x cannot be resolved or is not a field y cannot be resolved or is not a field

Main:

public class Main {
    public static void main(String [] args) {
        Foo<String, Location> foo = new Foo<>();
        foo.add("abc", new Location(5, 6));
    }
}

Location:

public class Location {
    public int x;
    public int y;

    public Location(int x, int y) {
        this.x = x;
        this.y = y;
    }

    @Override
    public String toString() {
        return "(" + x + "," + y + ")";
    }
}

Foo class:

public class Foo<K, T> {
    
    public void add(K name, T loc) {
        System.out.println(name + " " + loc.x + loc.y);
    }
    
    
}

When I try to run the program I get the “question title” error, I don’t know what that happens, and how to fix that.

Answer

One way is to use instanceof operator if you are expecting a type that you know. This is a demo.

public void add(K name, T loc) {
    if(loc instanceof Location) {
        System.out.println(name + " " + ((Location)loc).x + ((Location)loc).y);
    }
}

UPDATE

As Stephen C mentioned that Foo class potentially needs hard-coded knowledge of the capabilities of each of the possible types T. Which is (in theory) every Java reference type! The correct way to solve this is to declare <T extends SomeInterface> and provide getters in the interface to get the x and y attributes

So instead of knowing about all possible types of T we can make T extend SomeInterface and get the information by calling the getters which can be implemented in the respective type.

Make T extend LocationInfo interface to get info about x and y

class Foo<K, T extends LocationInfo> {
    public void add(K name, T loc) {
        System.out.println(name + " " + loc.getX() + loc.getY());
    }
}

interface LocationInfo {
    int getX();
    int getY();
}

And include the implementation in Location class

class Location implements LocationInfo {
    public int x;
    public int y;

    public Location(int x, int y) {
        this.x = x;
        this.y = y;
    }

    @Override
    public String toString() {
        return "(" + x + "," + y + ")";
    }
    
    public int getX() { return x; }
    
    public int getY() { return y; }
}