Array class, Math Class

Arrays类

java.util.Arrays is a utility class with an array of relevant, which provides a number of static methods used to implement an array of common operations.

public static String toString (array): The parameter array into a string (in accordance with the default format: [element 1, element 2, element 3 ...])

 public static void main(String[] args) {
        int[] intArray = {10, 20, 30};
        //将int[]数组按照默认格式变成字符串
        String intStr = Arrays.toString(intArray);
        System.out.println(intStr);//[10, 20, 30]
        //以下出了啥问题(报错)
//        Arrays arrays = new Arrays();
//        String str = arrays.toString(intArray);
//        System.out.println(str);
    }

public static void sort (array): ascending order by default (small to large) to sort the elements of the array.

int[] array1 = {1, 3, 5, 4, 9, 3};
Arrays.sort(array1);
System.out.println(Arrays.toString(array1));//[1, 3, 3, 4, 5, 9]
String[] array2 = {"bbb", "aaa", "ccc"};
Arrays.sort(array2);
System.out.println(Arrays.toString(array2));//[aaa, bbb, ccc]

Remarks:

1. If the value is, sort in ascending order from small to large default.
2. If a character, sort default in alphabetical order.
3. If it is a custom type, this custom class needs the support Comparable or Comparator interface. (Future learning)

Arrays Exercise:

1. Use API Arrays related to all the characters in the ascending order of the arrangement of a random string, and reverse printing.

    public static void main(String[] args) {
        String str = "kfnvkjnkjbkjb159ejbeb";
        char[] charArray = str.toCharArray();
        Arrays.sort(charArray);
        for (int i = charArray.length - 1; i >= 0; i--) {
            System.out.print(charArray[i]);//vnnkkkkjjjjfeebbbb951
        }
    }

Mathematics Math Tools

java.util.Math is a mathematics-related tools, which provides a number of static methods, to complete the operation related to mathematical operations.

public static double abs (double num): obtaining an absolute value. There are many overloaded.

public static void main(String[] args) {
    //获取绝对值
    System.out.println(Math.abs(3.14));//3.14
    System.out.println(Math.abs(0));//0
    System.out.println(Math.abs(-2.5));//2.5
}

public static double ceil（double num）：向上取整。

//向上取整
System.out.println(Math.ceil(3.9));//4.0
System.out.println(Math.ceil(3.1));//4.0
System.out.println(Math.ceil(3.0));//3.0

public static double floor (double num): rounded down.

//向下取整
System.out.println(Math.floor(3.1));//3.0
System.out.println(Math.floor(3.9));//3.0
System.out.println(Math.floor(3.0));//3.0

public static long round (double num): rounding.

//四舍五入
System.out.println(Math.round(20.4));//20
System.out.println(Math.round(20.5));//21

Representative Math.PI approximate constant pi (double).

Math Exercise:

1. Compute between -10.8 to 5.9, the absolute value of greater than 6 or less than 2.1 have an integer number?

public static void main(String[] args) {
    int count = 0;
    double min = -10.8;
    double max = 5.9;
    //这样处理，变量i就是区间之内所有的整数
    for (int i = (int)min; i < max; i++) {
        int abs = Math.abs(i);
        if (abs > 6 || abs < 2.1) {
            System.out.print(abs + " ");
            count ++;
        }
    }
    System.out.println("总个数有:" + count);
}