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Developing in Java
Published on Fri Oct 25 2019 08:00:00 GMT+0000

Java Language

Tiny notes

1B=8bit

Java have three systems：

  • Java SE（J2SE）（Java2 Platform Standard Edition）
  • Java EE (J2EE) (Java 2 Platform,Enterprise Edition)
  • Java ME (J2ME) (Java 2 Platform Micro Edition)

Java class

The define of a java class

You can see it’s like C++ class in some ways.

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class Employee
{
    //status(variable)
    private String name;
    private double salary;
    
    //constructor
    public Employee(String n,double s)
    {
        name = n;
        salary = s;
    }
    
    //method
    public String getName()
    {
        return name;
    }
}
Class Fields
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private String name;//private refers this variable can only access by it's own method.
private double salary;
Class constructor
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public Employee(String n , double s)
{
    name =n;
    salary =s;


//use constructor:
new Employee("Jamws Bond",10000);
//there can be serveral constructors in a class.
}

Constructor

Java offers several mechanisms of constructing a new object.

This function is called as overloading. If there’s several functions with same name but different parameters , the complicator will choose a function .This called overloading resolution .

Ok, let’s see constructor.

Default field initialize:

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Employee John = new Employee();

All the number will be 0, boolean will be false, object reference will be null.

Caution: This is not a good habit to initialize a new object with default field.

Constructor without parameter:

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public Employee()
{
    name ="";
    salary =0;
    hireDay =LocalDate.now();
}

Explicit field initialize

You can initialize field value in the declaration of a class.

Before using constructor.

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public class Employee
{
    private String name =" ";
    private static int nextId;
    private int id = assignId();
    
    
    private static int assignId()
    {
        int r= nextId;
        nextId++;
        return r;
	}
}

Calling another constructor

Use this to call another constructor when you are just using a constructor.

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public Employee(double s)
{
    this("Employee" + nextId,s);
    nextId++;
}

Initialization block

Initialize field values in the class define.

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class Employee
{
    //......
    private int Id;
    private String name;
    private double salary;
    //......
    
    //Use initialization block
    {
        id = nextId;
        nextId++;
    }
}

Summary

The Steps of Calling constrictor

1.If the constructor calls another, construct by that one.

2.a. All the field will be initialized by default.

b. According to the order in class defination, execute field initialize function and initialization block.

Class Operation

Use var to state a local variable
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var harry = new Employee("Hurry",5000);
Use clone to return a copy of an object
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return (Date) hireDay.clone();

Tips: a private field is accessible to an object of same class.

Static

A field or method which is defined as static means there is ONLY ONE in the same class.

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public static final double PI = 3.141592653;

A static method do not use an object. Such as:

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Math.pow(10,3);

A static method can not access fields in an object ,but can access static field.

Use static initialization block

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static
{
    var generator = new Radom();
    nextId = generator.nextInt(10000);
}

Main method

A class can have a main method, but it will not come into effect. It is used to TEST a class.

Class Date

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System.out.println(new Date());
String s = new Date.toString();
Date deadline;//However,this variable does not refer to any object.
deadline = new Date();//ok.
deadline = null;// refer to no object
deadline = birthday;//they refer to the same object

Class LocalDate

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package com.msfasr.java_startup;

import java.time.DayOfWeek;
import java.time.LocalDate;

public class Calendar {
    public static void main(String[] args)
    {
        LocalDate currentDate = LocalDate.now();
        System.out.println("Mon Tue Wed Thu Fri Sat Sun");
        int month=currentDate.getMonthValue();
        int today=currentDate.getDayOfMonth();
        currentDate = currentDate.minusDays(today-1);
        int dayOfWeek = currentDate.getDayOfWeek().getValue();
        for(int i=0;i<dayOfWeek-1;i++)
            System.out.printf("    ");
        while(currentDate.getMonthValue() == month)
        {
            if(currentDate.getDayOfMonth()==today)
            {
                System.out.printf("*");
            }
            else
                System.out.printf(" ");
            System.out.printf("%-3d",currentDate.getDayOfMonth());
            if(currentDate.getDayOfWeek().getValue()==7)
                System.out.println();
            currentDate = currentDate.plusDays(1);
        }
    }
}

Java class methods

Main method

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package com.msfasr.java_startup;//The code is included in a package

import java.util.Scanner;//Inport Scanner for input


public class HelloWorld {//The main method is contained in a class
    public static void main(String[] args) {//Be aware of the define of the main method
        System.out.println("Hello,world");
    }
}

Class Methods

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public static void printCalendar()
{
    ......
}//static method, do not operate fields

public void changeFormat()
{
    ......
}//this method must based on an object

Method parameters

Java programming language can only Call by value .

However, when the parameter is an OBJECT , methods can change values of OBJECTS.

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public static void tripleSalary(Employee x)
{
    x.raiseSalary(200);
}

So,a method can:

Change the status of an object.

Can’t:

Change values of variable.

Make an object refer to an new object.

Package

Java package is a sort of set contains several class.

Package standard name:

com.(company name).(project name)

Such as:

com.msfasr.javaStartup

Importing package

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import java.util.Scanner;//import a class
import java.util.*       //import a package

When there is a class name used by several package, you need to write the package name when using.

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import java.util.Date;
import java.sql.Date;

public class Main{
    public static void main(String[] args){
        var deadline = new java.util.Date();
        var today = new java.sql.Date(. . . );
    }

}

Static importing

By using static importing, you can import a sort of static method and field.

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import static java.lang.System.*;
//then, you can use:
out.println("123");//System.out
exit(0);//System.exit

Adding class into a package

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package com.msfasr.javaStartup;
//......

The class is now in the package.

Class annotation 

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/**
 * This class is built for Studying Java!
 * This means I will enter a new field!
 */

public class Main {
	//......
}

Class Designing tips

1.Guarantee that the data is private.

2.Warning: You must initialize the field!!!

3.Do not use so much basic type variable!!!

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//......
private String street;
private String city;
private String state;//bad
//......


private class address
{
    private String street;
	private String city;
	private String state;   //good
}

4.not all the fields are changeable.

5.Resolve the classes with so many responsibility.

6.The name should refer their responsibility.

7.Use classes unchangeable at first.

Class Inheritance

Class, Superclass, Subclass

A subclass define:

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public class Manager extends Employee
{
    private double bonus;
    
    private void setBonus(double aBonus)
    {
        bonus=aBonus;
    }
}

Covering methods

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public class Manager extends Employee
{
    //...
    public double getSalary()
    {
        int aSalary= super.getSalary();
        return aSalary + bonus;
    }
}

Subclass constructor

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public class Manager extends Employee
{
    public Manager(String name , double salary, int year, int month, int day)
    {
        super(name,salary,year, month, day);
        bonus = 0 ;
    }
}

Class Array with dynamic binding

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//...
Manager boss = new Manager("Makinohara",80000000,2001,4,13);
boss.setBonus(200000);

//then, define an array of employees

var staff[] = new Employee[3];

staff[0] = boss;
staff[1] = new Employee("Mai",20000,2000,1,2);
staff[2] = new Employee("Sakuta",10,2001,4,13);

for(Employee e : staff)//for:each circulation
{
    System.out.println(e.getName()+":"+e.getSalary);
}

The compiler will choose the right method to get salary, this is Dynamic Binding.

Polymorphism

In java, the object variable is polymorphic, it means a variable of an object can refer all the subclass object.

Java Method Calling

  1. The complier gets the method list of a class and its superclass.

  2. The complier confirms the parameter types of the method which is being called.

  3. The JVM calls the method.

Stop Inheriting : final.

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public final class Boss extends Manager//the class can't be inheriting
{
    //...
    public final String getName()//the method can't be covered.
    {
        //......
    }
}

Forced type conversion

In Class Array with dynamic binding, we created an array contains Manager and Employee. However, the array is a Employee array, so we need to use Forced type conversion to convert a Employee object to a Manager object.

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Manager boss = (Manager)staff[0];

But, when you are trying to convert a object from top to bottom, it won’t work.

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Manager boss = (Manager)staff[1];// Not work

//So,we can avoid this case by using:
if(staff[1] instanceof Manager)
{
    
    boss = (Manager)staff[1];
    
    //......
}

Abstract Class

Abstract class is a sort of class that is more general. Such as Person can contain Employee and Student.

The define of an abstract class

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public abstract class Person
{
    private String name;
    public Person (String name)
    {
        this.name = name;
	}
    
    public abstract String getName()
    {
        return name;
    }
}

Instantiation is not allowed in an Abstract class. It means you can’t :

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var aP = new Person("Deja vu");
// you can do this:

Person stu1 = new student("Deja vu","Computer Driving");

Access modifier

public : this method or field can be used by all classes.

private: this method or field can only used by the same class, not for subclasses.

protected: this method or field can used by this package and all subclass

no modifier: this method or field can be used by all the methods in the same package!!

Object: the superclass of all classes

You can use a variable whose type is Object to refer objects of all classes.

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Object obj = new Employee("Harry Potter",300000);

But, when you are trying to operate the object or using methods, you need to do type conversing .

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Employee e = (Employee)obj;

equals

equals is a method to compare whether two objects are equal.

The equals method in Class Object is used to compare two objects are equal or not, and if two objects reference are same, the method returns true.

Equals method writing
Standard:
  1. Reflexivity : x.equals(x) == true
  2. Symmetry : x.equals(y) == y.equals(x)
  3. Transitivity : x.equals(y) == true , y.equals(z) == true -> x.equals(z) == true.
  4. Consistency : If x and y did not change , the equality do not change.
  5. x.equals(null) == false.
Advice of designing an equals method

See Core Java Vol.1 , 11th edition , Page 178.

Hash code

hashcode() : returns an integer , the hash code , that refers the object.

Object.hash(Obj1,Obj2,……,ObjN) ;

Object.hashcode(Obj);

toString

Object.toString(); Print a string that consist of all fields. Needs you write yourself.

ArrayList

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ArrayList<Employee> staff = new ArrayList<Employee>();

//add an object:
staff.add(Object);
staff.add(new Employee(......));

staff.size();//returns the number of objects.

Compare: ArrayList and Array

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stuff.set(i,harry);//stuff[i] = hurry
Employee ep = stuff.get(i);// Employee ep = stuff[i]
stuff.add(i,E);//insert E to position i , move the rest. 
stuff.remove(i);//delete Element at position i, move the rest

Object Wrapper

Wrapper class is a sort of class that corresponding to basic data types.

Such as:

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var list = new ArrayList<Integer>();
list.add(3);          //Autoboxing
int n = list.get(i);  //Autounboxing

Interface

Realize an interface

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class Employee implements Comparable<Employee>	 //在实现接口时加上类名，可以可以防止对Object类型转换
{
    //......
    public int compareTo(Employee other)
    {
        return Double.compare(salary,other.salary);
    }
    //......
}
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public class Main {

    public static void main(String[] args)  {
        var staff = new Employee[3];
        staff[0] = new Employee("Mark",12000,2020,1,20);
        staff[1] = new Employee("Lin",10000,2020,1,20);
        staff[2] = new Employee("Coal",41230,2020,1,20);
        Arrays.sort(staff);
        return;

    }
}

接口不是一种类，所以不能使用new实例化接口。

接口的一个样例：

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public interface Comparable<T>{
	default int compareTo(T other){  //一种默认方法。
		return 0;
	}
}

接口与回调

一个Listener接口：

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public interface ActionListener
{
	void actionPerformed(ActionEvent event);
}

如果想要调用这个接口，则可以在一个类中实现：

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class TimePrinter implements ActionListener{
	public void actionPerformed(ActionEvent event){          //在类中实现这个接口
		System.out.println(123);
	}
}

实现回调的方法：

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var listener = new TimePrinter;
var timer = new Timer(1000,listener);
//......
//timer每1000毫秒通知实现了ActionListener接口的类，这个类就会实现接口提供的函数actionPerformed，完成一个回调。

Lambda 表达式

一种自定义代码块，用于实现某些计算或执行某些函数

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//比如，要计算a.length() - b.length()
(String a, String b)
	->a.length()-b.length()
//或者：
(String a, String b)->
{
	if(a.length<b.length)
		return -1;
	//......
}
//执行函数：
var timer = new Timer(1000,event ->System.out.println(event))

使用lambda来使方法实现函数式接口，避免多余的重写方法：

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package com.icsii.interfaceDemo;

public interface printer {
    void printString(String something);
}
public class print {
    public void printSomething(String something,printer aPrinter){
        aPrinter.printString(something);
    }

    public static void main(String[] args) {
        print Demo1 = new print();
        Demo1.printSomething("123",(String something)->System.out.println(something));
    }
}

(接口接收的参数)->{接口实现的方法}

Java I/O

Input(scanner)

Firstly, import a class scanner .

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import java.util.Scanner;

This class has these basic operations :

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Scanner sc = new Scanner(System.in);//new a scanner
String name = sc.nextLine();//Get a line
int age = sc.nextInt();//Get the next int
sc.close();//Close the scanner when used,or it will report an error.
Other measures
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String nextLine();//read next line;
String next();//read next word, use blank as separator.
int nextInt();//read next int
boolean hasNext();//if there's words in line
boolean hasNextInt();
Read files
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Scanner sc = new Scanner("Path",StandardCharsets.UTF_8);//read file
PrintWriter out = new PrintWriter("Path",StandardCharsets.UTF_8);//Write file
out.print();//Warning: this method will rewrite the file!!!

Output (System.out)

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System.out.println(1111);//Print and return
System.out.print(1111);//Print without returning
System.out.write(2222);//byte output
System.out.printf("%+8.3f\n", 3.14);//Print according to format

Java variable :

Variable types

Variable type value
int 4byte,-2147483648~2147483647
short 2byte
long 8byte
byte 1byte
float 4byte
double 8byte
char(UTF-16) 16byte(UTF-16 character )
boolean true/false

Caution: A variable must be initialized ,or the compiler will put an ERROR.

In java, key word final refers a variable is constant.

enum:

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enum Size {Small,Medium};//enum is a CLASS!
Size s = Size.Small;//it can only use values defined above.

String

str.substring can extract a part of string .

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String greeting = "Hello";
String s = greeting.substring(0,3);//s = "Hel"
String str = greeting+s;

str.equals compares two strings:

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"Hello".equals(greeting);

Caution: DO NOT USE == !

Other measures
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str.length(); //return length
str.blank(); // judge if the string is empty
str.join("/",str1,str2,str3......);// use / as separator , combine strings
String.format("%f",a);//returns a String with format

Big numbers

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BigInteger;
BigDecimal;
num1.add(num2);
//use add subtract multiply devide mod

Array

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int[] a = new int[100];
int[] a = new int[n];
int[] a = {1,3,7,7,9};
a = new int[] {1,3,5,7,9};
a.length();
Array.sort(a);
Array.binarySearch(xxx[] a,xxx v);
Array.binarySearch(xxx[] a,int start,int end,xxx v);
Array.fill(xxx[] a,xxx v);
int a[][] = new int[1][3];
int[] temp = a[1];
a[1]=a[2];
a[2]=temp;//exchange 2 lines
int[][] feb = new int[100][];
for(int n=0;n<100;n++)
    feb[n]=new int[n+1];//regard feb[n] as an array name.

Array copy:

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int[] a = {1};
int[] b = a;
a[0]=5;//now,b[0] is also 5
int[] ca = Arrays.copyOf(a,5);//then we get a new array, the size is 5.

Java sentences

For

For each

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for(int element:a)
	System.out.println(element);