Linked hash map in java

Question

I'm trying to make a project and I have the following issue: the project is something related to students. The class Student contains 3 fields id, name, grade. I have to represent the students collection using a linkedHashMap, id being the key. I have tried this but I'm not sure is ok because the name is a string and the grade an int.

Map<Integer,ArrayList> myMap = new LinkedHashMap<Integer,ArrayList>();

Could you give me some ideas how to represent this collection? I need to be able to add/remove a student.

• Brian Hoover: You probably want to look at a tree map - docs.oracle.com/javase/7/docs/api/java/util/TreeMap.html where the key is the student object and the values are an integer of the grades.

Answer 1

Why not just map from integer to student then?

Map<Integer,Student> myMap = new LinkedHashMap<Integer,Student>();

Add students via

myMap.put(student.id(), student);

A linked hash map preserves insertion order. A TreeMap would keep the students sorted by id; a plain HashMap avoids some overhead compared to a LinkedHashMap when no particular order is required.

Answer 2

You may include another LinkedHashMap inside first one.

Your declaration would be like:

Map<Integer, LinkedHashMap> my Map = new LinkedHashMap<Integer, LinkedHashMap>();

Or you might use a TreeMap instead.

Then you can just use the put, get and remove methods. Remember you can also use keySet and values to iterate and even containsKey.

• user3043278: I can't use TreeMap because LinkedHashMap is required for this project :) I've read about it
• diegoaguilar: i agree with the other answer, I even already implemented SAME thing in the same way with HashMaps, I may share the code if you want.
• user3043278: I could use an example to follow, if you want to share your code I'm opened

Answer 3

You can add custom classes inside a Map:

Usuario class

public class Usuario implements Serializable {
 private String nombre;
 private String ciudadOrigen;
 private String paisOrigen;
 private String idiomaMaterno;
 private int edad;
 public Usuario(String nombre, String ciudadOrigen, String paisOrigen, String idiomaMaterno, int edad) {
 this.nombre = nombre;
 this.ciudadOrigen = ciudadOrigen;
 this.paisOrigen = paisOrigen;
 this.idiomaMaterno = idiomaMaterno;
 this.edad = edad;
 }
 public Usuario(String nombre, String paisOrigen, String idiomaMaterno) {
 this.nombre = nombre;
 this.paisOrigen = paisOrigen;
 this.idiomaMaterno = idiomaMaterno;
 }
 public Usuario(String nombre, String ciudadOrigen, String paisOrigen, String idiomaMaterno) {
 this.nombre = nombre;
 this.ciudadOrigen = ciudadOrigen;
 this.paisOrigen = paisOrigen;
 this.idiomaMaterno = idiomaMaterno;
 }
 public String getIdiomaMaterno() {
 return idiomaMaterno;
 }
 public void setIdiomaMaterno(String idiomaMaterno) {
 this.idiomaMaterno = idiomaMaterno;
 }
 public String getNombre() {
 return nombre;
 }
 public void setNombre(String nombre) {
 this.nombre = nombre;
 }
 public String getCiudadOrigen() {
 return ciudadOrigen;
 }
 public void setCiudadOrigen(String ciudadOrigen) {
 this.ciudadOrigen = ciudadOrigen;
 }
 public String getPaisOrigen() {
 return paisOrigen;
 }
 public void setPaisOrigen(String paisOrigen) {
 this.paisOrigen = paisOrigen;
 }
 public int getEdad() {
 return edad;
 }
 public void setEdad(int edad) {
 this.edad = edad;
 }
 }

Saludo class

public class Saludo implements Serializable {
 private String saludo;
 private String idioma;
 public Saludo(String saludo, String idioma) {
 this.saludo = saludo;
 this.idioma = idioma;
 }
 public String getIdioma() {
 return idioma;
 }
 public void setIdioma(String idioma) {
 this.idioma = idioma;
 }
 public String getSaludo() {
 return saludo;
 }
 public void setSaludo(String saludo) {
 this.saludo = saludo;
 }
}

Implementation

public class ModeloDatos implements Serializable {
 private HashMap<Usuario, Saludo> usuariosSaludos = new HashMap<>();
 public void ingresar (Usuario usuario, Saludo saludo) {
 usuariosSaludos.put(usuario, saludo);
 }
 public String obtenerUsuarios () {
 StringBuilder stringBuilder;
 if (usuariosSaludos.isEmpty()) return "Lista vacía";
 else {
 stringBuilder = new StringBuilder("Lista de usuarios:\n");
 for (Usuario usuario : usuariosSaludos.keySet()) 
 stringBuilder.append(usuario.getNombre()).
 append(" de ").
 append(usuario.getPaisOrigen()).
 append(".\n");
 }
 return stringBuilder.toString();
 }
 public String obtenerSaludos () {
 StringBuilder stringBuilder;
 if (usuariosSaludos.isEmpty()) return "Lista vacía";
 else {
 stringBuilder = new StringBuilder("Lista de saludos:\n");
 for (Saludo saludo : usuariosSaludos.values()) 
 stringBuilder.append("Se dice ").
 append(saludo.getSaludo()).
 append(" en ").
 append(saludo.getIdioma()).
 append(".\n");
 }
 return stringBuilder.toString();
 }
 public String obtenerResultados() {
 StringBuilder stringBuilder;
 if (usuariosSaludos.isEmpty()) return "Lista vacía";
 else {
 stringBuilder = new StringBuilder("Resultados:\n\n");
 for (Usuario usuario : usuariosSaludos.keySet()) {
 Saludo s = usuariosSaludos.get(usuario);
 stringBuilder.append(usuario.getNombre()).
 append(" de ").
 append(usuario.getPaisOrigen()).
 append(".\n").
 append("Nos dice ").
 append(s.getSaludo()).
 append("en ").
 append(s.getIdioma()).append("\n\n");
 }
 }
 return stringBuilder.toString(); 
 }
}

Where:

• ingresar adds an entry to the map using put method.
• obtenerUsuarios iterates over the map's keys using keySet method.
• obtenerSaludos iterates over the map's values using values method.
• obtenerResultados is a combination of both methods.

• diegoaguilar: By the way, Usuario means "User" and Saludo means "Greeting". That's spanish ;)
• diegoaguilar: I use StringBuilder to construct Strings while iterating over the Map.
• user3043278: Thanks I will explore it deeply but now I don't really have the time to do it. I promise that after I do it I'll come back with an answer
• diegoaguilar: No worries, in your case you would only have an Integer as Key. So far would be exactly the same programm ;)
• user3043278: Thank you it helped me to fix my problem :)