[JAVA] How to use TreeSet

Introduction

Hello. It's Kecho. ** Do you use TreeSet? ** ** I saw it for the first time today. Have you ever used a similar HashSet or TreeMap?

What is TreeSet?

It is a class that does not allow duplication and sorts and retains when adding. It's a class that seems to be useful in competitive programming.

Try using

First, create an object to add to the TreeSet

class Product {
	private int id;
	private String name;

	public Product() {
	}

	public Product(int id, String name) {
		this.id = id;
		this.name = name;
	}

	public int getId() {
		return this.id;
	}

	public String getName() {
		return this.name;
	}

}

I created the id and name fields, the constructor, and the accessor.

Then add an instance

Set<Product> tree = new TreeSet<>();
tree.add(new Product(1, "kechong"));
tree.add(new Product(2, "me"));
tree.add(new Product(3, "you"));
for (Product product : tree) {
	System.out.print("id:" + product.getId() + ", ");
	System.out.print("name:" + product.getName());
	System.out.println();
}

Yes, I'll try it.

Exception in thread "main" java.lang.ClassCastException: Product cannot be cast to java.base/java.lang.Comparable
	at java.base/java.util.TreeMap.compare(TreeMap.java:1291)
	at java.base/java.util.TreeMap.put(TreeMap.java:536)
	at java.base/java.util.TreeSet.add(TreeSet.java:255)
	at Main.main(Main.java:26)

You got an error.

What kind of error?

You're throwing a ClassCastException. I'm angry that the Product can't be cast to Comparable. The place of occurrence is when you try to add to the TreeSet.

Why you are trying to cast

I don't have to cast it ... I think, but this is necessary for TreeSet.

TreeSet has the concept of order. When adding to a TreeSet, you need to decide the order to determine where to hold it. When I created the Product class myself, an error occurred because I didn't specify "how to compare Product objects".

Add the concept of comparison

Specifically, it implements the Comparable interface.

class Product implements Comparable<Product> {
	private int id;
	private String name;

	public Product() {
	}

	public Product(int id, String name) {
		this.id = id;
		this.name = name;
	}

	public int getId() {
		return this.id;
	}

	public String getName() {
		return this.name;
	}

	@Override
	public int compareTo(Product o) {
		return this.id - o.id;
	}

}

Yes, I implemented and overridden the compareTo method.

If you run it again ...

id:1, name:kechong
id:2, name:me
id:3, name:you

Yes, it looks good!

By the way

This time, I implemented the Comparable interface in the Product class. However, when instantiating TreeSet, the same output can be obtained by specifying the following in the argument of the constructor.

Set<Product> tree = new TreeSet<>(Comparator.comparing(Product::getId));

This makes it possible to determine the order by giving a comparison method to the TreeSet side.

Summary

If you want to add your own object to TreeSet, you need to do one of the following: -Implement a Comparable interface with your own object -Specify the comparison method in the constructor argument of TreeSet