HashSet、LinkedHashSet 和 TreeSet 的排序

基本观点

• HashSet：无序
• LinkedHashSet：按插入排序
• TreeSet：按自然顺序

主要讲 TreeSet 的自然顺序

基本类型的自然排序

TreeSet tree = new TreeSet();
tree.add(12);
tree.add(63);
tree.add(34);
tree.add(45);

Iterator iterator = tree.iterator();
System.out.print("Tree set data: ");
while (iterator.hasNext()) {
    System.out.print(iterator.next() + " ");
}
​

输出结果：Tree set data: 12 34 45 63

类对象的自然排序

class Dog {
  int size;

  public Dog(int s) {
	size = s;
  }

  public String toString() {
	return size + "";
  }
}
​

import java.util.Iterator;
import java.util.TreeSet;

public class TestTreeSet {
  public static void main(String[] args) {
	TreeSet dset = new TreeSet();
	dset.add(new Dog(2));
	dset.add(new Dog(1));
	dset.add(new Dog(3));

	Iterator iterator = dset.iterator();
	while (iterator.hasNext()) {
	System.out.print(iterator.next() + " ");
	}
  }
}
​

结果

Exception in thread "main" java.lang.ClassCastException: 
collection.Dog cannot be cast to java.lang.Comparable 
at java.util.TreeMap.put(Unknown Source) 
at java.util.TreeSet.add(Unknown Source) 
at collection.TestTreeSet.main(TestTreeSet.java:22)
​

Dog 类实现 Comparable 接口和 compareTo 方法

class Dog implements Comparable{
  int size;

  public Dog(int s) {
	size = s;
  }

  public String toString() {
	return size + "";
  }

  @Override
	public int compareTo(Dog o) {
	  return size - o.size;
  }
}
​

输出

1 2 3
​