# 【韩顺平讲Java】Chapter7 - 面向对象编程（基础部分）
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> **即使再小的帆也能遠航**

> 參考自[【零基础 快速学Java】韩顺平 零基础30天学会Java](https://www.bilibili.com/video/BV1fh411y7R8/?vd_source=c5074574112ef27dae243d70aa2175b8)

>###### tags: `韓順平講Java`
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# 類與對象
- 類是一種**自定義** 的 **==數據類型==**
- 對象是類的具體實例

- 有過程的 (P199)



# 成員方法
- 訪問修飾符有四 : public, protected, private, (默認)

package chapter7;

public class MethodExercise01 {
    public static void main(String[] args) {
        AA a = new AA();
        System.out.println(a.isOdd(2)); 
        a.printTable(4, 4, '#');
    }
}

class AA {
    public boolean isOdd (int num){
        // return num % 2 == 0 ? false : true;
        return num % 2 != 0; 
    }

    public void printTable(int row, int col, char chr){
        for (int i = 0; i < row; i++) {
            for (int j = 0; j < col; j++) {
                System.out.print(chr + "\t");
            }
            System.out.println();
        }
    }
}

# 成員方法傳參機制
- 很多內存圖、理解
# 方法遞歸調用
## 老鼠走迷宮

package chapter7;

public class MiGong {
    public static void main(String[] args) {
        // 思路
        // 1. 先創建迷宮，使用二維數組
        // 2. 規定元素值  : 0 表示可以走，1表示障礙物
        int[][] map =new int[8][7];

        // 3. 外層設為1
        for (int i = 0; i < 7; i++) {
            map[0][i] = 1;
            map[7][i] = 1;
        }
        for (int i = 1; i < 8; i++) {
            map[i][0] = 1;
            map[i][6] = 1;
        }

        // 4. 設置障礙物
        map[3][1] = 1; map[3][2] = 1; map[2][2] = 1;

        // 輸出當前迷宮
        System.out.println("====當前地圖====");
        for (int i = 0; i < map.length; i++) {
            for (int j = 0; j < map[i].length; j++) {
                System.out.print(map[i][j]+" ");
            }
            System.out.println();
        }

        // 使用findWay給老鼠找路
        T t1 = new T();
        t1.findWay(map, 1, 1);

        // 輸出當前迷宮
        System.out.println("====找路的情況如下====");
        for (int i = 0; i < map.length; i++) {
            for (int j = 0; j < map[i].length; j++) {
                System.out.print(map[i][j]+" ");
            }
            System.out.println();
        }
    }
}

class T{
    // 使用遞規回溯的思想來解決老鼠出迷宮

    // 老韓解讀
    // 1. findWay 就是專門來找出迷宮的路徑
    // 2. 如果找到，返回true，否則false
    // 3. map 就是二維數組，即表示迷宮
    // 4. (i,j)是老鼠的位置，初始位置是(1,1)
    // 5. 因為是遞歸的找路，所以得先規定map數值的含意
    //    0 表示可以走，1表示障礙物，2表示可以走，3表示走過，但是走不通
    // 6. 當map[6][5] = 2，就說明找到通路，可以退出，否則就繼續找
    // 7. 先確定老鼠找路的策略 下 -> 右 -> 上 -> 左
    public boolean findWay(int[][] map, int i, int j){
        if (map[6][5] == 2){    // 說明已經找到
            return true;
        } else {
            if (map[i][j] == 0) {   // 當前這個位置0，表示可以走
                // 我們假定可以走通
                map[i][j] = 2;
                // 使用找路的策略，來確定該位置是否可以走通
                // 下 -> 右 -> 上 -> 左
                if (findWay(map, i + 1, j)){
                    return true;
                } else if (findWay(map, i, j + 1)){
                    return true;
                } else if (findWay(map, i - 1, j)){
                    return true;
                } else if (findWay(map, i, j - 1)){
                    return true;
                } else {
                    map[i][j] = 3;
                    return false;
                }
            } else {   //map[i][j] == 1,2,3
                return false;   // 已經走過或走不通，不須再嘗試
            }
        }
    }
}

## 漢諾塔

package chapter7;

public class HanoiTower {
    public static void main(String[] args) {
        Tower tower = new Tower();
        tower.move(64, 'A', 'B', 'C');
    }
}

class Tower{
    // num 表示要移動的個數
    // a, b, c分別表示 初始, 過度, 目標塔
    public void move(int num, char a, char b, char c){
        // 如果只有一個盤
        if (num == 1){
            System.out.println(a + " -> " + c);
        }else{
            // 如果有多個盤，可以看成兩個，最下面的和上面的所有盤(num-1)
            // (1) 先移動上面的所有盤到b，借助c
            move(num - 1, a, c, b);
            // (2) 把最下面的這個盤，移動到c
            System.out.println(a + " -> " + c);
            // (3) 再把b塔的所有盤，移動到c，借助a
            move(num-1, b, a, c);
        }
    }
}

## 八皇后
- 自己寫的

package chapter7;

public class EightQueen {
    public static void main(String[] args) {
        int[] map = new int[8];
        // int[] test = {0,4,7,5,2,6,1,3};
        Solver solver = new Solver();
        
        for (int i = 0; i < map.length; i++) {
            solver.solve(map, 0, i);
        }
    }
}

class Solver{
    static int ways = 1;
    public void solve (int[] map, int row, int pos) {
        map[row] = pos;
        if (row == 7 && check(map,7)) {
            System.out.print("第"+ ways++ +"種方法: {");
            for (int i = 0; i < map.length; i++) {
                System.out.print(map[i] + " ");
            }
            System.out.println("}");
        } else {
            if (check(map, row)){
                for (int i = 0; i < map.length; i++) {
                    solve(map, row+1, i);
                }
            }
        }
        
    }

    /*
     * 檢查新加入(row)皇后對整個棋盤的影響，過去檢查的一定合格，無須重複確認
     * 新加入的一定在最後，所以檢查到row就行
     */
    public boolean check(int[] map, int row) {
        // 檢查重複
        for (int i = 0; i < row; i++) {
            if (map[i] == map[row]) {
                return false;
            }
        }
        // 向斜檢查
        for (int i = 1; i <= row; i++) {
            if (map[row]+i == map[row - i] || map[row]-i == map[row - i]){
                return false;
            }
        }
        return true;
    }
}

# 重載 (overload)

# 可變參數


# 作用域




# 構造器
- 主要作用是完成對**新對象的初始化**


- 可以使用javap反編譯.class文件
:::info
javap Dog.class
(.class可以不寫)
:::

- P245



# this



# 本章作業
## 1

package chapter7;

public class Homework01 {
    public static void main(String[] args) {
        double[] arr = {1,2,3,4,5,6,7,8,9};

        A01 a01 = new A01();
        System.out.println(a01.max(arr)); 
    }
}

class A01{
    public double max (double[] arr) {
        double max = arr[0];

        for (int i = 1; i < arr.length; i++) {
            if (max < arr[i]){
                max = arr[i];
            }
        }

        return max;
    }
}

## 2

package chapter7;

public class Homework02 {
    public static void main(String[] args) {
        String[] arr = {"Jerry","Tome","Amy","Elsa"};
        A02 a02 = new A02();
        System.out.println(a02.find(arr, "Elsa"));
    }
}

class A02{
    public int find(String[] arr, String findElm){       
        for (int i = 0; i < arr.length; i++) {
            if (findElm.equals(arr[i])){
                return i;
            }
        }
        return -1;
    }
}

## 3

class Book {
    private int price;
    public Book (){}
    public Book (int price){
        this.price = price;
    }
    
    public void updatePrice (Book book){
        if (book.price > 150){
            book.price = 150;
        } else {
            book.price = 100;
        }
    }
}

## 4

class A03 {
    public int[] copyArr (int[] arr){
        int[] ret = new int[arr.length];
        for (int i = 0; i < arr.length; i++){
            ret[i] = arr[i];
        }
        return ret;
    }
}

## 5

package chapter7;

public class Homework05 {
    public static void main(String[] args) {
        Circle circle = new Circle(2);
        circle.getArea();
        circle.getPerimeter();
    }
}

class Circle{
    double radius;
    public Circle(double radius){
        this.radius = radius;
    }

    public void getPerimeter (){
        if (radius < 0) {
            System.out.println("無效半徑");
        } else {
            System.out.println(2*Math.PI*radius);
        }
    }

    public void getArea (){
        if (radius < 0) {
            System.out.println("無效半徑");
        } else {
            System.out.println(Math.PI*radius*radius);
        }
    }
}

## 6

package chapter7;

public class Homework06 {
    public static void main(String[] args) {
        Cale c1 = new Cale(2.5, 0.5);
        System.out.println(c1.sum());
        System.out.println(c1.sub());
        System.out.println(c1.time());
        System.out.println(c1.div());
        Cale c2 = new Cale(2.7, 0);
        System.out.println(c2.sum());
        System.out.println(c2.sub());
        System.out.println(c2.time());
        System.out.println(c2.div());
    }
}

class Cale {
    double d1;
    double d2;

    public Cale(double d1, double d2) {
        this.d1 = d1;
        this.d2 = d2;
    }

    public double sum (){
        return d1 + d2;
    }

    public double sub (){
        return d1 - d2;
    }

    public double time (){
        return d1 * d2;
    }

    public Double div (){
        if (d2 == 0){
            System.out.println("無效除數");
            return null;
        } else {
            return d1 / d2;    
        }
    }
}

## 7

class Homework07{
    public static main (String[] args){
        Dog dog = new Dog("旺財","Yellow","10");
        dog.show();
    }
}

class Dog {
    String name;
    String color;
    int age;
    
    public Dog (String name, String color, int age){
        this.name = name;
        this.color = color;
        this.age = age;
    }
    
    public void show(){
        System.out.println("Dog name is "+ this.name);
        System.out.println("Dog color is "+ this.color);
        System.out.println("Dog age is "+ this.age);
    }
}

## 8

count1 = 10
count1 = 9
count1 = 10

## 9

class Music {
    String name;
    int times;
    
    public Music (String name, int times){
        this.name = name;
        this.times = times;
    }
    
    public void play (){
        System.out.println("音樂播放中...")
    }
    
    public void getInfo (){
        System.out.println("音樂名為"+name);
        System.out.println("音樂時長 "+times+" sec");
    }
}

## 10

i=101
j=100
101
101
## 11

public double method (double d1, double d2){...}

## 12
- 重點 : 如何**複用**

class Employee {
    String name, position;
    char gen;
    int age, sal;
/*    
    public Employee (String name, char gen, int age, String position, int sal){
        this.name = name;
        this.gen = gen;
        this.age = age;
        this.position = position;
        this.sal = sal;
    }*/
    
    public Employee (String name, char gen, int age){
        this.name = name;
        this.gen = gen;
        this.age = age;
        
    }
    
    public Employee (String position, int sal){
        this.position = position;
        this.sal = sal;
    }
    
    public Employee (String name, char gen, int age, String position, int sal){
        this(name, gen, age);
        
        // this(position, sal)    
        // 想法是好的，但構造器一定要放在第一個，所以會報錯
        
        this.position = position;
        this.sal = sal;
        
    }
}

## 13

package chapter7;

public class Homework13 {
    public static void main(String[] args) {
        Circle2 circle = new Circle2();
        PassObject passObject = new PassObject();
        passObject.printAreas(circle, 5);
    }
}

class Circle2{
    double radius;

    public double getArea (){
        return Math.PI*radius*radius;
    }
}

class PassObject {
    public void printAreas(Circle2 c, int times){
        System.out.println("Radius\tArea");
        for (int i = 1; i <= times; i++) {
            c.radius = i;
            System.out.println((double)i+"\t"+c.getArea());
        }
    }
}

## 14

Random r = new Random();
r.nextInt(3);