How can I add code to .class files in java

Question

I have a class file and I want to add some code to it. I decompiled it and got this

package net.minecraft.client.main;

import com.google.gson.Gson;
import java.util.List;
import joptsimple.OptionSet;
import java.lang.reflect.Type;
import com.mojang.authlib.properties.PropertyMap;
import com.google.gson.GsonBuilder;
import java.net.Authenticator;
import java.net.SocketAddress;
import java.net.URISyntaxException;
import java.net.InetSocketAddress;
import java.net.Proxy;
import joptsimple.OptionSpec;
import java.io.File;
import joptsimple.OptionParser;
import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;

public class Main
{
    public static void main(final String[] u2603) {

        System.setProperty("java.net.preferIPv4Stack", "true");
        final OptionParser optionParser = new OptionParser();
        optionParser.allowsUnrecognizedOptions();
        optionParser.accepts("demo");
        optionParser.accepts("fullscreen");
        optionParser.accepts("checkGlErrors");
        final OptionSpec<String> withRequiredArg = (OptionSpec<String>)optionParser.accepts("server").withRequiredArg();
        final OptionSpec<Integer> defaultsTo = (OptionSpec<Integer>)optionParser.accepts("port").withRequiredArg().ofType((Class)Integer.class).defaultsTo((Object)25565, (Object[])new Integer[0]);
        final OptionSpec<File> defaultsTo2 = (OptionSpec<File>)optionParser.accepts("gameDir").withRequiredArg().ofType((Class)File.class).defaultsTo((Object)new File("."), (Object[])new File[0]);
        final OptionSpec<File> ofType = (OptionSpec<File>)optionParser.accepts("assetsDir").withRequiredArg().ofType((Class)File.class);
        final OptionSpec<File> ofType2 = (OptionSpec<File>)optionParser.accepts("resourcePackDir").withRequiredArg().ofType((Class)File.class);
        final OptionSpec<String> withRequiredArg2 = (OptionSpec<String>)optionParser.accepts("proxyHost").withRequiredArg();
        final OptionSpec<Integer> ofType3 = (OptionSpec<Integer>)optionParser.accepts("proxyPort").withRequiredArg().defaultsTo((Object)"8080", (Object[])new String[0]).ofType((Class)Integer.class);
        final OptionSpec<String> withRequiredArg3 = (OptionSpec<String>)optionParser.accepts("proxyUser").withRequiredArg();
        final OptionSpec<String> withRequiredArg4 = (OptionSpec<String>)optionParser.accepts("proxyPass").withRequiredArg();
        final OptionSpec<String> defaultsTo3 = (OptionSpec<String>)optionParser.accepts("username").withRequiredArg().defaultsTo((Object)("Player" + ave.J() % 1000L), (Object[])new String[0]);
        final OptionSpec<String> withRequiredArg5 = (OptionSpec<String>)optionParser.accepts("uuid").withRequiredArg();
        final OptionSpec<String> required = (OptionSpec<String>)optionParser.accepts("accessToken").withRequiredArg().required();
        final OptionSpec<String> required2 = (OptionSpec<String>)optionParser.accepts("version").withRequiredArg().required();
        final OptionSpec<Integer> defaultsTo4 = (OptionSpec<Integer>)optionParser.accepts("width").withRequiredArg().ofType((Class)Integer.class).defaultsTo((Object)854, (Object[])new Integer[0]);
        final OptionSpec<Integer> defaultsTo5 = (OptionSpec<Integer>)optionParser.accepts("height").withRequiredArg().ofType((Class)Integer.class).defaultsTo((Object)480, (Object[])new Integer[0]);
        final OptionSpec<String> defaultsTo6 = (OptionSpec<String>)optionParser.accepts("userProperties").withRequiredArg().defaultsTo((Object)"{}", (Object[])new String[0]);
        final OptionSpec<String> defaultsTo7 = (OptionSpec<String>)optionParser.accepts("profileProperties").withRequiredArg().defaultsTo((Object)"{}", (Object[])new String[0]);
        final OptionSpec<String> withRequiredArg6 = (OptionSpec<String>)optionParser.accepts("assetIndex").withRequiredArg();
        final OptionSpec<String> defaultsTo8 = (OptionSpec<String>)optionParser.accepts("userType").withRequiredArg().defaultsTo((Object)"legacy", (Object[])new String[0]);
        final OptionSpec<String> nonOptions = (OptionSpec<String>)optionParser.nonOptions();
        final OptionSet parse = optionParser.parse(u2603);
        final List<String> values = (List<String>)parse.valuesOf((OptionSpec)nonOptions);
        if (!values.isEmpty()) {
            System.out.println("Completely ignored arguments: " + values);
        }
        final String hostname = (String)parse.valueOf((OptionSpec)withRequiredArg2);
        Proxy no_PROXY = Proxy.NO_PROXY;
        if (hostname != null) {
            try {
                no_PROXY = new Proxy(Proxy.Type.SOCKS, new InetSocketAddress(hostname, (int)parse.valueOf((OptionSpec)ofType3)));
            }
            catch (Exception ex) {}
        }
        final String u26032 = (String)parse.valueOf((OptionSpec)withRequiredArg3);
        final String u26033 = (String)parse.valueOf((OptionSpec)withRequiredArg4);
        if (!no_PROXY.equals(Proxy.NO_PROXY) && a(u26032) && a(u26033)) {
            Authenticator.setDefault((Authenticator)new Main.Main$1(u26032, u26033));
        }
        final int intValue = (int)parse.valueOf((OptionSpec)defaultsTo4);
        final int intValue2 = (int)parse.valueOf((OptionSpec)defaultsTo5);
        final boolean has = parse.has("fullscreen");
        final boolean has2 = parse.has("checkGlErrors");
        final boolean has3 = parse.has("demo");
        final String s = (String)parse.valueOf((OptionSpec)required2);
        final Gson create = new GsonBuilder().registerTypeAdapter((Type)PropertyMap.class, (Object)new PropertyMap.Serializer()).create();
        final PropertyMap propertyMap = (PropertyMap)create.fromJson((String)parse.valueOf((OptionSpec)defaultsTo6), (Class)PropertyMap.class);
        final PropertyMap propertyMap2 = (PropertyMap)create.fromJson((String)parse.valueOf((OptionSpec)defaultsTo7), (Class)PropertyMap.class);
        final File file = (File)parse.valueOf((OptionSpec)defaultsTo2);
        final File file2 = (File)(parse.has((OptionSpec)ofType) ? parse.valueOf((OptionSpec)ofType) : new File(file, "assets/"));
        final File file3 = (File)(parse.has((OptionSpec)ofType2) ? parse.valueOf((OptionSpec)ofType2) : new File(file, "resourcepacks/"));
        final String s2 = (String)(parse.has((OptionSpec)withRequiredArg5) ? withRequiredArg5.value(parse) : ((String)defaultsTo3.value(parse)));
        final String s3 = parse.has((OptionSpec)withRequiredArg6) ? ((String)withRequiredArg6.value(parse)) : null;
        final String s4 = (String)parse.valueOf((OptionSpec)withRequiredArg);
        final Integer n = (Integer)parse.valueOf((OptionSpec)defaultsTo);
        final avm avm = new avm((String)defaultsTo3.value(parse), s2, (String)required.value(parse), (String)defaultsTo8.value(parse));
        final bao bao = new bao(new bao.e(avm, propertyMap, propertyMap2, no_PROXY), new bao.a(intValue, intValue2, has, has2), new bao.b(file, file3, file2, s3), new bao.c(has3, s), new bao.d(s4, (int)n));
        Runtime.getRuntime().addShutdownHook((Thread)new Main.Main$2("Client Shutdown Thread"));
        Thread.currentThread().setName("Client thread");
        new ave(bao).a();
    }
    
    private static boolean a(final String u2603) {
        return u2603 != null && !u2603.isEmpty();
    }
}

I want to add some code at the start of the main function

String jarPath;
try {
    File BetterSettings = new File(System.getProperty("user.dir"), "BetterSettings.txt");
    
    jarPath = Main.class
             .getProtectionDomain()
             .getCodeSource()
             .getLocation()
             .toURI()
             .getPath();
     System.out.println("JAR Path : " + jarPath);
     try {
        PrintWriter printwriter = new PrintWriter(new FileWriter(BetterSettings));
        String[] texts= jarPath.split("/");
        String version = texts[texts.length-1];
        printwriter.println(jarPath);
        printwriter.println(version);
        printwriter.close();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

} catch (URISyntaxException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
}

I add the code in, but when I go to recompile it in cmd it doesn't work.

Is there any way for me to decompile 1 class file add code and recompile? The class file was part of a whole jar file. Do I need to decompile the whole jar file?