Java Indirect Access to Variables by Name at Runtime?

Question

This question has multiple parts, and will all make sense in the last part:

Is it possible to programmatically list any programmer-defined variables in a Java class at runtime..?

I recently wrote a utility procedure in VB.Net that lists all the forms in my project by name, so I can pick one to run from a listbox (just for development purposes). With help from elsewhere on StackOverflow, I was able to pin the following bit of code together, and this example is pinned together even more for illustrative purposes...
```
    System.Reflection.Assembly.GetExecutingAssembly().DefinedTypes(n).BaseType.FullName
```
Can something similar be done in Java to list variable names..? That being MyInteger, MyString, or MyObject, for example. It'd be like the function behind the "Locals" window in Visual Studio (but I'm using Eclipse for Java).
Somewhere in the past 20 years, in another programming language, I came across a function called Indirect(String varname). It was used to access a primitive variable by name. And the 'varname' parameter could itself be a variable; it didn't have to be a literal. Here's an example showing its use:
```
int N1, N2, N3;
// N1, N2, & N3 are all zero
for (int i = 1; i < 4; i++){
    Indirect("N" + i.toString) = 100
}
// N1, N2, & N3 are now all 100
```
Does Java have an equivalent to Indirect(), or is there a way to duplicate the same functionality..?

To glue all this together, and the reason for this entire forum post, is that I have the following procedure that I've written for debugging purposes, that I drop into my classes to output variable values in CSV format during development...

public String listVars(){
    Object input[] = {var1, var2, var3, var4, var5};    // list of vars
    String output = "";                     // output catcher
    int varCount = input.length;            // get the length of the array
    for (int i = 0; i < varCount; i++){     // loop through the array
        output += "\"" + input[i] + "\"";   // surround with quotes, append to output
        if (i < varCount-1){                // if the item NOT the last...
            output += ",";                  // append a comma to it
        }
    }
    return output;                          // return completed output
}

What I would like to do is automate the input to the Object array with code that automatically enumerates the variables, so I don't have to copy & paste them into the function. And I'm assuming that in so doing, they would need to be referred to indirectly once their names are determined, so their values can be accessed in code.

PS - Filtering the variables by scope would be nice, such as 'private", 'super', <classname>, 'public', etc, but I know this may stretch it a bit too far. =-)

Java is not a dynamic language. Once you compile to bytecode, you throw away most of the local variable names. So there is no Indirect, and no list of Locals. However, it can see member names of classes at runtime through reflection. — Andrew Williamson
Also, it sounds like you just want to see the objects at runtime. You could use a library like JSON, or the upcoming Java REPL project 'kulla'. — Andrew Williamson

radai radai · Accepted Answer · 2016-02-09T00:00:06

listing class fields/methods at runtime

yes, you can do that, with reflection. see the tutorial here:

for (Field f : getClass().getDeclaredFields()) {
    System.out.println("found " + f);
}

"Indirect" access

yes, using the above mechanism to find the field you wish to altar you can then set its value

Field f = ... //get it from somewhere
f.set(this, "new value");

what you want to achieve sounds very close to the java bean concept. the gist of it is that you keep to a getX()/setX() naming convention for your getter and setter methods, each such pair representing a property called "x" and you can then utilize bean-oriented libraries, such as apache commons beanutils to all all the heavy lifting (that i told you about above) for you, like so:

Employee employee = ...;
String firstName = (String)PropertyUtils.getSimpleProperty(employee, "firstName");
String lastName = (String)PropertyUtils.getSimpleProperty(employee, "lastName");
 ... manipulate the values ...
PropertyUtils.setSimpleProperty(employee, "firstName", firstName);
PropertyUtils.setSimpleProperty(employee, "lastName", lastName);

Java Indirect Access to Variables by Name at Runtime?

2 Answers

TL;DR