What are the main differences between PHP and Java that someone proficient in PHP but learning Java should know about?
Edit: I mean differences in the syntax of the languages, i.e their data types, how they handle arrays & reference variables, and so forth :)
I think these two languages (as well as their runtime systems) are too different to list all differences. Some really big ones that come to my head:
You will notice most differences when it's time to, but what's most important:
Not an exhaustive list, and I'm PHP developer who did a tour of Java a while back so Caveat Emptor.
Every variable in Java needs to be prepended with a data type. This includes primitive types such as boolean, int, double and char, as well as Object data-types, such as ArrayList, String, and your own objects
int foo = 36;
char bar = 'b';
double baz = 3.14;
String speech = "We hold these truths ...";
MyWidget widget = new MyWidget(foo,bar,baz,speech);
Every variable can only hold a value of its type. Using the above declarations, the following is not valid
foo = baz
Equality on objects (not on primitive types) checks for object identity. So the following un-intuitively prints false. Strings have an equality method to handle this.
//see comments for more information on what happens
//if you use this syntax to declare your strings
//String v1 = "foo";
//String v2 = "foo";
String v1 = new String("foo");
String v2 = new String("foo");
if(v1 == v2){
pritnln("True");
}
else{
println("False");
}
Arrays are your classic C arrays. Can only hold variables of one particular type, need to be created with a fixed length
To get around this, there's a series of collection Objects, one of which is named ArrayList that will act more like PHP arrays (although the holds one type business is still true). You don't get the array like syntax, all manipulation is done through methods
//creates an array list of strings
ArrayList<String> myArr = new ArrayList<String>();
myArr.add("My First Item");
ArrayLists still have numeric keys. There's another collection called HashMap that will give you a dictionary (or associative array, if you went to school in the 90s) like object.
ArrayLists and other collections are implemented with something called generics (the <String>). I am not a Java programmer, so all I understand about Generics is they describe the type of thing an Object will operate on. There is much more going on there.
Java has no pointers. However, all Objects are actually references, similar to PHP 5, dissimilar to PHP 4. I don't think Java has the (depreciated) PHP &reference &syntax.
All method parameters are passed by value in Java. However, since all Objects are actually references, you're passing the value of the reference when you pass an object. This means if you manipulate an object passed into a method, the manipulations will stick. However, if you try something like this, you won't get the result you expect
public void swapThatWontWork(String v1, String v2)
{
String temp = var1;
var1 = var2;
var2 = temp;
}
It's as good a time as any to mention that methods need to have their return type specified, and bad things will happen if an method returns something it's not supposed to. The following method returns an int
public int fooBarBax(int v1){
}
If a method is going to throw an exception, you have to declare it as such, or the compiler won't have anything to do with it.
public int fooBarBax(int v1) throws SomeException,AnotherException{
...
}
This can get tricky if you're using objects you haven't written in your method that might throw an exception.
You main code entry point in Java will be a method to a class, as opposed to PHPs main global entry point
Variable names in Java do not start with a sigil ($), although I think they can if you want them to
Class names in Java are case sensitive.
Strings are not mutable in Java, so concatenation can be an expensive operation.
The Java Class library provides a mechanism to implement threads. PHP has no such mechanism.
PHP methods (and functions) allow you have optional parameters. In java, you need to define a separate method for each possible list of parameters
public function inPHP($var1, $var2='foo'){}
public void function inJava($var1){
$var2 = "foo";
inJava($var1,$var2);
}
public void function inJava($var1,$var2){
}
PHP requires an explicit $this be used when an object calls its own methods methods. Java (as seen in the above example) does not.
Java programs tend to be built from a "program runs, stays running, processes requests" kind of way, where as PHP applications are built from a "run, handle the request, stop running" kind of way.
=== and !==. Java's implicit type conversion is primarily limited to auto-boxing of primitive types (PHP has no primitive types). This often comes up.Consider:
$val = 'a';
if (strpos('abcdefghij', $val)) {
// do stuff
}
which is incorrect and will have the block not executed because the return index of 0 is converted to false. The correct version is:
$val = 'a';
if (strpos('abcdefghij', $val) !== false) {
// do stuff
}
Java conditional statements require an explicit boolean;
$ and otherwise indistinguishable;Maps (eg HashMap). Associative arrays are ordered on insertion order and can be used like ordinary arrays (on the values). Theres one Map implementation that maintains insertion order in Java but this is the exception rather than the norm;$arr['foo'] = 'bar' insert or update an element in an associative array. Java must use Map.put() and Map.get();create_function()). Java must use anonymous classes for both, which is somewhat more erbose;global keyword in PHP;register_globals()) automatically result in global variables of the same name. They can alternatively be accessed via the $_POST global variable (and $_SESSION for session variables) whereas support for these things comes from a JEE add-on called the servlets API via objects like HttpServletRequest and HttpSession;function keyword whereas in Java you declare return types and parameter types;Compare:
function do_stuff($name = 'Foo') {
// ...
}
to
void doStuff() {
doStuff("Foo");
}
void doStuff(String what) {
// ...
}
-> operator. Java uses the . operator;__construct();this is implicit and only used to be explicit about scope and in certain cases with inner classes. In PHP5, $this is explicit;. operator on an instance (although this is discouraged it is syntactically valid) but generally the class name is used instead.These two are equivalent:
float f = 9.35f;
String s1 = String.valueOf(f);
String s2 = "My name is Earl".valueOf(f);
but the former is preferred. PHP uses the :: scope resolution operator for statics;
. operator to append strings. Java uses +;...) to declare variable length argument lists explicitly. All PHP functions are variable length;func_get_args(), func_get_arg() and/or func_num_args();you could use JavaDoc tool to autogenerate documentation on your software. But you need to write comments in specific way.
you can't run PHP on mobile phones :) There are a lot of run time environments and platforms. That means you need to think in advance which libraries there could be missing or which limitations there could be (screen size, memory limits, file path delimiter "/" or "\" e.g).