为什么提示不兼容的类型: int无法转换为boolean

import java.util.Scanner;
class  PT2
{
    public static void main(String[] args)
    {
      Scanner s = new Scanner(System.in);
      System.out.println("输入:");
       int angle = s.nextInt();

       if ( angle = 90)
       {
           System.out.println("The angle is a right angle");
       }else
           {
           System.out.println("The angle is not a right angle");
           }
       }
}