1 多态 & 虚方法调用

public class TestVirtualInvoke {
    void doStuff(Shape s) {
        s.draw();
    }
    public static void main(String[] args) {
         Circle c = new Circle();
        Triangle t = new Triangle();
        Line l = new Line();
        doStuff(c);
        doStuff(t);
        doStuff(l);
    }
}
class Shape {
     void draw() { System.out.println("Shape Drawing"); }   
}
class Circle extends Shape {
     void draw() { System.out.println("Draw Circle"); }   
}
class Triangle extends Shape {
     void draw() { System.out.println("Draw Three Lines"); }   
}
class Line extends Shape {
     void draw() { System.out.println("Draw Line"); }   
}
// 输出结果
// Draw Circle
// Draw Three Lines
// Draw Line

Java 中的 **instanceof** 关键词可以判断改对象是不是某个对象的实例：

Object[] things = new Object[3];
things[0] = new Integer(4);
things[1] = new Double(3.14);
things[2] = new String("2.09");
double res = 0;
for (int i = 0; i < things.length; ++i) {
     if (things[i] instanceof Integer)
        res += things[i].intValue();
    else if (things[i] instanceof Double)
        res += thing[i].doubleValue();
}

2 非虚方法

class JavaP3Methods {
     void fun();
    private void p() {}
    static void s() {}
    public static void main(String[] args) {
         JavaP3Methods obj = new JavaP3Methods();
        obj.fun();
        obj.p();
        obj.s();
    }
}

其汇编代码为：

invokevirtual #4
invokespecial #5
invokestatic #7

2.1 三种非虚的方法