Sequentially structured program statements can only be executed once. If you want to perform the same operation multiple times, you need to use a loop structure.
There are three main loop structures in Java:
while loop
do…while loop
for loop
In Java5 an enhanced for loop was introduced mainly for arrays.
while is the most basic loop, its structure is:
while(Boolean expression) {
//Loop content}As long as the Boolean expression is true, the loop body will continue to execute.
public class Test {
public static void main(String args[]) {
int x = 10;
while( x < 20 ) {
System.out.print("value of x : " + x );
x++;
System.out.print("n");
}
}
}The compilation and running results of the above example are as follows:
value of x: 10
value of x : 11
value of x : 12
value of x : 13
value of x : 14
value of x: 15
value of x: 16
value of x : 17
value of x : 18
value of x : 19For the while statement, if the condition is not met, the loop cannot be entered. But sometimes we need to execute it at least once even if the conditions are not met.
The do...while loop is similar to the while loop, except that the do...while loop will be executed at least once.
do {
//Code statement}while(Boolean expression);Note: The Boolean expression is after the loop body, so the statement block has been executed before the Boolean expression is detected. If the Boolean expression evaluates to true, the statement block is executed until the Boolean expression evaluates to false.
public class Test {
public static void main(String args[]){
int x = 10;
do{
System.out.print("value of x : " + x );
x++;
System.out.print("n");
}while( x < 20 ); } }The compilation and running results of the above example are as follows:
value of x: 10
value of x : 11
value of x : 12
value of x : 13
value of x : 14
value of x: 15
value of x: 16
value of x : 17
value of x : 18
value of x : 19Although all loop structures can be expressed with while or do...while, Java provides another statement - for loop, which makes some loop structures simpler.
The number of times the for loop is executed is determined before execution. The syntax format is as follows:
for(initialization; Boolean expression; update) {
//code statement}Here are a few notes about the for loop:
The initialization step is performed first. A type can be declared, but one or more loop control variables can be initialized, or it can be an empty statement.
Then, test the value of the Boolean expression. If true, the loop body is executed. If it is false, the loop terminates and execution of the statements following the loop body begins.
After executing the loop once, update the loop control variables.
Check the Boolean expression again. Perform the above process in a loop.
public class Test {
public static void main(String args[]) {
for(int x = 10; x < 20; x = x+1) {
System.out.print("value of x : " + x );
System.out.print("n");
}
}
}The compilation and running results of the above example are as follows:
value of x: 10
value of x : 11
value of x : 12
value of x : 13
value of x : 14
value of x: 15
value of x: 16
value of x : 17
value of x : 18
value of x : 19Java5 introduced an enhanced for loop mainly for arrays.
The Java enhanced for loop syntax format is as follows:
for (declaration statement: expression)
{
//code sentence}Declaration statement: declare a new local variable. The type of the variable must match the type of the array element. Its scope is limited to the loop statement block, and its value is equal to the value of the array element at this time.
Expression: The expression is the name of the array to be accessed, or a method that returns an array.
public class Test {
public static void main(String args[]){
int [] numbers = {10, 20, 30, 40, 50};
for(int x : numbers ){
System.out.print(x);
System.out.print(",");
}
System.out.print("n");
String [] names ={"James", "Larry", "Tom", "Lacy"};
for( String name : names ) {
System.out.print( name );
System.out.print(",");
}
}
}The compilation and running results of the above example are as follows:
10,20,30,40,50,
James,Larry,Tom,Lacy,break is mainly used in loop statements or switch statements to jump out of the entire statement block.
break jumps out of the innermost loop and continues executing the statements below the loop.
The usage of break is very simple, it is a statement in the loop structure:
break; public class Test {
public static void main(String args[]) {
int [] numbers = {10, 20, 30, 40, 50};
for(int x : numbers ) {
if( x == 30 ) {
break;
}
System.out.print(x);
System.out.print("n");
}
}
}The compilation and running results of the above example are as follows:
10
20continue applies to any loop control structure. The function is to allow the program to immediately jump to the next iteration of the loop.
In the for loop, the continue statement causes the program to jump immediately to the update statement.
In a while or do...while loop, the program immediately jumps to the judgment statement of the Boolean expression.
continue is a simple statement in the loop body:
continue; public class Test {
public static void main(String args[]) {
int [] numbers = {10, 20, 30, 40, 50};
for(int x : numbers ) {
if( x == 30 ) {
continue;
}
System.out.print(x);
System.out.print("n");
}
}
}The compilation and running results of the above example are as follows:
10
20
40
50