第十八课，理解修饰符的作用。修饰符（private、protected、public）是封装的基础，用于控制外部程序对对象内部信息的访问权限。看看这个程序，该类Rectangle有width字段和height字段以及构造函数，在main()方法中创建一个对象。

public class Main
{
	public static void main(String[] args)
	{
		Rectangle rect = new Rectangle(100, 200);
		
	}
}

class Rectangle
{
	int width;
	int height;
	
	Rectangle(int width, int height) 
	{
		this.width = width;
		this.height = height;
	}
	
	int getHeight()
	{
		return this.height;
	}
}

该类声明了方法getHeight()，并返回字段height

public class Main
{
	public static void main(String[] args)
	{
		Rectangle rect = new Rectangle(100, 200);
		
	}
}

class Rectangle
{
	int width;
	int height;
	
	Rectangle(int width, int height) 
	{
		this.width = width;
		this.height = height;
	}
	
	int getHeight()
	{
		return this.height;
	}
}

在main()方法中调用rect的getHeight（）方法并打印结果

public class Main
{
	public static void main(String[] args)
	{
		Rectangle rect = new Rectangle(100, 200);
		System.out.println(rect.getHeight());
	}
}

class Rectangle
{
	int width;
	int height;
	
	Rectangle(int width, int height) 
	{
		this.width = width;
		this.height = height;
	}
	
	int getHeight()
	{
		return this.height;
	}
}

在main()方法中访问rect的字段height并打印结果

public class Main
{
	public static void main(String[] args)
	{
		Rectangle rect = new Rectangle(100, 200);
		System.out.println(rect.getHeight());
        System.out.println(rect.height);
	}
}

class Rectangle
{
	int width;
	int height;
	
	Rectangle(int width, int height) 
	{
		this.width = width;
		this.height = height;
	}
	
	int getHeight()
	{
		return this.height;
	}
}

现在在height字段之前写上关键字private

public class Main
{
	public static void main(String[] args)
	{
		Rectangle rect = new Rectangle(100, 200);
		
		System.out.println(rect.getHeight());
		System.out.println(rect.height);
	}
}

class Rectangle
{
	int width;
	private int height;
	
	Rectangle(int width, int height) 
	{
		this.width = width;
		this.height = height;
	}
	
	int getHeight()
	{
		return this.height;
	}
}

现在显示Main类仍然有height字段的错误，private的修饰符使其无法被外部的类访问，删除height字段的println语句再运行程序

public class Main
{
	public static void main(String[] args)
	{
		Rectangle rect = new Rectangle(100, 200);
		
		System.out.println(rect.getHeight());
	}
}

class Rectangle
{
	int width;
	private int height;
	
	Rectangle(int width, int height) 
	{
		this.width = width;
		this.height = height;
	}
	
	int getHeight()
	{
		return this.height;
	}
}

该height字段被封装在类中，只能在这个类内部被访问

public class Main
{
	public static void main(String[] args)
	{
		Rectangle rect = new Rectangle(100, 200);
		
		System.out.println(rect.getHeight());
	}
}

class Rectangle
{
	int width;
	private int height;
	
	Rectangle(int width, int height) 
	{
		this.width = width;
		this.height = height;
	}
	
	int getHeight()
	{
		return this.height;
	}
}

现在在getHeight()方法之前添加public关键字，表明该方法可以从外部访问

public class Main
{
	public static void main(String[] args)
	{
		Rectangle rect = new Rectangle(100, 200);
		
		System.out.println(rect.getHeight());
	}
}

class Rectangle
{
	int width;
	private int height;
	
	Rectangle(int width, int height) 
	{
		this.width = width;
		this.height = height;
	}
	
	public int getHeight()
	{
		return this.height;
	}
}

标记字段width为private

public class Main
{
	public static void main(String[] args)
	{
		Rectangle rect = new Rectangle(100, 200);
		
		System.out.println(rect.getHeight());
		
	}
}

class Rectangle
{
	private int width;
	private int height;
	
	Rectangle(int width, int height) 
	{
		this.width = width;
		this.height = height;
	}
	
	
	
	public int getHeight()
	{
		return this.height;
	}
}

添加使用public修饰的getWidth()方法，并返回宽度值

public class Main
{
	public static void main(String[] args)
	{
		Rectangle rect = new Rectangle(100, 200);
		
		System.out.println(rect.getHeight());
		
	}
}

class Rectangle
{
	private int width;
	private int height;
	
	Rectangle(int width, int height) 
	{
		this.width = width;
		this.height = height;
	}
	
	public int getWidth(){
        return this.width;
    }
	
	public int getHeight()
	{
		return this.height;
	}
}

在main()方法中调用rect的getWidth()方法并打印结果

public class Main
{
	public static void main(String[] args)
	{
		Rectangle rect = new Rectangle(100, 200);
		
		System.out.println(rect.getHeight());
		System.out.println(rect.getWidth());
	}
}

class Rectangle
{
	private int width;
	private int height;
	
	Rectangle(int width, int height) 
	{
		this.width = width;
		this.height = height;
	}
	
	public int getWidth(){
        return this.width;
    }
	
	public int getHeight()
	{
		return this.height;
	}
}

public class Main
{
	public static void main(String[] args)
	{
		Rectangle rect = new Rectangle(100, 200);
		
		System.out.println(rect.getHeight());
		System.out.println(rect.getWidth());
	}
}

class Rectangle
{
	private int width;
	private int height;
	
	Rectangle(int width, int height) 
	{
		this.width = width;
		this.height = height;
	}
	
	public int getWidth(){
        return this.width;
    }
	
	public int getHeight()
	{
		return this.height;
	}
}

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