[```markmap
## 1. Primitive Conversion
- 1.1 Upcasting
- 1.1 Upcasting(Implicit Conversion / Promotion)

- 1.2 Downcasting

## 2. String Method
- 2.1 isBlank()
- 2.2 isEmpty()
- 2.3 substring
- 2.3.1 substring(int startIndex)
- 2.3.2 substring(int startIndex, int endIndex)
- 2.4 toLowerCase()
- 2.5 toUpperCase()
- 2.6 replace(char oldChar, char newChar)
- 2.7 replace(CharSequence target, CharSequence replacement)
- 2.8 contains(CharSequence sequence)
- 2.9 endsWith(String suffix)
- 2.10 startsWith(String prefix)
- 2.11 trim()
- 2.12 equals()
- 2.12 equalsIgnoreCase()
- 2.13 indexOf(char ch)
- 2.14 indexOf(String str)
- 2.15 lastIndexOf(char ch)
- 2.16 lastIndexOf(String str)
- 2.17 concat(String str)
- 1.18 compareTo(String str)

## 3. Wrapper Classes
- 3.1
- 3.1.1 Wrapper Class

- 3.2 AutoBoxing, UnBoxing
- ```java
// AutoBoxing
Integer autoBoxed = 100; // primitive to Wrapper
// UnBoxing
int autoUnboxed = autoBoxed; // Wrapper to primitive

- 3.2.1 Comparisons
- use equals() instead of ==

## 4. Array and Collection
- 4.1 Array
- 4.1.1 1D Array
- ```java
// Declaration and initialization of a one-dimensional array
int[] intArray = new int[5]; // An array of integers with a length of 5

// Accessing and modifying array elements
intArray[0] = 10;
intArray[1] = 20;
intArray[2] = 30;
intArray[3] = 40;
intArray[4] = 50;
- 4.1.2 2D Array
- ```java
// Declaration and initialization of a two-dimensional array
int[][] twoDArray = new int[3][3]; // A 3x3 array of integers
// Accessing and modifying two-dimensional array elements
twoDArray[0][0] = 1;
twoDArray[0][1] = 2;
twoDArray[0][2] = 3;
// ... similar for other rows and columns

- 4.2 Collection
- 4.2.1 List
- 4.2.1.1 ArrayList
- ```java
// Declaration and initialization of an ArrayList
ArrayList<String> stringList = new ArrayList<>();

// Adding elements to the ArrayList
stringList.add("Apple");
stringList.add("Banana");
stringList.add("Orange");

// Accessing elements in the ArrayList
String firstElement = stringList.get(0); // "Apple"

// Iterating through the ArrayList
for (String fruit : stringList) {
System.out.println(fruit);
}
- 4.2.1.2 LinkedList
- ```java
// Declaration and initialization of a LinkedList
LinkedList<Integer> integerList = new LinkedList<>();

// Adding elements to the LinkedList
integerList.add(10);
integerList.add(20);
integerList.add(30);

// Accessing elements in the LinkedList
int firstElement = integerList.getFirst(); // 10

// Iterating through the LinkedList
for (int number : integerList) {
System.out.println(number);
}
- 4.2.2 Set
- ```java
// Declaration and initialization of a Set (HashSet)
Set<String> stringSet = new HashSet<>();

// Adding elements to the Set
stringSet.add("Red");
stringSet.add("Green");
stringSet.add("Blue");

// Checking if an element exists in the Set
boolean containsBlue = stringSet.contains("Blue"); // true
- 4.2.3 Map
- ```java
// Declaration and initialization of a Map (HashMap)
Map<String, Integer> ageMap = new HashMap<>();

// Adding key-value pairs to the Map
ageMap.put("John", 25);
ageMap.put("Jane", 30);
ageMap.put("Bob", 22);

// Accessing values using keys
int johnsAge = ageMap.get("John"); // 25
- 4.2.4 Queue
- 4.2.4.1 Deque
- ```java
// Declaration and initialization of a Deque (ArrayDeque)
Deque<String> deque = new ArrayDeque<>();

// Adding elements to the front and back of the Deque
deque.addFirst("First");
deque.addLast("Last");

// Accessing elements in the Deque
String firstElement = deque.getFirst(); // "First"
String lastElement = deque.getLast(); // "Last"

// Iterating through the Deque
for (String element : deque) {
System.out.println(element);
}
- 4.2.4.2 PriorityQueue
- ```java
// Declaration and initialization of a PriorityQueue
PriorityQueue<Integer> priorityQueue = new PriorityQueue<>();

// Adding elements to the PriorityQueue
priorityQueue.add(3);
priorityQueue.add(1);
priorityQueue.add(2);

// Accessing and removing elements based on priority
int highestPriorityElement = priorityQueue.poll(); // 1

// Iterating through the PriorityQueue
for (int element : priorityQueue) {
System.out.println(element);
}

## 5. Useful Java Class
- Java Class
- 5.1 String Builder
- 5.2 Math
- 5.3 BigDecimal
- 5.4 LocalDate
- 5.5 Random
- 5.6 Scanner

graph TD;
  byte-->short;
  short-->int;
  char-->int;
  int-->long;
  long-->float;
  float-->double;