The source code is as follows:

        Point[] points = new Point[n];//Point is a class
         for ( int i=0;i<n;i++ ) { 
            System.out.print( "Please enter x:" ); 
            points[i]. setX(in.nextInt());//The 29th line 
            System.out.print( "Please enter y:" ); 
            points[i].setY(in.nextInt()); 
        }

The error message is as follows:

Exception in thread “main” java.lang.NullPointerException
at test.Main.zjd(Main.java:29)
at test.Main.main(Main.java:9)

analysis:

Point[] points=new Point[n];//After defining the class array, I thought that the above code could be used directly, but the above error was reported. 
So I set line 29 as a breakpoint, and 
when I started Debug and run to this point, I found the problem:

It is found in the variable list that the values ​​of points[0] and points[1] are all null

It seems that there is no problem at a glance, but when you think about it, each element in the array is an object. Since it is an object, how can we use it without allocating space (without new)?

Modified code:

        Point[] points= new Point[n];
         for ( int i=0;i<n;i++ ) {
            points[i] =new Point(); //Solution: apply for allocating space for the elements in the class array here . 
            System.out.print( "Please enter x:" ); 
            points[i].setX(in.nextInt()); 
            System.out.print( "Please enter y:" ); 
            points[i].setY(in .nextInt()); 
        }