1. Class#

Object-Oriented Programming (OOP)

Class: encapsulates instance fields + methods

Class >== Inheritance (is-a)/ Dependency (uses-a)/ Aggregation (has-a) ==> Class

2. Built-in Classes#

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// Math
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// Date
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Date date = null;    //类似于cpp的对象指针
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date = new Date();
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System.out.println(date.toString());

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// LocalDate
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System.out.println(LocalDate.now());
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LocalDate date = LocalDate.now();        //construct current date
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date = LocalDate.of("year","month","day");    //construct specified date
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int today = date.getDayOfMonth();
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int getYear();
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int getMonthValue();
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int getDayofMonth();
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int getDayofWeek();                //1~7
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// n days earlier, n days later
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date = date.minusDays(n);
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date = date.plusDays(n);
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// is leap year
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date.isLeapYear();
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// number of days in the current year and in the current month
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date.lengthOfYear();
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date.lengthOfMonth();

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public class chapter04main {
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    public static void main(String[] args) {
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        LocalDate date = LocalDate.now();
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        int month = date.getMonthValue();
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        int today = date.getDayOfMonth();
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        date = date.minusDays(today-1);     // first day of the month
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        DayOfWeek week = date.getDayOfWeek();
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        System.out.println("Mon Tue Wed Thu Fri Sat Sun");
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        for(int i=0;i<week.getValue();i++) System.out.printf("   ");
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        while (date.getMonthValue() == month){
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            int now = date.getDayOfMonth();
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            System.out.printf("%3d",now);                 // print date
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            if(now == today) System.out.printf("*");
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            else System.out.printf(" ");
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            date = date.plusDays(1);            // next day
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            if(date.getDayOfWeek().getValue() == 1) System.out.println();
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        }
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    }
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}

3. Custom Classes#

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class Test{
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  //field
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    private int t;
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    //constructor
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    public Test(/*...*/){
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        //...
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    }
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    // method
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    public void test(int n){
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        System.out.println("just test");
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    }
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}

• Run multiple source files

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javac Test*.java

• Instantiation

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// Using the constructor
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Test test1 = new Test();
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var test2 = new Test();
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// detect null
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t = Objects.requireNonNullElse(n,"unknown");    //warn but accept
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Objects.requireNonNull(n,"not to be null");      //reject directly

• Explicit parameter, implicit parameter

In test1.test(3), test1 is the implicit parameter, and 3 is the explicit parameter.

• Encapsulation

Do not return private objects of the class

• Access modifiers

public: public

private: private

• final

Must be initialized, and the reference cannot be changed to point to another object, though the object it points to can be modified.

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private final StringBuilder eva;
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eva = new StringBuilder();      //must be initialized
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eva.append("ok!\\n");        //legal

4. Static Methods and Static Fields#

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static int number = 1;            //static field, shared by the class
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static final double PI = 3.141592653589;  //static constant

• Static methods

static int getNum(){...}

There are no implicit instances; call directly on the class. e.g. Test.getNum()

No object state required; just access the class’s static fields

• Factory methods

Similar to the constructors of LocalDate and NumberFormat.

• main

The main method is also a static method.

5. Method Parameters#

• Call by value (Java default)
• Call by reference
• Call by name (used by Algol)

Call by value: parameters are copies (primitive data types).

Object references allow the parameter to refer to the object, but it is still passed by value. e.g., you cannot swap two objects.

6. Object Construction#

• Overloading

Same name, different parameter lists (not including return type)

• No-arg constructor

Initialize to default values.

P.S. When there is custom construction, you must provide a no-arg constructor yourself.

• Explicit field initialization

private String name = ""; initialize directly in the class.

• Using this to delegate to another constructor

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public Test(double s){
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  this("Test "+Double.toString(s));
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}

• Initialization blocks

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class Test{
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  private static int Aid = 1;
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  private int id;
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  // initialization block
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    // static, if present, runs when the class is loaded for the first time.
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  {
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    id = Aid;
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    Aid ++;
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  }
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  //...
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}

As long as you construct an object, the initialization block will run. It runs before the constructor.

• Destruction

Java can perform automatic garbage collection

7. Packages#

package

• Package name
• Importing classes

Directly use all classes within the package.

Use public classes from other packages.

Fully qualified name

java.time.LocalDate today = java.time.LocalDate.now();

• import

import java.time.*; | import java.time.LocalDate;

You can only use * to import a package,

• Static imports

import static java.lang.System.*;

You can directly use out.println();

• Packages

package cc.dreaife.chapter04;

If there is no package statement, the classes in the file belong to the unnamed package.

• Package access

If neither public nor private is specified, it can be accessed within the same package.

• Class path

Classes can be stored in JAR files, which can contain multiple class files and subdirectories.

JARs organize files and subdirectories using ZIP format.

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# Set the classpath
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java -classpath ${PATH(JAR)} ${className};
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export CLASSPATH=${PATH};
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set CLASSPATH=${PATH}

8. JAR Files#

• Creating a jar file

jar cvf jarFileName file1 file2 ...

• Manifest file

META-INF/MANIFEST.MF

jar cfm jarName manifestName ...

• Executable jar file

java -jar jarName

• Multi-version jar files

9. Documentation Comments#

javadoc => HTML files

• Comment placement

Public classes and interfaces at the module/package level, public or protected fields, public or protected constructors and methods

/**...*/ comments

@ param tag + free-form text (the first sentence should be a brief summary; HTML modifiers may be used)

• Class comments

After imports, before the class definition

• Method comments

@param

Parameter description, can span multiple lines, HTML modifiers may be used

• @return

Return value, can span multiple lines, HTML modifiers may be used

• @throws

Thrown exceptions

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/**
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     * testA
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     * @param s
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     * @param num
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     * @return string
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     */
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    public String testA(String s,int num){
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        return s + Integer.toString(num);
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    }

• Field comments
• General notes

@author name@since text@version text@see | @link (use hyperlinks to packages, classes, and methods)

• Package comments

Add a separate file within the package directory.

package-info.javapackage.html extract the text between …

• Comment extraction

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javadoc -encoding UTF-8 -d ../doc Chapter04    #extract package
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javadoc -encoding UTF-8 -d docTest test.java  #extract class

10. Class Design Tips#

• Ensure data is private
• Data initialization
• Don’t use too many primitive types
• Not everything needs get() and set()
• Avoid classes with too many responsibilities
• Class names and method names should reflect their responsibilities
• Prefer immutable classes