Java 14 类型信息

14 类型信息

运行是识别对象和类的信息 两种方式
RTTI 假定编译时已经知道所有的类型
反射 运行时发现和使用类的信息

1 RTTI

//多态 创建一个具体的对象(Circle Square Triangle) 向上转型为Shape 忽略对象具体类型
//在后面程序中使用匿名的Shape引用

abstract class Shape {
    void draw() {
        System.out.println(this + ".draw()");
    }

    abstract public String toString();
}

class Circle extends Shape {
    public String toString() {
        return "Circle";
    }
}

class Square extends Shape {
    public String toString() {
        return "Square";
    }
}

class Triangle extends Shape {
    public String toString() {
        return "Triangle";
    }
}

public class Shapes {
    public static void main(String[] args) {
        List<Shape> shapeList = Arrays.asList(new Circle(), new Square(), new Triangle());
        for (Shape shape : shapeList)
            shape.draw();
    }
}

2 Class 对象

class Candy {
    //在第一被加载时执行
    static {
        System.out.println("Loading Candy");
    }
}

class Gum {
    static {
        System.out.println("Loading Gum");
    }
}

class Cookie {
    static {
        System.out.println("Loading Cookie");
    }
}

public class SweetShop {
    public static void main(String[] args) {
        System.out.println("inside main");
        new Candy();
        System.out.println("After creating Candy");
        try {
            Class.forName("Gum");
        } catch (ClassNotFoundException e) {
            System.out.println("Couldn't find Gum");
        }
        System.out.println("After Class.forName(\"Gum\")");
        new Cookie();
        System.out.println("After creating Cookie");
    }
}

interface HasBatteries {
}

interface Waterproof {
}

interface Shoots {
}

class Toy {
    // Comment out the following default constructor
    // to see NoSuchMethodError from (*1*)
    Toy() {
    }

    Toy(int i) {
    }
}

class FancyToy extends Toy implements HasBatteries, Waterproof, Shoots {
    FancyToy() {
        super(1);
    }
}

public class ToyTest {
    static void printInfo(Class cc) {
        System.out.println("Class name: " + cc.getName() + " is interface? [" + cc.isInterface() + "]");
        System.out.println("Simple name: " + cc.getSimpleName());
        System.out.println("Canonical name : " + cc.getCanonicalName());
    }

    public static void main(String[] args) {
        Class c = null;
        try {
            c = Class.forName("FancyToy");
        } catch (ClassNotFoundException e) {
            System.out.println("Can't find FancyToy");
            System.exit(1);
        }
        printInfo(c);
        for (Class face : c.getInterfaces())
            printInfo(face);
        Class up = c.getSuperclass();
        Object obj = null;
        try {
            // Requires default constructor:
            obj = up.newInstance();
        } catch (InstantiationException e) {
            System.out.println("Cannot instantiate");
            System.exit(1);
        } catch (IllegalAccessException e) {
            System.out.println("Cannot access");
            System.exit(1);
        }
        printInfo(obj.getClass());
    }
}

2.1 类字面常量

class Initable {
    static final int staticFinal = 47;
    static final int staticFinal2 = ClassInitialization.rand.nextInt(1000);
    static {
        System.out.println("Initializing Initable");
    }
}

class Initable2 {
    static int staticNonFinal = 147;
    static {
        System.out.println("Initializing Initable2");
    }
}

class Initable3 {
    static int staticNonFinal = 74;
    static {
        System.out.println("Initializing Initable3");
    }
}

public class ClassInitialization {
    public static Random rand = new Random(47);

    public static void main(String[] args) throws Exception {
        Class initable = Initable.class;
        System.out.println("After creating Initable ref");
        // Does not trigger initialization:
        System.out.println(Initable.staticFinal);
        // Does trigger initialization:
        System.out.println(Initable.staticFinal2);
        // Does trigger initialization:
        System.out.println(Initable2.staticNonFinal);
        Class initable3 = Class.forName("Initable3");
        System.out.println("After creating Initable3 ref");
        System.out.println(Initable3.staticNonFinal);
    }
}