Generic List in Java
Jakob Jenkov |
Java's List interface (java.util.List) can be generified. In other words, instances of List can be given a type, so only instances of that type can be inserted and read from that List. Here is an example:
List<String> list = new ArrayList<String>;
This list is now targeted at only String instances, meaning only String instances can be put into this list. If you try to put something else into this List, the compiler will complain.
The generic type checks only exists at compile time. At runtime it is possible to tweak your code so that a String List has other objects that String's inserted. This is a bad idea, though.
Accessing a Generic List
You can get and insert the elements of a generic List like this:
List<String> list = new ArrayList<String>; String string1 = "a string"; list.add(string1); String string2 = list.get(0);
Notice how it is not necessary to cast the object obtained from the List.get() method call, as is normally necessary. The compiler knows that this List can only contain String instances, so casts are not necessary.
Iterating a Generic List
You can iterate a generic List using an iterator, like this:
List<String> list = new ArrayList<String>; Iterator<String> iterator = list.iterator(); while(iterator.hasNext()){ String aString = iterator.next(); } Notice how it is not necessary to cast the object returned from the iterator.next() next call. Because the List is generified (has a type), the compiler knows that it contains String instances. Therefore it is not necessary to cast the objects obtained from it, even if it comes from its Iterator.
You can also use the new for-loop, like this:
List<String> list = new ArrayList<String>; for(String aString : list) { System.out.println(aString); } Notice how a String variable is declared inside the parantheses of the for-loop. For each iteration (each element in the List) this variable contains the current element (current String).
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Jakob Jenkov | |
