1.两个类之前调用

例子来源：http://docs.oracle.com/javase/tutorial/java/concepts/class.html

Bicycle.java文件

class Bicycle{
    int cadence = 0;
    int speed = 0;
    int gear = 1;
    
    void changeCadence(int newValue){
        cadence = newValue;
    }
    
    void changeGear(int newValue){
        gear = newValue;
    }
    
    void speedUp(int increment){
        speed = speed + increment;
    }
    
    void applyBrakes(int decrement){
        speed = speed - decrement;
    }
    
    void printStates(){
        System.out.println("cadence:" + cadence + " speed:" + speed + " gear:" + gear);
    }
}

BicycleDemo.java

class BicycleDemo{
    public static void main(String[] args){
        Bicycle bike1 = new Bicycle();
        Bicycle bike2 = new Bicycle();
        
        bike1.changeCadence(50);
        bike1.speedUp(10);
        bike1.changeGear(2);
        bike1.printStates();
        
        
        bike2.changeCadence(50);
        bike2.speedUp(10);
        bike2.changeGear(2);
        bike2.changeCadence(40);
        bike2.speedUp(10);
        bike2.changeGear(3);
        bike2.printStates();
    }
}

控制台操作方法：

javac Bicycle.java              //编译Bicycle.java文件为字节码，生成Bicycle.class文件

javac BicycleDemo.java     //编译BicycleDemo.java文件为字节码，生成BicycleDemo.class文件

java BicycleDemo                      //运行BicycleDemo

F:\Code\java>java BicycleDemo
cadence:50 speed:10 gear:2
cadence:40 speed:20 gear:3