保存 Object 数组

"你好，阿米戈！今天，我们将学习另一个有趣的主题。具体来讲，就是保存和加载（重构）对象。假设我们有一个 Cat 类："

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class Cat
{
 public String name;
 public int age;
 public int weight;
}

假设我们要添加一种便捷机制来保存到文件以及从文件加载。

我们可能会这样做：

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class Cat {
    public String name;
    public int age;
    public int weight;

    public void save(PrintWriter writer) throws Exception {
        writer.println(name);
        writer.println(age);
        writer.println(weight);
        writer.flush();
    }

    public void load(BufferedReader reader) throws Exception {
        name = reader.readLine();
        age = Integer.parseInt(reader.readLine());
        weight = Integer.parseInt(reader.readLine());
    }
}

"哇！这么简单！我们只是写入每个参数的值，每行一个。加载文件时，我们以同样的顺序读取它们。这是一个完美的解决方法。"

"谢谢你，阿米戈。现在，你可以为这组类编写 save 和 load 方法："

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class Cat
{
 public String name;
 public int age;
 public int weight;
}
class Dog
{
 public String name;
 public int age;
}
class Human
{
 public Cat cat;
 public Dog dog;
}

你有一个 Human 对象，它可以包含一个 dog 和一个 cat。

"我有一个解决方法："

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class Cat {
    public String name;
    public int age;
    public int weight;

    public void save(PrintWriter writer) throws Exception {
        writer.println(name);
        writer.println(age);
        writer.println(weight);
        writer.flush();
    }

    public void load(BufferedReader reader) throws Exception {
        name = reader.readLine();
        age = Integer.parseInt(reader.readLine());
        weight = Integer.parseInt(reader.readLine());
    }
}

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class Dog {
    public String name;
    public int age;

    public void save(PrintWriter writer) throws Exception {
        writer.println(name);
        writer.println(age);
        writer.flush();
    }

    public void load(BufferedReader reader) throws Exception {
        name = reader.readLine();
        age = Integer.parseInt(reader.readLine());
    }
}

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public class Human {
    public Cat cat;
    public Dog dog;

    public void save(PrintWriter writer) throws Exception {
        cat.save(writer);
        dog.save(writer);
    }

    public void load(BufferedReader reader) throws Exception {
        cat.load(reader);
        dog.load(reader);
    }
}

"这个一个非常好的解决方法。但是，如果一个 human 只有 dog 而没有 cat，会发生什么呢？"

null 检查在哪里？

"我马上修改："

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public class Human {
    public Cat cat;
    public Dog dog;

    public void save(PrintWriter writer) throws Exception {
        if (cat != null)
            cat.save(writer);
        if (dog != null)
            dog.save(writer);
    }

    public void load(BufferedReader reader) throws Exception {
        cat = new Cat();
        cat.load(reader);
        dog = new Dog();
        dog.load(reader);
    }
}

"还是不太对。你有两处错误："

1) 一个人可能没有 cat 或 dog，但在调用 load 方法时仍会创建它们

2) 如果只保存 dog，则其数据在加载时将被 cat 读取。

"那我应该怎么做？"

"我们不能跳过变量编写，否则在读取过程中会遇到问题。你需要确保在保存操作期间为 null 的变量在加载操作期间被设置为 null。下面是我的版本："

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public class Human {
    public Cat cat;
    public Dog dog;

    public void save(PrintWriter writer) throws Exception {
        String isCatPresent = cat != null ? "yes" : "no";
        writer.println(isCatPresent);
        writer.flush();

        if (cat != null)
            cat.save(writer);

        String isDogPresent = dog != null ? "yes" : "no";
        writer.println(isDogPresent);
        writer.flush();

        if (dog != null)
            dog.save(writer);
    }

    public void load(BufferedReader reader) throws Exception {

        String isCatPresent = reader.readLine();
        if (isCatPresent.equals("yes")) {
            cat = new Cat();
            cat.load(reader);
        }

        String isDogPresent = reader.readLine();
        if (isDogPresent.equals("yes")) {
            dog = new Dog();
            dog.load(reader);
        }
    }
}

"嗯，我喜欢这个解决方法。"

"是的，这个方法很好。"