pass the SecureSystem object reference instead:

class ReferenceMonitor{
    ReferenceMonior(SecureSystem secureSystem) {
        secureSystem.getSubject().put("a", new int[] {1,2});
        secureSystem.getObject().put("b", new int[] {3,4});
    }
}

Also, please note that Java is NOT pass by reference.

Thomas Junk · Answer 3 · 2013-09-13T21:39:15.797

Did you in any place use a new() to initialize your Hashmaps? In case not. There is your answer.

What you intended was something like

class ReferenceMonitor{
    ReferenceMonior(HashMap<String, int[]> subject, HashMap<String, int[]> object) {
        SecureSystem ss = new SecureSystem();
        ss.subject=subject;
        ss.object=object;
        ss.subject.put("a", new int{1,2});
        ss.object.put("a", new int{3,4})
    }
}

Besides, that is awful code.

1) You should consider using patterns like builder pattern

2) Your use of public data-members is a violation of OOP's information hiding principle and is considered bad style. Please use setter methods.

how to pass by a reference in java

3 Answers3