Java on timerring

11 Multithreading and Locks

Sat, 13 Jan 2024 19:57:43 +0800

11.1 Multithreading #

11.1.1 Implementing Multithreading #

Approach 1: Extend the Thread class.

class Worker extends Thread {
 @Override
 public void run() {
 for (int i = 0; i < 10; i ++ ) {
 System.out.println("Hello! " + this.getName());
 try {
 Thread.sleep(1000);
 } catch (InterruptedException e) {
 throw new RuntimeException(e);
 }
 }
 }
}

public class Main {
 public static void main(String[] args) {
 Worker worker1 = new Worker();
 Worker worker2 = new Worker();
 worker1.setName("thread-1");
 worker2.setName("thread-2");
 worker1.start();
 worker2.start();
 }
}

10 Annotations and Reflection

Fri, 12 Jan 2024 21:32:06 +0800

10.1 Annotations #

(1) Annotations, also called metadata, describe packages, methods, fields, constructors, local variables, and other program elements.
(2) Annotations do not affect program logic, but can be processed at compile time or runtime.
(3) In Java SE, annotations serve relatively simple purposes, such as marking obsolete features or suppressing warnings. In Java EE, they play a more important role, such as configuring aspects of an application in place of the verbose code and XML configuration left over from older Java EE versions.

09 Exception Handling

Thu, 11 Jan 2024 11:37:58 +0800

1. Differences Between Error and Exception #

An Error is a failure that a program cannot handle, such as OutOfMemoryError or ThreadDeath. When these occur, the Java Virtual Machine (JVM) generally terminates the thread. Such failures are fatal to the program and cannot be caught and handled.

An Exception is an exceptional condition that the program itself can handle. Exceptions fall into two categories: runtime and non-runtime exceptions. Programs should handle these exceptions whenever possible.

08 Common Containers

Wed, 10 Jan 2024 20:49:57 +0800

8.1 List #

Interface: java.util.List<>.

Implementations:

java.util.ArrayList<>: resizable array
java.util.LinkedList<>: doubly linked list

Methods:

add(): add an element at the end
clear(): clear the list
size(): return its length
isEmpty(): check whether it is empty
get(i): get the ith element
set(i, val): set the ith element to val

Loop syntax:

// Standard
for(int i = 0; i < list.size(); i ++) { 
 System.out.println(list.get(i)); 
}
// Enhanced
for (Integer integer : list) { 
 System.out.println(integer); 
}

07 Classes and Interfaces

Tue, 09 Jan 2024 22:24:11 +0800

7.1 Classes and Objects #

A class defines an entirely new data type containing a group of variables and functions; an object is an instance of that class.

Defining Student as a class represents the abstract concept of a “student.” Each student is then an object (instance) of the Student class.

06 Functions

Mon, 08 Jan 2024 19:18:32 +0800

Function Basics #

A typical function definition includes the following parts: modifiers, return type, function name, a list of zero or more parameters, and the function body.

05 Strings

Sun, 07 Jan 2024 21:53:46 +0800

1. Characters and Integers — ASCII Codes #

Each commonly used character corresponds to a number in -128 ~ 127, and the two can be converted to each other. Note: currently, negative numbers have no corresponding characters.

import java.util.Arrays;

public class Main {
 public static void main(String[] args) {
 char c = 'a';
 System.out.println((int)c);

 int a = 66;
 System.out.println((char)a);
 }
}

Common ASCII values: 'A'- 'Z' are 65 ~ 90, 'a' - 'z' are 97 - 122, and 0 - 9 are 48 - 57.

In Java, character literals must be enclosed in single quotes and string literals in double quotes. They are distinct. (Python and JS do not make this distinction.)

Characters can participate in arithmetic; they are treated as integers during operations:

import java.util.Arrays;

public class Main {
 public static void main(String[] args) {
 int a = 'B' - 'A';
 int b = 'A' * 'B';
 char c = 'A' + 2;

 System.out.println(a); // 1
 System.out.println(b);
 System.out.println(c);
 }
}

04 Arrays

Sat, 06 Jan 2024 20:36:19 +0800

One-Dimensional Arrays #

Defining Arrays #

Array definitions are similar to variable definitions.

public class Main {
 public static void main(String[] args) {
	 int[] a;// Definition: define an array a
		a = new int[10]; // Initialization: initialize a as an array of length 10, meaning a contains 10 int variables; equivalent to =>int[] a = new int[10]// Define and initialize at the same time
 int[] a = new int[10], b;
 float[] f = new float[33];
 double[] d = new double[123];
 char[] c = new char[21];
 // Arrays can be defined for more than just primitive data types; String arrays can also be defined
 String[] strs = new String[10];// This array contains 10 String variables
 }
}

03 Loops

Fri, 05 Jan 2024 22:07:53 +0800

Use == to compare primitive data types for equality; for other types (such as strings and functions), use equals to compare them.

02 Conditional Statements

Thu, 04 Jan 2024 19:42:08 +0800

If Statements #

if-else statements can also contain other if-else statements.

import java.util.Scanner;

public class Main {
 public static void main(String[] args) {
 Scanner sc = new Scanner(System.in);
 int a = sc.nextInt(), b = sc.nextInt(), c = sc.nextInt();

 if (a > b) {
 if (a > c)
 System.out.println(a);
 else
 System.out.println(c);
 } else {
 if (b > c)
 System.out.println(b);
 else
 System.out.println(c);
 }
 }
}

Chained if-else statements

Enter a score between 0 and 100.
If it is at least 85, output A;
if it is at least 70 but less than 85, output B;
if it is at least 60 but less than 70, output C;
if it is less than 60, output D.

import java.util.Scanner;

public class Main {
 public static void main(String[] args) {
 Scanner sc = new Scanner(System.in);
 int s = sc.nextInt();

 if (s >= 85) {
 System.out.println("A");
 } else if (s >= 70) {
 System.out.println("B");
 } else if (s >= 60) {
 System.out.println("C");
 } else {
 System.out.println("D");
 }
 }
}

01 Variables, Operators, Expressions, Input and Output

Wed, 03 Jan 2024 21:14:37 +0800

Syntax Basics #

Variables #

In Java, outputting a variable without assigning it a value causes an error.
Built-in data types

Type	Bytes	Example	Additional notes
byte	1	123
short	2	12345	$2^{16}=65536$
int	4	123456789	$2^{32}=2147483648≈2.1*10^9$
long	8	1234567891011L	$2^{64}≈10^{18}$
float	4	1.2F	About 6 or 7 significant decimal digits; subsequent digits are imprecise approximations. To explicitly denote a float, append f or F.
double	8	1.2, 1.2D	About 15 or 16 significant decimal digits; subsequent digits are imprecise approximations. To explicitly denote a double, append d or D.
boolean	1	true, false
char	2	‘A’	ASCII code correspondences: a-97 A-65 0-48