JAVA学习笔记（二）-运算符、选择语句、循环语句

运算符

package com.blog.learning;

public class Test {

public static void main(String[] args) {
// TODO Auto-generated method stub
int xx;
xx = 100;
System.out.println(xx);

//使用final来定义Java中的常量。
//在声明的同时为常量名赋值。一旦赋值后，该常量名的值则不能修改。
final int BLOOD = 10;

//关于常量名的命名-全部大写，如果多个单词，中间用_分割
final int MY_AGE = 20;

//交换变量
//1-方法一
//      int temp = 0;
//      temp = a;
//      a = b;
//      b = temp;
//      System.out.println("a = " + a + ", b = " + b);

//2-方法2
//      a = a + b;//a = 15, b = 5;//注意，有可能出现数据溢出
//      b = a - b;//a = 15, b = 10;
//      a = a - b;//a = 5, b = 10;
//      System.out.println("a = " + a + ", b = " + b);

//3-方法3
//      a = a ^ b;//a=10^5  b=5
//      b = a ^ b;//b=10 a=10^5
//      a = a ^ b;//a=5, b = 10

/*
* 字符串拼接:区分哪些是可变的，哪些是不可变的，不可变的放入""内，可变的通过+连接
*/
System.out.println(10 + "aaa");//"10" + "aaa"="10aaa"
int a = 10;
int b = 5;
System.out.println("a + b = " + a + b);//"a + b = 105"
System.out.println(a + b + " = a + b");//"15 = a + b"
System.out.println("a + b = " + (a + b));//"a + b = 15"
System.out.println(a + " + " + b + " = " + (a + b));//10 + 5 = 15

System.out.println("(" + a + ") + (" + b + ") = [" + (a + b) + "]");

byte b1 = 10;
byte b2 = 10;
//      b1 = b1 + b2;//编译错误
//      b1 = (byte)(b1 + b2);
b1 += b2;// b1 = (byte)(b1 + b2);

a = 10;
b = 5;

int c = a > b? a : b;
System.out.println(c);

boolean flag = false;
int x = flag?10:20;
System.out.println(x);

String str = 10 < 20 ? "10 < 20" : "10 > 20";
System.out.println(str);
}

}

选择语句

package com.blog.learning;

import java.util.Scanner;

public class Test1 {

public static void main(String[] args) {
// TODO Auto-generated method stub
/*
* 题目：根据学生学号，打印输出名字。1，张三；2，李四；3，王五，其他，不存在。
*/
Scanner input=new Scanner(System.in);
int Id=input.nextInt();
/*  if(Id==1){
System.out.println("zhangsan");
}
else if(Id==2){
System.out.println("lisi");
}
else if(Id==3){
System.out.println("wangwu");
}
else {
System.out.println("no result");
}

switch(Id)  //表达式，case后的常量必须是byte,short,char,int,JDK1.5,enum,1.7 String
{
case 1:
System.out.println("zhang");
break;
case 2:
System.out.println("li");
break;
case 3:
System.out.println("wang");
break;
default:
System.out.println("no result");
break;
}
*/
/*
* 打印输出[100, 90],输出A；[70, 90), 输出B， [60, 70),输   出C，不然输出D
*/
int score=Id;
switch(score/10)
{
case 10:
case  9:
System.out.println("A");
break;
case  8:
case  7:
System.out.println("B");
break;
case  6:
System.out.println("C");
break;
default:
System.out.println("D");
break;
}
}

}

循环语句

package com.blog.learning;

public class Test2 {

public static void main(String[] args) {
// TODO Auto-generated method stub
/*
* 题目：打印输出1~100之间偶数的和
*/
int sum=0;
for(int i=1;i<=100;i++)
{
if((i%2)==0)
sum+=i;
}
System.out.println(sum);

/*
* 题目：打印输出1000~2050年中所有的闰年
*/
for(int year=1000;year<=2050;year++)
{
if(year%400==0||(year%100!=0&&year%4==0))
System.out.println(year);
}

/*
* 要求，使用*打印输出一个等腰三角形
*
5 - row
*   2*row - 1
***
*****
*******
*********
*
*/
for(int row=1;row<=5;row++)
{
for(int sp=1;sp<=5-row;sp++)
System.out.print(" ");
for(int x=1;x<=2*row-1;x++)
{
System.out.print("*");
}
System.out.println();
}

}

}