# TreeSet進行物件排序,Comparator的用法
**Question:**

class Vehicle{
	int vno;
	String name;
	public Vehicle (int vno, String name) {
		this.vno = vno;
		this.name = name;
	}
	public String toString () {
		return vno + ":" + name;
	}
}
public class Test {
	public static void main(String[] args) {
		Set<Vehicle> vehicle = new TreeSet<Vehicle>();
		vehicle.add(new Vehicle(10123, "Ford"));
		vehicle.add(new Vehicle(10124, "BMW"));
		System.out.println(vehicle);
	}
}

**Console結果如下:**

Exception in thread "main" java.lang.ClassCastException: exam.ocpjp.v1.q023.Vehicle cannot be cast to java.lang.Comparable
	at java.util.TreeMap.compare(TreeMap.java:1294)
	at java.util.TreeMap.put(TreeMap.java:538)
	at java.util.TreeSet.add(TreeSet.java:255)
	at exam.ocpjp.v1.q023.Test.main(Test.java:23)

**會產生ClassCastException
因為TreeSet的元素必須要實作Comparable介面，
或是在宣告TreeSet時，實作Comparator方法才可以進行自動排序。**

---
**以下是改良版:**

import java.util.Comparator;
import java.util.Set;
import java.util.TreeSet;

class Vehicle implements Comparable<Vehicle>{
	int vno;
	String name;
	public Vehicle (int vno, String name) {
		this.vno = vno;
		this.name = name;
	}
	public String toString () {
		return vno + ":" + name;
	}
	@Override
	public int compareTo(Vehicle other) {
		return this.vno-other.vno;
	}
}
public class Test {
	public static void main(String[] args) {
		Set<Vehicle> vehicle = new TreeSet<Vehicle>(new Comparator<Vehicle>() {
			@Override
			public int compare(Vehicle o1, Vehicle o2) {
				return o1.vno-o2.vno;
			}
		});
		vehicle.add(new Vehicle(10123, "Ford"));
		vehicle.add(new Vehicle(10124, "BMW"));
		System.out.println(vehicle);
	}
}

**Console結果如下:**

[10123:Ford, 10124:BMW]

**TreeSet可以正常排序了**
:::danger
:bulb: Comparable有個compareTo()方法要實作，它會傳入一個物件，你要傳回大於0、0或小於0的數。
傳回大於0表示，傳入的物件順序上比目前物件大，應該排在後面，傳回0表示兩個物件順序上相等，傳回小於0 表示傳入的物件順序上比目前物件小，應該排在前面。
:::

**Question:**

class Book implements Comparator<Book> {
	String name;
	double price;
	public Book(){
		
	}
	public Book(String name, double price) {
		this.name = name;
		this.price = price;
	}
	public int compare(Book b1, Book b2) {
		return b1.name.compareTo(b2.name);
	}
	public String toString() {
		return name + ":" + price;
	}
}
public class Test {

	public static void main(String[] args) {
		List<Book>books = Arrays.asList (new Book ("Beginning with Java", 2),new Book("A Guide to Java Tour", 3));
		Collections.sort(books, new Book());
		System.out.print(books);
	}

}

**Console:**

[A Guide to Java Tour:3.0, Beginning with Java:2.0]

###### tags: `ocpjp`