import java.util.HashMap;
import java.util.Map;
public class fibMemo {
  public static void main(String[] args) {
    Map<Integer, Integer> map = new HashMap<Integer, Integer>();
    System.out.println(fibMemo(10, map));
    map = new HashMap<Integer, Integer>();
    System.out.println(fibMemo(20, map));
  }
  public static int fibMemo(int n, Map<Integer, Integer> map) {
    return n;
  }
}
/*prints
10
20
*/

编译多个Java文件

//方法一
javac TransportBike.java
javac TransportCar.java
javac TransportationDriver.java
//方法二
javac TransportBike.java TransportCar.java TransportationDriver.java
//方法三
javac Transport*.java
//方法四  只编译有main方法的文件
javac TransportationDriver.java
//运行
java TransportationDriver

外部文件传递参数

//方法一
//TransportationDriver文件：
import java.io.*;
public class TransportationDriver {
  public static void main(String[] args) {
    Bike harley = new Bike();
    Car toyota = new Car();
    harley.wheels();
    toyota.wheels();
    System.out.println(args[0] + " and " + args[1] + " want to purchase a bike.");
    System.out.println(args[1] + " wants to purchase a car.");
  }
}
java TransportationDriver Customer1 Customer2
//方法二
import java.io.*;
import java.util.Scanner;
public class TransportationDriver {
  public static void main(String[] args) {
    Bike harley = new Bike();
    Car toyota = new Car();
    String fileName = args[0];
    Scanner input = new Scanner(fileName);
    String person1 = input.next();
    String person2 = input.next();
    harley.wheels();
    toyota.wheels();
    System.out.println(person1 + " and " + person2 + " want to purchase a bike.");
    System.out.println(person2 + " wants to purchase a car.");
  }
}
//input.txt 文件里放Customer1 Customer2
java TranportationDriver input.txt
//打印结果
/*A bike has two wheels.
  A car has four wheels.
  Customer1 and Customer2 want to purchase a bike.
  Customer2 wants to purchase a car.
*/

序列化

序列化接口用于储存文件中，内存中，数据库中对象的状态 (成员字段)

import java.io.Serializable;
public class Car implements Serializable{
  private String make;
  private int year;
//provide the JVM with a unique identifier when converting a serialized stream of bytes back into an object with a serialVersionUID
  private static final long serialVersionUID=1L;
}

FileOutputStream，它将帮助我们写入文件
ObjectOutputStream，它将帮助我们将可序列化对象写入输出流
在main()中初始化FileOutputStream对象，该对象将创建一个字节流，并将其写入名为person-file.txt的文件。
在main()中初始化ObjectOutputStream对象，这将帮助将对象序列化到指定的输出流。
在main()中使用objectOutputStream.writeObject()将michael和peter对象序列化到一个文件。

public class Person implements Serializable {
  private String name;
  private int age;
  private static final long serialVersionUID = 1L; 
  public Person(String name, int age) {
    this.name = name;
    this.age = age;
  }  
  public static void main(String[] args) throws FileNotFoundException, IOException{
    Person michael = new Person("Michael", 26);
    Person peter = new Person("Peter", 37);
      //将创建一个字节流，并将其写入名为person-file.txt的文件
    FileOutputStream fileOutputStream = new FileOutputStream("persons.txt");
      //将对象序列化到指定的输出流
    ObjectOutputStream objectOutputStream = new ObjectOutputStream(fileOutputStream);
      //将michael和peter对象序列化到一个文件。
    objectOutputStream.writeObject(michael);
    objectOutputStream.writeObject(peter);    
  }
} 
--------------------------------------------------------------------
import java.io.Serializable;
import java.io.FileOutputStream;
import java.io.ObjectOutputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
public class Car implements Serializable {
  private String make;
  private int year;
  private static final long serialVersionUID = 1L;
  public Car(String make, int year){
    this.make=make;
    this.year=year;
  }
  public static void main(String[] args) throws FileNotFoundException, IOException {
    Car toyota=new Car("Toyota",2021);
    Car honda=new Car("Honda",2020);
    FileOutputStream fileOutputStream = new FileOutputStream("cars.txt");
    ObjectOutputStream objectOutputStream = new ObjectOutputStream(fileOutputStream);
    objectOutputStream.writeObject(toyota);
    objectOutputStream.writeObject(honda); 
  }
}