TreeSet与TreeMap实现自定义比较器

1.常见集合

• 键值：Map集合等；

• 单值存储：数组，List，Set，Queue集合等；

• 我们知道部分的集合实现的原理不用，比如TreeMap底层采用红黑树，实现了排序的功能，其默认排序一般为从小到大排序数字，根据字典序排序字符等，但默认排序方式因为缺陷往往不能满足开发的需求，因此必须自己学会如何实现自定义比较器；

• TreeSet与TreeMap可以代表大部分集合实现自定义比较器的操作，因此以TreeSet与TreeMap为例；

2.TreeMap默认排序与自定义比较器实现

       1.TreeMap部分源码；

     // 调用compare比较两个key值
     cmp = cpr.compare(key, t.key);
     if (cmp < 0)
        t = t.left;
     else if (cmp > 0)
        t = t.right;
     else
        return t.setValue(value);
        
    final int compare(Object k1, Object k2) {
        // 调用默认的比较器，实现默认排序
        return comparator==null ? ((Comparable)k1).compareTo((K)k2)
            : comparator.compare((K)k1, (K)k2);
    }

    2.测试以Integer为键类型的默认排序；

package test;

import java.util.Map.Entry;
import java.util.TreeMap;

public class MyTreeMap {
	//public static TreeMap studentMap = new TreeMap<>();
	public static TreeMap intergerMap = new TreeMap<>();
	
	public static void main(String[] args) {
		// 给集合中添加10个student对象
		for (int i = 10; i > 0; --i) {
			//studentMap.put(new Student(i, "小王" + i), i);
			intergerMap.put(i, new Student(i, "小王" + i));
		}
		// 打印集合中的元素
		while(!intergerMap.isEmpty()) {
			Entry firstEntry = intergerMap.pollFirstEntry();
			System.out.println(firstEntry.getValue().toString());
		}
	}
}

class Student {
	int age;
	String name;

	public int getAge() {
		return age;
	}

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

	public String getName() {
		return name;
	}

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

	@Override
	public String toString() {
		return "Student [age=" + age + ", name=" + name + "]";
	}

	public Student(int age, String name) {
		super();
		this.age = age;
		this.name = name;
	}
}
        // 输出结果

        /*

        Student [age=1, name=小王1]
        Student [age=2, name=小王2]
        Student [age=3, name=小王3]
        Student [age=4, name=小王4]
        Student [age=5, name=小王5]
        Student [age=6, name=小王6]
        Student [age=7, name=小王7]
        Student [age=8, name=小王8]
        Student [age=9, name=小王9]
        Student [age=10, name=小王10]
        */

     由默认排序结果可以看出，IntegerMap根据键（Interger)从小到大进行排序；

3.自定义比较器，根据键从大到小进行排序

   1.初始化TreeMap时实现匿名内部类Comparator，实现根据Interger键从大到小排序

 

package test;

import java.util.Map.Entry;
import java.util.Comparator;
import java.util.TreeMap;

public class MyTreeMap {
	public static TreeMap intergerMap = new TreeMap<>(new Comparator() {

		@Override
		public int compare(Integer o1, Integer o2) {
			Integer integer1 = (Integer)01;
			Integer integer2 = (Integer)o2;
			return integer1 > integer2? 1:-1;
		}
	});
	public static void main(String[] args) {
		// 给集合中添加10个student对象
		for (int i = 1; i < 11; ++i) {
			//studentMap.put(new Student(i, "小王" + i), i);
			intergerMap.put(i, new Student(i, "小王" + i));
		}
		// 打印集合中的元素
		while(!intergerMap.isEmpty()) {
			Entry firstEntry = intergerMap.pollFirstEntry();
			System.out.println(firstEntry.getValue().toString());
		}
	}
}

class Student {
	int age;
	String name;

	public int getAge() {
		return age;
	}

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

	public String getName() {
		return name;
	}

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

	@Override
	public String toString() {
		return "Student [age=" + age + ", name=" + name + "]";
	}

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

}               
        
        // 测试结果
        /*
        Student [age=10, name=小王10]
        Student [age=9, name=小王9]
        Student [age=8, name=小王8]
        Student [age=7, name=小王7]
        Student [age=6, name=小王6]
        Student [age=5, name=小王5]
        Student [age=4, name=小王4]
        Student [age=3, name=小王3]
        Student [age=2, name=小王2]
        Student [age=1, name=小王1]
        */

 

      2.初始化TreeMap时，将comparator的实现类进行传参，实现根据Interger从大到小排序；

package test;

import java.util.Map.Entry;
import java.util.Comparator;
import java.util.TreeMap;

public class MyTreeMap {
	static MyCompare com = new MyCompare();
	public static TreeMap intergerMap = new TreeMap<>(com);
	
	public static void main(String[] args) {
		// 给集合中添加10个student对象
		for (int i = 1; i < 11; ++i) {
			//studentMap.put(new Student(i, "小王" + i), i);
			intergerMap.put(i, new Student(i, "小王" + i));
		}
		// 打印集合中的元素
		while(!intergerMap.isEmpty()) {
			Entry firstEntry = intergerMap.pollFirstEntry();
			System.out.println(firstEntry.getValue().toString());
		}
	}
}

class Student {
	int age;
	String name;

	public int getAge() {
		return age;
	}

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

	public String getName() {
		return name;
	}

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

	@Override
	public String toString() {
		return "Student [age=" + age + ", name=" + name + "]";
	}

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

} 
class MyCompare implements Comparator