code: @Override
      public void sawOpcode(int seen) {
          String methodInfo = null;
          try {
              stack.precomputation(this);
  
              switch (seen) {
                  case Const.INVOKEVIRTUAL:
                  case Const.INVOKEINTERFACE:
                  case Const.INVOKESTATIC:
                      String sig = getSigConstantOperand();
                      if (!sig.endsWith(Values.SIG_VOID)) {
                          methodInfo = getClassConstantOperand() + '@' + getNameConstantOperand() + getSigConstantOperand();
                      }
                  break;
  
                  case Const.POP:
                  case Const.POP2:
                      if (stack.getStackDepth() > 0) {
                          OpcodeStack.Item item = stack.getStackItem(0);
                          String mInfo = (String) item.getUserValue();
                          if (mInfo != null) {
                              for (Pattern p : IMMUTABLE_METHODS) {
                                  Matcher m = p.matcher(mInfo);
  
                                  if (m.matches()) {
                                      bugReporter.reportBug(new BugInstance(this, BugType.NPMC_NON_PRODUCTIVE_METHOD_CALL.name(), NORMAL_PRIORITY).addClass(this)
                                              .addMethod(this).addSourceLine(this).addString(mInfo));
                                      break;
                                  }
                              }
                          }
                      }
                  break;
              }
  
          } finally {
              stack.sawOpcode(this, seen);
              if ((methodInfo != null) && (stack.getStackDepth() > 0)) {
                  OpcodeStack.Item item = stack.getStackItem(0);
                  item.setUserValue(methodInfo);
              }
          }
      }
docs: implements the visitor to look for return values of common immutable method calls, that are thrown away.
  
  @param seen
  the opcode of the currently parsed instruction