More syntactic sugars for strings

The + operator is overloaded to concatenate strings. When + is used with one String operand and one non-String operand, the non-String operand will be sent the toString() message:

    Integer i = new Integer(7);
    System.out.println("i = " + i);

    System.out.println(2 + 3 + " = 1 + 4.");
    System.out.println("Other numbers are " + 5 + " and " + 2.1);

Hey, hold on...

What's tricky about the last two lines in the above code snippet? (They're tricky in different ways.)

First: Addition is left-associative, so in the next-to-last line, 2 + 3 gets evaluated first. So the overloaded operator + is resolved to the addition operation on integers, not the concatenation operation on strings.

Second: You can't send messages to primitive types! So, you can't just say 5.toString(). Therefore, when you try to concatenate a string with a value of primitive type, Java will implicitly create a new instance of the appropriate wrapper classes so that it can send the toString() message to it. So the last line above translates to:

    System.out.println("Other numbers are " 
                       + new Integer(5).toString()
                       + " and "
                       + new Float(2.1));

Arrays

Arrays are a special kind of object with syntactic support (and weird typechecking rules, but let's leave that till next week). Arrays of both primitives and objects are supported:

    int[] intArr = new int[10];

    Foo[] fooArr = new Foo[10];
    for (int i = 0; i < fooArr.length; i++)
        fooArr[i] = new Foo();

The important thing to remember is that, despite the syntax, Java arrays don't actually behave that much like C arrays:

• The brackets in the declaration after the type, where they belong, not after the array name.
• Since arrays are objects, they are handled by reference.
• Declaring an array reference does not create an array---you must create a new array instance explicitly.
• Allocating an array of objects does not create instances for each array index. Instead, all the elements will be initialized to null. You must allocate an instance for each array location yourself.
• All arrays have a publicly accessible instance variable named length, so you can do safe in-bounds iteration over any array.