// exceptions/CleanupIdiom.java

// (c)2016 MindView LLC: see Copyright.txt

// We make no guarantees that this code is fit for any purpose.

// Visit http://mindviewinc.com/Books/OnJava/ for more book information.

// Disposable objects must be followed by a try-finally

class NeedsCleanup { // Construction can't fail

private static long counter = 1;

private final long id = counter++;

public void dispose() {

System.out.println(

"NeedsCleanup " + id + " disposed");

}

}

class ConstructionException extends Exception {}

class NeedsCleanup2 extends NeedsCleanup {

// Construction can fail:

public NeedsCleanup2() throws ConstructionException {}

}

public class CleanupIdiom {

public static void main(String[] args) {

// (1):

NeedsCleanup nc1 = new NeedsCleanup();

try {

// ...

} finally {

nc1.dispose();

}

// (2):

// If construction cannot fail you can group objects:

NeedsCleanup nc2 = new NeedsCleanup();

NeedsCleanup nc3 = new NeedsCleanup();

try {

// ...

} finally {

nc3.dispose(); // Reverse order of construction

nc2.dispose();

}

// (3):

// If construction can fail you must guard each one:

try {

NeedsCleanup2 nc4 = new NeedsCleanup2();

try {

NeedsCleanup2 nc5 = new NeedsCleanup2();

try {

// ...

} finally {

nc5.dispose();

}

} catch(ConstructionException e) { // nc5 const.

System.out.println(e);

} finally {

nc4.dispose();

}

} catch(ConstructionException e) { // nc4 const.

System.out.println(e);

}

}

}

/* Output:

NeedsCleanup 1 disposed

NeedsCleanup 3 disposed

NeedsCleanup 2 disposed

NeedsCleanup 5 disposed

NeedsCleanup 4 disposed

*/