javap lets you examine java class files and jar files in a number of ways. See this web page for more information. For me, it’s an API reference. I use it in two ways:

1. When I’m coding, and I need to know the exact syntax of a method, I shell out: javap java.util.StringTokenizer. (Yes, I know that any modern IDE will do this for you without shelling out, but javap will work anywhere java is installed and with any editing tool. You trade portability for convenience.) One large catch is that inherited methods are not shown:

$ javap java.io.BufferedReader
Compiled from "BufferedReader.java"
public class java.io.BufferedReader extends java.io.Reader{
public int read();
throws java/io/IOException
static {};
public void close();
throws java/io/IOException
public void reset();
throws java/io/IOException
public boolean markSupported();
public boolean ready();
throws java/io/IOException
public void mark(int);
throws java/io/IOException
public long skip(long);
throws java/io/IOException
public int read(char[],int,int);
throws java/io/IOException
public java.io.BufferedReader(java.io.Reader);
public java.io.BufferedReader(java.io.Reader,int);
public java.lang.String readLine();
throws java/io/IOException
java.lang.String readLine(boolean);
throws java/io/IOException
}

Running javap on java.io.BufferedReader does not show the method read(char[]), inherited from java.io.Reader. (This example is from the J2SE 1.4 libraries.)

2. Sometimes, the javadoc is too up-to-date (or your jar files are too old) to answer questions about an API. For example, I’m working on a project with Jetspeed which depends on Turbine version 2.2. Unfortunately, this is an extremely old version of Turbine (release 16-Aug-2003), and the javadoc doesn’t appear to be available. (Updated Dec 11: It looks like the Turbine 2.2 javadoc is indeed online. Whoops.) Generating the javadoc with ant is certainly an possibility, and if I found myself going time and again to verify the API of Turbine 2.2, I’d do that. But for a quick one- or two-off question about an API that no web search turns up, javap can be very handy.

In short, if you have a quick question about an API, javap can help you out.


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