Java learning notes Day04

Posted by duall on Thu, 03 Mar 2022 15:20:21 +0100

day04 summary notes

01 array introduction

  • Introduction: array is a kind of container, which can store multiple values of the same data type
int[] arr = {10,20,'a'};
System.out.println(arr[2]);        // 97

double arr = {10,20,30};
System.out.println(arr[0]);        // 10.0

proposal: The same data type is stored in the array
  • Question: when to use arrays?

    • If there are multiple data to be operated, and multiple data belong to the same group of data, you can consider using array container

02 - definition format of array

1. data type[] Array name;

        int[] arr;

2. Data type array name[];
        double arr[];
  • Note: this definition format only defines variables of array type, and the container has not opened up space in memory

03 - static initialization of array

  • Initialization: the process of opening up space for array containers in memory and storing data into space
  • Static initialization:
1. Full format: data type[] Array name = new data type[]{Element 1, Element 2, Element 3...};

        int[] arr = new int[3]{11,22,33};

2. Simplified format: data type[] Array name = {Element 1, Element 2, Element 3...};

        int[] arr = {11,22,33};

be careful: Print array name, See the hexadecimal memory address of the array
        @ : Separator 
        [ : The current space is an array type, Several brackets, It's a few dimensional array.
        I : Type of container
        233ac4 : Hexadecimal address

04 - element access of array

  • Format: array name [index];
  • Index | subscript | subscript: the number corresponding to the space in the array. The number starts from 0 and increases by + 1 one by one
int[] arr = {11,22,33,44,55};

System.out.pritnln(arr[0]);        // Print

int result = arr[1] + 100;        // calculation

if(arr[2] % 2 == 0){            // judge


arr[3] = 66;                    // modify

for(int i = 1; i <= arr[4]; i++){    // loop

05 - traversal of array

  • Introduction: take out each element in the array
  • Traversal usage scenario: if you want to implement the requirements of the array, you need to traverse the array to operate on [Each] element in the array
for(int i = 0; i < arr.length; i++){
    // i: index
    // arr[i]: element

Array name.length : Get the length of the array dynamically (Number of elements)

06 - dynamic initialization of array

  • Introduction: when initializing the array, you only need to manually specify the length, and the system will allocate the default value
  • Format: data type [] array name = new data type [length];
int[] arr = new int[3];

System.out.println(arr[0]);        // 0
  • Classification of default values:
integer: 0
 decimal: 0.0
 Boolean: false
 character: '\u0000'  ---> Unicode character ----> Common white space characters
 Reference data type : null


Reference data type: array, class, Interface

null(Null value) : Can only be assigned to a reference data type
  • Differences between the two initialization methods:

    • Static initialization: specify elements manually, and the system will automatically calculate the length according to the number of elements
    • Dynamic initialization: specify the length manually, and the system will assign the default value
  • There are two usage scenarios for initialization:

    • Static initialization: if the data to be operated is specified in the requirements
    demand: The grade of students in the known class is 100 20 100 30 100
    int[] arr = {100,20,100,30,100};
    • Dynamic initialization: I only know how many numbers to save, but I don't know what it is
    demand: Enter five integers on the keyboard, Find the maximum value
     demand: Generate 10 1~100 Random number between, Find the minimum value

07 - memory diagram of array

  • Method area: the memory entered when the bytecode file (. class) is loaded
  • Stack: the memory entered when the method is running
  • Heap: the contents of new will enter the heap memory, and usually open up space to generate addresses

The following memory needs no attention

  • Local method stack:
  • Register:
requirement: Be sure to open it ppt, Sort out today's memory map.

08 array FAQ

  • ArrayIndexOufOfBoundsException: array index out of bounds exception

    • Reason: a nonexistent index was accessed
  • NullPointerException: null pointer exception

    • Reason: when the data type variable is referenced and null is recorded, the connection with heap memory will be cut off
    • If you still want to access heap memory data at this time, a null pointer exception will appear
  • Objective: when you see these two errors in the future, you can find the location of the code error and solve the code error according to the error prompt

09 - Introduction to two-dimensional array

  • Introduction: two dimensional array is also a kind of container, which stores one-dimensional array

    • Understanding: nesting of arrays
  • Usage scenario of two-dimensional array:

    • There are multiple groups of data to be operated
    • It is recommended to use multiple two-dimensional arrays for overall maintenance

10 - static initialization of two-dimensional array

  • Static initialization format:
data type[][] Array name = new data type[][]{
        {One dimensional array 1},
        {One dimensional array 2}

data type[][] Array name = {
        {One dimensional array 1},
        {One dimensional array 2}

int[][] arr = {
  • Element access of two-dimensional array:
Array name[m Indexes][n Indexes];

m Indexes : Which one-dimensional array do you want to access
n Indexes : Which element of a one-dimensional array do you want to access

System.out.println(arr[1][0]);        // 44

11 - two dimensional array traversal

  • Idea:

    • Traverse the two-dimensional array and get each one-dimensional array
    • Continue to traverse the one-dimensional array to get the specific elements
  • code:
for(int i = 0; i < arr.length; i++){
    // arr[i]: every one-dimensional array
    for(int j = 0; j < arr[i].length; j++){

12 - dynamic initialization of two-dimensional array

  • Format:
data type[][] Array name = new data type[m][n];

m : How many one-dimensional arrays can be stored
n : Each one-dimensional array, How many elements can be stored

int[][] arr = new int[3][1];

A two-dimensional array can store three one-dimensional arrays, Each one-dimensional array can store one element.

13 - two dimensional array memory diagram

  • Be sure to open the ppt and sort out the internal structure

Topics: Java