输入与输出
常见输入输出的一个示例：
import static java.lang.Math.*;
import java.io.*;
import java.nio.charset.StandardCharsets;
import java.nio.file.Path;
import java.util.Scanner;
public class hello {
public static void main(String[] args) throws IOException {
Scanner in = new Scanner(System.in); //创建一个读取系统输入的Scanner用于操作
System.out.println("What's your name?");
String name = in.nextLine(); //in接收系统输入，nextLine()接收一行字符串
System.out.println("What's your nickname?(one word)");
String nickname = in.next(); //next()接收一个单词()空格为分隔符
System.out.println("What's your age?");
int age = in.nextInt(); //nextInt()接收一个整数
//String message=String.format("Hello your name is %s,\nyour nickname is %s,\nnext year you will be %d\n",name,nickname,(age+1));
//format方法用于创建一个格式化字符串
//System.out.println(message);
System.out.printf("Hello your name is %s,\nyour nickname is %s,\nnext year you will be %d\n", name, nickname, (age + 1));
//printf函数格式化输出
}
}
结合文件读写：（依次读取文件中的int,double,word）
import static java.lang.Math.*;
import java.io.*;
import java.nio.charset.StandardCharsets;
import java.nio.file.Path;
import java.util.Scanner;
public class hello {
public static void main(String[] args) throws IOException {
Scanner in = new Scanner(Path.of("1.txt"), StandardCharsets.UTF_8); //第一个参数是Path
while(in.hasNextInt()) //按照顺序依次判定
{
System.out.println("1:");
System.out.println(in.nextInt());
}
while(in.hasNextDouble())
{
System.out.println("2:");
System.out.println(in.nextDouble());
}
while(in.hasNext())
{
System.out.println("3:");
System.out.println(in.next());
}
}
}
import static java.lang.Math.*;
import java.io.*;
import java.nio.charset.StandardCharsets;
import java.nio.file.Path;
import java.util.Scanner;
public class hello {
public static void main(String[] args) throws IOException {
PrintWriter out=new PrintWriter("1.txt",StandardCharsets.UTF_8);
out.println("写入");
out.printf("%d",1);
out.close(); //关闭数据流，以防数据丢失
}
}
带标签的break
import static java.lang.Math.*;
import java.io.*;
import java.nio.charset.StandardCharsets;
import java.nio.file.Path;
import java.util.Scanner;
public class hello {
public static void main(String[] args) throws IOException {
Scanner in = new Scanner(System.in);
int n=0;
ReadData:
for(int i=10;i>0;i--)
{
n=in.nextInt();
if(n<0)
{
break ReadData;//到达ReadData标签的末尾
}
}
if(n<0)
{
System.out.println("n<0");
}
}
}
大数
import static java.lang.Math.*;
import java.io.*;
import java.math.BigInteger;
import java.nio.charset.StandardCharsets;
import java.nio.file.Path;
import java.util.Scanner;
public class hello {
public static void main(String[] args) throws IOException {
BigInteger a= BigInteger.valueOf(1); //valueOf将普通的数值转换为大数
BigInteger b=new BigInteger("999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999");//字符串构造器
for(int i=1;i<=100;i++)
{
a=a.multiply(BigInteger.valueOf(i));//multiply函数用于乘法
}
b=b.add(a);//add函数用于加法
//不能直接用+和*运算符
System.out.println(a);
System.out.println(b);
}
}
数组
基本初始化和打印
import static java.lang.Math.*;
import java.io.*;
import java.math.BigInteger;
import java.nio.charset.StandardCharsets;
import java.nio.file.Path;
import java.util.Arrays;
import java.util.Scanner;
public class hello {
public static void main(String[] args) throws IOException {
int[] a=new int[100];//管理者和对象
int[] b={2,3,5,7,11,13,};//赋初值
int[] c={2,3,5,7,11,13,};//赋初值
b=new int[] {4,5,};//创建匿名数组写法，并且赋给变量
int[] d=new int[0];
int[] e={};
//d,e等价写法，允许出现长度为0的数组
System.out.println(Arrays.equals(d, e));
for(int element:b) //for each结构，更简洁并且不用操心下标
{
System.out.println(element);
}
System.out.println(Arrays.toString(c));
//打印c的值，显示[2, 3, 5, 7, 11, 13]
}
}
数组拷贝
import static java.lang.Math.*;
import java.io.*;
import java.math.BigInteger;
import java.nio.charset.StandardCharsets;
import java.nio.file.Path;
import java.util.Arrays;
import java.util.Scanner;
public class hello {
public static void main(String[] args) throws IOException {
int[] num={2,3,5,7,};
int[] num2=Arrays.copyOf(num,num.length);
int[] num3=Arrays.copyOf(num,num.length/2);
int[] num4=Arrays.copyOf(num,num.length*2);
//Arrays.copyOf重新创建了一个新的数组对象，前一参数是一组数组值，后一参数是新数组长度
//若新数组长度多于原数组，则多余空间值均为0;若少于则只复制一部分值
int[] num5=num;
//num5与num管理同一个数组
System.out.println(Arrays.toString(num));
num5[0]=999;
System.out.println(Arrays.toString(num2));
System.out.println(Arrays.toString(num3));
System.out.println(Arrays.toString(num4));
System.out.println(Arrays.toString(num5));
Arrays.sort(num);//数组排序方法，使用快排实现
System.out.println(Arrays.toString(num));
}
}
运行结果：
[2, 3, 5, 7]
[2, 3, 5, 7]
[2, 3]
[2, 3, 5, 7, 0, 0, 0, 0]
[999, 3, 5, 7]
[3, 5, 7, 999]
命令行参数：
import jdk.jshell.JShell;
import static java.lang.Math.*;
import java.io.*;
import java.math.BigInteger;
import java.nio.charset.StandardCharsets;
import java.nio.file.Path;
import java.util.Arrays;
import java.util.Scanner;
public class hello {
public static void main(String[] args) throws IOException {
//args数组是命令行参数构成的字符串数组
if(args.length==0||args[0].equals("-h"))
{
System.out.print("Hello,"); //print打印不会额外打印空行，println会
}
else if(args[0].equals("-g"))
{
System.out.print("Goodbye,");
}
//print command-line arguments
for(int i=1;i<args.length;i++)
{
System.out.print(" "+args[i]);
}
System.out.print("!");
}
}
运行结果：
不规则数组：
import static java.lang.Math.*;
import java.io.*;
import java.math.BigInteger;
import java.nio.charset.StandardCharsets;
import java.nio.file.Path;
import java.util.Arrays;
import java.util.Scanner;
public class hello {
public static void main(String[] args) throws IOException {
int[][] a = {
{1, 2,},
{5, 8, 9, 10},
{6, 7, 4,},
};//多维数组初始化，可以是不规则数组
//本质：“数组的数组”
for (int i = 0; i < a.length; i++) {
for (int j = 0; j < a[i].length; j++) {
System.out.print(a[i][j] + " ");
}
System.out.print("\n");
}
System.out.print("\n");
//另一种写法
for (int[] row : a) {
for (int value : row) {
System.out.print(value + " ");
}
System.out.print("\n");
}
System.out.print("\n");
System.out.println(Arrays.deepToString(a));
//Arrays.toString()方法在多维数组的扩展
int[] temp=a[0];
a[0]=a[2];
a[2]=temp;
//可以单独访问数组的某一行，所以两行交换
System.out.println(Arrays.deepToString(a));
}
}
运行结果：
1 2
5 8 9 10
6 7 4
1 2
5 8 9 10
6 7 4
[[1, 2], [5, 8, 9, 10], [6, 7, 4]]
[[6, 7, 4], [5, 8, 9, 10], [1, 2]]
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