Scanner sc = new Scanner(System.in);

int num = sc.nextInt();

int cot = 0;

if(num%2==0) {

for(int i=1;i<=num-1;i=i+2) {  //i每次循环加2，直到i等于num-1，停止

cot = cot + i;

}

System.out.println("n为偶数，从1加到n-1之和为:" + cot);

    }else {

    for(int i=1;i<=num;i=i+2) {

    cot = cot +i;

    }

    System.out.println("n为奇数，从1加到n-1之和为:" + cot);

}

上面n为偶数时，条件应该为i<=n-1；等于号忘写了

public class Test_if_else {

public static void main(String[] args) {

// TODO Auto-generated method stub

//打印1+3+5+...+n，当n为奇数，则累加到n，若为偶数，则累加到n-1，变量n的初始值在程序中指定

Scanner sc = new Scanner(System.in);

System.out.println("请输入数字：");

int n = sc.nextInt();

int sum=0;

if(n%2==0) {

for(int i=1;i<n-1;) {

sum+=i;

i+=2;

}

System.out.print(sum);

}else {

for(int i=1;i<=n;) {

sum+=i;

i+=2;

}

System.out.println(sum);

}

}

}

package com.imooc;

public class helloword {

 public static  void main (String[] args) {

int sum = 0;

for (int i = 1;i<=100;i++)

{

if ( i%2 ==0   ) {

continue;

}

sum = sum + i;

}

System.out.println("100以内基数和为"+sum);

}

}