Comparable 外部比較器  在java.lang包內  自然排序

以學生類為例，包含屬性，構造方法，hashcode，equals方法等

public class Student {
	public String name;
	public int id;

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

	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + id;
		result = prime * result + ((name == null) ? 0 : name.hashCode());
		return result;
	}

	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		Student other = (Student) obj;
		if (id != other.id)
			return false;
		if (name == null) {
			if (other.name != null)
				return false;
		} else if (!name.equals(other.name))
			return false;
		return true;
	}

	@Override
	public String toString() {
		return "姓名=" + name + ", 學號=" + id;
	}
}
 

import java.util.Comparator;
import java.util.Set;
import java.util.TreeSet;
import org.junit.Test;

public class TestStudent{

	@Test
	public void testComparable() {
		Set<Student> s1 = new TreeSet<>();
		s1.add(new Student("bb", 3));
		s1.add(new Student("bb", 3));
		s1.add(new Student("cc", 2));
		s1.add(new Student("aa", 1));
		System.out.println(s1);
	}
}

會有java.lang.ClassCastException: org.witer.Student cannot be cast to java.lang.Comparable異常；意思是學生型別轉換為Comparable型別失敗

讓學生類實現Comparable介面，重寫compareTo方法，程式碼如下

import java.util.Comparator;
import java.util.Set;
import java.util.TreeSet;
import org.junit.Test;

public class Student implements Comparable<Student> {
	public String name;
	public int id;

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

	// 計算hash值是否相等
	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + id;
		result = prime * result + ((name == null) ? 0 : name.hashCode());
		return result;
	}

	// 若hash值相等，用equals判斷是否相等
	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		Student other = (Student) obj;
		if (id != other.id)
			return false;
		if (name == null) {
			if (other.name != null)
				return false;
		} else if (!name.equals(other.name))
			return false;
		return true;
	}

	@Override
	public String toString() {
		return "姓名=" + name + ", 學號=" + id;
	}

	// 改寫compareTo方法
	@Override
	public int compareTo(Student o) {
		// 定義一個變數bj 將傳入的引數和自己比較
		int bj = this.id > o.id ? 1 : (this.id == o.id ? 0 : -1);
		// 若id值相同，則用姓名的自然排序
		if (bj == 0) {
			bj = this.name.compareTo(o.name);
		}
		return bj;
	}
}
 

Comparator 內部比較器  在java.util包內   定製排序

以人類為例，參照學生類，也可以實現Comparator介面，重寫compare方法，但是這樣浪費了資源（一個類）

所以可以在外部寫一個比較器，呼叫比較器解決問題

public class Person {
	private String name;
	private int age;

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public int getAge() {
		return age;
	}

	public void setAge(int age) {
		this.age = age;
	}

	public Person(String name, int age) {
		super();
		this.name = name;
		this.age = age;
	}

	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + age;
		result = prime * result + ((name == null) ? 0 : name.hashCode());
		return result;
	}

	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		Person other = (Person) obj;
		if (age != other.age)
			return false;
		if (name == null) {
			if (other.name != null)
				return false;
		} else if (!name.equals(other.name))
			return false;
		return true;
	}

	@Override
	public String toString() {
		return "姓名=" + name + ", 年齡=" + age;
	}
}

import java.util.Comparator;
import java.util.Set;
import java.util.TreeSet;
import org.junit.Test;

public class TestStudent{
	
	@Test
	public void testComparator(){
		//定義一個比較器
		Comparator<Person> com=new Comparator<Person>() {
			@Override
			//重寫compare方法
			public int compare(Person o1, Person o2) {
				//定義一個變數接收比較值
			int b=o1.getAge()>o2.getAge()?1:(o1.getAge()==o2.getAge()?0:-1);
				//如果年齡相等，按姓名的自然排序
				if(b==0){
					b=o1.getName().compareTo(o2.getName());
				}
				return b;
			}
		};
		//建立集合並呼叫比較器
		Set<Person>p1=new TreeSet<>(com);
		p1.add(new Person("cc", 15));
		p1.add(new Person("aa", 28));
		p1.add(new Person("aa", 12));
		p1.add(new Person("bb", 15));
		System.out.println(p1);
		
	}
}

Console顯示   [姓名=aa, 年齡=12, 姓名=bb, 年齡=15, 姓名=cc, 年齡=15, 姓名=aa, 年齡=28]

什麼時候使用定製排序？

答：1，當自然排序和定製排序都存在時，優先使用定製排序

       2，當集合不支援自身比較器或者不滿足程式設計師需求時