Java Basic Knowledge

Published: May 19, 2020

Simple Introduction to Java

JRE includes {JVM + Runtime Library}
JDK includes JRE + {Compiler,debugger,etc.}

Variable

short: 2 bytes;
int: 4 bytes;
long: 8 bytes;
float: 4 bytes;
double: 8 bytes;
char: 2 bytes
There are two types of java variables: basic type and reference type
int
float, double
boolean
char
String a = “hello”; (reference type)

Calculation

There might be some errors when calculating two float numbers:

public class Main{
    public static void main(String[] args){
        double x = 1.0/10;
        double y = 1-9.0/10;
        double r = Math.abs(x-y);
        if(r<0.00001){
            System.out.println("x=y");
        }
        else{
            System.out.println("x!=y");
        }
    }
}

HW1: The way to calculate a Quadratic Equation of One Variable:

public class Main {
    public static void main(String[] args) {
        double a = 1.0;
        double b = 3.0;
        double c = -4.0;
        double r1 = 0;
        double r2 = 0;
        
        r1 = (-b + Math.sqrt(b*b-4*a*c))/(2*a);
        r2 = (-b - Math.sqrt(b*b-4*a*c))/(2*a);
        
        System.out.println(r1);
        System.out.println(r2);
        System.out.println(r1 == 1 && r2 == -4 ? "right" : "wrong");
    }
}

The new way to represent a char:

char a = '\u3e3d'        // Use Hex to represent

Array

public class Main {
    public static void main(String[] args){
        int[] ns = new int[5];
        ns[0] = 1;
        ns[1] = 2;
        ns[3] = 3;
        ns[4] = 4;
        ns[5] = 5;
        System.out.println(ns.length)
    }
}

To be more simplify:

public class Main {
    public static void main(String[] args){
        int[] ns = {1,2,3,4,5};
        System.out.println(ns.length)
    }
}

The important thing about the array is that there is a ns points to the first element of the array; If using String array:

String[] names = {
    "ABC", "XYZ", "ZOO"
};

There is a “pointer” names pointing at an array, the elements of the array is the address of the string

Java Input and Output

Formalized Input and Output:

public class Main {
    public static void main(String[] args) {
        double d = 3.1425926;
        System.out.printf("%.2f\n",d);  //The result is 3.14 
    }
}

import java.util.Scanner;

public class Main {
    public static void main(String[] args){
        Scanner scanner = new Scanner(System.in);  //Create a scanner object
        System.out.print("Input your name: ");     
        String name = scanner.nextLine();          //Read a line of input and get the string
        System.out.print("Input your age: ");
        int age = scanner.nextInt();               //Read a line of input and get the integer
        System.out.printf("Hi, %s, you are %d\n",name,age);    
    }
}

Traverse the Array in the reverse way

public class Main{
    public static void main(String[] args){
        int[] ns = {1,2,3,4,5};
        for (int i = 0; i<5; i++){
            System.out.println(ns[4-i]);
        }  
    }
}

Sort the array

Despite there are a lot of ways to implement the array sorting

import java.util.Arrays;

public class Main {
    public static void main(String[] args){
        int[] ns = {1,2,3,4,5};
        Arrays.sort(ns);
        System.out.println(Arrays.toString(ns));    
    }
}

Object Oriented Programming

public class Main {
    public static void main(String[] args) {
        Person p = new Person("Xiao Ming", 15);
        System.out.println(p.getName());
        System.out.println(p.getAge());
    }
}

class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    
    public String getName() {
        return this.name;
    }

    public int getAge() {
        return this.age;
    }
}

Overload

The name of the methods are the same, but the parameters are different

class Hello {
    public void hello(){
        System.out.println("Hello, world!");    
    }
    public void hello(String name){
        System.out.println("Hello,"+name+"!");
    }
}

extends: If the student class has construction method, the person needs to have construction method and there should be super() in the student class.
The name of the variable cannot be the same, the name and parameter of the methods can be the same, the latter case is also called override.

Override

class Person {
    public void run() {
        System.out.println("Person.run");
    }
}
class Student extends Person {
    @Override
    public void run() {
        System.out.println("Student.run");
    }
}

Abstract class

public class Main {
    public static void main(String[] args) {
        Person p = new Student();
        p.run();
    }
}

abstract class Person {
    public abstract void run();
}

class Student extends Person {
    @Override
    public void run() {
        System.out.println("Student.run");
    }
}

Static Field and Static Method

Static Field and Static Methods have their own space

Java Core Class

String and Coding

public class Main {
    public static void main(String[] args){
        String s = "Hello";
        String s1 = "Hello";
        a = s.toUpperCase();          // Change the String to Upper Case; s.toLowerCase(): Change the String to Lower Case;
        b = s1.equals(s);             // To see if s1 is equal to s
        "Hello".contains("H");        // To see if "Hello" contains "H"
        "Hello".substring(1,3);       // The substring of "s[1],s[2]"
        "\u3000Hello\u3000".strip();  // The thing printed out: "Hello"    
    }
}

String to Char:

public class Main {
    public static void main(String[] args) {
        char[] cs = "Hello".toCharArray();
        String s = new String(cs);
        System.out.println(s);
        cs[0] = 'X';
        System.out.println(s);
    }
}

Java StringBuilder (The way to splice the string)

public class Main {
    public static void main(String[] args) {
        var s = new StringBuilder(1024);
        s.append("Mr ")
          .append("Bob")
          .append("!")
          .insert(0, "Hello, ");
        System.out.println(s.toString());
    }
}

The thing print out is: Hello, Mr Bob!

Java StringJoiner

public class Main {
    public static void main(String[] args) {
        String[] names = {"Bob", "Alice", "Grace"};
        var sj = new StringJoiner(", ", "Hello ", "!");
        for (String name : names) {
            sj.add(name);
        }
        System.out.println(sj.toString());
    }
}

Print out Hello Bob, Alice, Grace!

Java Exceptions and Errors

Use try and catch to get the Exceptions and Errors Use throw to make an error or Exception

try…catch example:

import java.io.UnsupportedEncodingException;
import java.util.Arrays;

try{
    return(x)
} catch(UnsupportedEncodingException e){
    System.out.println(e);
}

Java Input and Output

List all the directory and files, then print them

import java.io.File;

import java.io.IOException;

public class Main {

    public static void main(String[] args) throws IOException {

        File currentDir = new File("F:\\javaProject\\Documents");

        String tree = "";

        listDir(currentDir.getCanonicalFile(), tree);

    }

    static void listDir(File dir, String t) {

        // TODO: 递归打印所有文件和子文件夹的内容

        File[] fs = dir.listFiles();

        if (fs != null) {

            for (File f : fs) {

                if(f.isDirectory()) {

                    System.out.println(t + f.getName() + "/");

                    listDir(f, t + "    ");

                } else {

                    System.out.println(t + f.getName());

                    listDir(f, t + " ");

                }

            }

        }

    }

}

}

Zhenyu Yang

Simple Introduction to Java

Variable

Calculation

Array

Java Input and Output

Traverse the Array in the reverse way

Sort the array

Object Oriented Programming

Overload

Override

Abstract class

Static Field and Static Method

Java Core Class

String and Coding

Java StringBuilder (The way to splice the string)

Java StringJoiner

Java Exceptions and Errors

Java Input and Output