Decompiler tools modify the source code for .class file in java

Question

I used DJ JAVA DECOMPILER tool to get back the source code from .class file in java. What the source file it generated was having different code than what i coded earlier in the original source program.

What my doubt is:

1. Is this because of JVM does code optimization before creating the target code for better execution speed & reduce space and time complexity?
2. Or the decompiler tool modify the .class file code to generate the source program again?

See the original source program was this:

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.Scanner;


public class Test1 {
    /**
     * @param args
     * @throws SQLException
     * @throws ClassNotFoundException
     */
    public static void main(String args[]) throws SQLException, ClassNotFoundException{
        Class.forName("oracle.jdbc.driver.OracleDriver");
        Connection con= DriverManager.getConnection("jdbc:oracle:thin:@localhost:1521:choxx","choxx","choxx");
        if(con==null){
            System.out.println("not established");
        }else{
            System.out.println("established");
        }

        Statement st= con.createStatement();
        //st.executeQuery("create table student if not exists(sno number(10), name varchar2(30), addr varchar(20))");
        if(st!=null){
            System.out.println("table created..");
        }

        st.execute("delete from student where addr='hyderabad'");
        ResultSet rs= st.executeQuery("select * from student");
        while(rs.next()){
            System.out.println(rs.getInt(1)+"   "+rs.getString(2)+"   "+rs.getString(3));
        }


    }
}

After decompilation what i got is:

// Decompiled by DJ v3.12.12.99 Copyright 2015 Atanas Neshkov  Date: 29-03-2015 10:55:31
// Home Page:  http://www.neshkov.com/dj.html - Check often for new version!
// Decompiler options: packimports(3) 
// Source File Name:   Test1.java

import java.io.PrintStream;
import java.sql.*;

public class Test1
{

    public Test1()
    {
    }

    public static void main(String args[])
        throws SQLException, ClassNotFoundException
    {
        Class.forName("oracle.jdbc.driver.OracleDriver");
        Connection con = DriverManager.getConnection("jdbc:oracle:thin:@localhost:1521:choxx", "choxx", "choxx");
        if(con == null)
            System.out.println("not established");
        else
            System.out.println("established");
        Statement st = con.createStatement();
        if(st != null)
            System.out.println("table created..");
        st.execute("delete from student where addr='hyderabad'");
        for(ResultSet rs = st.executeQuery("select * from student"); rs.next(); System.out.println(rs.getInt(1) + "   " + rs.getString(2) + "   " + rs.getString(3)));
    }
}

Answer 1

Java Compiler does only, minimal set of optimization.

Most optimization is done by JIT compiler only.(Since it has the most information about the Platform and runtime environment.)

for-loops and while-loops use an identical pattern in byte code. This is not surprising because all while-loops can be re-written easily as an identical for-loop

http://blog.jamesdbloom.com/JavaCodeToByteCode_PartOne.html

Since in bytecode version,, the for loop and while loop looks same, the decompiler might decompile it into while loop.