java基础第四天_复习运算符、进制转化和数组
1.按位运算操作
答：
/**
* 运算符演示
*/
class CalcDemo{
//入口点
public static void main(String[] args){
//定义整型变量
int a = 3 ;
int b = 5 ;
//+-符号
System.out.println("****** 正负号 *********");
System.out.println("+a = " + (+a)) ;
System.out.println("-a = " + (-a)) ; 负数由正数取反加1
//算术运算符 + - * / %
System.out.println("****** 算术运算符 *********");
System.out.println("a + b = " + (a + b));
System.out.println("a - b = " + (a - b));
System.out.println("a * b = " + (a * b));
System.out.println("a / b = " + (a / b));
System.out.println("a % b = " + (a % b));
//赋值运算符 a += b ==> a = a + b
System.out.println("****** 赋值运算符 *********");
System.out.println("a += b = " + (a += b));
System.out.println("a -= b = " + (a -= b));
System.out.println("a *= b = " + (a *= b));
System.out.println("a /= b = " + (a /= b));
System.out.println("a %= b = " + (a %= b));
//比较运算符
System.out.println("****** 比较运算符 *********");
System.out.println("a == b = " + (a = b));
System.out.println("a != b = " + (a != b));
System.out.println("a > b = " + (a > b));
System.out.println("a >= b = " + (a >= b));
System.out.println("a < b = " + (a < b));
System.out.println("a <= b = " + (a <= b));
//逻辑运算符
boolean b1 = false ;
boolean b2= true ;
System.out.println("****** 逻辑运算符 *********");
System.out.println("b1 | b2 = " + (b1 | b2));//true
System.out.println("b1 & b2 = " + (b1 & b2));//false
System.out.println("b1 ^ b2 = " + (b1 ^ b2));//true
System.out.println("!b1 = " + !b1);//true
System.out.println("b1 || b2 = " + (b1 || b2));//true
System.out.println("b1 && b2 = " + (b1 && b2));//false
//位运算符
System.out.println("****** 逻辑运算符 *********");
a = -7 ; b = 3 ;//1111 1111 - 1111 1111
System.out.println("a << 2 = " + (a << 2));//-28
System.out.println("a >> 2 = " + (a >> 2));//-2
System.out.println("a >>> 2 = " + (a >>> 2));//1073741822
System.out.println("a | b = " + (a | b));//-5
System.out.println("a & b = " + (a & b));//1
System.out.println("a ^ b = " + (a ^ b));//-6
System.out.println("~a = " + (~a));//6
}
}
2.byte数到十六进制字符串表现形式程序设计原理和实现方式。
答：
/**
* 将字节类型数组转换成十六进制的字符串表现形式
*/
class Byte2Hex {
public static void main(String[] args) {
byte b = 108;
System.out.println(convert(b));
}
// 转换函数
public static String convert(byte b) {
// 1.取出字节b的低四位的数值
int low = b & 0x0F; // 低四位0-15
// 2.取出高四位的值,先将b向右移动4位
int high = (b >> 4) & 0x0F; // 高四位0-15
// 定义字符数组
char[] arr = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A',
'B', 'C', 'D', 'E', 'F' };
// 拼接串
return "" + arr[high] + arr[low];
}
}
3.定义函数，计算一位整型数组所有元素的和。
答：
/**
* 计算所有元素的总和
*/
class SumDemo{
public static void main(String[] args){
//int result = sum(new int[]{1,2,4,5,6});
System.out.println(result);
}
//计算数组的总和
public static int sum(int[] arr){
int sum = 0 ;
//循环数组
for(int i = 0 ; i < arr.length ; i ++){
//开始累加
sum = sum + arr[i] ;
}
return sum;
}
}
4.数组的拷贝。
答：
class ArrayCopyDemo {
public static void main(String[] args) {
int[] arr1 = {1,6,8,7,4};
int[] arr2 = new int[arr1.length];
copy(arr1, arr2);
out(arr2);
}
private static void copy(int[] arr1, int[] arr2) {
for (int i = 0; i < arr1.length; i++) {
arr2[i] = arr1[i];
}
}
private static void out(int[] arr) {
for (int i = 0; i < arr.length; i++) {
System.out.println(arr[i]);
}
}
}
5.堆内存默认是1/4,
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java -Xmx//设置堆内存最大值
-Xms//设置堆内存初始值
className//类名
答：堆内存的默认最大空间是物理内存的1/4,在运行java程序时，可以通过 -Xms初始堆内存的大小，-Xmx设置最大堆内存的大小；