How to print contents of a class object by field?

Question

I have a POJO class, Location that is used to map a JSON file to a class using Jackson. The current implementation can print out every Location object in the class by calling,Location's toString() but I'm wondering how I can print for example, just the location with id= "2", which would be name="Desert"

At the moment, I use a toString method like this to print all the contents of Location:

public String toString() {
        return "Location [location=" + Arrays.toString(location) + ", id=" + id
                + ", description=" + description + ", weight=" + weight
                + ", name=" + name + ", exit=" + Arrays.toString(exit)
                +"]";
    }

Does anyone know how I can print specific locations within the Location object based on a field id?

This is an example of what is stored in the Location class when I call toString() on it:

http://hastebin.com/eruxateduz.vhdl

An example of one of the Locations within the Location object:

[Location [location=null, id=1, description=You are in the city of Tiberius. You see a long street with high buildings and a castle.You see an exit to the south., weight=100, name=Tiberius, exit=[Exit [title=Desert, direction=South]]]

This is the POJO location class I use to map the JSON fields to the class:

public class Location {

    private Location[] location;

    private int id;

    private String description;

    private String weight;

    private String name;

    private Exit[] exit;

    private boolean visited = false;
    private boolean goalLocation;
    private int approximateDistanceFromGoal = 0;
    private Location parent;




    public Location[] getLocation() {
        return location;
    }

    public void setLocation(Location[] location) {
        this.location = location;
    }


    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getDescription ()
    {
        return description;
    }

    public void setDescription (String description)
    {
        this.description = description;
    }


    public String getWeight() {
        return weight;
    }

    public void setWeight(String weight) {
        this.weight = weight;
    }

    public String getName ()
    {
        return name;
    }

    public void setName (String name)
    {
        this.name = name;
    }

    public Exit[] getExit() {
        return exit;
    }

    public void setExit(Exit[] exit) {
        this.exit = exit;
    }


    public boolean isVisited() {
        return visited;
    }

    public void setVisited(boolean visited) {
        this.visited = visited;
    }

    public boolean isGoalLocation() {
        return goalLocation;
    }

    public void setGoalLocation(boolean goalLocation) {
        this.goalLocation = goalLocation;
    }

    public int getApproximateDistanceFromGoal() {
        return approximateDistanceFromGoal;
    }

    public void setApproximateDistanceFromGoal(int approximateDistanceFromGoal) {
        this.approximateDistanceFromGoal = approximateDistanceFromGoal;
    }

    public Location getParent() {
        return parent;
    }

    public void setParent(Location parent) {
        this.parent = parent;
    }

    @Override
    public String toString() {
        return "Location [location=" + Arrays.toString(location) + ", id=" + id
                + ", description=" + description + ", weight=" + weight
                + ", name=" + name + ", exit=" + Arrays.toString(exit)
                +"]";
    }



}

Answer 1

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-collections4</artifactId>
    <version>4.0</version>
</dependency>

You need the above dependency to define predicate unless you want to do it on your own.

public class Location {
   private int id;
   // more stuff here

   private Predicate<Integer> filter;
     public Location() {
          this.filter = TruePredicate.INSTANCE; 
     }
     public Location(int idFilter) {
             this.filter = new EqualPrediate(idFilter);
     }

     public String toString() {
       StringBuilder buffer = new StringBuilder();
          if(filter.apply(this.id)) { 
             buffer.append("Location [location=" + Arrays.toString(location) + ", id=" + id
                + ", description=" + description + ", weight=" + weight
                + ", name=" + name + ", exit=" + Arrays.toString(exit)
                +"]");
          }
       return buffer.toString();
     }

}

This code is a simplified Visitor Pattern where the

'Visitor' -> your predicate

'this' -> 'this.id'

This works because your toString() is invoking the toString() of the nested Location objects which also have their predicates for filtering set.

If you aren't in control of their construction where you can propogate the filter then you can take this approach:

 public String toString() {
           StringBuilder buffer = new StringBuilder();
           int i = 0;
           for(Location l = this; i < locations.length; l = locations[i++])
              if(filter.apply(l.id) { 
                 buffer.append("Location [location=" + Arrays.toString(location) + ", id=" + id
                    + ", description=" + description + ", weight=" + weight
                    + ", name=" + name + ", exit=" + Arrays.toString(exit)
                    +"]");
              }
           return buffer.toString();
         }