How to pass in any Object type into a method? Java

Question

Note: the NPE error has been fixed

I need a way to identify an Object when it is passed into a method. It can be by name or anything else unique to the Object, but not by type or value, since there may be other Objects with the same type or value in the HashMap.

In TOES.java:

import java.util.HashMap;

public class TOES{

    private HashMap<Object, HashMap<String, Object>> TOES = new HashMap<Object, HashMap<String, Object>>();

    public void add(Object foot, String tag, Object data){
        HashMap<String, Object> TOE = TOES.get(foot);
                if(TOE == null){
                         TOE = new HashMap<String, Object>;
                 }
        TOE.put(tag, data);
        TOES.put(foot, TOE);
    }

    public Object val(Object foot, String tag){
        return TOES.get(foot).get(tag);
    }

}

In TOESTest1.java:

public class TOESTest1{

    public static void main(String[] arg){
        TOES Toes = new TOES();
        Integer potatoes = 5;
        Integer eyes = 7;
        String tagname = "eyes";
        Toes.add(potatoes,tagname,eyes);
           potatoes = 3;
        System.out.println(potatoes);
        System.out.println(Toes.val(potatoes, "eyes"));
    }

}

When TOESTest1 is run, it shows an error for the last println and the return in the val method.

The output should be:

3
7

(Ignore the weird names, TOES is an acronym)

I am new to java, but not new to programming (I know C++), so...

That's not your code - it can't be, because you're using `class` as a variable name. Show us your *actual* code and we're more likely to be able to help. — Jon Skeet, Sep 11 '14 at 00:14
Did you try with `Integer` instead of `int`? I meet Jon Skeet opinion: please give us your actual code. — R2B2, Sep 11 '14 at 00:21
You're not going to get a `NullPointerException` if you're using a type instead of another, but because your reference (your variable) is *null*. If the type wasn't compatible (i.e. `ArrayList` vs `HashMap`), you'd get a `ClassCastException` instead. — watery, Sep 11 '14 at 00:51
And which line throws NPE? Can you also post the full text of the exception (including stack frame)? — markspace, Sep 11 '14 at 01:01

Ahsan Pérez · Answer 1 · 2020-12-30T14:01:02.380

You can do that by using a Map and the Optional class implemented since Java 8.

The Map is gonna contain the String identifier and the Object you want to pass.

Map<String, Optional<?>> mapToSendToMethod = new HashMap<String, Optional<?>>();
mapToSendToMethod.put("This is a string", Optional.of("This is a test"));
List<String> ListToSend = new ArrayList<>();
mapToSendToMethod.put("This is a List", Optional.of(ListToSend));
mapToSendToMethod.put("This is a number", Optional.of(123));`
someMethod(mapToSendToMethod);`

to get the object inside Optional you can use the method Optional.get() which it will return the object if there's any, otherwise null.

<?> is a shorthand for <? extends Object>, it's also known as an unbounded wildcard. So you can specify any type of object in your generic.

score 0 · Answer 2 · edited May 23 '17 at 12:31

0

int doesn't inherit from Object in Java. You can cast your ints to Integers, and then you'll be able to pass them to a function that takes Objects.

update

Of course you are getting NPE, you haven't initialized the TOE. Fix add method like this

public void add(Object foot, String tag, Object data){
    HashMap<String, Object> TOE = TOES.get(foot);
    if (TOE == null) {
        TOE = new HashMap<String, Object>();
    }
    TOE.put(tag, data);
    TOES.put(foot, TOE);
}

end of update

You are looking for equality by reference. Instead of using HashMap, uses IdentityHashMap

... in an IdentityHashMap, two keys k1 and k2 are considered equal if and only if (k1==k2). (In normal Map implementations (like HashMap) two keys k1 and k2 are considered equal if and only if (k1==null ? k2==null : k1.equals(k2)).)

Instead of comparing equality with .equals method, it's comparing with the reference (pointer address) of the object.

There is also a warning thought.

This class is not a general-purpose Map implementation! While this class implements the Map interface, it intentionally violates Map's general contract, which mandates the use of the equals method when comparing objects. This class is designed for use only in the rare cases wherein reference-equality semantics are required.

Okay, that fixes that problem (I had started to think about the problem being that, but I didn't really know how to go about it). Now I need a way to have it so that I can pass in the potatoes integer, change the value of potatoes, and still have the val method work if I pass in potatoes. It doesn't really matter how this is done, since I only really need this to work. I am going to modify the code to show what I mean. — blackbeltJR, Sep 11 '14 at 01:29

How to pass in any Object type into a method? Java

3 Answers3

update

end of update