密碼學 簡單加密

package 密碼學作業;

import java.util.Arrays;
import java.util.Scanner;

public class first {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner sc = new Scanner(System.in);
		char []alp_a = new char[26];
		char []alp_b = new char[26];
		char []alp_c = new char[26];
		char []alp_d = new char[6];
		char []MingWen = new char[100];
		char []MiWen = new char[100];
		String c ;
		String mingwen;
		for(int i=1;i<=26;i++) {
			 alp_a[i-1]=(char)(96+i);
			 alp_c[i-1]=(char)(96+i);
		}
		
		while(true) {
			System.out.println("請輸入密鑰1-6位數,不重複");
			 c = sc.nextLine();
			if(c.length()>0&&c.length()<7) {
				break;
			}
			else {
				System.out.println("位數不合理，請重新輸入");
				continue;
			}
		}
		
		
		for(int i=0;i<c.length();i++) {
			alp_d[i] = c.charAt(i);
		}
		
		for(int i=0;i<c.length();i++) {
			for(int j=i+1;j<c.length();j++) {
				if(alp_d[i]==alp_d[j]) {
					System.out.println("字符重複，程序結束");
					System.exit(0);
				}
					
			}
		}
		for(int i=0;i<c.length();i++) {
			for(int j=0;j<=25;j++) {
				if(alp_a[j]==alp_d[i]) {
					alp_a[j]='0';
				}
			}
		}
		
		for(int i=0;i<c.length();i++) {
			alp_b[i]=alp_d[i];
		}
		
		int temp=c.length();
		for(int i=0;i<=25;i++) {
			for(int j=temp;j<26;j++) {
				
				if(alp_a[i]!='0') {
					alp_b[j]=alp_a[i];
					temp = j+1;
					break; //i+1
					// continue; j+1
				}
					
			}
		}
		
		System.out.println("請輸入100位以下的明文");
		mingwen = sc.nextLine();
	
		for(int i=0;i<mingwen.length();i++) {
			MingWen[i] = mingwen.charAt(i);
		}

		for(int i=0;i<mingwen.length();i++) {
			for(int j=0;j<26;j++) {
				if(MingWen[i]==','||MingWen[i]=='.'||MingWen[i]==' '||MingWen[i]=='?') {
					MiWen[i]=MingWen[i];
				}
				else if(MingWen[i]==alp_c[j])
					MiWen[i]=alp_b[j];
			}
		}
		
		System.out.println("對照表");
		System.out.println(Arrays.toString(alp_c));
		System.out.println(Arrays.toString(alp_b));
		System.out.println("   ");
		System.out.print("明文：");
		for(int i=0;i<mingwen.length();i++) {
			System.out.print(MingWen[i]);
		}
		System.out.println("");
		System.out.print("密文：");
		for(int i=0;i<mingwen.length();i++) {
			System.out.print(MiWen[i]);
		}
		
		System.out.println("   ");
		System.out.println("是否需要譯回，需要請輸入1，不需要請輸入2");
		
		int input=sc.nextInt();
		switch(input) {
		case 1:
			for(int i=0;i<mingwen.length();i++) {
				for(int j=0;j<26;j++) {
					if(MiWen[i]==','||MiWen[i]=='.'||MiWen[i]==' '||MiWen[i]=='?') {
						MingWen[i]=MiWen[i];
					}
					else	if(MiWen[i]==alp_b[j])
					MingWen[i]=alp_c[j];
			}
		}
			System.out.println("   ");
			System.out.print("密文：");
			for(int i=0;i<mingwen.length();i++) {
				System.out.print(MiWen[i]);
			}
			System.out.println("");
			System.out.print("明文：");
			for(int i=0;i<mingwen.length();i++) {
				System.out.print(MingWen[i]);
			}
			break;
		case 2:
			System.out.println("程序終止");
			System.exit(0);
			
		default: 
			System.out.println("輸入錯誤，程序終止");
			System.exit(0);
		}
	}

}

需要注意的點：

1.使用next() 輸入帶有空格的句子，例如 i am a teacher,就會把第一個空格當成結束符，只輸入i，nextLine()的結束符是回車，所以如果輸入帶有空格的句子用nextLine()

2.判斷字符串中是否有重複的字符

https://blog.csdn.net/qq_14927217/article/details/73472572

https://blog.csdn.net/u010698072/article/details/52802179

3.判斷兩個字符是不是相等可以直接用==

4.程序正常結束System.exit(0)