Question

      Map<Object,String> mp=new HashMap<Object, String>();
      Scanner sc=new Scanner(System.in);
      System.out.println("Enter the instrument name");
      String name=sc.next();


        mp.put(name, "Control Valve");
        mp.put(name, "BDV");
        mp.put(name, "SDV");
        mp.put(name, "ON-OFF VALVE");
        mp.put(name,"Analyser");


        Set s=mp.entrySet();

        Iterator it=s.iterator();

        while(it.hasNext())
        {

            Map.Entry m =(Map.Entry)it.next();

            String key=(String)m.getKey();

            String value=(String)m.getValue();

            System.out.println("Instrument name :"+key+"  fields:"+value);
        }
    }
}

In this only last value is mapped to the key .i.e analyser to key .

How to map all the values to one key entered by the user .And also it has to ask user to enter values for each value field.

updated code -:It asks for instrument name but then shows exception “java.util.ArrayList cannot be cast to java.lang.String”

 Map<String,List<String>> mp=new HashMap<String,List<String>>();
      Scanner sc=new Scanner(System.in);
      System.out.println("Enter the instrument name");
      String name=sc.next();
      List<String> valList = new ArrayList<String>();
      valList.add("Control Valve");
      valList.add("BDV");
      valList.add("SDV");
      valList.add("ON-OFF VALVE");
      valList.add("Analyser");
      mp.put(name, valList);




        Set s=mp.entrySet();

        Iterator it=s.iterator();

        while(it.hasNext())
        {

            Map.Entry m =(Map.Entry)it.next();

            String key=(String)m.getKey();

            String value=(String)m.getValue();

            System.out.println("Instrument name :"+key+" fields:"+value);
        }
    }
}

Answer 1

I would suggest to use

Map<Object,List<String>> mp=new HashMap<Object, List<String>>();

So that you can maintain set of values to a given key.

if a list available for given key , get the list and add the new value to list.

UPDATED

Map<String,List<String>> mp=new HashMap<String,List<String>>();
      Scanner sc=new Scanner(System.in);
      System.out.println("Enter the instrument name");
      String name=sc.next();
      List<String> valList = new ArrayList<String>();
      valList.add("Control Valve");
      valList.add("BDV");
      valList.add("SDV");
      valList.add("ON-OFF VALVE");
      valList.add("Analyser");
      mp.put(name,valList);

      for(String key : mp.keySet()){
          System.out.print("Instrument name :"+key+" Values : ");
          for(String val : mp.get(key)){
              System.out.print(val+",");
          }


      }