1.包
2.运算符
public class Operator {
public static void main(String[] args) {
int a = 5;
System.out.println("a = " + a);
//a = -a; //+ -
System.out.println("a = " + a);
//+ 字符串链接
System.out.println("影分身" + "软件开发");
System.out.println(3 + 5 + "软件开发" + 6 + 8);
double b = a / 2;
System.out.println("a / 2 = " + (a / 2));
System.out.println("b = " + b);
int weith = 10;
int height = 6;
double area = 1.0 / 2 * weith * height;
System.out.println("area = " + area);
int c = a % 3;//取余数
System.out.println("c = " + c);
//int d = c++;// c++ c = c + 1; 先赋值 后自增
int d = ++c;//先自增 后赋值
System.out.println("d = " + d);
System.out.println("c = " + c);
//赋值
int e = 10; //-= *= /= %=
e += 1;// e = e + 1;
System.out.println("e = " + e);
//比较 > >= 5 >= 5 5 > 5 < <= ==
boolean isEquals = (1 == 1);
System.out.println("isEquals = " + isEquals);
int year = 2012;
boolean isLeap = (year % 4 == 0 && year % 100 != 0 || year % 400 == 0);
System.out.println(year +" 是否是闰年 " + isLeap);
System.out.println("1 != 2 ? " + (1 != 2));
//逻辑
boolean isTrue = false;
System.out.println("isTrue ? " + isTrue);
System.out.println("!isTrue ? " + !isTrue);
System.out.println("true || false ? " + (true || false));//或运算
System.out.println("true && false ? " + (true && false));//与运算
int score = 85;
//score > 60 及格 > 70 良 > 85 优秀
// javase > 60 && oracle > 70 && xml > 70
// javase > 60 || oracle > 60 || xml > 60
int move = 16;
System.out.println("move = " + (move >> 3));//100 10
System.out.println("move = " + (move << 1));
}
}
3.选择语句
import java.util.Random;//导入类
public class IfDemo {
public static void main(String[] args) {
Random ran = new Random();//new 创建对象
int score = ran.nextInt(101);//ran.nextInt(41) + 60;//随机数 0 ~ 100
System.out.println("score = " + score);
/*
if(score > 60) {//满足条件执行
System.out.println("============");
System.out.println(" 及格了..");
System.out.println("============");
}
*/
//if的第二种形式
/*
if(score > 60) {//满足条件执行
System.out.println("============");
System.out.println(" 及格了..");
System.out.println("============");
} else { //不满足条件执行
System.out.println("============");
System.out.println(" 很遗憾..");
System.out.println("============");
}
*/
//多分支 匹配其中一个条件
/*
if( score >= 90) {
System.out.println("成绩： A");
} else if ( score >= 80 /*&& score < 90) {
System.out.println("成绩： B");
} else if ( score >= 60 /*&& score < 80) {
System.out.println("成绩： C");
} else {
System.out.println("成绩： D");
}
*/
switch(score / 10) {//如果匹配 下面代码执行
case 10: //System.out.println("成绩： A");break;
case 9: System.out.println("成绩： A");break;
case 8: System.out.println("成绩： B");break;
case 7: System.out.println("成绩： C");break;
case 6: System.out.println("成绩： D");break;
default : System.out.println("成绩： 不及格");
}
//三目运算符
boolean isPass = ( score > 60 ? true : false);
System.out.println("成绩 === " + isPass);
String passed = score > 60 ? "及格" : "挂了";
System.out.println("成绩 ==== " + passed);
}
}
4.while循环
import java.util.Random;
public class LoopDemo {
public static void main(String[] args) {
int i = 0;//计数器
//循环 不满足条件不执行 可能执行0次
while( i < 10 ){
System.out.println( i + " Hello World!...");//执行代码
i++;//运行时 i值每次发生改变 i = 10
}
System.out.println( "i = " + i );
i = 0; // 0 9
do{//先执行 后判断 至少执行一次
System.out.println( i + " 快乐!...");
i++;
}
while (i < 10);
int a = ran.nextInt(101);
System.out.println( "a = " + a );
int i = 2;
while( i < a) {//67
if( a % i == 0) {
break;//跳出循环
}
i++;
}
System.out.println( "i = " + i );
if(i >= a)//正常退出循环
System.out.println( a +"是素数");
else
System.out.println( a +"不是素数");
}
}
5.for循环
import java.util.Random;
public class LoopDemo2 {
public static void main(String[] args) {
int k = 10; //方法中的变量 作用域 在方法中使用
//i 局部变量
for(int i = 0;i < 10 ; i++ ) {//三条语句
System.out.println( i + " Hello World!...");
}
//循环嵌套
/*
for(int i = 0; i < 5; i++) {
//k = k + i;
for(int j = 0; j < 3; j++) {
System.out.print(" * ");
}
System.out.print("
");//输出换行
}
*/
for(int i = 1; i < 6; i++) {
for(int j = 1; j <= i; j++) {
System.out.print(j + " * " + i + " = "+ i * j + " ");
}
System.out.print("
");
}
int sum = 0;
int i;
for(i = 1; i < 200; i++) {
sum += i;
if(sum >= 200){
break;
}
}
System.out.println("sum = " + sum);
System.out.println("i = " + i);
}
}
6.标记循环
import java.util.Random;
public class LoopDemo3 {
public static void main(String[] args) {
int sum = 0;
int i,k = -1;
loopi: for(i = 1; i < 200; i++) {
for(int j = 1; j < 200; j++) {
sum += j;
if(sum >= 200){
k = j;
break loopi;//指定跳出位置
}
}
System.out.println("k = " + k);
System.out.println("i = " + i);
}
System.out.println("sum = " + sum);
System.out.println("k = " + k);
for(int a = 1; a < 101; a++ ) {
if( a % 3 != 0){//不能被3整除
continue;//结束本次后面代码执行
}
System.out.print( a + " ");
}
}
}
7.双循环
public class LoopDemo4 {
public static void main(String[] args) {
int sum = 0;
for(int i = 1; i < 11; i++) {//控制的是行
for(int j = 1; j <= i; j++) {//控制的是列
System.out.print(" * ");
/**/
if(j > 5){ //1 2 3 4 5 6 7 8 9 10
break;//跳出内层循环
}
}
System.out.println();
}
}
}