一、Math 类(P481)
public class Demo {
public static void main(String[] args) {
double pow = Math.pow(2, 4);
System.out.println(pow);
double ceil1 = Math.ceil(-3.2);
double ceil2 = Math.ceil(3.2);
System.out.println(ceil1);
System.out.println(ceil2);
double sqrt = Math.sqrt(9.0);
System.out.println(sqrt);
double random1 = Math.random();
double random2 = 2 + Math.random() * 6;
}
}
二、Arrays 类(P482)
public class Demo {
public static void main(String[] args) {
Integer[] arr1 = {1, 20, 30};
System.out.println(Arrays.toString(arr1));
Integer[] arr2 = {1, -1, 7, 50};
Arrays.sort(arr2);
System.out.println(Arrays.toString(arr2));
Integer[] arr3 = {1, -1, 7, 50};
Arrays.sort(arr3, new Comparator<Integer>() {
@Override
public int compare(Integer o1, Integer o2) {
return o2 - o1;
}
});
System.out.println(Arrays.toString(arr3));
Integer[] arr4 = {-1, 1, 7, 50};
int index1 = Arrays.binarySearch(arr4, 1);
System.out.println(index1);
int index2 = Arrays.binarySearch(arr4, 5);
System.out.println(index2);
Integer[] arr5 = {-1, 1, 7, 50};
int len1 = arr5.length - 1;
Integer[] newArr1 = Arrays.copyOf(arr5, len1);
System.out.println(Arrays.toString(newArr1));
int len2 = arr5.length + 1;
Integer[] newArr2 = Arrays.copyOf(arr5, len2);
System.out.println(Arrays.toString(newArr2));
int len3 = -1;
Integer[] newArr3 = Arrays.copyOf(arr5, len3);
System.out.println(Arrays.toString(newArr3));
Integer[] arr6 = {-1, 1, 7, 50};
Arrays.fill(arr6,99);
System.out.println(Arrays.toString(arr6));
Integer[] arr7 = {-1, 1, 7, 50};
Integer[] arr8 = {-1, 1, 7, 50};
System.out.println(Arrays.equals(arr7,arr8));
Integer[] arr9 = {-1, 1, 7, 50};
List<Integer> aslist = Arrays.asList(arr9);
System.out.println(aslist.getClass());
}
}
三、System类(P486)
public class Demo {
public static void main(String[] args) {
Integer[] arr = {-1, 1, 7, 50};
Integer[] destArr = new Integer[4];
System.arraycopy(arr, 0, destArr, 0, arr.length);
}
}
四、Biglnteger 和 BigDecimal 类(P487)
public class Demo {
public static void main(String[] args) {
BigInteger bigInteger = new BigInteger("10000");
BigDecimal bigDecimal = new BigDecimal("20.88");
}
}
1. Biglnteger 和 BigDecimal 常见方法
五、日期类(P488)
1. 第一代日期类 Date
public class Demo {
public static void main(String[] args) throws ParseException {
Date date = new Date();
System.out.println(date);
SimpleDateFormat sdf = new SimpleDateFormat("yyyy年MM月dd日 hh:mm:ss E");
String format = sdf.format(date);
System.out.println(format);
Date parse = sdf.parse(format);
System.out.println(parse);
}
}
2. 第二代日期类 Calendar (日历)
public class Demo {
public static void main(String[] args) throws ParseException {
Calendar instance = Calendar.getInstance();
System.out.println("年:"+instance.get(Calendar.YEAR));
System.out.println("月:"+(instance.get(Calendar.MONTH)+1));
System.out.println("日:"+instance.get(Calendar.DAY_OF_MONTH));
System.out.println("小时(12):"+instance.get(Calendar.HOUR));
System.out.println("小时(24):"+instance.get(Calendar.HOUR_OF_DAY));
System.out.println("分钟:"+instance.get(Calendar.MINUTE));
System.out.println("秒:"+instance.get(Calendar.SECOND));
}
}
3. 第三代日期类
3.1 前面两代日期类的不足分析
3.2 第三代日期类常见方法
public class Demo {
public static void main(String[] args) throws ParseException {
LocalDateTime now = LocalDateTime.now();
LocalDate.now();
LocalTime.now();
System.out.println(now);
System.out.println(now.getYear());
System.out.println(now.getMonth());
System.out.println(now.getMonthValue());
System.out.println(now.getDayOfMonth());
System.out.println(now.getHour());
System.out.println(now.getMinute());
System.out.println(now.getSecond());
}
}
3.3 DateTimeFormatter格式日期类
public class Demo {
public static void main(String[] args) throws ParseException {
LocalDateTime now = LocalDateTime.now();
System.out.println(now);
DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
String format = dtf.format(now);
System.out.println(format);
}
}
3.4 Instant时间戳
public class Demo {
public static void main(String[] args) throws ParseException {
Instant now = Instant.now();
System.out.println(now);
Date date = Date.from(now);
Instant instant = date.toInstant();
}
}
3.5 第三代日期类更多方法
public class Demo {
public static void main(String[] args) throws ParseException {
LocalDateTime now = LocalDateTime.now();
System.out.println(now);
DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
System.out.println(dtf.format(now));
LocalDateTime ldt1 = now.plusDays(890);
System.out.println(dtf.format(ldt1));
LocalDateTime ldt2 = now.minusMinutes(180);
System.out.println(dtf.format(ldt2));
}
}