updating references in an expression with a nested assignment

问题

Looking at this example code similar to this question:

public class A {
    public static void main(String args[]){
        A a = new A();
        System.out.println(a.equals((a = null)));
    }
}

This prints false. Why doesn't it fail with a NullPointerException? The assignment has to get processed before the equals method can run, but somehow that doesn't affect the reference that equals is called on until after the whole line is evaluated?

I didn't see where in the Java language spec it describes this, did I miss it somewhere?

回答1:

From JLS:

At run time, method invocation requires five steps. First, a target reference may be computed. Second, the argument expressions are evaluated. Third, the accessibility of the method to be invoked is checked. Fourth, the actual code for the method to be executed is located. Fifth, a new activation frame is created, synchronization is performed if necessary, and control is transferred to the method code.

This shows that in a.equals((a = null)). the following steps occur:

a's reference is computed. This is sometimes called "binding the method call".
a=null is evaluated as part of evaluating arguments. It does not affect step 1.
The accessibility of equals(Object) is checked.
JVM internal code finds the actual bytecode.
Invocation occurs

Clearly, the reference of a is determined before a is nulled, averting an NPE.

回答2:

I believe the section of the JLS you are interested in is

15.12.4. Run-Time Evaluation of Method Invocation

At run time, method invocation requires five steps. First, a target reference may be computed. Second, the argument expressions are evaluated

Thus, the evaluation of the target occurs while a still has a non-null value, THEN a is set to null as the argument to the method invocation. So the equals method is invoked with a this reference to the object formerly known as a, which still exists, with a null argument.

来源：https://stackoverflow.com/questions/23835908/updating-references-in-an-expression-with-a-nested-assignment

标签

java

variable-assignment

expression-evaluation