From .java to .smali file

Ask: Why would you want to do that?
Answer: For learning how java language is represented in smali

$ javac Hello.java
$ dx --dex --output=Hello.dex Hello.class
$ baksmali Hello.dex

There'll be a directory out/ and a file in it Hello.smali

For example, the content of Hello.java

public class Hello {
        public static void main(String [] args) {
                System.out.println("Aa");
        }
}

Then, the content of out/Hello.smali

.class public LHello;
.super Ljava/lang/Object;
.source "Hello.java"

# direct methods
.method public constructor <init>()V
    .registers 1
    .prologue
    .line 1
    invoke-direct {p0}, Ljava/lang/Object;-><init>()V
    return-void
.end method
.method public static main([Ljava/lang/String;)V
    .registers 3
    .prologue
    .line 3
    sget-object v0, Ljava/lang/System;->out:Ljava/io/PrintStream;
    const-string v1, "Aa"
    invoke-virtual {v0, v1}, Ljava/io/PrintStream;->println(Ljava/lang/String;)V
    .line 4
    return-void
.end method

Note: The dx command was from Android SDK build tools, the version of dx and the version of Java must be compatible, or there'll be issues.