// Supposing this class is going to function as a KEY -- it should be immutable!  
//  (See numbered comments below that describe how we ensure this)

public final class ImmutableSbPair { // 1) declare class as final (so it can't be extended)
    
    private final StringBuilder x;   // 2) all instance vars should be private, so client has no direct access 
    
    private final StringBuilder y;   // 3) mutable instance variables should be marked "final"
                                     //    so their values can only be assigned once
    
    public ImmutableSbPair(StringBuilder x, StringBuilder y) {
        this.x = new StringBuilder(x.toString());           // 4) initialize all instance vars via a 
        this.y = new StringBuilder(y.toString());           //    constructor using defensive (and deep) copies
    }
    
    // 5) don't provide any mutator methods (note, none of the methods below change x or y)
    
    public String toString() {
        return "(" + x + "," + y + ")";
    }
    
    public int hashCode() {
        int h = 17;
        h = 31 * h + x.hashCode();
        h = 31 * h + y.hashCode();
        return h;
    }
    
    // 6) provide no accessor methods for mutable parts of the object -- if needed, return a deep copy instead
    
    public StringBuilder getX() {
        return new StringBuilder(x.toString());
    }
    
    public StringBuilder getY() {
        return new StringBuilder(y.toString());
    }

}