题解 | #整数与IP地址间的转换#

较为笨拙的一种方法：

import java.util.*;

public class Main{
    public static void main(String[] args){
        Scanner scan=new Scanner(System.in);
        String input1=scan.nextLine();
        String input2=scan.nextLine();
        System.out.println(trans(input1));
        System.out.println(resTrans(input2));
    }
    
    public static String trans(String input1) {
        String[] inputs=input1.split("\\.");
        StringBuilder sb=new StringBuilder();
        for(String item:inputs){
            String item2=Long.toBinaryString(Long.parseLong(item));
            while(item2.length()<8){
                item2="0"+item2;
            }
            sb.append(item2);
        }
        return Long.valueOf(sb.toString(),2).toString();
    }
    
    public static String resTrans(String input2){
        input2=Long.toBinaryString(Long.parseLong(input2));
        String[] strs=new String[4];
        while(input2.length()<32){
            input2="0"+input2;
        }
        for(int i=0;i<4;i++){
            String item=input2.substring(i*8,i*8+8);
            strs[i]=Long.valueOf(item,2).toString();
        }
        return String.join(".",strs);
    }
}