Day05
java语句
for语句
class Demo1_for
{
public static void main ( String[ ] args)
{
int i;
for ( i = 0 ; i < 10 ; i++ ) {
System. out. println ( "HelloWorld" ) ;
}
}
}
while语句
class Demo1_While
{
public static void main ( String[ ] args)
{
int x = 1 ;
while ( x <= 10 )
{ System. out. println ( "x = " + x) ;
x++ ;
}
}
}
do …while语句
/*
*A: do...while语句的基本格式
初始化语句
do{
循环体语句;
控制条件语句
}while(判断条件语句);
*B: while语句的执行流程
*a :初始化语句;
*b :执行循环体语句;'
*c :执行控制条件语句;
*d :执行条件判断语句,看其返回值是true还是false;
*true则继续执行;
*false 则结束循环;
*e :回到b继续执行;
*C: 案例演示:
输出1-10;
*/
class Demo1_DoWhile
{
public static void main(String[] args)
{ int x = 1;
do
{ System.out.println("x = " + x);
x++;
}
while (x <= 10);
}
}
for语句,do…while语句,while语句之间的区别:
*do...while语句至少执行一次循环体;
*而for语句和while语句循环体执行前,必须先判断条件是否成立;
*for语句执行后,变量会被释放;while语句执行结束后,变量不会被释放,可继续使用.
java语言中循环语句的注意事项:
A: 一定要注意控制条件语句的变量,弄对后容易造成死循环;
B: 两种简单的死循环模式:
*while(true)(.....);
*for( ; ;) {.......};
练习
class Demo1_ForFor
{
public static void main ( String[ ] args)
{
for ( int i = 1 ; i <= 4 ; i++ )
{
for ( int j = 1 ; j <= 5 ; j++ )
{
System. out. print ( "*" ) ;
}
System. out. println ( ) ;
}
}
}
class Test1_ForFor
{
public static void main ( String[ ] args)
{
for ( int i = 1 ; i <= 9 ; i++ )
{
for ( int j = 1 ; j <= i ; j++ )
{
System. out. print ( j + "*" + i + "=" + ( i* j) + '\t' ) ;
}
System. out. println ( ) ;
}
}
}
控制跳转语句
class Demo2_Continue
{
public static void main ( String[ ] args)
{
for ( int x = 1 ; x <= 10 ; x++ )
{
if ( x == 4 )
{
continue ;
}
System. out. println ( "x = " + x) ;
}
}
}
class Demo3_Mark
{
public static void main ( String[ ] args)
{
a: for ( int i = 1 ; i <= 10 ; i++ )
{
System. out. println ( "x = " + i) ;
b: for ( int j = 1 ; j <= 10 ; j++ )
{
System. out. println ( "j = " + j) ;
break a;
}
}
}
}
练习
class Test
{
public static void main ( String[ ] args)
{
for ( int i = 1 ; i <= 10 ; i++ )
{
if ( i % 3 == 0 )
{
System. out. println ( "怕是个傻子" ) ;
}
System. out. println ( "怕是个傻子" ) ;
}
}
}
class Demo4_Return
{
public static void main ( String[ ] args)
{
for ( int i = 1 ; i <= 10 ; i++ )
{
if ( i == 4 )
{
return ;
}
}
System. out. println ( "循环结束" ) ;
}
}
方法的概述和学习
A: 为什么要有方法?
提高代码的复用性;
B: 什么是方法?
完成特定功能的代码块;
C: 方法的格式
* 修饰符 返回值类型 方法名( 参数类型 参数名1 ; 参数类型 参数名2. . . . ) {
方法体语句;
return 返回值;
}
D: 方法的格式说明
* 修饰符: 开始阶段暂用public static , 后面继续学习;
* 返回值类型: 就是功能结果的数据类型;
* 参数:
实际参数: 就是实际参与运算的;
* 形式参数: 就是方法定义上的, 用于接收实际参数的;
* 参数类型: 就是参数的数据类型;
* 方法体语句: 就是完成功能的代码;
* return : 结束方法的;
* 返回值: 就是功能的结果. 有return 传递给调用者;
E: 注意事项
* 有返回值的方法的单独调用, 没有意义
* 方法不调用不执行,
* 方法与方法之间是平行关系, 不能嵌套;
* 方法定义的时候, 参数之间用逗号隔开;
* 方法调用的时候不用再传递参数;
* 如果方法有明确的返回, 一定要用return 返回.
* 如果返回值是void , return 可以省略
* 返回值是void 的方法只能单独调用
举例
import java. util. Scanner;
class Demo2_Sum
{
public static void main ( String[ ] args)
{
Scanner sc = new Scanner ( System. in) ;
System. out. println ( "请输入第一个字符串" ) ;
String x = sc. nextLine ( ) ;
System. out. println ( "请输入第二个字符串" ) ;
String y = sc. nextLine ( ) ;
String sum = add ( x, y) ;
System. out. println ( "两个字符串相连是" + '\n' + sum) ;
}
public static String add ( String a, String b) {
String sum = a + b;
return sum;
}
}
练习
import java. util. Scanner;
class Test1_Method
{
public static void main ( String[ ] args)
{
Scanner sc = new Scanner ( System. in) ;
System. out. println ( "请输入第一个整数" ) ;
int x = sc. nextInt ( ) ;
System. out. println ( "请输入第二个整数" ) ;
int y = sc. nextInt ( ) ;
int max = getMax ( x, y) ;
System. out. println ( "两个数中较大的是" + max) ;
boolean b1 = isEquals ( x, y) ;
System. out. println ( "两个数相等是" + b1) ;
}
public static int getMax ( int a , int b) {
return ( a > b ) ? a : b;
}
public static boolean isEquals ( int c , int d) {
return c == d;
}
}
import java. util. Scanner;
class Test4_Method
{
public static void main ( String[ ] args)
{
Scanner sc = new Scanner ( System. in) ;
System. out. println ( "请输入行数" ) ;
int row = sc. nextInt ( ) ;
System. out. println ( "请输入列数" ) ;
int column = sc. nextInt ( ) ;
print ( row, column) ;
}
public static void print ( int a , int b) {
for ( int i = 1 ; i <= a ; i++ )
{
for ( int j = 1 ; j <= b ; j++ )
{
System. out. print ( "*" ) ;
}
System. out. println ( ) ;
}
}
}
import java. util. Scanner;
class Test3_method
{
public static void main ( String[ ] args)
{
Scanner sc = new Scanner ( System. in) ;
System. out. println ( "请录入一个1~9之间的数" ) ;
int num = sc. nextInt ( ) ;
print99 ( num) ;
}
public static void print99 ( int a) {
for ( int i = 1 ; i <= a ; i++ )
{
for ( int j = 1 ; j <= i; j++ )
{
System. out. print ( i + "*" + j + "=" + ( i * j) + '\t' ) ;
}
System. out. println ( ) ;
}
}
}
方法重载
class Demo4_OverLoad
{
public static void main ( String[ ] args)
{
int sum1 = add ( 20 , 30 ) ;
double sum2 = add ( 30 , 40 , 50 ) ;
System. out. println ( sum1) ;
System. out. println ( sum2) ;
}
public static int add ( int a , int b) {
return a + b;
}
public static int add ( int a , int b, int c) {
return a + b + c;
}
}