Does TreeSet allows only comparable objects - class cast exception

[kiran nyala]

Hi

i would like to add Comparator objects to TreeSet, while doing ClassCastException throwing. Could you please help on this.

My question is when I implement my custom class with comparable every this goes fine. But what is the way to add Comparator objects to Treeset?

class Person implements Comparator<Person> {
	
	private int empId;
	private String empName;
	private int empSal;
	
	public Person(int id, String name, int sal) {
		this.empId = id;
		this.empName = name;
		this.empSal =  sal;
	}
	
	public int getEmpId() {
		return empId;
	}

	public String getEmpName() {
		return empName;
	}

	public int getEmpSal() {
		return empSal;
	}

	public void setEmpId(int empId) {
		this.empId = empId;
	}

	public void setEmpName(String empName) {
		this.empName = empName;
	}

	public void setEmpSal(int empSal) {
		this.empSal = empSal;
	}

	@Override
	public int compare(Person o1, Person o2) {
		if(o1.getEmpSal() > o2.getEmpSal())
			return 1;
		if(o1.getEmpSal() < o2.getEmpSal())
			return -1;
		else 
			return 0;
	}
	
	public String toString() {
		return this.empId +"---"+this.empName+"--"+this.empSal;
	}
	
}

public class ClassCastExceptionTest {

	public static void main(String[] args) {
		
		Person p1= new Person(101,"ABC", 4000);
		Person p2= new Person(101,"AB", 6000);
		Person p3= new Person(101,"ABCD", 1000);
		Person p4= new Person(101,"A", 8000);
		
		
		SortedSet<Person> personSet = new TreeSet<Person>();
		personSet.add(p1);
		personSet.add(p2);
		personSet.add(p3);
		personSet.add(p4);
		Iterator iter = personSet.iterator();
		while (iter.hasNext()) {
		    System.out.println(iter.next());
		}
	}
	
}

Exception in thread "main" java.lang.ClassCastException: com.exceptions.Person cannot be cast to java.lang.Comparable

[Junilu Lacar]

Look at the JavaDocs for TreeSet. There's a constructor that takes a Comparator.

[Campbell Ritchie]

I presume you have read the documentation for TreeSet; you must either pass an object implementing Comparable<T> or you must pass a Comparator<T> as a separate argument (I think to the constructor).

No, that is not the right way to use a Comparator. Create it in a different class, as an anonymous class, or as a λ. You might do well to change your class to implement the Comparable<T> interface, which is probably the easiest solution at present.
Why is your class called person if it has a salary field and an id field? Surely you have written an Employee class there?

[Junilu Lacar]

BTW, it makes no sense for Person to implement Comparator<Person>. Comparators should be separate entities because they are like a third party arbiter, taking two Person objects and evaluating how they compare to each other. It may make sense in the real world that every Person could potentially serve in that capacity (that's why we have judges and juries, after all) but in an object-oriented programming context, it doesn't. That's just going to be confusing. Look for examples of how to write a Comparator

[Campbell Ritchie]

kiran nyala wrote:. . . com.exceptions.Person cannot be cast to java.lang.Comparable

Don't use that package name, unless you own the www.exceptions.com website. More details in the Java™ Tutorials.

[kiran nyala]

Thanks for all your response.

I did with TreeSet constructor with comparator. thank you