Java:修改作为引用传递的参数

Question

我必须实现一个具有如下签名的接口(interface)方法:

public int runMethod(final int key, Reference <String> result);

我必须在方法返回之前更新 result 参数的值。例如调用方法时 result 的值为 ABC ，我需要将其修改为 DEF 并返回给调用者。有人可以建议我如何实现它吗？

Answer 1

您不能修改传递给方法的变量。例如:

public int runMethod(final int key, Reference <String> result) {
    result = null; // Only changed the method's version of the variable, and not the variable that was passed to the method
}
...
Reference<String> ref = ...
runMethod(0, ref);
// ref is still what you originally assigned it to

但是，您可以修改字段并调用您传递的对象的方法。

public int runMethod(final int key, Reference <String> result) {
    result.someField = ...; // Here we are changing the object, which is the same object as what was passed to the method.
}
...
Reference<String> ref = ...
runMethod(0, ref);
// ref.someField has now been changed

另一种方法是将方法的返回类型更改为 Reference<String>并返回更新后的值。

public Reference<String> runMethod(final int key, Reference <String> result) {
    return ...;
}
...
Reference<String> ref = ...
ref = runMethod(0, ref);
// ref is now whatever you returned from the method