tostring() is implicitly called… how?

问题

In the following code, how is toString() is implicitly called?

class Payload {
    private int weight;
    public Payload (int w) {
        weight = w; 
    }
    public void setWeight(int w) {
        weight = w; 
    }
    public String toString() {
        return Integer.toString(weight); 
    }
}

public class testpayload {
    static void changePayload(Payload p) {
        p.setWeight(420);
    } 
    public static void main(String[] args) {
        Payload p = new Payload(200);
        p.setWeight(1024);
        changePayload(p);
        System.out.println("p is " + p);
    }
}

回答1:

This line:

System.out.println("p is " + p);

uses string concatenation, which is specified in section 15.18.1 of the JLS, starting with:

If only one operand expression is of type String, then string conversion (§5.1.11) is performed on the other operand to produce a string at run time.

Section 5.1.11 has:

Any type may be converted to type String by string conversion.

...

Now only reference values need to be considered:

If the reference is null, it is converted to the string "null" (four ASCII characters n, u, l, l).

Otherwise, the conversion is performed as if by an invocation of the toString method of the referenced object with no arguments; but if the result of invoking the toString method is null, then the string "null" is used instead.

回答2:

You're calling "p is " + p, which effectively is compiled to

new StringBuffer("p is").append(p)

This code calls p.toString() within .append() as p is Object.

Specified by:
http://docs.oracle.com/javase/7/docs/api/java/lang/StringBuffer.html#append(java.lang.Object)

回答3:

This is just a language feature which is available for free. See Concatenating strings section:

Such a concatenation can be a mixture of any objects. For each object that is not a String, its toString() method is called to convert it to a String.

来源：https://stackoverflow.com/questions/24720321/tostring-is-implicitly-called-how

标签

java

tostring

object-to-string