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readme.md

ArrayList

Initializing ArrayLists

Method 1: Initialization using Arrays.asList

ArrayList<Type> obj = new ArrayList<Type>(
        Arrays.asList(Object o1, Object o2, Object o3, ....so on));

Method 2: Anonymous inner class method to initialize ArrayList

ArrayList<T> obj = new ArrayList<T>(){{
		   add(Object o1);
		   add(Object o2);
		   add(Object o3);
                   ...
                   ...
		   }};

Method 3: Normal way of ArrayList initialization (use .add(‘x’) after initializing the array)

Method 4: Use Collections.ncopies (ArrayList<T> obj = new ArrayList<T>(Collections.nCopies(count, element));)

public class Details {
   public static void main(String args[]) {
	   ArrayList<Integer> intlist = new ArrayList<Integer>(Collections.nCopies(10, 5));
	  System.out.println("ArrayList items: "+intlist);
   }
} // → ArrayList items: [5, 5, 5, 5, 5, 5, 5, 5, 5, 5]

Key methods:

Retrieving length of ArrayList .size()

int numberofitems = obj.size();

Retrieving values by index .get(i)

String str= obj.get(2);

Checking if element exists .contains()

obj.contains("Steve"); //returns a boolean value

Adding and removing elements

obj.add(‘x’)
obj.remove(‘x’)

Adding/Removing element by index

obj.add(0, "Rahul");
obj.remove(3); When given an integer, .remove() will read it as an index
obj.set(2, "Tom"); this replaces the element at the given index

Getting index of element

int pos = obj.indexOf("Tom");

Emptying the arraylist

obj.clear(); //remove everything from the arraylist

Looping

Apart from for, forEach and while, we can also use the iterator

Iterator iter = arrlist.iterator();
      while (iter.hasNext()) {
         System.out.println(iter.next());
      }

We can also use the Enumerator interface

import java.util.Enumeration;
// Get the Enumeration object
      Enumeration<String> e = Collections.enumeration(arrayList);

      // Enumerate through the ArrayList elements
      System.out.println("ArrayList elements: ");
      while(e.hasMoreElements())
      System.out.println(e.nextElement());

Sorting

Sorting strings and numbers - .sort()
Reversing the sort: call .sort() and then .reverse()
Sorting objects using Comparable:

public class Student implements Comparable {
    private String studentname;
    private int rollno;
    private int studentage;

    public Student(int rollno, String studentname, int studentage) {
        this.rollno = rollno;
        this.studentname = studentname;
        this.studentage = studentage;
    }
    //getter and setter methods same as the above example

    @Override
    public int compareTo(Student comparestu) {
        int compareage=((Student)comparestu).getStudentage();
        /* For Ascending order*/
        return this.studentage-compareage;

        /* For Descending order do like this */
        //return compareage-this.studentage;
    } // after doing this, we can then call Collections.sort(arraylist);

    @Override
    public String toString() {
        return "[ rollno=" + rollno + ", name=" + studentname + ", age=" + studentage + "]";
    }

}

Comparing two ArrayLists

You have to loop through one array and use the .contains(Object o) method within the for loop .contains(...) returns true if the list contains the Object o else it returns false.

for (String temp : al1)
              al3.add(al2.contains(temp) ? "Yes" : "No");
          System.out.println(al3);

Other frequently-used ArrayList methods:

toString() When we are dealing with ArrayList of Objects then it is must to Override the toString() method in order to get the output in desired format. Otherwise, using toString() will return this: loremipsum.com.Student@10b28f30
Join/combine two ArrayLists .addAll()

arraylist3.addAll(arraylist1);
arraylist3.addAll(arraylist2);

Clone an ArrayList to another ArrayList

ArrayList<String> al = new ArrayList<String>();
// add stuff to it
ArrayList<String> al2 = (ArrayList<String>)al.clone();

Empty an arraylist

.clear() // Method 1. recommended. Time complexity O(N)
al1.clear();

.removeAll()  // Method 2. Time complexity O(N2)
al2.removeAll(al2);

Check if an array is empty - al2.isEmpty();
Memory optimisation - al.trimToSize() If the array had the initial size of 50, but only has 5 elements now, you can call trimToSize() to free up the memory Converting ArrayList to String[]
synchronize() ArrayList (i skipped this)

java-gitbook/02-data-structures/arraylist at master · davified/java-gitbook · GitHub

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