Java Classes

www.w3schools.com/java/java_class_methods.asp

Java Class Methods

 

 

 

Static vs. Non-Static

스태틱은 오브젝트의 생성과 상관없이 접근가능함.

 

www.w3schools.com/java/java_modifiers.asp

Java Modifiers

 

 

 

 

Date & time 은 별로 필요없을것 같긴하지만..

 

www.w3schools.com/java/java_date.asp

Java Date and Time

 

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www.w3schools.com/java/java_arraylist.asp

Java ArrayList

 

c++의 벡터와 차이점이 뭐지?

 

정렬은

ArrayList<String> cars = new ArrayList<String>();

cars.add("Volvo");

Collections.sort(cars);

 

 

www.w3schools.com/java/java_linkedlist.asp

Java LinkedList

ArrayList vs. LinkedList

The LinkedList class is a collection which can contain many objects of the same type, just like the ArrayList.

The LinkedList class has all of the same methods as the ArrayList class because they both implement the List interface. This means that you can add items, change items, remove items and clear the list in the same way.

However, while the ArrayList class and the LinkedList class can be used in the same way, they are built very differently.

How the ArrayList works

The ArrayList class has a regular array inside it. When an element is added, it is placed into the array. If the array is not big enough, a new, larger array is created to replace the old one and the old one is removed.

How the LinkedList works

The LinkedList stores its items in "containers." The list has a link to the first container and each container has a link to the next container in the list. To add an element to the list, the element is placed into a new container and that container is linked to one of the other containers in the list.

When To Use

It is best to use an ArrayList when:

• You want to access random items frequently
• You only need to add or remove elements at the end of the list

It is best to use a LinkedList when:

• You only use the list by looping through it instead of accessing random items
• You frequently need to add and remove items from the beginning or middle of the
list

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LinkedList Methods

For many cases, the ArrayList is more efficient as it is common to need access to random items in the list, but the LinkedList provides several methods to do certain operations more efficiently:

MethodDescriptionTry it

addFirst()	Adds an item to the beginning of the list.	Try it »
addLast()	Add an item to the end of the list	Try it »
removeFirst()	Remove an item from the beginning of the list.	Try it »
removeLast()	Remove an item from the end of the list	Try it »
getFirst()	Get the item at the beginning of the list	Try it »
getLast()	Get the item at the end of the list	Try it »

 

 

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www.w3schools.com/java/java_hashmap.asp

Java HashMap

// Import the HashMap class
import java.util.HashMap;

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

    // Create a HashMap object called people
    HashMap<String, Integer> people = new HashMap<String, Integer>();


    // Add keys and values (Name, Age)
    people.put("John", 32);
    people.put("Steve", 30);
    people.put("Angie", 33);

    for (String i : people.keySet()) {
      System.out.println("key: " + i + " value: " + people.get(i));
    }
  }
}

 

 

 

import java.util.HashSet;

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

    // Create a HashSet object called numbers
    HashSet<Integer> numbers = new HashSet<Integer>();

    // Add values to the set
    numbers.add(4);
    numbers.add(7);
    numbers.add(8);

    // Show which numbers between 1 and 10 are in the set
    for(int i = 1; i <= 10; i++) {
      if(numbers.contains(i)) {
        System.out.println(i + " was found in the set.");
      } else {
        System.out.println(i + " was not found in the set.");
      }
    }
  }
}

 

 

 

 

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

public class MyClass {
  public static void main(String[] args) {
    ArrayList<Integer> numbers = new ArrayList<Integer>();
    numbers.add(12);
    numbers.add(8);
    numbers.add(2);
    numbers.add(23);
    Iterator<Integer> it = numbers.iterator();
    while(it.hasNext()) {
      Integer i = it.next();
      if(i < 10) {
        it.remove();
      }
    }
    System.out.println(numbers);
  }
}

 

 

Primitive Data TypeWrapper Class

byte	Byte
short	Short
int	Integer
long	Long
float	Float
double	Double
boolean	Boolean
char	Character

Sometimes you must use wrapper classes, for example when working with Collection objects, such as ArrayList, where primitive types cannot be used (the list can only store objects):

 

 

public class MyClass {
  public static void main(String[] args) {
    Integer myInt = 5;
    Double myDouble = 5.99;
    Character myChar = 'A';
    System.out.println(myInt.intValue());
    System.out.println(myDouble.doubleValue());
    System.out.println(myChar.charValue());
  }
}

 

 

public class MyClass {
  public static void main(String[] args) {
    Integer myInt = 100;
    String myString = myInt.toString();
    System.out.println(myString.length());
  }
}

 

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class MyClass {
  public static void main(String[] args) {
    Pattern pattern = Pattern.compile("w3schools", Pattern.CASE_INSENSITIVE);
    Matcher matcher = pattern.matcher("Visit W3Schools!");
    boolean matchFound = matcher.find();
    if(matchFound) {
      System.out.println("Match found");
    } else {
      System.out.println("Match not found");
    }
  }
}
// Outputs Match found

 

 

 

www.w3schools.com/java/java_threads.asp

Java Threads

thread 와 concurrency 도 다시보기

 

www.w3schools.com/java/java_lambda.asp

Java Lambda Expressions

www.w3schools.com/java/java_files.asp

Java Files

www.w3schools.com/java/java_files_create.asp

Java Create and Write To Files

 

 

www.w3schools.com/java/java_files_read.asp

Java Read Files

 

 

 

www.w3schools.com/java/java_files_delete.asp

Java Delete Files

 

www.w3schools.com/java/java_howto_add_two_numbers.asp

Java How To Add Two Numbers

 

 

www.w3schools.com/java/java_ref_string.asp

Java String Reference

www.w3schools.com/java/java_ref_math.asp

Java Math Reference

 

 

www.w3schools.com/quiztest/result.asp

https://www.w3schools.com/quiztest/result.asp

25문제 나오는 문제까지 다 풀어버렸넹

 

너무 간단한 내용들이었다..

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갑자기 이걸 왜 본건지는 모르겠으나...

 

dzone.com/articles/20-useful-open-source-libraries-for-java-programme

20 Useful Libraries Java Programmers Should Know - DZone Java