6/19/2008

java pkg.Main or java pkg/Main?

Which format do you use to run a java application? I always pass the fully-qualified class name to java command, namely, java com.whatever.Main. In fact, either way is acceptable:

build/classes > java javahowto.Member
build/classes > java javahowto/Member
I've tried java command with both forms in JDK 1.4, JDK 5, and JDK 6 on Linux without any error. On Windows, you can use either pkg.Main, or pkg/Main, but not pkg\Main:
C:\javahowto\build\classes>java javahowto.Main
In Main.main(String[])
C:\javahowto\build\classes>java javahowto/Main
In Main.main(String[])
C:\javahowto\build\classes>java javahowto\Main
Exception in thread "main" java.lang.NoClassDefFoundError: javahowto\Main (wrong name: javahowto/Main)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:620)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
at java.net.URLClassLoader.access$000(URLClassLoader.java:56)
at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:276)
at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
In any event, do not include .class in the class name. When running from the root of class directory on Unix, one can use the shell auto-completion to fill the class name, instead of typing the FQN. That's a big plus for the the second format. On Windows, auto-complete doesn't help in this case, since \ is class name is not accepted. I guess from now on, I will be using the slash form more often.

Unlike java command, javac operates on java source files and always takes file paths as arguments.

6/08/2008

Java generics examples -- parameterized class

Not only can Java generics be used in fields and methods, it can also be used in class definitions to produce parameterized classes. Since JDK 1.5, many built-in Java classes have been retrofitted to parameterized forms. An example is java.lang.Class, which is declared to be public final class Class<T>, the parameter type <T> represents the actual type modeled by this Class object. One benefit is that some of its methods can take or return specific types, avoiding casting while enforcing type-check.

It's also easy to write your own parameterized class. The following is a Member class whose Id field can be parameterized to String, Integer, etc:

package javahowto;
public class Member<T> {
private T id;
public Member(T id) {
this.id = id;
}
public T getId() {
return id;
}
public void setId(T id) {
this.id = id;
}
public static void main(String[] args) {
Member<String> mString = new Member<String>("id1");
mString.setId("id2");
System.out.printf("id after setting id: %s%n", mString.getId());
//output: id after setting id: id2

Member<Integer> mInteger = new Member<Integer>(1);
mInteger.setId(2);
System.out.printf("id after setting id: %d%n", mInteger.getId());
//output: id after setting id: 2
}