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case expressions must be constant expressions  

2013-03-11 20:41:36|  分类: android |  标签: |举报 |字号 订阅

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Non-constant Fields in Case Labels

In a regular Android project, constants in the resource R class are declared like this:
public static final int main=0x7f030004;

However, as of ADT 14, in a library project, they will be declared like this:
public static int main=0x7f030004;

In other words, the constants are not final in a library project. The reason for this is simple: When multiple library projects are combined, the actual values of the fields (which must be unique) could collide. Before ADT 14, all fields were final, so as a result, all libraries had to have all their resources and associated Java code recompiled along with the main project whenever they were used. This was bad for performance, since it made builds very slow. It also prevented distributing library projects that didn't include the source code, limiting the usage scope of library projects.

The reason the fields are no longer final is that it means that the library jars can be compiled once and reused directly in other projects. As well as allowing distributing binary version of library projects (coming in r15), this makes for much faster builds.

However, it has one impact on the source code of the library. Code of the following form will no longer compile:

int id = view.getId();
switch (id) {
    case R.id.button1:
        action1();
        break;
    case R.id.button2:
        action2();
        break;
    case R.id.button3:
        action3();
        break;
}

That's because the switch statement requires all the case labels, such as R.id.button1, to be constant at compile time (such that the values can be directlycopied into the .class files).

The solution for this is simple: Convert the switch statement into an if-else statement. Fortunately, this is very easy in Eclipse. Just place the caret on the switch keyword, and press Ctrl-1 (or Cmd-1 on Mac):
case expressions must be constant expressions - 姑射道人 - 姑射道人的博客
In the above scenario, it will turn the switch statement into this:
int id = view.getId();
if (id == R.id.button1) {
    action1();
} else if (id == R.id.button2) {
    action2();
} else if (id == R.id.button3) {
    action3();
}

This is typically in UI code and the performance impact is negligible.

We have a detector which finds these errors (non-constant case labels referencing an R field) and provides a brief explanation of the problem (and points to this page for more information.)

P.S. If your switch statement looks like this:

switch (view.getId()) {

then you end up with an inefficient if/else chain where each if check repeats the view.getId() call. Just extract this expression first (using the "Extract Local Variable" refactoring keystroke), then convert the switch statement.
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