Presents your 
JAVA E-NEWSLETTER for August 5, 2002

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USE JAVAP FOR MORE THAN JUST DOCUMENTATION

Javadoc allows you to view documentation on Java, but it's primarily
aimed towards Web clients. Fortunately, there are other ways to see
documentation on Java, such as the javap command.

Javap is a Java class dissassembler. It can disassemble a class file
into a bytecode representation, which gives you some understanding of 
what
an implementation is doing without actually decompiling it. Unlike
javadoc, it uses the class file and not the source file. This means you 
can use
it on the classes in a jar without having any source available.

The standard way to run javap is to type:

javap <classname>

on the command line. For example:

javap java.lang.Object

will output the following:

Compiled from Object.java
public class java.lang.Object {
    public java.lang.Object( );
    public final native java.lang.Class getClass( );
    public native int hashCode( );
    public boolean equals(java.lang.Object);
    protected native java.lang.Object clone( ) throws
java.lang.CloneNotSupportedException;
    public java.lang.String toString( );
    public final native void notify( );
    public final native void notifyAll( );
    public final native void wait(long) throws
java.lang.InterruptedException;
    public final void wait(long, int) throws
java.lang.InterruptedException;
    public final void wait( ) throws java.lang.InterruptedException;
    protected void finalize( ) throws java.lang.Throwable;
    static { };
}

Disassembling code can prove to be useful, such as when the 
Object.wait(
) method is run through 'javap -c':

Method void wait( )
   0 aload_0
   1 lconst_0
   2 invokevirtual #16 <Method void wait(long)>
   5 return

Without even having any great understanding of the disassembled code,
it's easy to see that Object.wait( ) simply calls Object.wait(long).
However, the actual value that's passed to the wait(long) method is not
available.

While disassembling is a useful feature, the most common usage of javap
is as a quick, easy way to see what methods are available on a class.
Even with the source and javadoc available, it's often easier to use javap
from the command line.

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