In Java, all objects are passed to methods ‘by reference’, and there is no way to mark them as immutable. So strictly speaking, they’re all ‘out variables’. This is the cause of a lot of mistakes in Java, where you eg. pass a list to a method, which then mutates it in some way. That will change the original that the caller passed in, which is normally unintended and may break class invariants. So Java tends to have an absurd number of ‘safety copies’ and immutable wrappers of collections.
I’d probably describe the inability to mark things immutable as the main problem with Java. The golden rule of concurrency is that if you share mutable state, you must use an appropriate synchronisation primitive. It’s not easy to mark things immutable (final doesn’t do what const does in C++) and although you can make class internals private if you like, the junior devs at my work will come along and add accessor methods.
tl:dr; yes it does. Passing an AtomicBoolean as a method argument will do as a built-in ‘mutable object that holds a boolean and can be checked by caller’, although it’ll be slower than your own custom object since it does sync you won’t need.
I’m from C# that has the following for your list example:
// If you add an item to this list, it will effect usages outside. You can't reassign it though.publicvoidExample(List<string> exampleParam)
//Full passing by ref, if you re-assign it to a complete new object, outside usages will be effectedpublicvoidExample(ref List<string> exampleParam)
//Output only, this acts as if the method assigned a variable named exampleParam. publicvoidExample(out List<string> exampleParam)
In Java, all objects are passed to methods ‘by reference’, and there is no way to mark them as immutable. So strictly speaking, they’re all ‘out variables’. This is the cause of a lot of mistakes in Java, where you eg. pass a list to a method, which then mutates it in some way. That will change the original that the caller passed in, which is normally unintended and may break class invariants. So Java tends to have an absurd number of ‘safety copies’ and immutable wrappers of collections.
I’d probably describe the inability to mark things immutable as the main problem with Java. The golden rule of concurrency is that if you share mutable state, you must use an appropriate synchronisation primitive. It’s not easy to mark things immutable (
finaldoesn’t do whatconstdoes in C++) and although you can make class internalsprivateif you like, the junior devs at my work will come along and add accessor methods.tl:dr; yes it does. Passing an
AtomicBooleanas a method argument will do as a built-in ‘mutable object that holds a boolean and can be checked by caller’, although it’ll be slower than your own custom object since it does sync you won’t need.I’m from C# that has the following for your list example:
// If you add an item to this list, it will effect usages outside. You can't reassign it though. public void Example(List<string> exampleParam) //Full passing by ref, if you re-assign it to a complete new object, outside usages will be effected public void Example(ref List<string> exampleParam) //Output only, this acts as if the method assigned a variable named exampleParam. public void Example(out List<string> exampleParam)