多态以及 LeetCode 每日一题

1 多态

1.1 多态性

Java 引用变量有两个类型：一个是编译时类型，一个是运行时类型。前者是代码中声明这个变量时的类型，后者是由实际对象的类型决定的。当编译类型和运行类型不一样时，产生多态。

class BaseClass{
    public int book = 6;
    public void base(){
        System.out.println("Father loves you!");
    }
    public void test(){
        System.out.println("Father's method is covered");
    }
}
public class SubClass extends BaseClass{
    public String book = "Marvel comic book";
    public void test(){
        System.out.println("Son's method covers Father's");
    }
    public void sub(){
        System.out.println("Son");
    }
    public static void main(String[] args){
        //多态
        BaseClass nc = new SubClass();
        System.out.println(nc.book);//输出6，对象的实例变量不具备多态性
        nc.test();//输出Son's method covers father's ，执行的是SubClass中重写的方法
        nc.base();//Father loves you，subclass继承的方法
        //nc.sub();  会报错，因为BaseClass 不具备sub方法
    }
}

这个例子中 nc 的编译时类型是 BaseClass ， 运行类型是 SubClass，在调用 nc 的方法时，总是显现除 SubClass 的行为特征。但是注意，实例变量不具有多态性。

可以这么理解，子类对象建立时其实也创建了一个父类类型的对象，若编译时类型是父类，运行时类型是子类，该变量运行子类和父类拥有的共同的方法依然保持子类的特征（依然使用子类重写的方法），无法使用子类独有的方法，而且只能使用父类的实例变量。
引用变量在编译时，只能调用其编译时类型所具有的方法，但运行时则执行它运行时类型所具有的方法；在访问其包含的实例变量时，系统总是访问它编译时类型所定义的成员变量，而不是运行时类型。

1.2 强制转换

引用类型的转换只能发生在具有继承关系的两个类型之间，如果试图把一个父类的实例转换成子类类型时，则这个对象必须实际上是子类实例才行（运行时类型是子类）。子类转换成父类总是可以成功的。但父类转换成子类不一定。

类型转换之前，最好先通过 instanceof 运算符判断。

if (obj1 instanceof Type2){
    Type2 obj2 = (Type2)obj1;
}

1.3 关于继承和多态的个人理解

1.3.1 继承

在学习继承和多态的过程中，发现了一个现象，当子类声明了一个和父类同名的变量时，子类的实例其实同时拥有了这两个实例变量，只是默认情况下父类的那个变量会被隐藏，接下来用代码测试展示。

class Father {
    int i = 0;

    public int getI() {
        return i;
    }

    public void setI(int i) {
        this.i = i;
    }
    
    public void print() {
        System.out.println("i:" + i);
    }
}

public class Son extends Father {
    int i = 1;
    
    public void print() {
        System.out.println(i);
    }
    
    public static void main(String[] args) {
        Son son = new Son();
        son.setI(2);
        son.print();
        System.out.println(son.i);
        System.out.println(son.getSuper());
        son.superPrint();
    }
    
    public int getSuper() {
        return super.i;
    }
    public void superPrint() {
        super.print();
    }
}

在这个程序里，父类声明 i 时赋值为 0，子类也声明了一个 i 赋初值为1，子类重写了父类的 print（）方法，为表示区分，父类的print（）方法加了字符串“i:”。

1. main 方法中，新建了一个子类对象后，从子类对象调用继承来的 setter 方法改变 i 的值为 2，接着调用子类重写的 print（）方法。打印的值依然为 “1”；
2. 用常规的 println（）方法打印 son.i 的值，也是“1”；
3. 用子类的 getSuper（） 方法获得父类的 i 的值，并将其打印，打印的值为 “2”；
4. 用子类的 superPrint（）方法，调用父类的 print（）方法，打印值为“i: 2”；

由我之前写的一批文章中可知，当子类重写了父类的方法后，父类的方法其实只是被覆盖了，可以用 super 限定调用，其实实例变量也是一样的。

另外值得提出的是，系统在执行方法时，查找某一变量的顺序：

1. 查找该方法中是否有名为 a 的局部变量；
2. 查找当前类中是否包含名为 a 的成员变量；
3. 查找 a 的直接父类中是否有名为 a 的成员变量，依次上溯到 a 的所有父类，这种查找只会往继承树的上方找。

所以我们可以解释上述程序的现象了。当我们调用 setter 方法时，setter 在其当前类也就是父类中找到了成员变量 i ，于是改变其值为 2；调用 son 的 print（）方法时，在其当前类也就是子类找到了 成员变量 i ，所以打印值为 1；用常规方法打印 son.i 时，父类的 i 已经被覆盖，所以打印值也是 1；用 getSuper（）方法获得父类的 i 并将其打印发现，其值确实被修改成了 2；用 superPrint（）方法调用父类的 print（）方法，该方法在当前类找到了变量 i，于是将其打印，结果为：“i: 2”。

在继承的学习中，我们知道，当我们创建一个子类实例时，系统会从继承树的顶端开始，依次往下初始化类，接着创建各个类的实例。根据上文的例子，我的理解是，与其说继承后子类“拥有”了父类的属性和方法，倒不如说是“借用”。当我们由子类实例调用父类的方法或访问父类的成员变量（子类中没有重写的方法\没有再次声明的成员变量），系统在子类下没有找到同名的方法或变量，于是由下往上在其父类中寻找并调用\访问，这就造成了子类“拥有”了父类的成员变量及方法的假象。一旦子类中有同名的成员，父类成员就被覆盖。

1.3.2 多态

利用上面继承所说的“覆盖”的特性，也可以解释一些多态的现象。

class Father{
    int i = 1;
    public void print(){
        System.out.println("Father");
    }
    public void printNum(){
        System.out.println(i);
    }
}
public class Son extends Father{
    int i = 2;

    public void print(){
        System.out.println("Son");
    }
    public void printNum(){
        System.out.println(i);
    }
    public static void main(String[] args) {
        Father val = new Son();
        val.print();
        System.out.println(val.i);
        val.printNum();
    }
}

程序的输出是：

第一条和第二条我们由上文“多态性”中不难理解，总结一句话就是：“方法有多态性，实例变量没有多态性”，寻找变量时系统总是由编译时类型出发，而寻找方法时系统总是由运行时类型出发。而第三条输出结果，我们就可以借由上文所说的“隐藏”特性以及“多态性”一同理解：当 val 调用 printNum（）方法时，系统由其运行时类型出发，找到了子类的重写的 printNum（）方法，该方法又由当前类（子类）出发寻找变量 i ，于是找到了被隐藏了的值为 2 的变量 i 。

2 LeetCode

38.报数

报数序列是一个整数序列，按照其中的整数的顺序进行报数，得到下一个数。其前五项如下：

1.     1
2.     11
3.     21
4.     1211
5.     111221

1 被读作 "one 1" ("一个一") , 即 11。
11 被读作 "two 1s" ("两个一"）, 即 21。
21 被读作 "one 2", "one 1" （"一个二" , "一个一") , 即 1211。

给定一个正整数 n（1 ≤ n ≤ 30），输出报数序列的第 n 项。

注意：整数顺序将表示为一个字符串。

示例 1:

输入: 1
输出: "1"

示例 2:

输入: 4
输出: "1211"

题目比较晦涩难懂。。。（外国人的脑洞）反正就是数组第一项是1，之后每一项都是把前一项像报数一样念出来，再将念出来的数变成数字序列。（好像我也没说清楚。。）

我的回答是：

class Solution {
    public String countAndSay(int n) {
        String[] array = new String[n];
        array[0] = "1";
        StringBuilder tmp = new StringBuilder();
        int count = 1;
        for(int i = 0; i < n - 1; i++){
            for(int j = 0; j < array[i].length(); j++){
                if(j < array[i].length() - 1 && array[i].charAt(j) == array[i].charAt(j + 1)){
                    count++;
                    continue;
                }
                tmp.append(count);
                tmp.append(array[i].charAt(j));
                count = 1;
            }
            array[i + 1] = tmp.toString();
            tmp.delete(0, tmp.length());
        }
        return array[n-1];       
    }
}

另附评论找到的最快方法（手动滑稽），其实是方便大家理解题意：

class Solution {
    public String countAndSay(int n) {
        switch(n){
            case 1:
                return "1";
            case 2:
                return "11";
            case 3:
                return "21";
            case 4:
                return "1211";
            case 5:
                return "111221";
            case 6:
                return "312211";
            case 7:
                return "13112221";
            case 8:
                return "1113213211";
            case 9:
                return "31131211131221";
            case 10:
                return "13211311123113112211";
            case 11:
                return "11131221133112132113212221";
            case 12:
                return "3113112221232112111312211312113211";
            case 13:
                return "1321132132111213122112311311222113111221131221";
            case 14:
                return "11131221131211131231121113112221121321132132211331222113112211";
            case 15:
                return "311311222113111231131112132112311321322112111312211312111322212311322113212221";
            case 16:
                return "132113213221133112132113311211131221121321131211132221123113112221131112311332111213211322211312113211";
            case 17:
                return "11131221131211132221232112111312212321123113112221121113122113111231133221121321132132211331121321231231121113122113322113111221131221";
            case 18:
                return "31131122211311123113321112131221123113112211121312211213211321322112311311222113311213212322211211131221131211132221232112111312111213111213211231131122212322211331222113112211";
            case 19:
                return "1321132132211331121321231231121113112221121321132122311211131122211211131221131211132221121321132132212321121113121112133221123113112221131112311332111213122112311311123112111331121113122112132113213211121332212311322113212221";
            case 20:
                return "11131221131211132221232112111312111213111213211231132132211211131221131211221321123113213221123113112221131112311332211211131221131211132211121312211231131112311211232221121321132132211331121321231231121113112221121321133112132112312321123113112221121113122113121113123112112322111213211322211312113211";
            case 21:
                return "311311222113111231133211121312211231131112311211133112111312211213211312111322211231131122211311122122111312211213211312111322211213211321322113311213212322211231131122211311123113223112111311222112132113311213211221121332211211131221131211132221232112111312111213111213211231132132211211131221232112111312211213111213122112132113213221123113112221131112311311121321122112132231121113122113322113111221131221";
            case 22:
                return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
            case 23:
                return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
            case 24:
                return "3113112221131112311332111213122112311311123112111331121113122112132113121113222112311311221112131221123113112221121113311211131122211211131221131211132221121321132132212321121113121112133221123113112221131112311332111213213211221113122113121113222112132113213221232112111312111213322112132113213221133112132123123112111311222112132113311213211221121332211231131122211311123113321112131221123113112221132231131122211211131221131112311332211213211321223112111311222112132113212221132221222112112322211211131221131211132221232112111312111213111213211231132132211211131221232112111312211213111213122112132113213221123113112221133112132123222112111312211312112213211231132132211211131221131211132221121311121312211213211312111322211213211321322113311213212322211231131122211311123113321112131221123113112211121312211213211321222113222112132113223113112221121113122113121113123112112322111213211322211312113211";
            case 25:
                return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
            case 26:
                return "1113122113121113222123211211131211121311121321123113213221121113122123211211131221121311121312211213211321322112311311222113311213212322211211131221131211221321123113213221121113122113121113222112131112131221121321131211132221121321132132211331121321232221123113112221131112311322311211131122211213211331121321122112133221121113122113121113222123211211131211121311121321123113111231131122112213211321322113311213212322211231131122211311123113223112111311222112132113311213211221121332211231131122211311123113321112131221123113111231121113311211131221121321131211132221123113112211121312211231131122113221122112133221121113122113121113222123211211131211121311121321123113213221121113122113121113222113221113122113121113222112132113213221232112111312111213322112311311222113111221221113122112132113121113222112311311222113111221132221231221132221222112112322211213211321322113311213212312311211131122211213211331121321123123211231131122211211131221131112311332211213211321223112111311222112132113213221123123211231132132211231131122211311123113322112111312211312111322111213122112311311123112112322211213211321322113312211223113112221121113122113111231133221121321132132211331121321232221123123211231132132211231131122211331121321232221123113112221131112311332111213122112311311123112112322211211131221131211132221232112111312111213111213211231132132211211131221131211221321123113213221123113112221131112211322212322211231131122211322111312211312111322211213211321322113311213211331121113122122211211132213211231131122212322211331222113112211";
            case 27:
                return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
            case 28:
                return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
            case 29:
                return "11131221131211132221232112111312111213111213211231132132211211131221232112111312211213111213122112132113213221123113112221133112132123222112111312211312112213211231132132211211131221131211132221121311121312211213211312111322211213211321322113311213212322211231131122211311123113223112111311222112132113311213211221121332211211131221131211132221231122212213211321322112311311222113311213212322211211131221131211132221232112111312111213322112131112131221121321131211132221121321132132212321121113121112133221121321132132211331121321231231121113112221121321133112132112211213322112311311222113111231133211121312211231131122211322311311222112111312211311123113322112132113212231121113112221121321132122211322212221121123222112111312211312111322212321121113121112131112132112311321322112111312212321121113122112131112131221121321132132211231131122111213122112311311222113111221131221221321132132211331121321231231121113112221121321133112132112211213322112311311222113111231133211121312211231131122211322311311222112111312211311123113322112132113212231121113112221121321132122211322212221121123222112311311222113111231133211121312211231131112311211133112111312211213211312111322211231131122111213122112311311222112111331121113112221121113122113121113222112132113213221232112111312111213322112311311222113111221221113122112132113121113222112311311222113111221132221231221132221222112112322211211131221131211132221232112111312111213111213211231132132211211131221232112111312211213111213122112132113213221123113112221133112132123222112111312211312111322212321121113121112133221132211131221131211132221232112111312111213322112132113213221133112132113221321123113213221121113122123211211131221222112112322211231131122211311123113321112132132112211131221131211132221121321132132212321121113121112133221123113112221131112311332111213211322111213111213211231131211132211121311222113321132211221121332211213211321322113311213212312311211131122211213211331121321123123211231131122211211131221131112311332211213211321223112111311222112132113213221123123211231132132211231131122211311123113322112111312211312111322111213122112311311123112112322211213211321322113312211223113112221121113122113111231133221121321132132211331121321232221123123211231132132211231131122211331121321232221123113112221131112311332111213122112311311123112112322211211131221131211132221232112111312211322111312211213211312111322211231131122111213122112311311221132211221121332211213211321322113311213212312311211131211131221223113112221131112311332211211131221131211132211121312211231131112311211232221121321132132211331121321231231121113112221121321133112132112211213322112312321123113213221123113112221133112132123222112311311222113111231132231121113112221121321133112132112211213322112311311222113111231133211121312211231131112311211133112111312211213211312111322211231131122111213122112311311221132211221121332211211131221131211132221232112111312111213111213211231132132211211131221232112111312211213111213122112132113213221123113112221133112132123222112111312211312111322212311222122132113213221123113112221133112132123222112311311222113111231133211121321132211121311121321122112133221123113112221131112311332211322111312211312111322212321121113121112133221121321132132211331121321231231121113112221121321132122311211131122211211131221131211322113322112111312211322132113213221123113112221131112311311121321122112132231121113122113322113111221131221";            
            case 30:
                return "3113112221131112311332111213122112311311123112111331121113122112132113121113222112311311221112131221123113112221121113311211131122211211131221131211132221121321132132212321121113121112133221123113112221131112212211131221121321131211132221123113112221131112311332211211133112111311222112111312211311123113322112111312211312111322212321121113121112133221121321132132211331121321132213211231132132211211131221232112111312212221121123222112311311222113111231133211121321321122111312211312111322211213211321322123211211131211121332211231131122211311123113321112131221123113111231121123222112111331121113112221121113122113111231133221121113122113121113221112131221123113111231121123222112111312211312111322212321121113121112131112132112311321322112111312212321121113122122211211232221121321132132211331121321231231121113112221121321132132211322132113213221123113112221133112132123222112111312211312112213211231132132211211131221131211322113321132211221121332211231131122211311123113321112131221123113111231121113311211131221121321131211132221123113112211121312211231131122211211133112111311222112111312211312111322211213211321223112111311222112132113213221133122211311221122111312211312111322212321121113121112131112132112311321322112111312212321121113122122211211232221121321132132211331121321231231121113112221121321132132211322132113213221123113112221133112132123222112111312211312112213211231132132211211131221131211322113321132211221121332211213211321322113311213212312311211131122211213211331121321123123211231131122211211131221131112311332211213211321223112111311222112132113213221123123211231132132211231131122211311123113322112111312211312111322111213122112311311123112112322211213211321322113312211223113112221121113122113111231133221121321132132211331222113321112131122211332113221122112133221123113112221131112311332111213122112311311123112111331121113122112132113121113222112311311221112131221123113112221121113311211131122211211131221131211132221121321132132212321121113121112133221123113112221131112311332111213122112311311123112112322211322311311222113111231133211121312211231131112311211232221121113122113121113222123211211131221132211131221121321131211132221123113112211121312211231131122113221122112133221121321132132211331121321231231121113121113122122311311222113111231133221121113122113121113221112131221123113111231121123222112132113213221133112132123123112111312211322311211133112111312211213211311123113223112111321322123122113222122211211232221121113122113121113222123211211131211121311121321123113213221121113122123211211131221121311121312211213211321322112311311222113311213212322211211131221131211221321123113213221121113122113121113222112131112131221121321131211132221121321132132211331121321232221123113112221131112311322311211131122211213211331121321122112133221121113122113121113222123112221221321132132211231131122211331121321232221121113122113121113222123211211131211121332211213111213122112132113121113222112132113213221232112111312111213322112132113213221133112132123123112111311222112132113311213211221121332211231131122211311123113321112131221123113112221132231131122211211131221131112311332211213211321223112111311222112132113212221132221222112112322211211131221131211132221232112111312111213111213211231131112311311221122132113213221133112132123222112311311222113111231132231121113112221121321133112132112211213322112111312211312111322212321121113121112131112132112311321322112111312212321121113122122211211232221121311121312211213211312111322211213211321322123211211131211121332211213211321322113311213211322132112311321322112111312212321121113122122211211232221121321132132211331121321231231121113112221121321133112132112312321123113112221121113122113111231133221121321132122311211131122211213211321222113222122211211232221123113112221131112311332111213122112311311123112111331121113122112132113121113222112311311221112131221123113112221121113311211131122211211131221131211132221121321132132212321121113121112133221123113112221131112311332111213213211221113122113121113222112132113213221232112111312111213322112132113213221133112132123123112111312211322311211133112111312212221121123222112132113213221133112132123222113223113112221131112311332111213122112311311123112112322211211131221131211132221232112111312111213111213211231132132211211131221131211221321123113213221123113112221131112211322212322211231131122211322111312211312111322211213211321322113311213211331121113122122211211132213211231131122212322211331222113112211";
        }
    }
}