新增 Java 章节内容：39-隐藏的 this 自引用，包含参数和返回值的详细示例

Author: AndersonHJB

src/.DS_Store

@@ -32,6 +32,292 @@ footer: 长期招收编程一对一学员！微信：Jiabcdefh, <a href="https:/
 - 看代码，学习参数传递；
 - 看代码，学习返回值的传递；
 
+## 2. 代码示例
+
+::: code-tabs
+
+@tab RunLittleSupperMarketAppMainV2
+
+```java
+package com.geekbang;
+
+import com.geekbang.supermarket.LittleSuperMarket;
+import com.geekbang.supermarket.MerchandiseV2;
+
+public class RunLittleSupperMarketAppMainV2 {
+    public static void main(String[] args) {
+        // 创建一个小超市类
+        LittleSuperMarket littleSuperMarket = new LittleSuperMarket();
+        // 依次给超市的名字，地址，停车位赋值
+        littleSuperMarket.superMarketName = "有家小超市";
+        littleSuperMarket.address = "浦东新区世纪大道666号";
+        littleSuperMarket.parkingCount = 100;
+        // 给超市200种商品
+        littleSuperMarket.merchandises = new MerchandiseV2[3];
+        // 统计用的数组
+        littleSuperMarket.merchandiseSold = new int[littleSuperMarket.merchandises.length];
+
+        // 为了使用方便，创建一个商品数组引用，和littleSuperMarket.merchandises指向同一个数组对象
+        MerchandiseV2[] all = littleSuperMarket.merchandises;
+
+        MerchandiseV2 giftNoodle = new MerchandiseV2();
+        giftNoodle.name = "赠品-面条";
+        giftNoodle.count = 2000;
+        giftNoodle.purchasePrice = 5;
+        giftNoodle.soldPrice = 0.05;
+        giftNoodle.id = "GIFT001";
+
+        MerchandiseV2 giftBowl = new MerchandiseV2();
+        giftBowl.name = "赠品-碗";
+        giftBowl.count = 2000;
+        giftBowl.purchasePrice = 8;
+        giftBowl.soldPrice = 0.08;
+        giftBowl.id = "GIFT002";
+
+        // 遍历并给200种商品赋值
+        for (int i = 0; i < all.length; i++) {
+            // 创建并给商品的属性赋值
+            MerchandiseV2 m = new MerchandiseV2();
+            m.name = "商品" + i;
+            m.count = 200;
+            m.purchasePrice = (i + 1) * 100;
+            m.soldPrice = m.purchasePrice * (1 + Math.random());
+            m.id = "ID" + i;
+            m.gift = giftNoodle;
+            // 用创建的商品，给商品数组的第i个引用赋值，all和小超市的商品数组引用指向的是同一个数组对象
+            all[i] = m;
+            m.describe();
+        }
+
+        int index = 0;
+
+        MerchandiseV2 m = littleSuperMarket.merchandises[index];
+
+        int c = 7;
+
+        // --------- 参数 ---------
+
+        // >> TODO 参数的传递，其实就是赋值。左边是形参，右边是括号里的实参。
+        //    TODO 类似于buy(int countToBuy = (c + 2) * 5)
+        // >> TODO 参数本身可以是一个表达式，只要表达式的值类型可以和参数类型匹配就可以
+        double totalCost = m.buy((c + 2) * 5);
+        System.out.println("商品总价为：" + totalCost);
+
+        // >> TODO 对于引用类型，参数同样是一个表达式
+        boolean biggerThan = m.totalValueBiggerThan(littleSuperMarket.merchandises[index + 1]);
+        System.out.println(biggerThan);
+
+        // >> TODO 方法里的代码并不能改变实参的值。
+        //    TODO 方法里使用的参数相当于一个局部变量。使用方法前，会用实参给局部变量赋值。
+        MerchandiseV2 paramRef = littleSuperMarket.merchandises[2];
+        int paramPrime = 7;
+
+        System.out.println("调用前");
+        System.out.println(paramPrime);
+        System.out.println(paramRef);
+
+        m.willOutsideValueChangeIfParameterValueChangeHerePrime(paramPrime);
+        // 效果类似如下代码
+        // 形参实参赋值：int intVal = paramPrime;
+        // 方法里执行：intVal = 99999999;
+        m.willOutsideValueChangeIfParameterValueChangeHereRef(paramRef);
+        // 效果类似如下代码
+        // 形参实参赋值：MerchandiseV2 m2 = paramRef;
+        // 方法里执行：m2 = gift;
+
+        System.out.println("调用后");
+        System.out.println(paramPrime);
+        System.out.println(paramRef);
+
+        // ------ 返回值 -------
+
+        // >> TODO 可以把有返回值的方法，当成一个成员变量，当成一个类型为返回值类型的成员变量
+        //    TODO 关注于返回值并忽略方法执行的部分，使用返回值，就好像在使用一个成员变量
+        double soldPrice = m.getSoldPrice();
+
+        System.out.println(soldPrice);
+        m.describe();
+
+
+        // >> TODO 给返回值赋值，并不会影响用来充当返回值的变量
+
+        System.out.println("调用getGiftAndHowCanOutsideChangeIt和getSoldPrice之前");
+
+        m.describe();
+
+        MerchandiseV2 giftOfM = m.getGiftAndHowCanOutsideChangeIt();
+
+        giftOfM = giftBowl;
+
+        double mSoldPrice = m.getSoldPrice();
+
+        mSoldPrice = 9999999;
+
+        System.out.println("能改变m本身的gift吗？能改变m的soldPrice吗？");
+
+        m.describe();
+    }
+}
+```
+
+@tab MerchandiseV2
+
+```java
+package com.geekbang.supermarket;
+
+public class MerchandiseV2 {
+
+    public String name;
+    public String id;
+    public int count;
+    public double soldPrice;
+    public double purchasePrice;
+
+    // describe方法变了一下
+    public void describe() {
+        System.out.println("商品名字叫做" + name + "，id是" + id + "。 商品售价是" + soldPrice
+            + "。商品进价是" + purchasePrice + "。赠品是" + gift.name + "。价值" + gift.purchasePrice);
+    }
+
+    public double calculateProfit() {
+        double profit = soldPrice - purchasePrice;
+        if (profit <= 0) {
+            return 0;
+        }
+        return profit;
+    }
+
+    // >> TODO 参数是定义在方法名字后面的括号里的
+    // >> TODO 参数定义的规范和变量一样，都是类型名字加标识符，这里的标识符我们叫做参数名。
+    // >> TODO 方法体中的代码可以使用参数
+    // >> TODO 参数的值在调用方法的时候需要给出，有的资料叫做实参（实际参数）
+    // >> TODO 对应的，方法定义这里的参数，叫做形参（形式参数）
+    public double buy(int countToBuy) {
+        if (count < countToBuy) {
+            System.out.println("商品库存不够");
+            return -1;
+        }
+
+        System.out.println("商品单价为" + purchasePrice);
+
+        int fullPriceCount = countToBuy / 2 + countToBuy % 2;
+        int halfPriceCount = countToBuy - fullPriceCount;
+        double totalCost = purchasePrice * fullPriceCount + halfPriceCount * purchasePrice / 2;
+
+        count -= countToBuy;
+
+        return totalCost;
+    }
+
+    // >> TODO 一个方法可以有多个参数，多个参数之间用逗号隔开
+    public double buyAndPrintLeft(int countToBuy, boolean printLeft) {
+        if (count < countToBuy) {
+            System.out.println("商品库存不够");
+            if (printLeft) {
+                System.out.println("商品剩余库存为" + count);
+            }
+            return -1;
+        }
+
+        System.out.println("商品单价为" + purchasePrice);
+
+        int fullPriceCount = countToBuy / 2 + countToBuy % 2;
+        int halfPriceCount = countToBuy - fullPriceCount;
+        double totalCost = purchasePrice * fullPriceCount + halfPriceCount * purchasePrice / 2;
+
+        count -= countToBuy;
+
+        if (printLeft) {
+            System.out.println("商品剩余库存为：" + printLeft);
+        }
+
+        return totalCost;
+    }
+
+    // >> TODO 参数可以是任何类型，包括自定义类型，甚至是自己的类型都没问题
+    public boolean totalValueBiggerThan(MerchandiseV2 merchandiseV2) {
+        return count * purchasePrice > merchandiseV2.purchasePrice * merchandiseV2.count;
+    }
+
+    // >> TODO 参数可以是任何类型，包括自定义类型，甚至是自己的类型都没问题
+    public boolean isTheBiggestTotalValueOne(LittleSuperMarket littleSuperMarket) {
+        double totalValue = count * purchasePrice;
+        for (int i = 0; i < littleSuperMarket.merchandises.length; i++) {
+            MerchandiseV2 m = littleSuperMarket.merchandises[i];
+            double newTotalValue = m.count * m.purchasePrice;
+            if (totalValue < newTotalValue) {
+                // >> TODO 执行到return的时候，方法直接结束，不管是不是在循环中，是在第几层循环中。
+                return false;
+            }
+        }
+        return true;
+    }
+
+    // ---------------------------新增加的内容---------------------------
+
+    public MerchandiseV2 gift;
+
+    public double getSoldPrice() {
+        return soldPrice;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public MerchandiseV2 getGiftAndHowCanOutsideChangeIt() {
+        return gift;
+    }
+
+
+    public void willOutsideValueChangeIfParameterValueChangeHereRef(MerchandiseV2 m2) {
+        m2 = gift;
+    }
+
+    public void willOutsideValueChangeIfParameterValueChangeHerePrime(int intVal) {
+        intVal = 99999999;
+    }
+
+    public void changeToTheSameGift(MerchandiseV2 m2) {
+        m2.gift = gift;
+    }
+
+}
+```
+
+@tab LittleSuperMarket
+
+```java
+package com.geekbang.supermarket;
+
+public class LittleSuperMarket {
+    public String superMarketName;
+    public String address;
+    public int parkingCount;
+    public double incomingSum;
+    public MerchandiseV2[] merchandises;
+    public int[] merchandiseSold;
+
+    public MerchandiseV2 getBiggestProfitMerchandise() {
+        MerchandiseV2 curr = null;
+        for (int i = 0; i < merchandises.length; i++) {
+            MerchandiseV2 m = merchandises[i];
+            // 这个逻辑有问题吗？相同的利润怎么判断？
+            if (curr == null || curr.calculateProfit() < m.calculateProfit()) {
+                curr = m;
+            }
+        }
+        return curr;
+    }
+}
+```
+
+
+
+:::
+
+
+