Java类有两个具有相同函数签名但返回类型不同的方法

Question

AFAIK it's not possible to have a method with the same call signature. However:

$ javap -public java.time.LocalTime  | grep "minus" | grep "Temporal" | grep -v "long"
    public java.time.LocalTime minus(java.time.temporal.TemporalAmount);
    public java.time.temporal.Temporal minus(java.time.temporal.TemporalAmount);

These clearly show multiple methods with the same call signature.

1. How does Java resolve the function call?
2. Why are there multiple functions?

EDIT: Simplified the question by keeping only the relevant bit.

Answer 1

This is due to how Java implements covariant return types. java.time.LocalTime has a minus method with signature

LocalTime minus(TemporalAmount amountToSubtract)

but this method implements an interface method from java.time.temporal.Temporal with signature

Temporal minus(TemporalAmount amount)

This is permitted due to covariant return typing, but due to the way method lookup works, a lookup at runtime for the method that returns a Temporal won't find the method that returns a LocalTime . Thus, the compiler creates an ordinarily-forbidden method with the same signature, but returning a Temporal . This method calls the version that returns a LocalTime . At runtime, calls that want a Temporal return type find the bridge method, and everything works out.

This bridge method is normally invisible, but it shows up in the javap output, leading to your current confusion.

Source: http://www.artima.com/weblogs/viewpost.jsp?thread=354443

Here's the javap -c disassembly for one of the bridge methods from StringBuilder , showing how it calls the method with the same signature, but a more specific return type:

  public java.lang.Appendable append(java.lang.CharSequence) throws java.io.IOException;
    Code:
       0: aload_0       
       1: aload_1       
       2: invokevirtual #6                  // Method append:(Ljava/lang/CharSequence;)Ljava/lang/StringBuilder;
       5: areturn       

Answer 2

Java forbids two methods in the same class or interface with the same name and signature .

However, two different classes, interfaces, or enums can have the same method signature; for example, this is valid:

public java.time.LocalTime minus(long, java.time.temporal.TemporalUnit);
public java.time.temporal.Temporal minus(long, java.time.temporal.TemporalUnit);

One minus method belongs to the LocalTime class, and the other belongs to the Temporal interface. Since LocalTime implements Temporal , there must be a signature match between those two or the contract is not fulfilled, which would lead to a compilation error.

Answer 3

Temporal是由LocalTime实现的接口，因此如果要检查尝试，它们的方法具有相同的签名

javap java.time.LocalTime