Notes on basic Java learning

Keywords: Java

1, Array

1. Declaration array

// Two ways to declare arrays
int array[];
int[] array2;// This is recommended

// Open up space
array = new int[3]; // Open up 3 spaces (array with length of 3)
// Note: after opening up a space, the default value of this type is saved

double[] array3 = new double[5];// Declare a double array of length 5

// Two ways to declare and assign values
int[] array4 = {1, 2, 3};
int[] array5 = new int[]{1, 2, 3, 4};

array2 = new int[5];
// Assign a value to the first element
array2[0] = 5;

2. Precautions

  • The type and length of the array cannot be modified

  • Only numeric values of the specified type can be stored in the array

  • The subscript of an array cannot exceed the maximum length of the array

3. Get array length

int[] arr = {1, 2, 3};
int length = arr.length; // Array length, result: 3

4. Array tool class Arrays

Arrays is a tool class used to operate arrays in java.util. Through this tool class, we can easily complete some operations on arrays. The following describes some methods that may be used

a. equals

Judge whether all elements are equal when two arrays are not null

int[] arr = {1, 2, 3};
int[] arr2 = {1, 2, 3};
boolean equals = Arrays.equals(arr, arr2); // Judge whether arr and arr2 are equal. Result: true

b. sort

Sort arrays

int[] arr = {1, 3, 2};
Arrays.sort(arr); // Sort arr, result: [1, 2, 3]

c. toString

Concatenate arrays into strings

int[] arr = {1, 2, 3};
System.out.println(Arrays.toString(arr)); // Concatenate the arr array into a string, and the result: [1, 2, 3]

**Note: * * if you directly output an array, the toString method of the array will be called by default. The default toString method of the array is as follows, so you may get something similar to[ I@1be6f5c3 Results

public String toString() {
    return getClass().getName() + "@" + Integer.toHexString(hashCode());
}

d. copyOf

Copy the data of the specified length from the original array, and make up the default value of the data type for the excess

int[] arr = new int[]{1, 2, 3, 4};
int[] arr1 = Arrays.copyOf(arr, 3); // Copy 3 elements from arr and assign them to arr1
System.out.println(Arrays.toString(arr1)); // Result: [1, 2, 3]

e. fill

Assigns all elements in the array to the specified value

int[] arr = new int[3];
Arrays.fill(arr, 5); // Assign a value of 5 to all elements in the arr
System.out.println(Arrays.toString(arr)); // Result: [5, 5, 5]

5. Array copy

int[] arr = {1, 2, 3, 4, 5, 6, 7, 8, 9, 0};
int[] arr2 = new int[8];
/*
 Data copied from one array to another array at a specified position and length
 arr Is the initial array,
 0 Represents that the replication starts from zero out in the following table,
 arr2 Is the target array,
 3 Represents the location copied to the target array
 4 Represents the length of data to be copied (cannot exceed the initial array size)
*/
System.arraycopy(arr, 0, arr2, 3, 4);
System.out.println(Arrays.toString(arr2)); // Result: [0, 0, 0, 1, 2, 3, 4, 0]

2, Assemble

There are many kinds of collections in Java. Next, we will only explain the two commonly used collections (List and Map)

1. List

List is an ordered collection. We can create a list collection through the following statement

List list = new ArrayList();

However, when creating a List, we often need to specify the generic type of the List (that is, specify that the data saved in the List can only be the specified type). If the String generic type is specified below, the List can only save the data of String type

List<String> list = new ArrayList<>();

Let's introduce the commonly used List method

a. add

With add, we can add data to the List

// Adding data directly is appended to the end of the List set
list.add("data"); // Result: [data]
// Add data to the specified location by subscript
list.add(0, "insert data"); // Result: [insert data, data]

b. size

Gets the number of elements in the List collection

list.size(); // Results: 2

c. contains

Determine whether the List collection contains an element

list.contains("data"); // Result: true

d. get

Get the data in the List collection by subscript

list.get(1); // Results: Data

e. clear

Empty the current List collection

list.clear(); // Result: []

f. addAll

Adds all the elements of another List collection to the current List collection

List<String> list2 = new ArrayList<>();
list2.add("Data 1");
list2.add("Data 2");
list2.add("Data 3");
list.addAll(list2); // Result: [data 1, data 2, data 3]

g. remove

Remove elements from the List collection

// Remove by data
list.remove("Data 1"); // Result: [data 2, data 3]
// Remove by subscript
list.remove(1); // Result: [data2]

h. removeAll

Removes all the same elements of another List collection from the current List collection

list.removeAll(list2); // Result: []

2. Map

The data in the Map is saved in the form of key value pairs. We can create a Map in the following ways

Map map = new HashMap();

However, it is often necessary to specify generics. As follows, we specify that the key and value of Map must be of String type

Map<String, String> map = new HashMap<>();

Let's introduce the commonly used Map methods

a. put

By put ting, we can add key value pairs to the Map

map.put("Data 1", "Data 1 content"); // Result: {data 1 = data 1 content}

**Note: * * if you add the same key to the Map, the latter will overwrite the former

map.put("Data 1", "Data 1 modification"); // Result: {data 1 = data 1 modification}

b. size

Used to obtain the number of key value pairs in the Map

map.size(); // Results: 1

c. get

Gets the value corresponding to the specified key in the Map

map.get("Data 1"); // Result: data 1 modification

d. clear

Empty Map

map.clear(); // Result: {}

e. putAll

Add all key value pairs in another Map to the current Map

Map<String, String> map2 = new HashMap<>();
map2.put("Data 1", "Data 1 content");
map2.put("Data 2", "Data 2 content");
map.putAll(map2); // Result: {data 1 = data 1 content, data 2 = data 2 content}

f. remove

Deletes the specified element

map.remove("Data 1"); // result:

1, Conditional judgment

1. if

The programming language is almost a general conditional statement, but in Java, if there is only one execution statement, the brackets can be omitted (but sometimes this writing method is often not recommended for the readability of the code)

int a = 3;
// if statement judgment
if (a > 4) {
    System.out.println("a Up to 4");
}
// if...else... Statement judgment
if (a < 5) {
    System.out.println("a Less than 5");
} else {
    System.out.println("a Greater than or equal to 5");
}
// if...else if...else statement judgment
int age = 23;
if (age >= 0 && age <= 8) {
    System.out.println("children");
} else if (age > 8 && age <= 12) {
    System.out.println("juvenile");
} else if (age > 12 && age <= 35) {
    System.out.println("youth");
} else if (age > 35 && age <= 60) {
    System.out.println("middle age");
} else {
    System.out.println("old age");
}
// Omit brackets
int age = 23;
if (age >= 0 && age <= 8)
    System.out.println("children");
else if (age > 8 && age <= 12)
    System.out.println("juvenile");
else if (age > 12 && age <= 35)
    System.out.println("youth");
else if (age > 35 && age <= 60)
    System.out.println("middle age");
else
    System.out.println("old age");

2. switch

In Java, switch can match byte, short, int, char, String and enumeration types, but if the switch is of char type and the case value is int, the matching is ASCII

int a = 2;
switch (a) {
    case 1:
        // Run from this line when a == 1
        System.out.println(1);
        break;// The code block where the end is located, that is, switch
    case 2:
        System.out.println(2);
        break;
    default:
        // The code that runs when none of the other items match, default can be omitted
        System.out.println("The value you entered is illegal!");
        break;
}

// Multiline matching
switch (a) {
        //Input 1 or 2 is output to class a
    case 1:
    case 2:
        System.out.println("The class you entered is" + a + ",To go a class");
        break;
        //Input 3 or 4 is output to shift b
    case 3:
    case 4:
        System.out.println("The class you entered is" + a + ",To go b class");
        break;
        //Enter other values to c shift
    default:
        System.out.println("You belong to another class, go c class");
        break;
}

Posted by k0z on Mon, 22 Nov 2021 10:51:35 -0800