javadoc: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64/bin/javadoc - from package: gcj-4.8-jdk
/usr/lib/jvm/java-1.5.0-gcj-4.9-amd64/bin/javadoc - from package: gcj-4.9-jdk
/usr/lib/debug/usr/lib/jvm/java-7-openjdk-amd64/bin/javadoc - from package: openjdk-7-dbg
/usr/lib/jvm/java-7-openjdk-amd64/bin/javadoc - from package: openjdk-7-jdk

You may install the required package with command apt-get install {package}

Error in other languages:
javadoc: bevel nie gevind nie
javadoc: comandă negăsită
javadoc: comando não encontrado
javadoc: commande introuvable
javadoc: command not found
javadoc: käsku ei ole
javadoc: không tìm thấy lệnh
javadoc: komanda nerasta
javadoc: Kommando nicht gefunden.
javadoc: kommandot finns inte
javadoc: komut yok
javadoc: nie znaleziono polecenia
javadoc: níor aimsíodh an t-ordú
javadoc: no se encontró la orden
javadoc: no s'ha trobat l'ordre
javadoc: opdracht niet gevonden
javadoc: parancs nem található
javadoc: perintah tidak ditemukan
javadoc: príkaz nenájdený
javadoc: příkaz nenalezen
javadoc: команда не найдена
javadoc: командата не е открита
javadoc: 命令找不到
javadoc: コマンドが見つかりません