StringBuffer

String Buffer helps you modify string very easily and in a efficient way. Whenever you have to modify a string repeatatively, using StringBuffer instead of simple String is highly recommended.

class Main {
  public static void main(String[] args) {
    
    /* String Buffer (Most commonly used methods) */

    String initialString = "z";
    StringBuffer sb=new StringBuffer(initialString);  
    sb.append("Hello");     // zHello
    sb.insert(1,"World");    // zWorldHello
    sb.replace(1,6,"HEY");     // zHEYWorld  NOTE: 1 to 5 is replaced(6th is excluded)
    sb.delete(1,4);    // zHello  NOTE: 1 to 3 is replaced(4nd is not deleted)
    sb.deleteCharAt(0);     //Hello
    sb.reverse();     // olleH
    sb.setCharAt(4, 'h');    // olleh 
    String s = sb.substring(1, 4);     // s=lle   NOTE: 1 included 4 excluded
    int x = sb.indexOf("l");    // x = 1
    x = sb.lastIndexOf("l");    // x = 2
    x = sb.lastIndexOf("some_string_which_is_not_present_in_the_main_string");  // x = -1
    
    
    System.out.println("Modified String: "+ sb.toString());
  }
}

ArrayList

Knowing ArrayList will make your life a lot easier because you can use it as Array, Linkedlist, queue, dequeue, stack etc.

void add(int index, E element)
boolean add(E e)
boolean addAll(Collection<? extends E> c)
boolean addAll(int index, Collection<? extends E> c)
void clear()
E get(int index)
boolean isEmpty()
int indexOf(Object o)
int lastIndexOf(Object o)
boolean contains(Object o)
E remove(int index)
boolean remove(Object o)
boolean removeAll(Collection<?> c)
E set(int index, E element)
Collections.sort(ArrayListName);

HashMap

/* HashMap (Most commonly used methods) */
HashMap<Integer,String> map=new HashMap<Integer,String>();
V put(Object key, Object value)
void putAll(Map map)
V putIfAbsent(K key, V value)
V remove(Object key)
boolean remove(Object key, Object value)
boolean containsValue(Object value)
boolean containsKey(Object key)
V get(Object key)
boolean isEmpty()
V replace(K key, V value)
int size()

//Iterating over map
for (Map.Entry<Integer,String> entry : map.entrySet()){
  int key = entry.getKey();
  String value = extry.getValue();
  System.out.println("KEY:  "+key + "      VALUE:   "+value);
}

Set

import java.util.Set;
import java.util.Arrays;
import java.util.HashSet;
class Main {
  public static void main(String[] args) {
    Set<String> set = new HashSet<String>();
    System.out.println("Is set empty?  "+set.isEmpty()+"\n");
    set.add("Hello");
    set.add("World");
    set.add("How");
    set.add("a");
    set.add("b");
    set.add("c");

    System.out.println("After adding elements :  "+set);
    System.out.println("Is set empty now?  "+set.isEmpty()+"\n");
    
    System.out.println("Now lets remove element \"Hello\" :  ");
    set.remove("Hello");
    System.out.println("After removing :"+set+"\n");

    System.out.println("Now lets remove element list {\"a\",\"c\",\"something\"} :  ");
    String removeMeList[] = { "a","c","something" };
    set.removeAll(Arrays.asList(removeMeList));

    System.out.println("After removing the element list : "+set+"\n");

    System.out.println("To add element list use \"set.addAll(arrayname)\".");
    System.out.println("Lets add String array {\"coding\",\"is\",\"love\"} to set.");
    set.addAll(Arrays.asList(new String[]{"coding","is","love"}));
    System.out.println("After adding the element list : "+set+"\n");
    
    System.out.println("\n----- Iterating over the set -----");
    System.out.println("Size of set now:"+set.size()+"\n");
    for(String x: set){
      System.out.println(x);
    }
  }
}