Java notes

From OriWiki

JAR

create a new archive

jar cvf my_archive.jar *.class 

extract an archive

jar xvf my_archive.jar

In order to create a jar that contains a package, one needs to archive the complete folder with its internal hierarchy:

jar cvf my_package.jar my_package/

or

jar cvf my_package.jar my_package/path/to/files/*.class

(this still preserves the folder's hierarchy)

Class path

The environment variable CLASSPATH defines all the paths that javac and java treats as root_dir for packages.

Note that in DOS the set command shouldn't involve spaces:

set CLASSPATH=%CLASSPATH%;c:\my\new\path

and not:

set CLASSPATH = %CLASSPATH%;c:\my\new\path

One can add path also at compilation & execution:

javac -cp c:\progra~1\dtn\iqfeed LookupClient.java

java -cp c:\progra~1\dtn\iqfeed;. LookupClient

== ArrayList == (generics?) import java.util.*;

   ArrayList<A> lst = new ArrayList<A>();
   lst.add(new A());
   A a = lst.get(0); // note: no need for downcast due to <A>
   a.foo();

reflection

http://java.sun.com/developer/technicalArticles/ALT/Reflection/

   A a = new A();
   System.out.println(a.getClass().getName());

   static public void main(String[] args) throws Exception {
   Class c = Class.forName("A");
   Method[] ms = c.getDeclaredMethods();
   for(int i=0; i<ms.length; ++i)
       System.out.print(ms[i]+",");
   }

    Class ca = Class.forName("A"), cb = Class.forName("B");
    A a = new A();
    B b = new B();
    if(cb.isInstance(a))
        System.out.println("a is instacne of B");
    if(ca.isInstance(b))
        System.out.println("b is instance of A");

  import java.lang.reflect.*;

  public class Check {
     private int f1(
      Object p, int x) throws NullPointerException
     {
        if (p == null)
           throw new NullPointerException();
        return x;
     }
      
     public static void main(String args[])
     {
        try {
          Class cls = Class.forName("Check");
      
           Method methlist[] = cls.getDeclaredMethods();
           for (int i = 0; i < methlist.length; ++i) { 
              Method m = methlist[i];
              System.out.println("name = " + m.getName());
              System.out.println("decl class = " +  m.getDeclaringClass());
              Class pvec[] = m.getParameterTypes();
              for (int j = 0; j < pvec.length; j++)
                 System.out.println(" param #" + j + " " + pvec[j]);
              Class evec[] = m.getExceptionTypes();
              for (int j = 0; j < evec.length; j++)
                 System.out.println("exc #" + j + " " + evec[j]);
              System.out.println("return type = " + m.getReturnType());
              System.out.println("-----");
           }
        }
        catch (Throwable e) {
           System.err.println(e);
        }
     }
  }

import java.lang.reflect.*;

  public class Check {
     private double d;
     public static final int i = 37;
     String s = "testing";
      
     public static void main(String args[]) throws Throwable
     {
           Class cls = Class.forName("Check");
      
           Field fieldlist[] = cls.getDeclaredFields();

           for (int i = 0; i < fieldlist.length; i++) {
              Field fld = fieldlist[i];
              System.out.println("name = " + fld.getName());
              System.out.println("decl class = " +
                          fld.getDeclaringClass());
              System.out.println("type = " + fld.getType());
              int mod = fld.getModifiers();
              System.out.println("modifiers = " + mod + "," +
                         Modifier.toString(mod));
              System.out.println("-----");
           }
      }
  }

invoke mwthods

import java.lang.reflect.*;
       
public class Check {
   
    public static void main(String args[]) throws Throwable {
    Class<?> c = Class.forName("A"); // the <?> is required to avoid a generic warning..
    Class[] params = {Integer.TYPE,Integer.TYPE};
    Method m = c.getMethod("add",params);
    Object[] as = {new Integer(2), new Integer(3)};
    A a = new A();
    Object o = m.invoke(a,as);
    System.out.println(o);
    }
}

class A {
    public int add(int a, int b) {
    return a + b;
    }
}

jdk download

• I've downloaded 1.5 from here