JavaSE

prelude

learning method

record

Write more code, take more notes and write more articles

Communication, thinking, skills

Share, ask (what's wrong), think (why)

insist

Blog = = effective memory

Summary and thinking

Writing organization ability

Learning and summarizing ability

Logical thinking ability

Help others and make friends

Rome wasn't built in a day

Blog platform - wordpress, typecho

Blog Garden

Preparatory course

1. io device

i – > input input

o – > output · output

1. dos command

Environmental installation

Configure environment variables

1. Add JAVA_HOME – > value of variable = installation path (parent directory of bin)

2. Configure path – >% Java_ HOME%\bin

​ %JAVA_HOME%\jre\bin

cmd – > whether the Java - version test is successful

Hello World

public class Hello{
    public static void main(String[] args)
        System.out.print("Hello World!");
}

Compile – > javac

Run – > java

The file name is consistent with the class name

Program operation mechanism

IDEA installation

IDE – > integrated development environment

introduction

All identifiers should be in case, $_ start

Naming rules

Package name: lowercase

Class name: hump. All words are capitalized

Variable name and method name: small hump lastNameAge

Constant name: upper case, middle plus_

data type

Basic data type

Integer type

int

byte

short

long–>l

Floating point type

float–>f

double

Character type

char

String – > is not a keyword, but a class

Boolean type

Reference data type

class

Interface

array

expand

//Integer extended binary 0b decimal octal 0 hex 0x
        int i=10;
        int i2=010;//8
        int i3=0x10;//16  0~9  A~F
        //Floating point extension
        //Rounding error divisor of float finite discrete
        // Avoid comparing floating-point numbers BigDecimal math tool class
        //double
        float f=0.1f;
        double d=1.0/10;
        System.out.println(f);
        System.out.println(d);
        System.out.println(f==d);
        //0.1
        //0.1
        //false
        float d1=212114343134431f;
        float d2=d1+1;
        System.out.println(d1==d2);
        //true
        //Character class expansion
        char c1='a';
        char c2='in';
        System.out.println((int)c1);
        System.out.println((int)c2);
        //All characters are numbers in nature
        //Encoding Unicode 2 bytes 65536 2 16
        //U0000 UFFFF
        char c3='\u0061';
        System.out.println(c3);
        //Escape character
        // \n -- line break \ t -- tab
        String sa = new String("hello world");
        String sb = new String("hello world");

        System.out.println(sa==sb);
        String sc = "hello world";
        String sd = "hello world";
        System.out.println(sc==sd);

        //Boolean extension
        boolean flag = true;
        //if (flag==true) is the same as if (flag)
        //Less is More!  The code should be concise and easy to read

Data type conversion

byte，short，char–>int–>long–>float–>double

Different data types are converted to the same type before operation

Force conversion

Variable from low to high (data type)

Automatic conversion

Automatic conversion

variable

Local variable – > in method

Instance variable – > outside the method and inside the class, it belongs to the object

public class Day02 {
    String name;
    int age;
    public static void main(String[] args) {
        Day02 day02 = new Day02();
        System.out.println(day02.age);
    }
}
//If the numeric type is not initialized, the default value is 0.0
//The Boolean default is false
//All but the basic types are null

Class variable stasic

constant

final constant name = numeric

operator

++a operation before assignment

a + + assignment before operation

Logical operator

And & &--- > are both true, and the result is true;

Or |--- > one of the two variables is true, and the result is true;

Not! --- > If it is true, it becomes false, and vice versa;

//Short circuit operation
        int a = 5;
        boolean d = (a<4)&&(a++<4);
        System.out.println(d);
        System.out.println(a);

Bit operation

//Bit operation
        /*
                                   A = 0011 1100
                                   B = 0000 1101
        If both are 1, it is 1, otherwise it is 0, a & B = 0000 1100
        One 1 is 1 a|b = 0011 1101
        0 for the same, 1 for the different, a ^ B = 0011 0001
        Reverse ~ B = 1111 0010
         */

Shift left < < = = * 2

Move right > > = = / 2

Base system

In order to understand what is binary, let's talk about decimal first.
We usually use hexadecimal, as the name suggests, full 10 into 1,
That is, when the low value is enough to 10, add 1 to the high value, and the standard becomes 0.
Binary is the same. When counting, full 2 will enter 1,
When the low position is full of 2, it is in the high position + 1, and the low position becomes 0.

String connector+

If a string appears on one side of the + side, other types will be converted to string type connection

Note: operation order

	int a = 10;
    int b = 20;
	System.out.println(""+a+b);
	System.out.println(a+b+"");
1020
30

Conditional operator

? : ----> x ? y : z

If x is true, the result is y, otherwise z

Priority questions ()

import package

com.baidu.www

Process control

Scanner

if selection structure

1. if
2. if......else
3. if......else if......else

Select one of them to execute, and then jump out of the program. However, the basic structure of Java is sequential structure

if nested structure

switch multiple selection structure

switch(expression)

	case value:......

		break；

	case value:......

		break；

	case value:......

		break；

	default:......

Cyclic structure

1. while

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2. do...while

   The do... While loop is similar to the while loop. The difference is that the do... While loop is cycled at least once (recirculation is performed first, while the reverse is true. Recirculation is judged first)

3. Enhanced for loop

   int[] numbers = {12,55,53,34,88,22};
   for (int n:numbers){
       System.out.println(n);
   }
   //Assign the value of numbers to n
   

4. for

Example 1:

//The for loop outputs the number between 1-1000 that can be divided by 5, and 3 are displayed in each line
        for (int i = 1  ; i <= 1000; i++) {
            if (i % 5 == 0) {
                System.out.print(i+"   ");
            }
            if (i%(5*3)==0) {
                System.out.println();
            }
        }

Example 2:

//Print 99 multiplication table
for (int i = 1; i <= 9; i++) {
            for (int j = 1; j <= i; j++) {
                System.out.print(i+"*"+j+"="+(i*j) + "\t");
            }
            System.out.println();
        }

1*1=1	
2*1=2	2*2=4	
3*1=3	3*2=6	3*3=9	
4*1=4	4*2=8	4*3=12	4*4=16	
5*1=5	5*2=10	5*3=15	5*4=20	5*5=25	
6*1=6	6*2=12	6*3=18	6*4=24	6*5=30	6*6=36	
7*1=7	7*2=14	7*3=21	7*4=28	7*5=35	7*6=42	7*7=49	
8*1=8	8*2=16	8*3=24	8*4=32	8*5=40	8*6=48	8*7=56	8*8=64	
9*1=9	9*2=18	9*3=27	9*4=36	9*5=45	9*6=54	9*7=63	9*8=72	9*9=81

//Own error demonstration   
for (int i = 1; i <= 1; i++) {
            int y = 1;
            int sum = y * i;
            System.out.println(i+"*"+y+"="+sum);
        }

        for (int i = 1; i <= 2; i++) {
            int e = 2;
            int sum = e * i;
            System.out.print(i+"*"+e+"="+sum+"    ");
        }
        System.out.println();

        for (int i = 1; i <= 3; i++) {
            int s = 3;
            int sum = s * i;
            System.out.print(i+"*"+s+"="+sum+"    ");
        }
        System.out.println();

        for (int i = 1; i <= 4; i++) {
            int si = 4;
            int sum = si * i;
            System.out.print(i+"*"+si+"="+sum+"    ");
        }
        System.out.println();

        for (int i = 1; i <= 5; i++) {
            int wu = 5;
            int sum = wu * i;
            System.out.print(i+"*"+wu+"="+sum+"    ");
        }
        System.out.println();

        for (int i = 1; i <= 6; i++) {
            int l = 6;
            int sum = l * i;
            System.out.print(i+"*"+l+"="+sum+"    ");
        }
        System.out.println();

        for (int i = 1; i <= 7; i++) {
            int q = 7;
            int sum = q * i;
            System.out.print(i+"*"+q+"="+sum+"    ");
        }
        System.out.println();

        for (int i = 1; i <= 8; i++) {
            int b = 8;
            int sum = b * i;
            System.out.print(i+"*"+b+"="+sum+"    ");
        }
        System.out.println();

        for (int i = 1; i <= 9; i++) {
            int j = 9;
            int sum = j * i;
            System.out.print(i+"*"+j+"="+sum+"    ");
        }
        System.out.println();

1. break/continue

   break force exit loop body

   continue exits the current loop and executes the next loop

practice:

//Print triangles
for (int i = 1; i <= 5; i++) {
    for (int j = 5; j >= i; j--) {
        System.out.print(" ");
    }
    for (int j = 1; j <= i; j++) {
        System.out.print("*");
    }
    for (int j = 1; j < i; j++) {
        System.out.print("*");
    }
    System.out.println();

}

method

retun can end the method

heavy load

Variable parameter

There can only be one, which must be put at the end

practice:

//Self written calculator
public class Demo04 {
    public double add(double a,double b){
        double sum = a + b;
        return sum;

    }
    public double sub(double a,double b){
        double sum = a - b;
        return sum;
    }
    public double mul(double a,double b){
        double sum = a * b;
        return sum;
    }
    public double div(double a,double b){
        double sum = a / b;
        return sum;

    }

    public static void main(String[] args) {
        Demo04 mythod = new Demo04();

        Scanner scanner = new Scanner(System.in);

        for (int i = 1;; i++){
        System.out.println("Please enter the number to calculate");
        double a = scanner.nextDouble();
        double b = scanner.nextDouble();

        System.out.println("Input calculation method");
        String method = scanner.next();

            switch (method) {
                case "+":
                    System.out.println(mythod.add(a, b));
                    break;
                case "-":
                    System.out.println(mythod.sub(a, b));
                    break;
                case "*":
                    System.out.println(mythod.mul(a, b));
                    break;
                case "/":
                    System.out.println(mythod.div(a, b));
                    break;
            }
    }
    }
}

array

//Declaration array
int[]  num01;
//Create array
array = new int[10]
    
//Declare and create an array
int[]  array = new int[length];

//assignment
	int[] num = {0,1,2,3,4,5,6,7,8,9};
	num[1] = 1;

Array principle

ArrayIndexOutOfBoundsException array index out of bounds exception

The length is definite

The type must be the same in the array

The elements in the array can be of any type

Array variables belong to the reference type. Arrays can be regarded as objects. Each element in the array is equivalent to the member variable of the object

The array itself is an object, and the object in Java is in the heap. Therefore, whether the original type or other object types are saved, the array object itself is in the heap

Use of arrays

public class Demo03 {
    public static void main(String[] args) {
        int[] array = {1,2,3,4,5,6};
        //printArry(array);
        int [] reverse = reverse(array);
        printArry(reverse);


//        for (int i : array) {
//            System.out.println(i);
//        }
    }
    //Print array elements
    public static void printArry(int[] arrays){
        for (int i = 0; i < arrays.length; i++) {
            System.out.println(arrays[i]);

        }
    }
⭐⭐//Invert array as return value
    public static int[] reverse(int[] arrays){
        int[] result = new int[arrays.length];
        //Reverse operation
        for (int i = 0,j = result.length-1; i < arrays.length; i++,j--) {
            result[j] = arrays[i];

        }

        return result;
    }
}

Multidimensional array

//Loop nested output two-dimensional array
int[][] array = {{1,2},{2,3},{3,4},{4,5}};
for (int i = 0; i < array.length; i++) {
    for (int j = 0; j < array[i].length; j++) {
        System.out.print(array[i][j]+"  ");
    }
}

//Output two-dimensional array with enhanced for loop
int[][] array1 = new int[11][11];
        array1[1][2] = 1;
        array1[2][3] = 2;
        
        System.out.println("Output original checkerboard array");
        for (int[] ints : array1) {
            for (int anInt : ints) {
                System.out.print(anInt+"\t");
            }
            System.out.println();
        }

Array class

Bubble sorting

One of the eight sorts

//Bubble sorting
    public static int[] sort(int[] array) {
        int temp = 0;
        //The outer loop determines how many times the program runs
        for (int i = 0; i < array.length-1; i++) {
            //Inner loop, compare the size of two numbers
            for (int j = 0; j < array.length-1; j++) {
                if (array[j+1]<array[j]){
                    temp = array[j];
                    array[j] = array[j+1];
                    array[j+1] = temp;
                }
            }
        }
    return array;
}

Sparse array

public class Demo08 {
    public static void main(String[] args) {
        int[][] array1 = new int[11][11];
        array1[1][2] = 1;
        array1[2][3] = 2;

        System.out.println("Output original checkerboard array");
        for (int[] ints : array1) {
            for (int anInt : ints) {
                System.out.print(anInt+"\t");
            }
            System.out.println();
        }
        System.out.println("=================================");
//Convert to sparse array save

        //Get the number of valid values
        int sum = 0;
        for (int i = 0; i < 11; i++) {
            for (int j = 0; j < 11; j++) {
                if (array1[i][j]!=0){
                    sum++;
                }
            }
        }
        System.out.println("altogether"+sum+"Valid values");
//Create a sparse array
        int[][] array2 = new int[sum+1][3];

        array2[0][0] = 11;
        array2[0][1] = 11;
        array2[0][2] = sum;
        //Traverse the two-dimensional array and assign the value to the sparse array
        int count = 0;
        for (int i = 0; i < array1.length; i++) {
            for (int j = 0; j < array1[i].length; j++) {
               if (array1[i][j]!=0){
                   count++;
                   array2[count][0] = i;
                   array2[count][1] = j;
                   array2[count][2] = array1[i][j];
               }
            }
        }
//Output sparse array
        System.out.println("Output sparse array");
        for (int i = 0; i < array2.length; i++) {
            System.out.println(array2[i][0]+"\t"
                            +array2[i][1]+"\t"
                            +array2[i][2]+"\t");
        }
//Restore sparse array
        //Read sparse array
        int[][] array3 =new int[array2[0][0]][array2[0][1]];
        //Restore value
        for (int i = 1; i < array2.length; i++) {
            array3[array2[i][0]][array2[i][1]] = array2[i][2];
        }
        System.out.println("==================================");
        for (int[] ints : array3) {
            for (int anInt : ints) {
                System.out.print(anInt+"\t");
            }
            System.out.println();
        }

    }
}

Memory

1. Heap:

   new objects and arrays

   It can be shared by all threads and will not store other object references

2. Stack

   Store basic variable types (including specific values of basic types)

   The variable of the reference object (the specific address where the reference is stored in the heap)

3. Method area

   Can be shared by all threads

   Contains all class and static variables

Object oriented programming OOP

Essence: organize code in the form of classes and encapsulate data in the form of objects

Abstract = common ground

Note: there are only properties and methods in the class

※ characteristics:

1. Encapsulate the internal data operation details of the private "high cohesion and low coupling" ---- > class, and only expose a small number of methods for external use

Property private get/set

2. The essence of inheriting extensions is to abstract a certain batch of buildings, so as to realize better modeling of the world (abstraction again)

Inheritance is a relationship between classes (in addition to dependency, composition, aggregation, etc.). In Java, there is only single inheritance without multiple inheritance (a subclass can only have one parent class)

Under public, after the subclass inherits the parent class, there will be all the methods of the parent class (private can also be used, and the public method get/set needs to be set)

CTRL + H to view the inheritance tree

Object class ----- > all classes in Java inherit object class from each other or indirectly

​ super

Method rewrite

3. Polymorphism

From the perspective of code operation, there are classes first and then objects. Classes are the template of objects

Objects, concrete things. Class, abstract, an abstraction of an object.

constructor

1. Even if a class doesn't write anything, it will have a method = = constructor

2. Must be the same as the name of the class

3. There must be no return value and void cannot be written

4. Used to initialize values