Java-->Arrays类、选择排序、二分查找

 

• 初识Arrays类的API：

    public static void main(String[] args) {
        int[] arr = {5, 4, 6, 8, 0, 1, 3, 2};
        System.out.println("数组地址：" + arr);
        System.out.println("数组内容：" + Arrays.toString(arr));
        Arrays.sort(arr);
        System.out.println("排序（升序）：" + Arrays.toString(arr));
        System.out.println("二分查找位置：" + Arrays.binarySearch(arr, 2));
        System.out.println("二分查找位置：" + Arrays.binarySearch(arr, 10));
    }

示例结果：

 

•  Comparator接口对应的比较器对应用：

public static void main(String[] args) {
        //1、使用API完成数组的升序
        int[] ages = {21,22,31,15};
        Arrays.sort(ages);
        System.out.println(Arrays.toString(ages));
        System.out.println("-------API只能升序，如何降序？-------");
        //自定义排序(只能支持引用类型)：降序
        Integer[] ages2 = {12,15,45,30,6};
        /**
         * 参数一 : 被比较的数组
         * 参数儿 : 匿名内部欸，代表一个比较器对象
         */
        Arrays.sort(ages2, new Comparator<Integer>() {
            @Override
            public int compare(Integer o1, Integer o2) {
//                if(o1 > o2){
//                    return 1;
//                }
//                else if(o1 < o2){
//                    return -1;
//                }
//                else {
//                    return 0;
//                }
                //return o1 - o2;
                return -(o1 - o2);
            }
        });
        System.out.println(Arrays.toString(ages2));

        System.out.println("-----------------");
        Student[] students = new Student[3];
        students[0] = new Student("张三",21,1.75);
        students[1] = new Student("李四",19,1.68);
        students[2] = new Student("王五",20,1.81);
        System.out.println(Arrays.toString(students));

        //Arrays.sort(students)  //会程序崩溃
        Arrays.sort(students, new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
                //return o1.getAge() - o2.getAge(); //按照年龄升序排序
                //return o2.getAge() - o1.getAge(); //按照年龄降序排序
                //return Double.compare(o1.getHeight(),o2.getHeight()); //按照身高升序
                return Double.compare(o2.getHeight(),o1.getHeight()); //按照升高降序
            }
        });
        System.out.println(Arrays.toString(students));
    }

Student类：

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

    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 double getHeight() {
        return height;
    }

    public void setHeight(double height) {
        this.height = height;
    }

    public Student() {
    }

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

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

示例结果：

 

•  常见算法（选择排序、二分查找）
• 选择排序：

public static void main(String[] args) {
        int[] arr = {1,5,3,2};
        for (int i = 0; i < arr.length - 1; i++) { //外层循环控制轮数
            // i = 0  j: 0, 1, 2, 3
            // i = 1  j: 1, 2, 3
            // i = 2  j: 2, 3
            // i = 3  j: 3
            //相邻两数选出较小值排在左边
            for (int j = i+1; j < arr.length; j++) {
                if(arr[i] > arr[j]){
                    int tmp = arr[i];
                    arr[i] = arr[j];
                    arr[j] = tmp;
                }
            }
        }
        System.out.println(Arrays.toString(arr));
    }

示例结果：

 

•  二分查找：

public static void main(String[] args) {
        int[] arr = {1,6,51,98,121,150,194,201,205,255};
        int key = 201;
        //定义左右边界
        int left = 0;
        int right = arr.length - 1;
        //循环条件
        while (left <= right){
            int midIndex = (left + right) / 2;
            //中间元素的值大于所要查找的数据
            if(arr[midIndex] > key){
                right = midIndex - 1;
            }
            //中间元素的值小于所要查找的数据
            else if(arr[midIndex] < key){
                left = midIndex + 1;
            }
           //中间元素的值等于所要查找的数据
            else{
                System.out.println("找到了，索引为：" + midIndex);
                break;
            }
        }
        if(left > right){
            System.out.println("找不到");
        }
    }