Postive or negative null check in if-statement

I do a lot of null checking in my code, and usually I will check for not null before going on to my for or more if/else statements.

For example:

if(x!=null) {
     for(int y:x) {
         if(y.property1 != null) {
             if( y.property < n) {
             }
         } else {
         }
     }
}

Is the following better practice?

 if(x ==null) {
     //exit or w/e you want to do
 }
 for(int y:x) {
      if(y ==null){
      }
      if(y<0) {
      }
 }