Java学习个人笔记

 

    写这个个人笔记完全是出于记录自己所学，方便本人日后参考，可能很零散。这个笔记是建立在C语言编程基础上，本人学习Java只学习它与C语言不同的地方，或者我在C编程过程中很少用到的地方。所用的教材是Youtube一位达人做的视频：Java编程简明教程 by Thenewboston（Youtube）

    每次开一贴，学习Java笔记都记在这里。所写内容都是个人菜鸟级的理解，肯定错误百出。

 

   ---------- Feb 04, 2013 ---------

    个人理解：Java 的基本架构是通过class建立。class相当于C函数集，每个Class 里面的 method 相当于C的函数。

    1. 关于Class 中对别的class中的method的使用，创建：

        题目：main method在apple.java apple class中，引用tuna class中的println(); method, 输出一行字。
理解：对象的创建与使用。System.out.println() 的使用。

//file: project/apples.java
class apples{
    public static void main(String args[]){
        tuna tunaObj = new tuna();    //create an obj: "tuna" - class name, tunaObj-Obj name,
        tunaObj.MysimpleMessage();
    }
}

//file: project/tuna.java
public class tuna{
    public void MysimpleMessage(){
    System.out.println("This is class tuna");
    }
}

    2. 题目：创建和使用Scanner对象，以从键盘获取输入：

//file: project/apples.java
import java.util.Scanner;

class apples{
    public static void main(String args[]){
        Scanner input = new Scanner(System.in);  //creat an scanner obj
        tuna tunaObj = new tuna();    //create an obj: "tuna" - class name, tunaObj-Obj name,

        System.out.println("Enter your name here: ");
        String name = input.nextLine();

        tunaObj.MysimpleMessage(name);
    }

}

//file: project/tuna.java
public class tuna{
    public void MysimpleMessage(String name){
    System.out.println("Hello" + name);
    }
}

 

    3. 从main method传递值到别的class的method中：
要求：通过 GetName method 把 name 传递进class tuna中，通过tuna中的saying method 内部调用ret_name method 打印出 name.
本实例理解：1、在method内部声明变量和在class内声明变量有何不同。
2、class 内部互相调用method
3、跨 class 调用method

//----------------------- tutorial 16
//file: project/apples.java
import java.util.Scanner;

class apples{
    // Create a method named "main" Type"void":
    public static void main(String args[]){
        //Create method body:
        //Create a Scanner Obj:
        Scanner input = new Scanner(System.in);
        //Create a tuna Obj:
        tuna tunaObj = new tuna();
        String name;

        System.out.println("Enter your name here: ");
        //wait usr to input a string:
        name = input.nextLine();
        tunaObj.GetName(name);

        //call class, use method MysimpleMessage();
        tunaObj.saying();
    }
}

//file: project/tuna.java
public class tuna {
        private String girlname;

        public void GetName(String name){
            girlname = name;
        }

        public String retName(){
            return girlname;
        }

        public void saying(){
            System.out.printf("your name: %s",retName());
        }
}

    4. Constructor的使用，用于创建对class的使用时，同时设置class的初始参数。比如一个班的学生的姓名，年龄等。

//----------------------- tutorial 17
//file: project/apples.java
class apples{
    // Create a method named "main" Type"void":
    public static void main(String args[]){
        tuna tunaObj1 = new tuna("Amily",16);
        tuna tunaObj2 = new tuna("Banry",19);

        tunaObj1.saying();
        tunaObj2.saying();

    }
}

//file: project/tuna.java
public class tuna{
    private String girlname;
    private int girlage;

    public tuna(String name, int age){    //Create constructor, this "tuna" is exactly the name of this class
        girlname = name;
        girlage  = age;
    }

/*  public void GetNmae(String name){
        girlname = name;
    }*/

    public String retName(){
        return girlname;
    }

    public void saying(){
        System.out.printf("Your name is: %s, Age: %d \n", retName(), girlage);

    }
}

 

     5.  Java中使用 " （（条件判断） ？ 语句1 ：语句2） " 替代并简化if条件判断语句：

//----------------------- tutorial 36/37 Display Regurlar Time
//file: project/apples.java

class apples{
    // Create a method named "main" Type"void":
    public static void main(String args[]){

        tuna tunaObj = new tuna();
        tunaObj.setTime(14, 56, 20);
        System.out.println(tunaObj.dispTime());
        System.out.println(tunaObj.dispRegTime());

    }
}

//file: project/tuna.java
public class tuna{
    private int hour;
    private int minute;
    private int second;

    public void setTime(int h, int m, int s){
        hour = ((h>=0 && h<=24) ? h : 0);    //equivalent with"  if (h>=0 && h<=24)  {hour = h;} else {hour = 0;}
        minute = ((m>=0 && m<=60) ? m : 0);
        second = ((s>=0 && s<=60) ? s : 0);
    }

    public String dispTime(){
        return String.format("%02d:%02d:%02d", hour, minute, second);
    }
    public String dispRegTime(){
        return String.format("%d:%02d:%02d  %s",
                ((hour<=12) ? hour : hour%12), minute, second, ((hour>12) ? "PM" : "AM"));
    }
}

    6.  Java中this关键字，public /private概念。当constructor 中arguments 和 variable 的名称完全一样时候，用this可以指明调用哪一个值。

//----------------------- Tutorial - 38 - Public, Private and this
//file: project/apples.java
class apples{
    // Create a method named "main" Type"void":
    public static void main(String args[]){

        tuna tunaObj = new tuna();

        System.out.println(tunaObj.dispTime());
        //System.out.println(tunaObj.dispRegTime());

        tunaObj.setTime(14, 56, 20);

        System.out.println(tunaObj.dispTime());
        //System.out.println(tunaObj.dispRegTime());

    }
}

//file: project/tuna.java
public class tuna{
    private int hour = 1;
    private int minute = 2;
    private int second = 3;

    public void setTime(int hour, int minute, int second){
        this.hour = hour;
        this.minute = 5;    // Use the value of 5 when minute is called by dispTime() method
        second = 6;         // Use the value of 3 when second is called by dispTime() method

//      hour = ((h>=0 && h<=24) ? h : 0);
//      minute = ((m>=0 && m<=60) ? m : 0);
//      second = ((s>=0 && s<=60) ? s : 0);
    }

    public String dispTime(){
        return String.format("%02d:%02d:%02d", hour, minute, second);
    }
    public String dispRegTime(){
        return String.format("%d:%02d:%02d  %s",
                ((hour<=12) ? hour : hour%12), minute, second, ((hour>12) ? "PM" : "AM"));
    }
}

 

    7. Constructor 的嵌套使用，适用于constructor输入参数个数不确定的情形：

//----------------------- tutorial 40/41 - Building Objects for Constructors
//file: project/apples.java
class apples{
    public static void main(String[] args){
        //create 4 diff objs as there are 4 diff constructor
        tuna tunaObj0 = new tuna();
        tuna tunaObj1 = new tuna(5);
        tuna tunaObj2 = new tuna(5,13);
        tuna tunaObj3 = new tuna(5,13,14);

        //use the objs
        System.out.printf("%s\n",tunaObj0.toMilitary());
        System.out.printf("%s\n",tunaObj1.toMilitary());
        System.out.printf("%s\n",tunaObj2.toMilitary());
        System.out.printf("%s\n",tunaObj3.toMilitary());
    }

}

//file: project/tuna.java
public class tuna{
    private int hour;
    private int minute;
    private int second;

    public tuna(){
        this(0,0,0);
    }
    public tuna(int h){
        this(h,0,0);
    }
    public tuna(int h, int m){
        this(h,m,0);
    }
    public tuna(int h, int m, int s){
        setTime(h,m,s);
    }

    public void setTime(int h, int m, int s){
        setHour(h);
        setMinute(m);
        setSecond(s);
    }
    public void setHour(int h){
        hour = ((h>=0 && h<24)? h: 0);
    }
    public void setMinute(int m){
        minute = ((m>=0 && m<60)? m: 0);
    }
    public void setSecond(int s){
        second = ((s>=0 && s<60)? s: 0);
    }
    public int getHour(){
        return hour;
    }
    public int getMinute(){
        return minute;
    }
    public int getSecond(){
        return second;
    }
    public String toMilitary(){
        return String.format("%02d:%02d:%02d",getHour(),getMinute(),getSecond());
    }
}

 

     8. 比较复杂一个例子。

1）关于toString() method的使用。在potpie class中，每次声明使用potpie class时，由于其中constructor 语句里具有一个“

System.out.printf("The constructor for this is %s \n", this);

” ，所以每次声明都会打印一句The constructor for this is .xxx ,xxx的内容就调用this. this 会指向the following "toString()" method. 

        2）在tuna class中声明并使用了一个 potpie object，这个object是由第6行的语句传递进去的，第16行的（potpie birthday）语句定义了它的入口。

        3）tuna class 继续使用toString method来服务 apples class 中 main method 中 System.out.println(tunaObj);语句对它的调用。

        明显感觉我还对toString() method 的使用理解不够深入。包括 this 和 toString 的配合使用。

//----------------- tutorial 42 toString methods, 43-Composition
//file: project/apples.java
class apples{
    public static void main(String[] args){
        potpie potObj = new potpie(4,5,2006);
        tuna tunaObj = new tuna("Jack",potObj); //take the potObj just built up
        System.out.println(tunaObj);
    }
}

//file: project/tuna.java
public class tuna{
    private String name;
    private potpie date;

    public tuna(String theName, potpie birthday){
        name = theName;
        date = birthday;
    }

    public String toString(){
        return String.format("Your name is %s, and birthday is %s", name, date);
    }
}

//file: project/potpie.java
public class potpie {

    private int month;
    private int day;
    private int year;

    public potpie(int m, int d, int y){
        month = m;
        day   = d;
        year  = y;

        System.out.printf("The constructor for this is %s \n", this);
        //this is a reference to the obj we just built whenever we call this class
    }
    public String toString(){
        return String.format("%d/%d/%d",month,day,year);
    }
}

    

    9. 枚举类型enum的使用。

//----------------------- tutorial 44/45 Enumeration 枚举
//file: project/apples.java
//import java.utile.EnumSet;   // enable to use EnumSet.rang(from,to);
class apples{
    public static void main(String[] args){
        for (tuna people : tuna.values())
            System.out.printf("%s\t%s\t%s \n",
                    people, people.getDesc(),people.getYear());
        System.out.println("\n And now for the range of constants!\n");

//      for (tuna people: EnumSet.range(tuna.kelsey,tuna.candy))   // pick up part of the members from tuna
//          System.out.printf("%s\t%s\t%s \n",
//                  people, people.getDesc(),people.getYear());


    }
}

//file: project/tuna.java
public enum tuna{            // change class into enum
    // enu constants/obj:
    bucky("nice","22"),
    kelsey("cutie","10"),
    julia("bigmistake", "12"),
    nicole("italian","13"),
    candy("different","14"),
    erin("iwish","16");

    private final String desc;  // variable(String)
    private final String year;

    // build enum constructor
    tuna(String description, String birthday){
        desc = description;
        year = birthday;
    }

    public String getDesc(){
        return desc;
    }
    public String getYear(){
        return year;
    }
}

 

 

10. 关于Static的用法：（此处包含变量的static和method的static）

//----------------------- tutorial 46/47 Static
//file: project/apples.java
//file: project/apples.java
class apples{
    public static void main(String[] args){
        tuna member1 = new tuna("Megan","Fox");
        tuna member2 = new tuna("Natalie","Portman");

        System.out.println("----------------");

        System.out.println(member1.getFirst());
        System.out.println(member1.getLast());
        System.out.println(member1.getMembers());
        System.out.println(   tuna.getsMembers()); //getsMembers() is a static method

    }
}

//file: project/tuna.java
public class tuna{
    private String first;
    private String last;
    private static int members = 0;   //static variable, like "in-class globel variable"

    public tuna(String fn,String ln) {
        first = fn;
        last  = ln;
        members++;
        System.out.printf("Constructor for %s %s, members in the club: %d\n",first,last,members);
    }
    public String getFirst(){
        return first;
    }
    public String getLast(){
        return last;
    }
    public int getMembers(){
        return members;
    }
    public static int getsMembers(){
        return members;
    }
}

 

    11. final的理解和用法：

//----------------------- tutorial 48 final
//file: project/apples.java
class apples{
    public static void main(String[] args){
        tuna tunaObj = new tuna(10);
        tuna tunaObj2= new tuna(1);

        for(int i = 0; i<3 ; i++){
            tunaObj.add();
            System.out.printf("%s",tunaObj);
        }
        System.out.println("-------------");
        for(int i = 0; i<3 ; i++){
            tunaObj2.add();
            System.out.printf("%s",tunaObj2);
        }
    }
}

//file: project/tuna.java
public class tuna{
    private int sum;
    private final int NUMBER;

    public tuna(int x){
        NUMBER = x;  // you can only modify it once in one class
        //NUMBER = 5;  //can not change its value again
    }
    public void add(){
        sum+=NUMBER;
    }
    public String toString(){
        return String.format("sum = %d\n",sum);
    }
}

 

    12. Inheritance -- 继承 ，这是我接触到的新概念。 什么是java 的继承？  1）被继承的class叫做super class, super class 中private的变量和method不能被继承。  2）继承的class中如果method名与super class中的method名重合，则使用继承object时，运行继承class中重合的method代码，即以继承class 为准。此谓overwrite.

//----------------------- tutorial 49 Inheritance 继承
//----------------------- tutorial 49 Inheritance 继承
//file: project/apples.java
class apples{
    public static void main(String[] args){
        tuna tunaObj = new tuna();
        potpie potpieObj = new potpie();

        System.out.println("--- tunaObj.eat() ---");
        tunaObj.eat();
        System.out.println("--- potpieObj.drink() ---");
        potpieObj.drink();
        System.out.println("--- potpieObj.potpiemethod() ---");
        potpieObj.potmethod();
    }
}

//file: project/food.java  //super class
public class food{
    public void eat(){
        System.out.println("I am eat methods from food");
    }
    public void drink(){
        System.out.println("I am drink methods from food");
    }
    private void bite(){     //private methods cannot be inheritance
        System.out.println("I am bite methods from food");
    }
}

//file: project/tuna.java
public class tuna extends food{ // Inheriance from food
    public void eat(){
        System.out.println("tuna overwritten eas() of food");
    }
}

//file: project/potpie.java
public class potpie extends food{ // Inheritance from food
    public void potmethod(){
        System.out.println("I am potmethod from potpie");
    }
}

    13.