Use BufferedReader to read a file

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;

public class BufferedReaderTest {
   
    public static void main(String[] args) throws Exception{
        String fileName = "c:\\tmp\\test.txt";
        File file = new File(fileName);
        FileReader fr = new FileReader(file);
        BufferedReader br = new BufferedReader(fr);
        String line = "";
        while ((line = br.readLine())!=null){
            System.out.println("line="+line);
           
        }
        br.close();
        fr.close();
    }

}

Use BufferedWriter write to a file

import java.io.BufferedWriter;
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.io.Writer;

public class BufferedWriterTest {
   
    public static void main(String[] args) throws IOException{
        String fileName = "C:\\tmp\\test.txt";
        File file = new File(fileName);    // File needs fileName
        FileWriter fw =  new FileWriter(file);    // so here use Writer implementing class - FileWriter
        BufferedWriter bw = new BufferedWriter(fw); //Writer can not be initialized b/c it's abstract class
        try {
            bw.write("this is my test file");
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } finally{
            bw.close();    // when close, the content can be written to the file
            fw.close();
        }

       
    }

}

Collections.synchronizedMap when map is used in thread

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

// HashMap is not synchronized

public class MapTest {
    public static void main(String[] args){
        Map<String, String> map = Collections.synchronizedMap(new HashMap<String, String>());
        map.put("1", "a");
        map.put("2", "b");
        map.put("3", "c");
       
        for (Map.Entry<String, String> entry:map.entrySet()){
           
            System.out.println("key="+entry.getKey()+" value="+entry.getValue());
           
        }
       
    }

}

OUTPUT:
key=3 value=c
key=2 value=b
key=1 value=a

Reverse Number

// how to write program: 
// give an example
// identify number by variable
// convert to program
// verify program by example

// if number to reverse is 123
// 12 3     0x10+3
// 1   2     3x10+2
// 0   1     32x10+1

public class ReverseNumberTest {
   
    public int reverseNumber(int number){
       
        int reverse = 0;
        while (number!=0){
            reverse = reverse*10 + number % 10;
            number = number / 10;
        }
       
        return reverse;
    }
   
    public static void main(String[] args){
        ReverseNumberTest rnt = new ReverseNumberTest();
        System.out.println("reverse number="+rnt.reverseNumber(123));
    }

}

Reverse String (recursive)

public class ReverseStringTest {
   
    public String reverseString(String str){
       
        // str.substring(0,2) returns index of 0 and 1 of str
        return (str.length()==1? str: str.charAt(str.length()-1) + reverseString(str.substring(0,str.length()-1)));
       
    }
   
    public static void main(String[] args){
        ReverseStringTest rst = new ReverseStringTest();
        System.out.println(rst.reverseString("abcd"));
    }

}

OUTPUT:
dcba

ArrayList Loop (for, Iterator, ListIterator)

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

// Iterator vs ListIterator
// 1. Iterator traverse List and Set; ListIterator traverse List
// 2. Iterator can go next; ListIterator can go next and previous
// 3. Iterator can not get Index; ListIterator can get Index
// 4. Iterator can NOT add item when traverse; ListIterator can add when traverse
// 5. Iterator can NOT replace item when traverse; ListIterator can replace when traverse

public class ArrayListTest {
   
    public static void main(String[] args){
       
        List<String> list = new ArrayList<String>();
        list.add("A");
        list.add("B");
        list.add("C");
       
        // use enhanced for to loop
        for (String str : list){
            System.out.println(str);
        }
       
        // use Iterator to loop
        Iterator it = list.iterator();
        while (it.hasNext()){
            System.out.println(it.next());
        }
       
        // use ListIterator to loop
        ListIterator li = list.listIterator();
        while (li.hasNext()){
            System.out.println(li.next());
        }
               
    }

}

OUTPUT:

A
B
C
A
B
C
A
B
C

java.lang.Math.random() and java.util.Random

import java.util.Random;

public class RandomTest {
   
    public static void main(String[] args){
       
        // 1) java.util.Random to generate random number
        Random random = new Random(1000); // 1000 is seed
        for (int i = 0; i<3; i++){
            System.out.println(random.nextInt()); // generates same random number because of seed is fixed
        }
       
        // 2) java.lang.Math.random() to generate random number
        for (int i=0; i<3; i++){
            System.out.println(Math.random()); // random double between 0 and 1, 0 is inclusive
        }
    }

}

OUTPUT:

-1244746321   (same)
1060493871    (same) 
-1826063944   (same)
0.22284876712156954   (different)
0.19407514115469882   (different)
0.2939945031187672     (different)

Loop Map using Map.Entry in map.entrySet()

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

public class MapEntryTest {
   
    public static void main(String[] args){
       
        Map<String, String > map = new HashMap<String, String>();
       
        //initialize map
        map.put("1", "a");
        map.put("2", "b");
        map.put("3", "c");

        // entry in entrySet
        for (Map.Entry<String, String> entry : map.entrySet()){
            System.out.println("key="+entry.getKey()+" value="+entry.getValue());
        }
    }

}

OUTPUT:
key=3 value=c
key=2 value=b
key=1 value=a

HashMap

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

public class FindDuplicateInString {
   
    public void findDuplicateInString(String str){
        char[] chars = str.toCharArray();
        Map<Character,Integer> map = new HashMap<Character,Integer>();
       
        // constructs map
        for (char ch : chars){
            if (map.containsKey(ch)){
                map.put(ch, map.get(ch)+1);
            }else{
                map.put(ch,1);
            }
        }
       
        // print out duplicate char
        Set<Character> keys = map.keySet();
        for (Character key : keys){
            if (map.get(key)>1){
                System.out.println("duplicate char: "+key);
            }
        }
       
       
    }
   
    public static void main(String[] args){
        FindDuplicateInString duplicate = new FindDuplicateInString();
        duplicate.findDuplicateInString("java");
    }

}
OUTPUT:
duplicate char: a

ListIterator

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

public class ListIteratorTest {
   
    public static void main(String[] args){
        List<Integer> list = new ArrayList<Integer>();
        list.add(1);
        list.add(2);
        list.add(3);
        ListIterator<Integer> li = list.listIterator();
       
        while (li.hasNext()){
            System.out.println(li.next());
        }
       
        while (li.hasPrevious()){
            System.out.println(li.previous());
        }
       
    }

}
OUTPUT:
1
2
3
3
2
1