Can I add a object to TreeMap without reference

Question

I have basic Rectangle class. Can I use the object without reference? I'tried but it seems like only adding the last one.

public class Rectangle implements Comparable {

int a1;
int a2;

public Rectangle (int a1, int a2) {
    this.a1= a1;
    this.a2= a2;
}

    TreeMap<Rectangle, String > rectangleStringTreeMap = new TreeMap<>();
    rectangleStringTreeMap.put(new Rectangle(2,5),"This is first");
    rectangleStringTreeMap.put(new Rectangle(3,7),"This is second");
    rectangleStringTreeMap.put(new Rectangle(4,8),"This is third");

Answer 1

If your class is primarily meant to be a transparent immutable carrier of data, define your class as a record. In a record, the compiler implicitly creates the constructor, getters, equals & hashCode, and toString by considering each and every member field.

Make your class implement Comparable using generics rather than as a raw type.

Define a Comparator with two clauses, to do the comparing work.

Something like this untested code.

public record Rectangle ( int a1 , int a2 ) implements Comparable < Rectangle >
{
    static private Comparator < Rectangle > comparator = 
            Comparator
                .comparingInt( Rectangle :: a1 )
                .thenComparingInt( Rectangle :: a2 ) ;
    
    @Override
    public int compareTo( Rectangle other )
    {
        return Rectangle.comparator.compare( this , other ) ;
    }
}

Proceed with your map. But declare your map as the more general interface NavigableMap rather than the concrete class TreeMap.

NavigableMap < Rectangle, String > rectangleStringNavMap = new TreeMap<>();
rectangleStringNavMap.put( new Rectangle(2,5), "This is first" );
rectangleStringNavMap.put( new Rectangle(3,7), "This is second" );
rectangleStringNavMap.put( new Rectangle(4,8), "This is third" );

If working with the map across threads, use ConcurrentNavigableMap interface, and ConcurrentSkipListMap class.