1 public final native Class<?> getClass()    //Return to this Object Runtime classes
 2  
 3 public native int hashCode()    //Returns the hash code of the object
 4  
 5 public boolean equals(Object obj)    //Determine whether other objects are "equal" to this object
 6  
 7 protected native Object clone() throws CloneNotSupportedException    //Create and return a copy of this object
 8  
 9 public String toString()    //Returns the string representation of the object
10  
11 public final native void notify()    //Wake up a single thread waiting on this object monitor
12  
13 public final native void notifyAll()    //Wake up all threads waiting on this object monitor
14  
15 public final native void wait(long timeout) throws InterruptedException    //Make the thread of the current object wait timeout Duration
16  
17 public final void wait(long timeout, int nanos) throws InterruptedException    //Make the thread of the current object wait timeout Duration, or other threads interrupt the current thread
18  
19 public final void wait() throws InterruptedException    //Make the thread of the current object wait 

 1 /**
 2  * 1.Objects Class: java.util.Objects
 3  * JDK1.7 Started to add a tool class
 4  * It is used to calculate the hashcode of an object, return the string representation of the object, compare two objects, etc
 5  * These methods are all null pointer safe
 6  */
 7 String s = null;
 8 String s1 = "s1";
 9 
10 //report errors: java.lang.NullPointerException，because s by null So no method can be called
11 //System.out.println(s.equals(s1));
12 
13 System.out.println(Objects.equals(s,s1));   //false

1 /**
2  * 2.Date Class: date time class, java.util.Date
3  * Accurate to milliseconds
4  */
5 System.out.println(new Date());     //Show current date
6 System.out.println(new Date(1000000000L));   //Convert millisecond value to date
7 System.out.println(new Date().getTime());   //Convert the current system time to millisecond display
8 System.out.println(System.currentTimeMillis()); //Returns the current system time in milliseconds

 1 /**
 2  * 3.DateFormat Class: an abstract class for time / date formatting, java.text.DateFormat
 3  * effect:
 4  *      Format: Date > text, parse: text > date
 5  * Member method:
 6  *      String format(Date date): Convert date to string
 7  *      Date parse(String source): Parse string to date
 8  *
 9  * Since the DateFormat class is an abstract class, we use the SimpleDateFormat class to use the
10  * SimpleDateFormat Construction method:
11  *      SimpleDateFormat(String pattern)
12  * Parameters:
13  *      String pattern: Specified mode of delivery
14  * Mode: case sensitive
15  *      y  Year, m month, d day, H hour, m minute, s second
16  * General default mode:
17  *      yyyy-MM-dd HH:mm:ss
18  * The letter of the pattern cannot be modified, but the connector can be modified
19  *      yyyy Mm / dd / yyyy HH H mm min ss s
20  */
21 //format:
22 System.out.println(new SimpleDateFormat("yyyy year MM month dd day HH Time mm branch ss second").format(new Date()));
23 //Resolution:
24 System.out.println(new SimpleDateFormat("yyyy year MM month dd day HH Time mm branch ss second").parse("2019 December 26, 2008 18:33:35"));

 1 /**
 2  * 4.Calendar Class: calendar class, java.util.Calendar , abstract class, providing methods for operating calendar
 3  * Use the static method getInstance() to return a subclass object of the Calendar class
 4  * Common member methods:
 5  *      public int get(int field): Returns the value of the given calendar field
 6  *      public void set(int field,int value): Set the given calendar field to the given value
 7  *      public abstract void add(int field, int amount): Add or subtract the specified amount of time from the calendar field according to the rules of the calendar
 8  *      public Date getTime(): Returns a Date object representing this Calendar time value
 9  * Member method parameters:
10  *      int feild: The field of calendar class can be obtained by using the static member variable of Calendar Class
11  *      YEAR = 1  year
12  *      MONTH = 2 month
13  *      DATE = 5  day
14  *      DAY_OF_MONTH = 5 One day of the month
15  *      HOUR = 10       Time
16  *      MINUTE = 12     branch
17  *      SECOND = 13     second
18  */
19 Calendar c = Calendar.getInstance();
20 
21 //Return to current year
22 System.out.println(c.get(Calendar.YEAR));
23 
24 //Set current year
25 c.set(Calendar.YEAR,9999);
26 System.out.println(c.get(Calendar.YEAR));
27 
28 //Increase year
29 c.add(Calendar.YEAR,3);
30 System.out.println(c.get(Calendar.YEAR));
31 
32 //get date
33 System.out.println(c.getTime());

 1 /**
 2  * 5.System Class: get system related information or operating system operation, java.lang.System
 3  *
 4  */
 5 //Returns the system time in milliseconds
 6 System.out.println(System.currentTimeMillis());
 7 
 8 //Copy the data specified in the array to another array
 9 int[] src = {1,2,3,4,5};
10 int[] dest = {6,7,8,9,0};
11 System.arraycopy(src,0,dest,0,3);
12 System.out.println(Arrays.toString(dest));      //[1, 2, 3, 9, 0]

 1 /**
 2  * 6.StringBuilder Class: string buffer, variable string, initial value is 16 characters
 3  * String Class immutable string
 4  */
 5 //Empty constructor
 6 StringBuilder bu1 = new StringBuilder();
 7 System.out.println(bu1);
 8 
 9 //Constructor with parameters
10 StringBuilder bu2 = new StringBuilder("abc");
11 System.out.println(bu2);
12 
13 //Chain programming
14 bu2.append(1).append(true).append("Hello");
15 System.out.println(bu2);
16 
17 //StringBuilder->String
18 String bu2tos = bu2.toString();
19 System.out.println(bu2tos);
20 
21 //String->StringBuilder
22 bu2.append(bu2tos);
23 System.out.println(bu2);

 1 /**
 2  * 7.Packaging: 8, java.lang in
 3  * Because the basic type is easy to use but there is no corresponding method to operate the basic type,
 4  * We define a class, encapsulate the basic data type and define some methods in this class for operation
 5  * Basic type packaging
 6  * byte                 Byte
 7  * short                Short
 8  * int                  Integer
 9  * long                 Long
10  * float                Float
11  * double               Double
12  * char                 Character
13  * boolean              Boolean
14  *
15  *
16  * Starting from JDK 1.5, it supports automatic packing and unpacking
17  * Packing: basic type - > Packing class
18  * Unpacking: packing type - > basic type
19  */
20 //int -> Integer
21 Integer i = Integer.valueOf(11111);
22 System.out.println(i);
23 
24 //String -> Integer
25 i = Integer.valueOf("22222222");
26 System.out.println(i);
27 
28 //Integer -> int
29 int num = i.intValue();
30 System.out.println(num);
31 
32 //Because generics must be non basic
33 ArrayList<Integer> list = new ArrayList<>();
34 
35 //Auto boxing: list.add(new Integer(1))
36 list.add(1);
37 //Automatic unpacking: list.get(0).intValue();
38 int a = list.get(0);
39 
40 //Basic type to string
41 String ss = 100 + "";
42 System.out.println(ss);
43 ss = Integer.toString(123);
44 System.out.println(ss);
45 ss = String.valueOf(444);
46 System.out.println(ss);
47 
48 //String to basic type
49 int nn = Integer.parseInt(ss);
50 System.out.println(nn);

1 UUID u;
2 for (int i = 0; i < 100; i++) {
3     u = UUID.randomUUID();
4     System.out.println(u.toString().replaceAll("-",""));
5 }

1 double[] limits = {3, 4, 5, 6, 7, 8, 9};
2 String[] formats = {"Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"};
3 ChoiceFormat format = new ChoiceFormat(limits, formats);
4 System.out.println(format.format(-100));    //Monday
5 System.out.println(format.format(3.6));     //Monday

 1 // Take integer part
 2 String s1 = new DecimalFormat("0").format(12345.1512134567);
 3 System.out.println(s1);//12345
 4 
 5 // 1 decimal place,rounding
 6 String s2 = new DecimalFormat("0.0").format(12345.1512134567);
 7 System.out.println(s2);//12345.2
 8 
 9 // Take 3 decimal places after the decimal point, and take 0 for the insufficient part
10 String s3 = new DecimalFormat("0.000").format(12345.1);
11 System.out.println(s3);//12345.100
12 
13 // percentage
14 String s4 = new DecimalFormat("0.0%").format(12345.1512134567);
15 System.out.println(s4);// 1234515.1%
16 
17 // Scientific counting
18 String s5 = new DecimalFormat("0.00E0").format(12345.1512134567);
19 System.out.println(s5);//1.23E4
20 
21 // Each three digits are separated by commas
22 String s6 = new DecimalFormat(",000.000").format(12345.1512134567);
23 System.out.println(s6);//12,345.151
24 
25 //3 decimal places after the decimal point, if it is 0, it will not be displayed
26 String s7 = new DecimalFormat("#.###").format(123.300);
27 System.out.println(s7);//123.3
28 
29 //Embed formatting in text
30 System.out.println(new DecimalFormat("Per second#.###Meters.").format(123.300));//123 per second.3 Meters.

 1 public class ArraysTest {
 2 
 3     public static void main(String[] args) {
 4         int a[] = new int[]{18, 62, 68, 82, 65, 9};
 5 
 6         //1.Arrays copy
 7         int[] b = Arrays.copyOfRange(a, 0, 3);
 8         for (int i : b) {
 9             System.out.print(i + " ");
10         }
11         System.out.println();
12 
13         //2.System copy
14         System.arraycopy(a, 0, b, 0, 3);
15         for (int i : b) {
16             System.out.print(i + " ");
17         }
18         
19     }
20 }

 1 public class ArraysTest {
 2 
 3     public static void main(String[] args) {
 4         //1.Returns the string form of a one-dimensional array
 5         int[] a = new int[]{18, 62, 68, 82, 65, 9};
 6         System.out.println(Arrays.toString(a));
 7 
 8         //2.Returns the string form of a multidimensional array
 9         int[][][] b = {{{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}, {{11, 12, 13}, {21, 22, 23}}};
10         System.out.println(Arrays.deepToString(b));
11         System.out.println(Arrays.toString(b));
12 
13         /**
14          * Output:
15          * [18, 62, 68, 82, 65, 9]
16          * [[[1, 2, 3], [4, 5, 6], [7, 8, 9]], [[11, 12, 13], [21, 22, 23]]]
17          * [[[I@135fbaa4, [[I@45ee12a7]
18          */
19     }
20 }

 1 public class ArraysSortTest {
 2 
 3     public static void main(String[] args) {
 4         //1.Arrays Own ascending sort
 5         Integer[] nums1 = {5, 2, 1, 3, 4, 9, 0, 7, 8, 6};
 6         Arrays.sort(nums1);
 7         System.out.println(Arrays.toString(nums1));
 8 
 9         //2.Custom class
10         Integer[] nums2 = {5, 2, 1, 3, 4, 9, 0, 7, 8, 6};
11         Comparator<Integer> comparator = new MyComparator();
12         Arrays.sort(nums2, comparator);
13         System.out.println(Arrays.toString(nums2));
14 
15         //3.Anonymous Inner Class 
16         Integer[] nums3 = {5, 2, 1, 3, 4, 9, 0, 7, 8, 6};
17         Arrays.sort(nums3, new Comparator<Integer>() {
18             @Override
19             public int compare(Integer a, Integer b) {
20                 return b - a;
21             }
22         });
23         System.out.println(Arrays.toString(nums3));
24 
25         //4.lambda expression
26         Integer[] nums4 = {5, 2, 1, 3, 4, 9, 0, 7, 8, 6};
27         Arrays.sort(nums4, (a, b) -> b - a);
28         System.out.println(Arrays.toString(nums4));
29     }
30 }
31 
32 
33 class MyComparator implements Comparator<Integer> {
34     @Override
35     public int compare(Integer a, Integer b) {
36         return b - a;
37     }
38 }

 1 public class ArraysTest {
 2 
 3     public static void main(String[] args) {
 4         int a[] = new int[]{18, 62, 68, 82, 65, 9};
 5 
 6         Arrays.sort(a);
 7         System.out.println(Arrays.toString(a));
 8 
 9         //Must sort before binary search
10         System.out.println("Where the number 62 appears in the array"+Arrays.binarySearch(a,62));
11 
12         /**Output:
13          * [9, 18, 62, 65, 68, 82]
14          * Number 62 in array position 2
15          */
16     }
17 }

 1 public class ArraysTest {
 2 
 3     public static void main(String[] args) {
 4         int a[] = new int[]{18, 62, 68, 82, 65, 9};
 5         int b[] = new int[]{18, 62, 68, 82, 65, 9};
 6         int c[] = new int[]{99, 62, 68, 82, 65, 8};
 7 
 8         System.out.println(Arrays.equals(a,b));
 9         System.out.println(Arrays.equals(a,c));
10         System.out.println(Arrays.equals(b,c));
11 
12         /**
13          * Output:
14          * true
15          * false
16          * false
17          */
18     }
19 }

 1 public class ArraysTest {
 2 
 3     public static void main(String[] args) {
 4         int[] a = new int[10];
 5 
 6         //1.Fill all
 7         Arrays.fill(a, 5);
 8 
 9         System.out.println(Arrays.toString(a));
10 
11         /**
12          * Output:
13          * [5, 5, 5, 5, 5, 5, 5, 5, 5, 5]
14          */
15 
16         //2.Fill subscript[2,4]Is 1
17         Arrays.fill(a, 2, 5, 1);
18         System.out.println(Arrays.toString(a));
19 
20         /**
21          * Output:
22          * [5, 5, 1, 1, 1, 5, 5, 5, 5, 5]
23          */
24     }
25 }