【java】TreeSet、Comparable、Comparator、内部类、匿名类
1 package com.tn.treeSet; 2 3 public class Student { 4 private String name; 5 private int age; 6 public Student(){} 7 public Student(String name, int age) { 8 super(); 9 this.name = name; 10 this.age = age; 11 } 12 public String getName() { 13 return name; 14 } 15 public void setName(String name) { 16 this.name = name; 17 } 18 public int getAge() { 19 return age; 20 } 21 public void setAge(int age) { 22 this.age = age; 23 } 24 @Override 25 public String toString() { 26 return "Student [name=" + name + "]"; 27 } 28 }
1 package com.tn.treeSet; 2 3 import java.util.TreeSet; 4 5 public class TreeSetDemo { 6 public static void main(String[] args){ 7 TreeSet<Student> students=new TreeSet<>();//后一个尖括号内可以省略类型 8 Student student1=new Student("武松",30); 9 Student student2=new Student("林冲",31); 10 Student student3=new Student("鲁智深",29); 11 students.add(student1); 12 students.add(student2); 13 students.add(student3); 14 System.out.println(students); 15 } 16 }
运行结果：
原因:TreeSet中加入的对象需要能进行比较，即实现Comparable接口
改造一：在Student类中实现Comparable接口
1 package com.tn.treeSet; 2 3 public class Student implements Comparable<Student> { 4 private String name; 5 private int age; 6 public Student(){} 7 public Student(String name, int age) { 8 super(); 9 this.name = name; 10 this.age = age; 11 } 12 public String getName() { 13 return name; 14 } 15 public void setName(String name) { 16 this.name = name; 17 } 18 public int getAge() { 19 return age; 20 } 21 public void setAge(int age) { 22 this.age = age; 23 } 24 @Override 25 public String toString() { 26 return "Student [name=" + name + "]"; 27 } 28 @Override 29 public int compareTo(Student o) { 30 if(!this.equals(o)){ 31 // return this.name.compareTo(o.name); 32 return o.name.compareTo(this.name);//和上面语句打印顺序颠倒。 33 } 34 return 0; 35 } 36 }
1 package com.tn.treeSet; 2 3 import java.util.TreeSet; 4 5 public class TreeSetDemo { 6 public static void main(String[] args){ 7 TreeSet<Student> students=new TreeSet<>();//后一个尖括号内可以省略类型 8 Student student1=new Student("武松",30); 9 Student student2=new Student("林冲",31); 10 Student student3=new Student("鲁智深",29); 11 students.add(student1); 12 students.add(student2); 13 students.add(student3); 14 System.out.println(students); 15 };//方法体结结尾大括号后有;不会报错 16 };//类体最后一个大括号后有;不会报错
改造二：用内部类实现Comparator接口
1 package com.tn.treeSet; 2 3 public class Student{ 4 private String name; 5 private int age; 6 public Student(){} 7 public Student(String name, int age) { 8 super(); 9 this.name = name; 10 this.age = age; 11 } 12 public String getName() { 13 return name; 14 } 15 public void setName(String name) { 16 this.name = name; 17 } 18 public int getAge() { 19 return age; 20 } 21 public void setAge(int age) { 22 this.age = age; 23 } 24 @Override 25 public String toString() { 26 return "Student [name=" + name + "]"; 27 } 28 }
1 package com.tn.treeSet; 2 3 import java.util.Comparator; 4 import java.util.TreeSet; 5 6 public class TreeSetDemo { 7 public static void main(String[] args){ 8 Student student1=new Student("武松",30); 9 Student student2=new Student("林冲",31); 10 Student student3=new Student("鲁智深",29); 11 //TreeSet构造时用Comparator作为构造函数参数 12 TreeSet<Student> students=new TreeSet<Student>(new ComparatorDemo()); 13 students.add(student1); 14 students.add(student2); 15 students.add(student3); 16 System.out.println(students); 17 } 18 static class ComparatorDemo implements Comparator<Student>{ 19 // 内部类要写在类体里，但不能写进类中方法体内。 20 @Override 21 public int compare(Student o1, Student o2) { 22 return o1.getName().compareTo(o2.getName()); 23 } 24 } 25 }
改造三：用匿名类实现Comparator接口
1 package com.tn.treeSet; 2 3 public class Student{ 4 private String name; 5 private int age; 6 public Student(){} 7 public Student(String name, int age) { 8 super(); 9 this.name = name; 10 this.age = age; 11 } 12 public String getName() { 13 return name; 14 } 15 public void setName(String name) { 16 this.name = name; 17 } 18 public int getAge() { 19 return age; 20 } 21 public void setAge(int age) { 22 this.age = age; 23 } 24 @Override 25 public String toString() { 26 return "Student [name=" + name + "]"; 27 } 28 }
1 package com.tn.treeSet; 2 3 import java.util.Comparator; 4 import java.util.TreeSet; 5 6 public class TreeSetDemo { 7 public static void main(String[] args) { 8 Student student1 = new Student("武松", 30); 9 Student student2 = new Student("林冲", 31); 10 Student student3 = new Student("鲁智深", 29); 11 12 TreeSet<Student> students = new TreeSet<Student>( 13 new Comparator<Student>() { 14 @Override 15 public int compare(Student o1, Student o2) { 16 return o1.getName().compareTo(o2.getName()); 17 } 18 }); 19 students.add(student1); 20 students.add(student2); 21 students.add(student3); 22 System.out.println(students); 23 } 24 }
总结：
TreeSet容器中的对象要能排序，两种实现排序方法：
1.TreeSet使用无参构造函数，容器中的对象实现Comparable接口，见改造一；
2.TreeSet构造时使用Comparator作为构造函数参数；
比较方法如果返回0，则对象不能重复加入。
TreeSet底层是TreeMap
