1. StringTokenizer example

In the old days, Java developers like to use the StringTokenizer class to split a string. This is because the StringTokenizer class is available since JDK 1.0 and the String.split() is available since JDK 1.4

package com.mkyong.test

import java.util.StringTokenizer;

public class TestSplit {

 public static void main(String[] args) {

  String test = "abc.def.123";

  StringTokenizer token = new StringTokenizer(test, ".");

  while (token.hasMoreTokens()) {
   System.out.println(token.nextToken());
  }
  
 }

}

Output

abc
def
123

reference: www.mkyong.com/java/java-how-to-split-a-string/ 

2. HASHMAP

package com.mkyong.test;

import java.util.HashMap;
import java.util.Map;

public class TestMap {

 public static void main(String[] args) {
  
  Map<String, Integer> fruits = new HashMap<>();
  fruits.put("apple", 1);
  fruits.put("orange", 2);
  fruits.put("banana", 3);
  
  if(fruits.containsKey("apple")){
   //key exists
   System.out.println(fruits.get("apple"));
  }else{
   //key not exists
  }
  
 }

}

3. Integer.parseInt() Examples

Example to convert a String “10” to an primitive int.

String number = "10"; 
int result = Integer.parseInt(number); 
System.out.println(result);
 
Output
10 

4. Java Leap Year Example

Java example to determine if the given year is a leap year.

package com.mkyong.utils;

public class DateTimeExample {

    public static void main(String[] args) {

 DateTimeExample obj = new DateTimeExample();
 System.out.println("1993 is a leap year : " + obj.isLeapYear(1993));
 System.out.println("1996 is a leap year : " + obj.isLeapYear(1996));
 System.out.println("2012 is a leap year : " + obj.isLeapYear(2012));

    }

    public boolean isLeapYear(int year) {

 if ((year % 400 == 0) || ((year % 4 == 0) && (year % 100 != 0))) {
  return true;
 } else {
  return false;
 }
    }
 
} 

5 How to join two Lists in Java

List.addAll()

package com.mkyong.example;

import java.util.ArrayList;
import java.util.List;

public class JoinListsExample {

 public static void main(String[] args) {
 
  List<String> listA = new ArrayList<String>();
  listA.add("A");
  
  List<String> listB = new ArrayList<String>();
  listB.add("B");
  
  List<String> listFinal = new ArrayList<String>();
  listFinal.addAll(listA);
  listFinal.addAll(listB);
  
  //same result
  //List listFinal = new ArrayList(listA);
  //listFinal.addAll(listB);
  
  System.out.println("listA : " + listA);
  System.out.println("listB : " + listB);
  System.out.println("listFinal : " + listFinal);
  
 }

}
 

6 How to count duplicated items in Java List

 1.

        Map<String, Integer> map = new HashMap<String, Integer>();

 for (String temp : list) {
  Integer count = map.get(temp);
  map.put(temp, (count == null) ? 1 : count + 1);
 }
 printMap(map); 

 2

        System.out.println("\nSorted Map");
 Map<String, Integer> treeMap = new TreeMap<String, Integer>(map);
 printMap(treeMap);

7 How to loop ArrayList in Java

package com.mkyong.core;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

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

  List<String> list = new ArrayList<String>();
  list.add("Text 1");
  list.add("Text 2");
  list.add("Text 3");

  System.out.println("#1 normal for loop");
  for (int i = 0; i < list.size(); i++) {
   System.out.println(list.get(i));
  }

  System.out.println("#2 advance for loop");
  for (String temp : list) {
   System.out.println(temp);
  }

  System.out.println("#3 while loop");
  int j = 0;
  while (list.size() > j) {
   System.out.println(list.get(j));
   j++;
  }

  System.out.println("#4 iterator");
  Iterator<String> iterator = list.iterator();
  while (iterator.hasNext()) {
   System.out.println(iterator.next());
  }
 }
}
 

8 Convert char array to String in Java

 

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

  char[] charArrays = new char[] {'1', '2', '3', 'A', 'B', 'C'};
  
  String newString1 = new String(charArrays);
  System.out.println("newString1 : " + newString1);
       
  String newString2;
  newString2 = String.valueOf(charArrays);
  System.out.println("newString2 : " + newString2);
 }
} 

9 How to sort an ArrayList in java

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class SortArrayList{
 
 public static void main(String args[]){
  
  List<String> unsortList = new ArrayList<String>();
  
  unsortList.add("CCC");
  unsortList.add("111");
  unsortList.add("AAA");
  unsortList.add("BBB");
  unsortList.add("ccc");
  unsortList.add("bbb");
  unsortList.add("aaa");
  unsortList.add("333");
  unsortList.add("222");
  
  //before sort
  System.out.println("ArrayList is unsort");
  for(String temp: unsortList){
   System.out.println(temp);
  }
  
  //sort the list
  Collections.sort(unsortList);
  
  //after sorted
  System.out.println("ArrayList is sorted");
  for(String temp: unsortList){
   System.out.println(temp);
  }
 }
 
}
 

10 How to sort a Map in Java

http://www.mkyong.com/java/how-to-sort-a-map-in-java/

package com.mkyong;

import java.util.HashMap;
import java.util.Map;
import java.util.TreeMap;

public class SortMapOnKeyStringExample {

 public static void main(String[] args) {

  Map<String, String> unsortMap = new HashMap<String, String>();
  unsortMap.put("Z", "z");
  unsortMap.put("B", "b");
  unsortMap.put("A", "a");
  unsortMap.put("C", "c");
  unsortMap.put("D", "d");
  unsortMap.put("E", "e");
  unsortMap.put("Y", "y");
  unsortMap.put("N", "n");
  unsortMap.put("J", "j");
  unsortMap.put("M", "m");
  unsortMap.put("F", "f");

  System.out.println("Unsort Map......");
  printMap(unsortMap);

  System.out.println("\nSorted Map......");
  Map<String, String> treeMap = new TreeMap<String, String>(unsortMap);
  printMap(treeMap);

 }

 public static void printMap(Map<String, String> map) {
  for (Map.Entry<String, String> entry : map.entrySet()) {
   System.out.println("Key : " + entry.getKey() 
                                      + " Value : " + entry.getValue());
  }
 }

} 

11 How to loop a Map in Java

Map<String, String> map = new HashMap<String, String>();
 map.put("1", "Jan");
 map.put("2", "Feb");
 map.put("3", "Mar");
 
 //loop a Map
 for (Map.Entry<String, String> entry : map.entrySet()) {
  System.out.println("Key : " + entry.getKey() + " Value : " + entry.getValue());
 }
  
 //Java 8 only, forEach and Lambda
 map.forEach((k,v)->System.out.println("Key : " + k + " Value : " + v));

12 How to convert List to Set (ArrayList to HashSet)

Convert List to Set

Set set = new HashSet(list);

Convert Set to List

List list = new ArrayList(set);

13 Java HashMap example

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

   Map mMap = new HashMap();
   mMap.put("PostgreSQL", "Free Open Source Enterprise Database");
   mMap.put("DB2", "Enterprise Database , It's expensive");
   mMap.put("Oracle", "Enterprise Database , It's expensive");
   mMap.put("MySQL", "Free Open SourceDatabase");

   Iterator iter = mMap.entrySet().iterator();

   while (iter.hasNext()) {
    Map.Entry mEntry = (Map.Entry) iter.next();
    System.out.println(mEntry.getKey() + " : " + mEntry.getValue());
   }

   mMap.put("Oracle", "Enterprise Database , It's free now ! (hope)");

   System.out.println("One day Oracle.. : " + mMap.get("Oracle"));

  } catch (Exception e) {
   System.out.println(e.toString());
  }
 }
}