code: private String set(ChaincodeStub stub, List<String> args) {
          if (args.size() != 2) {
              throw new RuntimeException("Incorrect arguments. Expecting a key and a value");
          }
          stub.putStringState(args.get(0), args.get(1));
          return args.get(1);
      }
docs: set stores the asset (both key and value) on the ledger. If the key exists,
  it will override the value with the new one
  
  @param stub {@link ChaincodeStub} to operate proposal and ledger
  @param args key and value
  @return value