2014-12-29

13 totally true facts every C++ programmer should know about Java

Java has a lot of similarity with C++. However the designers of Java set out to create a language that is safer, simpler, and more portable than C++.

Here is a quote from Java's original designer, James Gosling, explaining the difference of language philosophy.
JAVA omits many rarely used, poorly understood, confusing features of C++ that in our experience bring more grief than benefit. This primarily consists of operator overloading (although it does have method overloading), multiple inheritance, and extensive automatic coercions.

Below I've tried to highlight some of the major differences.

0. In Java variables hold either values or pointers to objects

In Java data is held in variables and variables hold either values belonging to primitive types or pointers to objects that are on the heap.

Variables include

  • local variables (including parameters),
  • fields of objects,
  • items of arrays,
  • static fields --these are fields that belong to a class rather than an object--, and
  • temporary variables that hold results from method calls and results of operations such as addition or multiplication.
All these sorts of variables can only hold either a value of a primitive type or a pointer to objects that are on the heap. In particular variables can not hold objects themselves.



By the way, in Java terminology, pointers are called references. So, if you are reading a book on Java, you probably won't see pointers mentioned, but lots of mentions of references. When a Java programmer speaks of a reference what they mean is what a C++ programmer would call a pointer.



Just to make things even more confusing, C++ has something C++ programmers call references, which are significantly different from pointers. As we will see, Java has no equivalent to C++'s references.



For the rest of this note, I'm going to use the word pointer to
mean a value that identifies a particular object: i.e., what a C++
programmer calls a pointer and what a Java programmer calls a reference.

1. In Java objects exist only on the heap and so can only be accessed via pointers.

In C++, as you know, objects can be held in local variables or global variables, they can be returned from subroutines as values, they can be passed to subroutines as parameters. None of these things are possible in Java because in Java objects exist only on the heap. Since objects are on the heap, we can never refer to them directly (just as is true of C++ objects on the heap); they are always accessed via pointers.

2. In Java types are either primitive or reference types

There are exactly 8 primitive types in Java and you can't define any others. They are boolean, char, byte, short, int, long, float, and double.



All other types are reference types, which means their values are pointers to objects and a special value 'null'. You can define new reference types by defining new classes and interfaces.



'What about arrays?' you may wonder. Well arrays are objects in Java, so they are accessed through pointers too. And so arrays can only exist on the heap.

3. There is no dereference operator in Java

It's time to look at some code.

This code does the following: (a) It creates a new object of class 'Car' on the heap. (b) It creates a new local variable 'p' of type 'Vehicle' and initializes it with a pointer to the new object. (c) It sends a message 'start' to the object.

In C++ we would write

In C++ the declaration and the use of 'p' include the derefernce operator  '*'; this is an operator that turns a pointer to an object into an object. You can see that, in Java, there is no dereference operator either in the declaration of 'p' or the use of 'p'. In fact in Java, there is no way to write an expression that simply refers to an object; we always refer to objects using pointers.



The best way to look at it is that the arrow operator '->' of C++ translates to a dot '.' in Java.



Let's extend the example:

Here 'q' is a pointer and it is initialized to the value held in 'p', which is a pointer to the object. Then a message is sent to the object that 'q' points to. This is the same object that 'p' points to. So ... both messages go to the same object.

It's important to understand that when we declare a pointer with like this

or this

we are declaring variables that hold pointers to objects, not variables that hold objects.

4. In Java all methods are virtual

In C++ a method (member function) can be either virtual or nonvirtual. Nonvirtual methods are dispatched based on the static type of the expression representing the recipient object (the type that the compiler know the object to have) whereas virtual methods are dispatched based on the dynamic type, i.e. based on the class of the actual object receiving the message. In general the dynamic type can only be known at run-time. In Java, all methods are dispatched based on the type of object that receives the message.



Let's look at an example. Suppose we have the following classes

If the following code is executed

then the version of 'start' in class 'Car' will be executed.



Here are two examples that illustrate why the compiler can not know the dynamic type of an object. Consider the following (rather artificial) example:

In general the compiler can't know what sort of vehicle 'p' will point to on the last line. The decision as to whether to call the 'start' methods of 'Car' or the 'start' method of 'Bicycle' is made at execution time. The most common situation where this comes up is when polymorphic methods are involved. Consider a method

and the code

When 'p' is passed to 'initialize', the 'start' method of 'Car' is called; when 'q' is passed to 'initialize', the 'start' method of 'Bicycle' is called.




The only exception is static methods. But these aren't really methods at all; they are used to send messages to classes, not to objects.

5. There is no explicit deleting of objects in Java

Java's approach to memory management is very different from that of C++. In Java, once an object is no longer accessible to the running program, it becomes eligible for garbage collection. Garbage collection is the process of finding objects that are no longer usable and making the space they occupy available for new objects. Thus there is no need to delete objects in Java. Once an object is no longer needed, just ensure that it is no longer accessible.



Also Java has no equivalent to C++'s destructors. If you want an object to clean up when it is no longer needed, write a method called 'cleanUp' and call it explicitly.



(There is a method in Java that is a little like a destructor. It is called 'finalize' and is called if and when the object is collected by the garbage collection process. It is almost always a bad idea to override this method; don't do it.)

6. Java has one class hierarchy and it's a tree

In C++ classes can inherit from more (or less) than one base class. In Java every class extends exactly one class; that class is called its direct superclass. There is one exception which is the class called 'java.lang.Object'; this class has no direct superclass. Thus the set of classes in a program forms a tree with 'java.lang.Object' at the root of the tree.



By the way it's not always obvious that a class has a direct superclass. Consider the definitions.

The direct superclass of 'Car' is 'Vehicle', but what is the direct superclass of 'Vehicle'? The default direct superclass is 'java.lang.Object'; so that is the direct superclass of 'Vehicle'.



Since every class inherits from 'java.lang.Object', there are a few methods we can count on every object supporting. For example every object has an 'equals' method, a 'toString' method, and a few others.



Why does Java allow only single inheritance between classes? Mostly it is a matter of simplicity. Multiple inheritance has a number of issues that make it complex to implement and confusing for the programmer.

7. Java allows a limited form of multiple inheritance

As mentioned earlier, no class can have more than one direct superclass. This can be inconvenient. Suppose we want to have a type of objects that can be displayed on the screen. We'll call this type 'Displayable' and all classes that inherit from 'Displayable' have a method called 'display' that takes an argument of type 'Screen'. Thus, if 'displayList' is a list of 'Displayable' objects, we can write code like this:

So far so good. But suppose we want 'Car' objects to be displayable, but not 'Vehicle' objects in general. We need 'Car' to inherit from 'Displayable' and from 'Vehicle'.  We could solve this by having Car extend 'Displayable' and 'Displayable' extend 'Vehicle'. Alas, we also want 'Pedestrian' to inherit from 'Displayable', but not from 'Vehicle'. We need multiple inheritance.



The solution is to use interfaces. An interface is a type very much like a class. But interfaces are restricted; interfaces can not have fields, method implementations, or constructors.  A class can only inherit directly from one class, but it can inherit from any number of interfaces. We can solve our problem like this


8. In Java, parameters are always passed by value and results are always returned by value

As mentioned above, variables can only hold values of primitive type (numbers, booleans, chars) or (possibly null) pointers to objects. Since parameters are just a special kind of variable, the same applies to parameters. The same also applies to the results of methods.



There is no concept in Java that corresponds to C++'s references and so there are no reference parameters or reference results.



In C++, passing an object by value will cause the object to be cloned via its copy constructor. In Java we can't pass object by value; all we can do is pass a pointer to the object and in that case (as in C++) there is no explicit copy of the object made. Here is an example. Suppose we have a method

and we call the method like this

For the duration of the method call the parameter 'v' points to the same object as 'p' does. No copy is made.
This is no different from the following in C++



If we want to get the effect of C++'s pass by copy construction, we would have to make an explicit copy like this

For this to work, class 'Car' needs a constructor that takes a pointer to a 'Car' and constructs a copy.


9. There is no operator overloading in Java

Operators in Java (e.g. +, =, and ==) have fixed meanings. You can't overload them to apply to your own class.



There is no need to overload assignment (=), since, as we've seen, variables can't hold objects anyway. If you really want a copy of an object it should be made explicitly.



Similarly you can't use == to compare objects as the value of its operands can not be objects.  If we have

we are comparing pointers, not objects. In this example, the pointers initially point to different objects and so, unless a pointer has been reassigned, they can't possibly be equal. If we want to compare objects, we can use the 'equals' method inherited from 'java.lang.Object'.

For this to work properly, the 'equals' method should be overridden. By default it will simply compute p==q. If we want to say that two cars are identical if they have the same 'vin' field we would override equals as follows.



A special case to look out for is strings since, in Java, strings are objects. The code

is almost certainly wrong. I should have written



10. In Java, arrays are objects too

As mentioned above, arrays in Java are objects too. This means they are always allocated on the heap and always dealt with via pointers. For example, consider the the following sequence of commands:

Here we declare 'a' to be a local variable of type pointer to an array of pointers to objects of type 'Vehicle'. Read that sentence again to be sure you've got it. Next, an array object is created on the heap. Initially the items of the array all have value 'null', that is, they hold the null pointer. Next 'a' is initialized to point to this new object. On line 3, a local variable 'b' is created and initialized to the same value as 'a', that is it points to the same array as 'a' points to. Line 4 creates two 'Car' objects and assigns pointers to them to the items of the array. The last two lines, print the values of the two 'Car' objects. The values of variables 'a' and 'b' point to the same array object and so 'a[0]' and 'b[0]' refer to exactly the same array item.



Arrays really are objects in Java. They have fields and methods. Well one field anyway. For example, consider:

The second last line reads a field of the array. In the last line a message is sent to the array.



By the way, the items of arrays are initialized when the array is allocated (In C++, the items initially have undefined values.) In the previous example, since 'a' is an array of pointers its items are initialized to 'null'. For number types (byte, short, int, long, float, and double) arrays are initialized to zero. For boolean arrays, items are initialized to 'false'. And for char arrays, items are initialized to the character with unicode encoding 0.


11. In Java, array lengths are not part of the type

In C++ we can have a variable whose type is pointer to array of 10 int items.

Such a variable can only contain the pointers to arrays of size 10. In Java the closest we can get to this declaration is

This declares 'm' to be a pointer to an array of ints, but it doesn't specify the size of the array.


12. Java has no 2-dimensional arrays

In C++ we can declare an array whose items are arrays. For example
declares 'm' to point to arrays that have 10 items, each of which is an array of 5 ints.

Java has no 2-dimensional arrays. Nor does it have 3-dimensional arrays. And so on. This is a natural consequence of the fact that objects must be on the heap, they can't be items of arrays. In Java we can declare
This means that 'm' points to an array of pointers, each of which points to an array of integers.
We can see this by running the following code

This will print 'true'. Since the items of an newly created array of pointers are initialized to 'null'.

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