DataStructures : ArrayList, HashMap, Set

ArrayList

- Values could be duplicated

- Use to store multiple "elements" together, represented by single variable name

- The "elements" could be of different Data types such as int, double, Objects
- No need to specify size while declaration, size = Dynamic
- Element can be accessed using array "index"
- Array index starts from zero [0]

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Ex. 1: ArrayList 

Source: TestArrayList.java

import java.util.ArrayList;

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


        ArrayList list = new ArrayList();
   
        list.add("Tokyo");
        list.add("Edogawa ku");
        list.add("Minato ku");
        list.add("Toshima ku");


        Customer cust = new Customer();
        list.add(cust); // object can be added to ArrayList
   
        System.out.println(list.get(3));
    }
}

Output: 

Toshima ku

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HashMap

- Use to store data in key and value pair 

- Key should be unique and of reference data type

- No need to specify size while declaration, size = Dynamic

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Ex. 2: HashMap 

Source: TestHashMap.java

import java.util.HashMap;

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


        HashMap hash = new HashMap();
        //key and value pair (phone/name)
        hash.put("5678", "Yamada");
        hash.put("8934", "Suzuki");
        hash.put("1234", "John");
   
        System.out.println(hash.get("8934"));
    }
}

Output: 

Suzuki


======================================================

TreeSet

- Use to store unique values, no duplicates

- Use to store multiple "elements" together, represented by single variable name

- The "elements" could be of different Data types such as int, double, Objects

- No need to specify size while declaration, size = Dynamic
- Element can be accessed using array "index"
- Array index starts from zero [0]

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Ex. 3: TreeSet 

Source: TestTreeSet.java

import java.util.TreeSet;

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

        TreeSet tset = new TreeSet();        

        tset.add(1);
        tset.add(2);
        tset.add(3);
        tset.add(3);
   
        System.out.println(tset.toString());    
   }

}

Output: 

[1,2,3]