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算数运算符
需要注意的：
1、取余结果的正负跟左操作的数有关，左操作数(被除数)是正，结果为正；如果左操作数
(被除数)
是负数，结果就为负。
2、对于除号“/”，它的整数除和小数除是有区别的：整数之间做除法时，只保留整数部分而舍弃小数部分；小数的话则会保留小数位
下边看例子：
class Demo
{
public static void main(String[] args)
{
//加法
int num1 = 2 + 3;
int num2 = 2 + num1;
int num3 = num1 + num2;
System.out.println("num3="+num3);
//+，-，*，/，跟上学的时候一样，有优先级
System.out.println(3+3-2+9);
System.out.println(3+9/3+9);
System.out.println(3+3*9);
//除法
int a = 10;
int b = 0;
// System.out.println("a/b = "+a/b);//运算时异常，不能除以0
int c = 3;
System.out.println("a/c = "+a/c); //整数除法，结果取整
System.out.println("a/4 = "+a/4); //不四舍五入
double d1 = 10;
double d2 = 0.0;
System.out.println("d1/d2 = " +d1/d2);//输出Infinity，无穷大
double d3 = 4;
System.out.println("d1/d3 = " +d1/d3);
//取余
int n1 = 10;
int n2 = 5;
int n3 = 3;
System.out.println("n1 % n2 = "+n1 % n2);
System.out.println("n1 % n3 = "+n1 % n3);
int n4 = 0;
//System.out.println("n1 % n4 = "+n1 % n4);//运行时异常不能除以0
int n5 = -3;
int n6 = 2;
int n7 = -2;
//取余结果的正负跟被除数有关，被除数是正结果为正；被除数为负结果为负。
System.out.println("n1 % n5 = "+n1 % n5); //1
System.out.println("n5 % n6 = "+n5 % n6); //-1
System.out.println("n5 % n7 = "+n5 % n7); //-1
double db1 = 10;
double db2 = 0;
System.out.println("db1 % db2 = "+db1 % db2); //NaN(Not a Number),不属于异常
//丢失精度
double dv1 = 64.77;
double dv2 = 58.62;
System.out.println("dv1 + dv2 = " + dv1 + dv2);
//++,--
int t1 = 10;
t1++;
System.out.println(t1); //11
++t1;
System.out.println(t1); //12
int t2 = 10;
int t3 = ++t2;
System.out.println(t2);
System.out.println(t3);
int t4 = 10;
int t5 = t4++;
System.out.println(t4);
System.out.println(t5);
int t6 = 10;
int t7 = 20;
int t8 = t6++ + ++t7;//10 + 21 -> t8 = 31
System.out.println(t8);
System.out.println(t7);
System.out.println(t6);
int t9 = 10;
int t10 = 20;
System.out.println(t9--);//10
System.out.println(t9); //9
//字符串连接，用+运算符
String s1 = "a"+"b";
System.out.println(s1);
s1 = "a" + 10 + 20; //左右两边有一个字符串，结果就是字符串连接
System.out.println(s1);//a1020
s1 = 5 + 10 + "a";
System.out.println(s1);//15a
s1 = 3 + "" + 2;
System.out.println(s1);//32
System.out.println("5 + 5=" + 5 + 5);//5 + 5 = 55
//字符类型和整型的兼容性
char c1 = 'a'; //ascii码 a 是97
int t11 = c1 + 10;
System.out.println(t11); //107
System.out.println((char)t11); //k 107对应ascii中的k
System.out.println("" + c1); //a
System.out.println("" + c1 + 10); //a10
System.out.println(c1 + 10 + ""); //107
}
}