Java EE Basics (v)

1. Java Language Foundation (array overview and Definition Format description)

• A: Why do we have arrays (containers)
  • To store multiple values of the same data type

• B: Array Concept

  • An array is a collection of multiple elements that store the same data type. can also be seen as a container.
  • Arrays can store either the base data type or the reference data type.

• C: Array definition Format data type [] Array name = new data type [length of array];

2. Java Language Foundation (initialization of arrays initialized dynamically)

• A: What is the initialization of an array
  • is to open up contiguous memory space for arrays and assign values to each array element
• B: How to initialize an array
  • A: Dynamic initialization only specifies the length, the system gives the initialization value
    • int[] arr = new INT[5];
  • B: Static initialization gives the initialization value, the system determines the length of the
• C: Dynamic initialization format:
  • data type [] Array name = new data type [array length];
• D: Case Demo
  • Output array names and arrays elements

3. Java Language Foundation (memory allocation in Java and differences between stacks and heaps)

• A: Stack (Master)
  • Store local Variables
• B: Heap (Master)
  • Storing new arrays or objects
• C: Method Area
  • Object-oriented section explanation
• D: Local Method area
  • and system-related
• E: Register
  • For CPU use

4. Java Language Foundation (array of memory plots 11 arrays)

• A: Drawing Demo
  • An array

5. Java Language Foundation (array of memory plots 22 arrays)

• A: Drawing Demo
  • Two different arrays

6. Java Language Foundation (array of memory plots 33 references two arrays)

• A: Drawing Demo
  • Three references, two array references to the same address

7, Java Language Foundation (initialization of the array static initialization and memory diagram)

• A: The Static initialization format:
  • Format: data type [] Array name = new data type []{element 1, element 2,...};
  • Simplified format:
    • data type [] Array name = {element 1, element 2,...};
• B: Case Demo
  • Explanation of the logarithm group
  • Output array names and arrays elements
• C: Drawing Demo
  • An array

8. Java Language Basics (two common minor problems with array operations out of bounds and null pointers)

• A: Case Demo
  • A:arrayindexoutofboundsexception: Array index out-of-bounds exception
    • Reason: You have accessed an index that does not exist.
  • B:nullpointerexception: null pointer exception
    • Cause: The array is no longer pointing to heap memory. You also use the array name to access the element.
    • Int[] arr = {A-i};
    • arr = null;
    • System.out.println (Arr[0]);

9, the Java Language Foundation (array operation 1 times calendar)

• A: Case Demo

  • Array traversal: Is the output of each element in the array sequentially.
  • Properties of the array: length of the arr.length array

  • Maximum index of the array: arr.length-1;

    public static void print(int[] arr) {    for (int i = 0;i < arr.length ;i++ ) {        System.out.print(arr[i] + " ");    }}

10, the Java Language Foundation (array of Operations 2 get the most value)

• A: Case Demo

  • Array gets the maximum value (gets the minimum value in the array)

    public static int getMax(int[] arr) {    int max = arr[0];    for (int i = 1;i < arr.length ;i++ ) {          //从数组的第二个元素开始遍历        if (max < arr[i]) {                         //如果max记录的值小于的数组中的元素            max = arr[i];                           //max记录住较大的        }    }    return max;}

11. Java Language Foundation (operation of Array 3 inversion)

• A: Case Demo

  • array element inversion (that is, swapping elements)

    public static void reverseArray(int[] arr) {    for (int i = 0;i < arr.length / 2 ; i++) {        //arr[0]和arr[arr.length-1-0]交换        //arr[1]和arr[arr.length-1-1]交换        //arr[2]和arr[arr.lentth-1-2]        //...        int temp = arr[i];        arr[i] = arr[arr.length-1-i];        arr[arr.length-1-i] = temp;    }}

12, the Java Language Foundation (array of Operations 4 look up the table method)

• A: Case Demo

  • Array Lookup Table method (according to the keyboard input index, find the corresponding week)

    public static char getWeek(int week) {    char[] arr = {‘ ‘,‘一‘,‘二‘,‘三‘,‘四‘,‘五‘,‘六‘,‘日‘};     //定义了一张星期表    return arr[week];                                           //通过索引获取表中的元素}

13, the Java Language Foundation (array of Operations 5 basic lookup)

• A: Case Demo

  • Array element Lookup (finds the index of the first occurrence of the specified element in the array)

    public static int getIndex(int[] arr,int value) {    for (int i = 0;i < arr.length ;i++ ) {              //数组的遍历        if (arr[i] == value) {                          //如果数组中的元素与查找的元素匹配            return i;        }    }    

14, the Java Language Foundation (two-dimensional array overview and Format 1 explanation)

• A: A Two-dimensional array overview
• B: two-dimensional array format 1
  • int[][] arr = new int[3][2];
• C: Interpretation of the two-dimensional array format 1

• D: Precautions

  • A: The following format can also represent a two-dimensional array
    • 1: Data type array name [] = new data type [m][n];
    • 2: Data type [] array name [] = new data type [m][n];
  • B: Note the differences defined below

  • int x;int y;int x,y;int[] x;int[] y[];int[] x,y[];    x是一维数组,y是二维数组

• E: Case Demo
  • Define two-dimensional arrays, output two-dimensional array names, one-dimensional array names, one element

15, Java Language Foundation (two-dimensional array format 1 memory plot)

• A: Drawing Demo
  • Drawing explains the problem of the above two-dimensional array name, one-dimensional array name, and the value of an element

16, Java Language Foundation (two-dimensional array format 2 explanation and memory plot)

• A: Two-dimensional array format 2
  • int[][] arr = new int[3][];
• B: Interpretation of the two-dimensional array format 2
• C: Case Demo
  • Explain formats, output data, and draw memory graphs

17, Java Language Foundation (two-dimensional array format 3 explanation and memory plot)

• A: Two-dimensional array format 3
  • Int[][] arr = {{1,2,3},{4,5},{6,7,8,9}};
• B: Interpretation of the two-dimensional array format 3
• C: Case Demo
  • Explain formats, output data, and draw memory graphs

18, Java Language Foundation (two-dimensional array exercises 1 times calendar)

• A: Case Demo

  • Requirements: two-dimensional array traversal

  • The outer loop controls the length of the two-dimensional array, which is actually the number of one-dimensional arrays.

  • The inner loop controls the length of a one-dimensional array.

    int[][] arr = {{1,2,3},{4,5},{6,7,8,9}};for (int i = 0;i < arr.length ;i++ ) {          //获取到每个二维数组中的一维数组    for (int j = 0;j < arr[i].length ;j++ ) {   //获取每个一维数组中的元素        System.out.print(arr[i][j] + " ");    }    System.out.println();}

19. Java Language Basics (two-dimensional array exercises 2 summation)

• A: Case Demo

• 需求：公司年销售额求和某公司按照季度和月份统计的数据如下：单位(万元)第一季度：22,66,44第二季度：77,33,88第三季度：25,45,65第四季度：11,66,99int[][] arr = {{22,66,44},{77,33,88},{25,45,65},{11,66,99}};int sum = 0;                                    //定义变量,记录每次相加的结果for (int i = 0;i < arr.length ;i++ ) {          //获取每一个一维数组    for (int j = 0;j < arr[i].length ;j++ ) {   //获取每一个一维数组中的元素        sum = sum + arr[i][j];                  //累加    }}System.out.println(sum);

20, the Java Language Foundation (study questions Java in the parameter transfer problem and diagram)

• A: Case Demo

• 看程序写结果，并画内存图解释public static void main(String[] args) {    int a = 10;    int b = 20;    System.out.println("a:"+a+",b:"+b);    change(a,b);    System.out.println("a:"+a+",b:"+b);    int[] arr = {1,2,3,4,5};    change(arr);    System.out.println(arr[1]);}public static void change(int a,int b) {    System.out.println("a:"+a+",b:"+b);    a = b;    b = a + b;    System.out.println("a:"+a+",b:"+b);}public static void change(int[] arr) {    for(int x=0; x<arr.length; x++) {        if(arr[x]%2==0) {            arr[x]*=2;        }    }}

Java EE Basics (v)