day1-java basics

java knowledge points

type of data

Data types are divided into basic types and reference types. The difference between basic data types and reference types is that basic data types are allocated on the stack, while reference types are allocated on the heap.

Stack concept
? . . . .

basic type

• byte
• short
• int
• long
• chart
• float
• double
• boolean

Reference type

Apart from the basic types, the rest are reference types, which are stored in the heap. (Stack difference)

String

Characters and strings

Characters are basic types, use ''representations, strings are reference types, use ""representations, and can store characters of any length.

Array

int[] arr = new int[5];// 未初始化
// int[] ns = new int[] { 68, 79, 91, 85, 62 }; // 创建时候直接初始化
// int[] ns = { 68, 79, 91, 85, 62 }; // 简写
System.out.println(arr);
int len = 0;
for(; len < arr.length; len++) {
    System.out.println(arr[len]);
}

• Create an array using the new keyword, the timing of initialization is optional
• Not initialized, the system will assign default values
• Array access through index, error will be reported if the array length is exceeded

variable

Create variable

Type name variable name [= value], can also be named after initialization

String a;
a = "s";

Scope

Process control

Logical Operators

> < == & | ....

Flow control statement

Conditional statements

• if
• switch

loop statement

• while
• for

public class HelloWorld {
    public static void main(String[] args) {
        int i = 10;
        // 循环
        // while(i > 0) {
        //     System.out.println(i);
        //     i -= 1;
        // }

        // for(; i > 0; i--) {
        //     System.out.println(i);
        // }

        for(int b = 3; b > 0; b--) {
            System.out.println(b);
        }

        // 条件

        if (i > 10) {
            System.out.println(i);
        } else {
            System.out.println("x");
        }

        switch (i) {
            case 10:
                System.out.println(i);
                break;
        
            default:
                System.out.println("x");
                break;
        }
    }
}

break和continue

class

Keyword class

Properties and methods

Modifier

• public
• private
• static
• protected
• final
• abstract

Construction method

Same as the class name, it will be called only when the object is instantiated, no return value, no default constructor will be specified, multiple constructors can be specified, use overloading to determine the call based on the parameters

Method overloading

In a class, we can define multiple methods. If there are a series of methods, their functions are similar, only the parameters are different, then you can make this group of method names into the same name method, and judge the execution according to the parameters

inherit

java only supports single inheritance

interface

Only public or abstract methods, or public static final fields

package

Maintain namespace

abnormal

java exception

Exception catch

Custom exception

Assertion

Abnormality when the log is saved