Unknown behaviour of Java Program

Question

 System.out.println(symbol_b.symbolName + " " + symbol_b.pointsTo.size());
 // change the fieldpointsTo set of all above symbols
 for (int i = 0; i < samePointsTo.size(); i++) 
 {
 for (int j = 0; j < samePointsTo.elementAt(i).size(); j++) 
 {
 Symbol symbol = samePointsTo.elementAt(i).elementAt(j);
 System.out.println(symbol.symbolName);
 // if field information is already present
 if (symbol.fieldPointsTo.containsKey(fieldObj)) 
 {
 symbol.fieldPointsTo.get(fieldObj).clear();
 symbol.fieldPointsTo.get(fieldObj).addAll(symbol_b.pointsTo);
 } 
 else 
 {
 // we have to create field information
 symbol.fieldPointsTo.put(fieldObj, symbol_b.pointsTo);
 }
 }
 }
 System.out.println(symbol_b.symbolName + " " + symbol_b.pointsTo.size());

Before the for loop the size of symbol_b.pointsTo is 2, but just after the for loop its value is changed from 2 to 0. How it is happening can anyone tell me. I am not doing anything to symbol_b.pointsTo and symbol_b.pointsTo is a vetor.

• Richard JP Le Guen: What are the declared and/or run time types of these variables?
• neel: fieldPointsTo is a hashTable, pointsTo is a vector, and symbol,symbol_a, symbol_b is of type Symbol which is a user defined class

Answer 1

Perhaps pointsTo is not only referred to by that variable name, but is also a value returned by symbol.fieldPointsTo.get(fieldObj), so that when you call .clear() on that, it removes all the vector's elements?

• NPE: I think this is the most likely explanation (+1)

Answer 2

Could it be the case that:

if (symbol.fieldPointsTo.containsKey(fieldObj)) 
{
 symbol.fieldPointsTo.get(fieldObj).clear();
 symbol.fieldPointsTo.get(fieldObj).addAll(symbol_b.pointsTo);
}

the symbol variable points to symbol_b, symbol_b is not in the fieldPointsTo so it calls clear() on it?