Way to print the variable name and variable value in java

Question

String activityState = "resume";
DebugLog(activityState)
void DebugLog(String obj1) {}

How to make the DebugLog to print like this:

activityState : resume

I used to write many print statement as logs at many places while debugging. I will write statements like

System.out.println("activityState : " + activityState);

I want a method to print the variable name and variable value. In C++, it can be done like the below:

#define dbg(x) cout<< #x <<" --> " << x << endl ;

Is there any way to do this?

Thanks in advance.

• Suresh Atta: Not possible to get the variable name unless you use reflection. Do you want to ?
• NPE: @sᴜʀᴇsʜᴀᴛᴛᴀ: Are you implying this can be done with reflection?
• Suresh Atta: @NPE I'm opposed to use reflection :) Just telling the fact.
• A4L: To get the field for a member variable using reflection you have to know its name, or you have to get them all. btw there is nothing wrong with reflection all modern frameworks use it extensively...
• Bathsheba: You can't do anything more elegant than DebugLog("activityState", activityState). You could use reflection to convert "activityState" to activityState but you'd still have to pass a context parameter to tell your function whether activityState is a field etc. So a one-argument version does not exist. So best keep it simple!

Answer 1

There's no direct solution to get the variable name.

However, in a context where you have many fields and don't want to manually print their state, you can use reflection.

Here's a quick example:

class MyPojo {
 public static void main(String[] args) {
 System.out.println(new MyPojo());
 }
 int i = 1;
 String s = "foo";
 @Override
 public String toString() {
 StringBuilder result = new StringBuilder();
 for (Field f: getClass().getDeclaredFields()) {
 try {
 result
 .append(f.getName())
 .append(" : ")
 .append(f.get(this))
 .append(System.getProperty("line.separator"));
 }
 catch (IllegalStateException ise) {
 result
 .append(f.getName())
 .append(" : ")
 .append("[cannot retrieve value]")
 .append(System.getProperty("line.separator"));
 }
 // nope
 catch (IllegalAccessException iae) {}
 }
 return result.toString();
 }
}

Output

i : 1
s : foo

Answer 2

You can use java Reflection to get the variable name and the value. Here's an example code;

public class Example{
 String activityState = "resume";
 public static void main(String[] args) {
 Example example = new Example();
 Class<?> c = example.getClass();
 Field field = c.getDeclaredField("activityState");
 System.out.println(field.getName());
 System.out.println(field.get(example));
 } 
}

Answer 3

Since this is for debugging you could use instrumentation with aspectj, your code remains clean from the the debugging output statements and you can waeve the aspect as needed.

Define a set(FieldPattern) point cut to catch all field assignements (join points)

public aspect TestAssignmentAspect {
 pointcut assigmentPointCut() : set(* *);
 after() : assigmentPointCut() {
 System.out.printf("%s = %s%n", thisJoinPoint.getSignature().getName(),
 String.valueOf(Arrays.toString(thisJoinPoint.getArgs())));
 }
}

Here is Test class

public class Test {
 public static String activityState = "stopped";
 public static void main(String[] args) {
 activityState = "start";
 doSomething();
 activityState = "pause";
 doSomeOtherthing();
 activityState = "resume";
 System.out.printf("the end!%n");
 }
 private static void doSomeOtherthing() {
 System.out.printf("doing some other thing...%n");
 }
 private static void doSomething() {
 System.out.printf("doing something...%n");
 }
}

If you run this example with the aspect weaved the output will be

activityState = [stopped]
activityState = [start]
doing something...
activityState = [pause]
doing some other thing...
activityState = [resume]
the end!

Explanation

pointcut assigmentPointCut() : set(* *);

set point cut to catch assignments, the point joins, to any variable with any name, could also in the example be

pointcut assigmentPointCut() : set(String activityState);

The advice, the desired behavior when the given point cut matches

after() : assigmentPointCut() { ... }

Informations about the point join can be accessed using the special reference thisJoinPoint.

• Henry: Nobody upvoted this back when the question was first asked but I like this solution!