1. Sequential sentences
Statements: Code separated by semicolons is called a statement
Note: When no code is written but a semicolon, it is also a statement, called an empty statement
Two, judgment (if… else)
Note: If there is only one statement in the curly braces, the curly braces can be omitted without writing
Format one:
if(judgment condition){
Executed code block 1;
Executed code block 2;
…；
the executed code block n;
}
Format two:
if(judgment condition){
Executed code block 1;
…；
the executed code block n;
}else{
Executed code block 1;
…；
the executed code block n;
}
Format three:
if(judgment condition 1){
Executed code block 1;
} else if (judgment condition 2) {
execute statement;
} else if (judgment condition 3) {
execute statement;
}
example:
import java.util.Scanner;
//Import the package import java.util.Scanner, use the Scanner class, the fixed usage Scanner sc=new Scanner(System.in), int nextInt = sc.nextInt() to get the number entered by the user
public class Demo1 {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.println("Please enter a number: ");
int nextInt = sc.nextInt ();
if (nextInt %2 == 0) {
System.out.println(nextInt + ": is an even number");
} else {
System.out.println(nextInt + ": is an odd number");
}
System.out.println("over");
}
}
//operation result
[root@bch04 java]# javac Demo1.java
[root@bch04 java]# java Demo1
Please enter a number:
3
3: is an odd number
over
[root@bch04 java]# cat Demo2.java
import java.util.Scanner;
public class Demo2 {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.println("Please enter a number: ");
double score = sc.nextDouble();
char grade;
if (score >=90) {
grade = 'A';
}
else if (score >= 80) {
grade = 'B';
}
else if (score >= 60) {
grade = 'C';
}
else {
grade = 'D';
}
System.out.println(grade);
}
}
//operation result
[root@bch04 java]# java Demo2
Please enter a number:
78.4
C
import java.util.Scanner;
public class Demo3 {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.println("Please enter the year: ");
int year = sc.nextInt();
boolean isLeapYear = (year %4 == 0);
isLeapYear = isLeapYear && (year %100 != 0);
isLeapYear = isLeapYear || (year %400 ==0);
if (isLeapYear) {
System.out.println(year +": is the run year");
} else {
System.out.println(year +": is a normal year");
}
}
}
//operation result
[root@bch04 java]# java Demo3
Please enter the year:
2018
2018: is a normal year
3. Select the judgment statement (switch)
Format:
switch(expression) { case takes value 1: execute statement; break； case takes value 2: execute statement; break； …... default: execute statement; break； } |
Features of switch statement:
1) There are only four types selected by the switch statement: byte, short, int, char;
2) There is no order between cases and default, first judge all cases, and execute default if there is no matching case;
3) The condition for the switch statement to stop is: encounter the break keyword or the braces that end the switch statement;
4) If the matching case or default does not have a corresponding break, then the program will continue to execute downward until it encounters the break or the end of the switch;
5) The value in the switch case must have the same data type as the value of the switch expression
example:
[root@bch04 java]# cat Demo4.java
import java.util.Scanner;
public class Demo4 {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
double x = sc.nextDouble();
String z = sc.next();
double y = sc.nextDouble();
switch (z) {
case "+":
System.out.println("x+y=" +(x+y));
break;
case "-":
System.out.println("x-y=" +(x-y));
break;
case "*":
System.out.println("x*y=" +(x*y));
break;
case "/":
System.out.println("x/y=" +(x/y));
break;
default:
System.out.println("Not reliable");
break;
}
}
}
//operation result
[root@bch04 java]# java Demo4
4
+
5
x+y=9.0
Fourth, while loop
Description: Judge the condition first, and execute the loop body only if the condition is met
Format:
while(conditional expression)
{
execute statement;
}
example
[root@bch04 java]# cat Demo5.java
import java.util.Scanner;
public class Demo5 {
public static void main(String[] args) {
int num = (int)(Math.random()*100)+1;
Scanner sc = new Scanner(System.in);
int guessNum = -1;
while (guessNum != num) {
System.out.println("Please enter a number between 1-100: ");
guessNum = sc.nextInt();
if ( guessNum == num) {
System.out.println("In!");
} else if ( guessNum > num) {
System.out.println("The value is too big!");
} else {
System.out.println("The value is too small!");
}
}
}
}
//operation result
[root@bch04 java]# javac Demo5.java
[root@bch04 java]# java Demo5
Please enter a number between 1-100:
12
Value is small!
Please enter a number between 1-100:
Five, do while loop
Description: Execute the loop body first, then judge the condition, if the condition is satisfied, then continue to execute the loop body
Features: Whether the condition is met or not, the loop body is executed at least once
Format
do { execute statement; }while(conditional expression); |
example
[root@bch04 java]# cat Demo6.java
import java.util.Scanner;
public class Demo6 {
public static void main(String[] args) {
int num = (int)(Math.random()*100)+1;
Scanner sc = new Scanner(System.in);
int guessNum = -1;
int count = 0;
do {
System.out.println("Please enter a number between 1-100: ");
guessNum = sc.nextInt();
if ( guessNum == num) {
System.out.println("In!");
} else if ( guessNum > num) {
System.out.println("The value is too big!");
} else {
System.out.println("The value is too small!");
}
count++;
} while ( guessNum != num);
System.out.println("The number to guess is: " +num +" You guessed a total of: " +count + "times");
}
}
//operation result
[root@bch04 java]# javac Demo6.java
[root@bch04 java]# java Demo6
Please enter a number between 1-100:
10
Great value!
Please enter a number between 1-100:
5
Great value!
Please enter a number between 1-100:
4
Great value!
Please enter a number between 1-100:
2
bingo!
The number to guess is: 2 You guessed a total of: 4 times
Six, for loop
Format:
for(initialization expression; loop condition expression; operation expression after loop) {
execute statement;
}
example:
[root@bch04 java]# cat Demo7.java
public class Demo7 {
public static void main(String[] args) throws InterruptedException {
for (int i=1;i<=9;i++) {
for (int j=1;j<=i;j++) {
System.out.print(i +"*" +j +"=" +(i*j) +"\t");
Thread.sleep(1000);
}
System.out.println(" ");
}
}
}
//operation result
[root@bch04 java]# javac Demo7.java
[root@bch04 java]# java Demo7
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
Seven, break, continue keywords
break: the statement following the termination statement
continue: jump out of this loop and execute the next loop
Note: if continue appears at the end of the loop (the last statement), then it can be omitted