在if else语句中使用可选的类类型定义变量

Question

I need to change the class / stream type depending on whether a param (opath) is set, but when I declare in an if-else statement, netbeans complains that the variable oos cannot be found.

I don't understand this as the var oos is always set and there's no way that it is ever undefined ??

if(_param.get("opath") != null) {
    FileOutputStream oos = new FileOutputStream(_param.get("opath").toString());
} else {
    OutputStream oos = csocket.getOutputStream();
}

do something with oos...

Answer 1

Change you code to following

OutputStream oos;    
if(_param.get("opath") != null) {
    oos = new FileOutputStream(_param.get("opath").toString());
} else {
    oos = csocket.getOutputStream();
}
//do something with oos

It is just about scope and making the object available to the code where you want to use it

Answer 2

local varibales's scope is limited it's definition block, in this case, the if or else block, so it's unaccessible from outside.

you can move it out:

OutputStream oos;

if(_param.get("opath") != null) {
    oos = new FileOutputStream(_param.get("opath").toString());
} else {
    oos = csocket.getOutputStream();
}

do something with oos...

Answer 3

You wouldn't encounter this issue using the ternary operator which is a " shorthand for an if-then-else statement ":

OutputStream oos = _param.get("opath") != null ?
  new FileOutputStream(_param.get("opath").toString()) :
  csocket.getOutputStream();

In this case oos is declared and initialized at once.

Additionally this allows to define the variable oos even final which isn't possbile using the regular if-else-statement.