“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:”
代码
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:
代码
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:”
代码
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:”
代码
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());
}
}
代码
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());
}
}
代码
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:”
代码
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:”
代码
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。”
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