1: Sequential structure
1.1. The sequence structure is relatively simple Execute line by line in the order in which the code is written
public class TestDemo {
public static void main(String[] args) {
System.out.println("aaa");
System.out.println("bbb");
System.out.println("ccc");
//aaa
//bbb
//ccc
}
}
1.2. If you adjust the writing order of the code, the execution order also changes
public class TestDemo {
public static void main(String[] args) {
System.out.println("aaa");
System.out.println("ccc");
System.out.println("bbb");
//aaa
//ccc
//bbb
}
}
2: Branching structure
2.1 if statement
2.1.1 three forms of basic syntax of if statement
① Basic syntax form of if statement 1
if(Boolean expression){
//Execute code when conditions are met
}
② Basic syntax form of if statement 2
if(Boolean expression){
//Execute code when conditions are met
}else{
//Execute code when conditions are not met
}
③ Basic syntax form of if statement 3
if(Boolean expression){
//Execute code when conditions are met
}else if(Boolean expression){
//Execute code when conditions are met
}else{
//Execute code when none of the conditions are met
}
2.1.2 specific code demonstration of if statement
public class TestDemo {
public static void main(String[] args) {
//Determine whether an even number or an odd number
int num=10;
if(num%2==0) {
System.out.println(num + "It's an even number");
}else{
System.out.println(num+"It's an odd number");
}
}
}
public class TestDemo {
public static void main(String[] args) {
//Determine whether a number is positive or negative
int num = 10;
if (num > 0) {
System.out.println(num+"Is a positive number");
} else if (num < 0) {
System.out.println(num+"Is a negative number");
} else {
System.out.println(num+"Is 0");
}
}
}
2.1.3 precautions for if statement
① Drape else problem
public class TestDemo {
public static void main(String[] args) {
int x=10;
int y=10;
if(x==5){
if(y==10){
System.out.println("LOL");
}
else{
System.out.println("hahaha");
}
}
}
}
None of the above code will be output because else matches the closest if at this time, but we don't recommend it in actual development.
② Code style issues
In Java, it is more recommended to use: {on the same line as if / else. As shown in the above code
③ Semicolon problem
public class TestDemo {
public static void main(String[] args) {
int x = 20;
if (x == 10); {
System.out.println("hehe");
}
}
}
An extra semicolon is written here, resulting in the semicolon becoming the statement body of the if statement, and the code in {} has become a code block irrelevant to an if. Program output hehe
2.2 switch statement
2.2.1 basic syntax of switch statement
switch(integer|enumeration|character|character string){
case Content 1 : {
Execute statement when content is satisfied;
[break;]
}
case Content 2 : {
Execute statement when content is satisfied;
[break;]
}
...
default:{
Execute the statement when the content is not satisfied;
[break;]
}
}
2.2.2 code examples
public class TestDemo {
public static void main(String[] args) {
int day=1;
switch(day){
case 1:
System.out.println("Monday");
break;
case 2:
System.out.println("Tuesday");
break;
case 3:
System.out.println("Wednesday");
case 4:
System.out.println("Thursday");
break;
case 5:
System.out.println("Friday");
break;
case 6:
System.out.println("Saturday");
break;
case 7:
System.out.println("Sunday");
break;
default:
System.out.println("Incorrect input");
break;
}
}
}
Depending on the value in switch, the corresponding case statement will be executed. The case statement will end when a break is encountered.
If the value in switch does not have a matching case, the statement in default will be executed.
It is recommended that a switch statement should be accompanied by default.
2.2.3 precautions for switch statement
① Do not omit break, otherwise you will lose the effect of "multi branch selection"
② Note 2 the value in switch can only be integer | enumeration | character | string
③ Note 3 switch cannot express complex conditions
④ Note 4 although switch supports nesting, it is ugly~
// For example, if the value of num is between 10 and 20, print hehe // Such code is easy to express using if, but cannot be expressed using switch
3: Cyclic structure
3.1 while cycle
3.1.1 basic syntax format of while statement
while(Cycle condition){
Circular statement;
}
If the loop condition is true, execute the loop statement; Otherwise, end the cycle
3.1.2 code examples
//Use the while loop to calculate 1+ 2! + 3! + 4! + 5!
int i=1;
int sum=0;
while(i<=5){
//It is important to reset fac j to 1
int fac=1;
int j=1;
while(j<=i){
fac*=j;
j++;
}
sum+=fac;
i++;
}
System.out.println(sum);
3.1.3 precautions for while loop statement
① Similar to if, the statement under while can not write {}, but only one statement can be supported when it is not written It is suggested to add {}
② Similar to if, the {suggestion after while is written on the same line as while
③ Do not write multiple semicolons after if, which may lead to incorrect execution of the loop
3.2 for loop
3.1.1 basic syntax of for loop
for(Expression 1;Expression 2;Expression 3){
Circulatory body;
}
Expression 1: used to initialize loop variables
Expression 2: loop condition
Expression 3: update loop variable
Compared with the while loop, the for loop combines these three parts together and is not easy to miss when writing code
3.1.2 code examples
//Use the for loop to calculate 1+ 2! + 3! + 4! + 5!
int i=1;
int sum=0;
for(i=1;i<=5;i++){
int j=1;
int fac=1;
for(j=1;j<=i;j++){
fac*=j;
}
sum+=fac;
}
System.out.println(sum);
3.1.3 precautions for loop statement
① Similar to if, the statement below for can not write {}, but only one statement can be supported when it is not written It is suggested to add {}
② Similar to if, the {suggestion after for is written on the same line as while
③ Similar to if, do not write more semicolons after for, otherwise the loop may not execute correctly
3.3 do while loop (minor)
3.1.1 basic syntax of do while statement
do{
Circular statement;
}while(Cycle condition);
Execute the loop statement first, and then determine the loop condition
3.1.2 code examples
//Print 1-10 with do while loop
int i=1;
do{
System.out.println(i);
i++;
}while(i<=10);
3.1.3 precautions for do while loop statement
① Don't forget the semicolon at the end of the do while loop
② Generally, do while is rarely used, and for and while are more recommended
3.4 break
The function of break is to end the cycle ahead of time
//Find the multiple of the first 3 in 100 - 200
int num = 100;
while (num <= 200) {
if (num % 3 == 0) {
System.out.println("A multiple of 3 was found, by:" + num);
break;
}
num++;
}
Executing break will end the loop
3.5 continue
The function of continue is to skip this cycle and immediately enter the next cycle
//Find multiples of all 3 in 100 - 200
int num = 100;
while (num <= 200) {
if (num % 3 != 0) {
num++; // Don't forget the + + here! Otherwise, there will be a dead cycle
continue;
}
System.out.println("A multiple of 3 was found, by:" + num);
num++;
}
When the continue statement is executed, it will immediately enter the next cycle (determine the cycle conditions), so it will not execute the following print statement