When declaring an object in Java:
Runtime rt = Runtime.getRuntime();
Process lsProc = rt.exec("who -q");
InputStream in = lsProc.getInputStream();
Why is lsproc not declared this way -> Process lsproc = new Process?
How can an object lsproc hold the value of another function?
Process lsProc = rt.exec("who -q");
That line alone means,
That means the method exec of Runtime Class returning an instance IsProc of type Process.
Look at the source code of exec method
public Process exec(String[] cmdarray, String[] envp, File dir)
609 throws IOException {
610 return new ProcessBuilder(cmdarray)
611 .environment(envp)
612 .directory(dir)
613 .start();
614 }
It's returning the instance of ProcessBuilder which is of type Process, that means Process is an abstract class and ProcessBuilder is it's concrete class.
Now you might encounter a question that how
Process pro = new ProcessBuilder(..);
is valid, that routes you to read about Polymorphism.
The method rt.exec("who -q") can return a reference to an instance of type Process. It might create that instance itself by calling new Process (..) (or actually, since Process is an abstract class, by creating an instance of a sub-class of Process), or it may retrieve it from elsewhere.
Following the chain of calls of the Runtime.exec(String) method leads to this call :
return new ProcessBuilder(cmdarray)
.environment(envp)
.directory(dir)
.start();
The start method calls ProcessImpl.start which actually creates the Process instance.
Process is an abstract class, so it can't be instantiated. The actual class being instantiated is ProcessImpl.Because it returns a Process. See here #exec(java.lang.String)
