Collection is a set of objects that hold references to other objects in the program. In doing the same we already have studied a data structure- HashMap which internally works out the same way. So we come up with one of the approaches to print a Collection in java that is through HashMap. Now a step further we encounter- a collection framework.
The toString method is inherited by all the Classes in Java in order to print any collection in Java by overriding the toString method. After overriding, we can iterate through the collection using a for-each loop to print all the objects of the collection
Collections in java can be printed through 2 approaches which are:
- Printing a user-defined ArrayList
- Printing a user-defined HashMap
Approach 1: Printing a user-defined ArrayList
- Create an ArrayList of the user-defined objects and populate the ArrayList.
- Overrider the toString() method in the user-defined class to print the item of the ArrayList in the desired format.
- Run a for-loop to print the objects.
Example
Java
// Java Program to print an arraylist of an// user-defined collectionÂ
import java.util.*;Â
class GFG {Â
    String name;    int rollNo;Â
    // constructor of class GFG    GFG(String s, int n)    {        name = s;        rollNo = n;    }Â
    // over-riding the toString method    // to print the collection    public String toString()    {        return "Name : " + name + " | Roll No : " + rollNo;    }Â
    // Driver Main Method    public static void main(String[] args)    {        // creating an arraylist of user-defined collection        ArrayList<GFG> arr = new ArrayList<GFG>();Â
        // creating objects of class GFG        GFG t1 = new GFG("John", 101);        GFG t2 = new GFG("Paul", 102);        GFG t3 = new GFG("Jack", 103);        GFG t4 = new GFG("Jose", 104);Â
        // adding objects to arraylist        arr.add(t1);        arr.add(t2);        arr.add(t3);        arr.add(t4);Â
        // printing the collection        for (GFG c : arr)            System.out.println(c);    }} |
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Output
Name : John | Roll No : 101 Name : Paul | Roll No : 102 Name : Jack | Roll No : 103 Name : Jose | Roll No : 10
Approach 2: Printing a user-defined HashMap
- Create a hash map with user-defined key and value pair and fill the hash map using put() method.
- Make sure to Override the toString() method in the user-defined class to print the items in the desired format.
- Iterate over the hash map using the EntrySet() for loop and print the elements.
Example
Java
// Java program printing ArrayList// of an user-defined collectionÂ
// Importing Classes/Filesimport java.util.*;Â
public class GFG {Â Â Â Â String firstName;Â Â Â Â String lastName;Â
    // Constructor    GFG(String fn, String ln)    {        firstName = fn;        lastName = ln;    }Â
    // Function- toString()    public String toString()    {        // Over-riding the toString method to print the        // collection        return "| First Name : " + firstName            + " | LastName : " + lastName;    }Â
    // Driver Main Method    public static void main(String[] args)    {        // Creating a hashmap with key as ID and        // value as user defined class        HashMap<Integer, GFG> hm            = new HashMap<Integer, GFG>();Â
        // creating objects        GFG p1            = new GFG("Mohit", "Singh");        GFG p2            = new GFG("Tarun", "Anand");        GFG p3            = new GFG("Madhu", "Singh");        GFG p4            = new GFG("Rohit", "Ahuja");Â
        // adding mappings        hm.put(101, p1);        hm.put(102, p2);        hm.put(103, p3);        hm.put(104, p4);Â
        // Iterating HashMap through for loop and printing the collection        for (Map.Entry m : hm.entrySet())            System.out.println(m.getKey() + " "                               + m.getValue().toString());    }} |
Output
101 | First Name : Mohit | LastName : Singh 102 | First Name : Tarun | LastName : Anand 103 | First Name : Madhu | LastName : Singh 104 | First Name : Rohit | LastName : Ahuja
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