Understanding Method Overloading and Type Promotion in Java

回答：

Method overloading in Java is a feature that allows a class to have more than one method with the same name, but with different parameter lists. The methods can differ in the number of parameters, the types of parameters, or both. This allows for more readable and maintainable code, as you can use the same method name for similar operations that differ in input types or counts.

Type Promotion in Method Overloading

Type promotion comes into play when you call an overloaded method. Java has a set of rules for how it determines which method to invoke when there are multiple candidates. If the arguments passed to the method do not match the parameter types exactly, Java will attempt to promote the arguments to match one of the overloaded methods.

How Type Promotion Works

Widening Primitive Conversion: If a method accepts a parameter of a wider type, Java will automatically promote a narrower type to that wider type. For example, an int can be promoted to a long, float, or double.
Reference Type Promotion: If a method accepts a parameter of a superclass type, an object of a subclass type can be passed to that method.
Varargs: If a method is defined with varargs (variable-length argument lists), it can accept zero or more arguments of a specified type.

Example of Method Overloading with Type Promotion

Here’s a simple example to illustrate method overloading and type promotion:

class OverloadExample {
    
    // Method with int parameter
    void display(int a) {
        System.out.println("Integer: " + a);
    }
    
    // Method with double parameter
    void display(double a) {
        System.out.println("Double: " + a);
    }
    
    // Method with String parameter
    void display(String a) {
        System.out.println("String: " + a);
    }
}

public class Main {
    public static void main(String[] args) {
        OverloadExample obj = new OverloadExample();
        
        obj.display(5);          // Calls display(int a)
        obj.display(5.5);        // Calls display(double a)
        obj.display("Hello");    // Calls display(String a)
        
        // Type promotion example
        byte b = 10;
        obj.display(b);          // Calls display(int a) due to type promotion
    }
}

Explanation of the Example

Method Overloading: The display method is overloaded with three different parameter types: int, double, and String.
Type Promotion: When obj.display(b) is called with a byte argument, Java promotes the byte to an int because int is the closest match that can accommodate the byte value. Therefore, the display(int a) method is invoked.

Conclusion