The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
The Java compiler also erases type parameters in generic method arguments. Consider the following generic method:
// Counts the number of occurrences of elem in anArray.
//
public static <T> int count(T[] anArray, T elem) {
int cnt = 0;
for (T e : anArray)
if (e.equals(elem))
++cnt;
return cnt;
}
Because T is unbounded, the Java compiler replaces it with Object:
public static int count(Object[] anArray, Object elem) {
int cnt = 0;
for (Object e : anArray)
if (e.equals(elem))
++cnt;
return cnt;
}
Suppose the following classes are defined:
class Shape { /* ... */ }
class Circle extends Shape { /* ... */ }
class Rectangle extends Shape { /* ... */ }
You can write a generic method to draw different shapes:
public static <T extends Shape> void draw(T shape) { /* ... */ }
The Java compiler replaces T with Shape:
public static void draw(Shape shape) { /* ... */ }