Overview of the Java language
Java language running mechanism and running process
- Features of the Java language
JVMS vary from operating system to operating system.
TFboys held concerts in Beijing, Shanghai and Gansu. JVM is equivalent to the stage in different places, and TFboys’ singing and dancing is equivalent to JAVA code, which can be performed on different stages.
- Java Core Mechanism
Java Environment Setup
Companies generally do not exceed 8.
JDK, JVM, and JRE
The official version
- Download and install
- Configuring environment Variables
Initial development experience
class HelloChina {
public static void main(String[] args) {
System.out.println("hello world"); }}Copy the codeannotation
The API documentation
Java8 API documentation
www.matools.com/api/java8
First programming summary
Good programming style
Java common integrated development environment
Java basic syntax
Keywords and reserved words
identifier
variable
- Basic data types
- Conversion between basic data type variables
- Basic data type and String conversion
- Conversion from base to base
Binary to decimal:
For example: 11000101 1*2^0+0+1*2^2+0+0+0+1*2^6=1+4+64=69
Find the decimal value of 10111011:
—-> The bottom layer of the computer is stored in the way of complement, so 10111011 is the complement. On the far left is the negative digit, which means the digit is a negative number.
—-> complement -1, inverse 10111010
—-> In addition to the sign bit, inverse, source code 11000101
—-> Switch to base 10, 1*2^0+0+1*2^2+0+0+0+1*2^6=1+4+64=69
—-> minus sign, -69
Why is the result -128 when 128 is strong converted to byte?
int 128 —-> 0000 0000 0000 0000 0000 0000 1000 0000
Force conversion to byte —-> Remove 24 bits of the signature, resulting in 1000 0000, or -128
Decimal to binary:
Take the inverse mod by 2, e.g. 13.
Binary to octal/hexadecimal:
16 / octal to binary:
Transfer function
The operator
1.False x=2,ture y=2
2. False = 2 x, y = 1
3. True x = 2, false y = 2 – > x = 7
4. If x=2, y=1 — >x= 2
Program flow control
Branching structure
- if-else
- swith-case
Output season according to the specified month :(case conditions can be output in this way)
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
int month = scan.nextInt();
switch (month){
case 3:
case 4:
case 5:
System.out.println("Spring");
break;
case 6:
case 7:
case 8:
System.out.println("Summer");
break;
case 9:
case 10:
case 11:
System.out.println("Autumn");
break;
case 12:
case 1:
case 2:
System.out.println("Winter");
break; }}}Copy the code
Loop structure
- The for loop
public static void main(String[] args) {
for(int i=1; i <=150; i++){ System.out.print(i+"");
if(i%3= =0){
System.out.print("foo ");
}
if(i%5= =0){
System.out.print("biz ");
}
if(i%7= =0){
System.out.print("baz "); } System.out.println(); }}Copy the code
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
System.out.println("Please enter two positive integers");
int positiveIntOne = scan.nextInt();
int positiveIntTwo = scan.nextInt();
int min = (positiveIntOne<=positiveIntTwo)? positiveIntTwo:positiveIntOne;
// Define the greatest common divisor
int greatestCommonDivision = 0;
for(int i=1; i<=min; i++){if(positiveIntOne%i==0&&positiveIntTwo%i==0){ greatestCommonDivision = i; }}// Define the least common multiple
int leastCommonMultiple = 0;
for(int i=1; i<=positiveIntOne*positiveIntTwo; i++){if(i%positiveIntOne==0&&i%positiveIntTwo==0){
leastCommonMultiple = i;
break;
}
}
System.out.println("Greatest common divisor:"+greatestCommonDivision);
System.out.println("Least common divisor:"+leastCommonMultiple);
}
Copy the code- The while loop
- The do while loop
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
System.out.println("Please enter an integer. Zero means the input is complete.");
int positive_count=0;
int negative_count=0;
while (true) {int num = scan.nextInt();
if (num>0){
System.out.println("This is a positive number.");
positive_count++;
}
if (num<0){
System.out.println("This is a negative number.");
negative_count++;
}
if (num==0){
System.out.println("End of program");
break;
}
}
System.out.println("The positive numbers are:"+positive_count+"Zero, negative numbers are:"+negative_count+"个");
Copy the codefor(;;) It means always loop. Equivalent to while (true).
- Nested loop
99 times table
public static void main(String[] args) {
for(int i=1; i<=9; i++){// Control the number of rows
for(int j=1; j<=i; j++){// Control column number
System.out.print(i+"*"+j+"="+i*j+"\t");
}
System.out.println();
}
Copy the codeAll primes up to 100
public static void main(String[] args) {
Boolean isFlag =true;// if I is divided by j, change its value
for(int i=2; i<=100; i++){// iterate over the number up to 100
for (int j=2; j<=i-1; j++){//i%j
if (i%j==0){
isFlag = false; }}if(isFlag==true){
System.out.print(i+"");
}
// Reset isFlag at the end of the inner loop
isFlag=true; }}Copy the codePrime code optimization
- break continue
Marked break continue
public static void main(String[] args) {
label:for (int i=1; i<=4; i++){for (int j=1; j<=10; j++){if (j%4= =0) {// break; // The current loop, the inner loop, is broken out by default
// continue; // By default, the current loop of the current loop, i.e., the inner loop, is skipped
// break label; // Break out of the loop that specifies the label
continue label; // Break the loop of the layer that defines the label} System.out.print(j); } System.out.println(); }}Copy the code
Prime code optimization – two
Measure the merits of a code
-
Function correctly
-
readability
-
Robustness (handling of abnormal input)
-
High efficiency vs. low storage (time complexity, space complexity (measure algorithm))