Collections Data-structure in Java

Any group of individual objects which are represented as a single unit is known as the collection of the objects. In Java, a separate framework named the “Collection Framework” has been defined in JDK 1.2 which holds all the collection classes and interface in it.

The Collection interface (java.util.Collection) and Map interface (java.util.Map) are the two main “root” interfaces of Java collection classes.

//import util package[all classes]
import java.util.*;
public class StartCode {
    public static void main(String[] args) {
        System.out.println("Welcome to code java collection framework");
       /*
       creating collection
       1)Type Safe- same type of elements(objects) are added to collection
       2)Un Type safe- different types of elements can be added to collection
        */
        //Type safe collection
        ArrayList<String> names = new ArrayList<String>();
        names.add("Abdul");
        names.add("Sitara");
        names.add("Roshni");
        names.add("Vandna");
        names.add("Vandna");
        System.out.println(names);
        //get
//        System.out.println(names.get(0));
//        System.out.println(names.get(1));
        //un type safe
//        LinkedList list = new LinkedList();
//        list.add("Sachin");
//        list.add(101);
//        list.add(625.5124);
//        list.add(true);
//        System.out.println(list);
        //remove
        names.remove("Roshni");
        System.out.println(names);
        //size
        System.out.println("SIZE = " + names.size());
        //check item is there or not
        // Type ka equals method call karta hai checking karne ke lie
        //Student
        System.out.println(names.contains("Durgesh Kumar"));
        //check for empy
        System.out.println(names.isEmpty());
        //setting value..
        //names.set(1,"Ram");
        names.add(1, "Ram");
        System.out.println(names);
        //remove all elements
//        names.clear();
        System.out.println(names);
        Vector<String> vector = new Vector<>();
        vector.addAll(names);
        System.out.println("VECTOR " + vector);
        System.out.println("_____________________________________");
        HashSet<Double> nms = new HashSet<>();
        nms.add(14.14);
        nms.add(34.12354);
        nms.add((2354.235));
        nms.add(99.3);
        nms.add(99.3);
        nms.add(5.3);
        System.out.println(nms);
        TreeSet<Double> tset = new TreeSet<>();
        tset.addAll(nms);
        System.out.println(tset);
    }
}

TraverseExample.java

import java.util.*;
import java.util.function.Consumer;
public class TraverseExample {
    public static void main(String[] args) {
        ArrayList<String> names = new ArrayList<String>();
        names.add("Abdul");
        names.add("Sitara");
        names.add("Roshni");
        names.add("Vandna");
        names.add("Vandna");
        names.add("ABC");
        //for each loop:
        for(String str:names)
        {
            System.out.print(str +"\t"+str.length()+"\t");
            StringBuffer br=new StringBuffer(str);
            System.out.println(br.reverse());
        }
        System.out.println("________________________");
        //traversing using ITERATOR: Forward traversing
        Iterator<String> itr = names.iterator();
        while(itr.hasNext())
        {
            String next = itr.next();
            System.out.println(next);
        }
        System.out.println("+++++++++++++++++++++++");

        //backward traversal of collection LISTITERATOR
        ListIterator<String> litr = names.listIterator(names.size());
        while(litr.hasPrevious())
        {
            String previous = litr.previous();
            System.out.println(previous);
        }
        //Enumeration
        System.out.println("++++++++FOR EACH METHOD+++++++++++++");
        //for each method
        names.forEach(str->{
            System.out.println(str);
        });

        System.out.println("SORTING OF ELEMENTS");
        TreeSet<String> set=new TreeSet<>();
        set.addAll(names);
        set.forEach(e->{
            System.out.println(e);
        });
//        Comparable
//        Comparator
    }
}

MapExample

import java.util.HashMap;
import java.util.function.BiConsumer;

public class MapExample {
    public static void main(String[] args) {

        HashMap<String,Integer>  courses=new HashMap<>();

        //adding elements
        courses.put("Core Java",4000);
        courses.put("Basic Python",3500);
        courses.put("Spring",8000);
        courses.put("Android",4000);
        courses.put("Android",6000);
        courses.put("PHP",2414);
        System.out.println(courses);
//        courses.forEach((e1,e2)->{
//            System.out.println(e1+"=>"+e2);
//        });
        courses.forEach((key,value)->{
            System.out.print(key);
            System.out.print("=>");
            System.out.println(value);
            System.out.println();
        });
        //entry set
        //key set
        System.out.println(courses.get("Core Java"));
    }
}