Java中的类类型转换

类型转换是将一种原始数据类型的值评估为另一种类型。在Java中，有两种类型的转换，即向上转换和向下转换，如下所示：

1. 向上转换是在继承树的向上方向上将子类型转换为超类型。这是一个自动过程，在子类对象由超类引用变量引用的情况下，无需为此付出任何努力。人们可以将其与动态多态性联系起来。
   • 隐式转换是指由编译器在没有转换语法的情况下完成的类类型转换。
   • 显式转换是指程序员使用转换语法完成的类类型转换。
2. 向下转换是指子类类型引用父类的对象时的过程称为向下转换。如果直接执行，编译器会报错，因为在运行时抛出 ClassCastException。只有使用 instanceof运算符才能实现 已经向上转换的对象，该对象只能向下转换。

为了执行类类型转换，我们必须遵循以下两条规则：

1. 类必须是“IS-A-关系”
2. 一个对象必须具有它要转换的类的属性。

执行：

(A)向上转型

示例 1

Java

// Importing input output classes
import java.io.*;
 
// Class 1
// Parent class
class Parent
{
 
  // Function
  void show()
  {
 
    // Print message for this class
    System.out.println("Parent show method is called");
  }
} 
 
// Class 2
// Child class
class Child extends Parent
  {
 
    // Overriding existing method of Parent class
    @Override
 
    // Same Function which will override
    // existing Parent class function
    void show()
    {
 
    // Print message for this class
    System.out.println("Child show method is called");
    }
 
  }
 
// Class3
// Main class
class GFG
{
 
    // Main driver method
    public static void main(String[] args)
    {
      // Creating a Parent class object
      // but referencing it to a Child class
       Parent obj = new Child();
       
      // Calling the show() method to execute
       obj.show();
    }
}

输出

Child show method is called

输出说明：这里调用了父类对象，但引用了子类对象。因此，人们可以将其与动态多态性或函数覆盖联系起来。

(B)向下转型

示例 2

Java

// Java Program to illustrate Downcasting
 
// Importing input output classes
import java.io.*;
 
// Class 1
// Parent class
class Vehicles {
}
 
// Class 2
// Child class
class Car extends Vehicles {
    static void method(Vehicles v)
    {
 
        //
        if (v instanceof Car) {
 
            // Downcasting
            Car c = (Car)v;
 
            // Display message
            System.out.println("Downcasting performed");
        }
    }
    // Main driver method
    public static void main(String[] args)
    {
        // Creating an object of Vehicle class
        // and referring it to Car class
        Vehicles v = new Car();
        Car.method(v);
    }
}

输出

Downcasting performed

NOTE : Without perform upcast if we try to downcast , ClassCastException will be thrown.

• It is a runtime exception or unchecked exception.
• It is class, present in java.lang package.
• It can be avoided by using a operator known as ‘instanceof’.

示例 3

Java

// Java Program showing ClassCastException
 
// Importing input output classes
import java.io.*;
 
// Class 1
// Parent class/ Member class
class Member {
 
    // Member variable of this class
    String name;
    long phone;
 
    // Member function of this class
    void chat()
    {
 
        // Print message of Member/ Child class
        System.out.println(
            name + " : chatting in whatsapp group");
    }
}
 
// Class 2
// Child class/ Admin class
class Admin extends Member {
 
    // Member function of this class
    void addUser()
    {
 
        // Print message of Admin/ Parent class
        System.out.println(
            name
            + " : adding a new user in whatsapp group");
    }
}
 
// Class3 - Main class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
 
        // Creating an object Ad
        Member mem = new Admin();
 
        // Upcasting access only general property of
        // superclass
 
        // Custom entry for Member class
        mem.name = "Sneha";
        mem.phone = 9876543210l;
        // Calling function
        mem.chat();
 
        Admin ad = (Admin)mem;
 
        // Downcast to access specific property of subclass
        ad.addUser();
    }
}

输出

Sneha : chatting in whatsapp group
Sneha : adding a new user in whatsapp group