HashSet、LinkedHashSet以及TreeSet

HashSet与LinkedHasgSet在存储自定义对象时，该类需要重写HashCode()与equals()方法
若不重写上述方法，会造成存储元素不唯一。

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

    public Student() {
    }

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

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return age == student.age && Objects.equals(name, student.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }

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

TreeSet在存储自定义对象时，当使用内部比较器时，该类需重写Comparable接口中compareTo()方法，当使用外部比较器时，该类需重写Comparator接口中compare()方法。
若不重写以上方法，会直接报错。

内部比较器

@Override
    public int compareTo(Student o) {
        return this.age-o.age;
    }

外部比较器

public class StuNameCompare implements Comparator<Student> {
    @Override
    public int compare(Student stu1, Student stu2) {
        return stu1.getName().compareTo(stu2.getName());
    }
}

//在主函数中的实现与应用
 Comparator<Student> comp=new StuNameCompare();

 Set<Student> set2=new TreeSet<>(comp);

通过匿名内部类实现

```java
Comparator<Student> comp1=new Comparator<Student>(){
            @Override
            public int compare(Student stu1, Student stu2) {
                if (stu1.getAge()>stu2.getAge())
                    return 1;
                if (stu1.getAge()<stu2.getAge())
                    return -1;
                else
                    return stu1.getName().compareTo(stu2.getName());
            }
        };

        Set<Student> set3=new TreeSet<>(comp1);

若有错误，请留言！！！