Collections工具类

Collections常用方法：

                      1.public static <T> boolean addAll(Collection<T> c,T...elements)          往集合中添加一些元素

public class Demo02 {
    public static void main(String[] args) {
        ArrayList<Integer> list = new ArrayList<>();
        Collections.addAll(list,1,3,2,4,5);
        System.out.println(list);
    }
}
输出的就是[1, 3, 2, 4, 5]

                      2.public static void shuffle（List<?> list）                打乱集合顺序

public class Demo02 {
    public static void main(String[] args) {
        ArrayList<Integer> list = new ArrayList<>();
        Collections.addAll(list,1,3,2,4,5);
        Collections.shuffle(list);
        System.out.println(list);
    }
}
输出的就是它们打乱之后的样子。比如[4, 1, 2, 3, 5]

                      3.public static <T> void sort(List<T> list)                将集合中的元素按照默认（升序）规则排序

public class Demo02 {
    public static void main(String[] args) {
        ArrayList<Integer> list = new ArrayList<>();
        Collections.addAll(list,1,3,2,4,5);
        Collections.shuffle(list);
        Collections.sort(list);
        System.out.println(list);
    }
}
输出的就是[1, 2, 3, 4, 5]

                      4.public static <T> void sort(List<T> list,Comparator<? super T>)       将集合中元素按照指定规则排序

Comparable  和    Comparator比较器：

第一种方法：     继承Comparable<Student>接口

定义一个Student学生类：

public class Student implements Comparable<Student> {
    private String name;
    private int age;
    private float score;

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

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

    @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 &&
                Float.compare(student.score, score) == 0 &&
                Objects.equals(name, student.name);
    }

    @Override
    public int hashCode() {

        return Objects.hash(name, age, score);
    }

    @Override
    public int compareTo(Student o) {
        if (this.score > o.score) {
            return -1;
        } else if (this.score < o.score) {
            return 1;
        }else{
            if (this.age > o.age) {
                return 1;
            } else if (this.age < o.age) {
                return -1;
            } else {
                return 0;
            }
        }
    }
}
注意：成绩是按照降序排列的，年龄是升序排列的

定义一个实现类：

public class Test {
    public static void main(String[] args) {
        ArrayList<Student> list = new ArrayList<>();
        Student stu1 = new Student("liu", 20, 90.0F);
        Student stu2 = new Student("zhang", 22, 90.0F);
        Student stu3 = new Student("guan", 20, 99.0F);
        Student stu4 = new Student("ma", 22, 100.0F);
        Collections.addAll(list, stu1, stu2, stu3, stu4);
        Collections.sort(list);
        for (Student s : list) {
            System.out.println(s);
        }
    }
}
结果为
      Student{name='ma', age=22, score=100.0}
      Student{name='guan', age=20, score=99.0}
      Student{name='liu', age=20, score=90.0}
      Student{name='zhang', age=22, score=90.0}

第二种方法：   使用Comparator比较器

1.定义一个学生类：

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

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

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

    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 float getScore() {
        return score;
    }

    public void setScore(float score) {
        this.score = score;
    }
}

定义一个实现类

public class Test {
    public static void main(String[] args) {
        ArrayList<Student> list = new ArrayList<>();
        list.add(new Student("liu", 20, 90.0F));
        list.add(new Student("zhang", 22, 90.0F));
        list.add(new Student("liu", 20, 99.0F));
        list.add(new Student("liu", 22, 100.0F));
        Collections.sort(list, new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
                if (o1.getScore() > o2.getScore()) {
                        return -1;
                } else if (o1.getScore() < o2.getScore()){
                    return 1;
                }else{
                    if (o1.getAge() > o2.getAge()) {
                        return 1;
                    } else if (o1.getAge() < o2.getAge()) {
                        return -1;
                    }else{
                        return 0;
                    }
                }
            }
        });
        for (Student s : list) {
            System.out.println(s);
        }
    }
}
中间那个为比较器
输出结果为
          Student{name='liu', age=22, score=100.0}
          Student{name='liu', age=20, score=99.0}
          Student{name='liu', age=20, score=90.0}
          Student{name='zhang', age=22, score=90.0}