/usr/sbin/update-java-alternatives is in java-common 0.58.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 | #! /bin/bash
shopt -s dotglob
prog=$(basename $0)
usage()
{
    rv=$1
    cat >&2 <<-EOF
	usage: $prog [--jre-headless] [--jre] [--plugin] [-v|--verbose]
	           -l|--list [<jname>]
	           -s|--set <jname>
	           -a|--auto
	           -h|-?|--help
	EOF
    exit $rv
}
action=
uaopts='--quiet'
while [ "$#" -gt 0 ]; do
    case "$1" in
      -a|--auto)
	[ -z "$action" ] || usage 1
	action=auto;;
      -hl|--jre-headless)
	hlonly=yes;;
      -j|--jre)
	jreonly=yes;;
      --plugin)
	pluginonly=yes;;
      -l|--list)
	[ -z "$action" ] || usage 1
	action=list
	if [ "$#" -gt 0 ]; then
	    shift
	    jname=$1
	fi
	;;
      -s|--set)
	[ -z "$action" ] || usage 1
	action=set
	shift
	[ "$#" -gt 0 ] || usage 1
	jname=$1
	;;
      -v|--verbose)
	verbose=yes
	uaopts="${uaopts/--quiet/}";;
      -h|-?|--help)
	usage 0;;
      -*)
	echo "X: $1"
	usage 1;;
      *)
	break;;
    esac
    shift
done
[ "$#" -eq 0 ] || usage 1
[ -n "$action" ] || usage 1
which='^(hl|jre|jdk|jdkhl|plugin|DUMMY) '
if [ -n "$hlonly$jreonly$pluginonly" ]; then
    which='^('
    [ -n "$hlonly" ] && which="${which}hl|"
    [ -n "$jreonly" ] && [ -n "$hlonly" ] && which="${which}jre|"
    [ -n "$jreonly" ] && [ -z "$hlonly" ] && which="${which}hl|jre|"
    [ -n "$pluginonly" ] && which="${which}plugin|"
    which="${which}DUMMY) "
fi
top=/usr/lib/jvm
if [ -n "$jname" ]; then
    arch=$(dpkg --print-architecture)
    if [ ! -x $top/$jname/bin/java ] && [ -x $top/$jname-$arch/bin/java ]; then
	echo >&2 "$prog: obsolete <jname>, please use $jname-$arch instead"
	jname=$jname-$arch
    fi
    case "$jname" in
	/*) jdir=$jname; jname=$(basename $jdir);;
	*)  jdir=$top/$jname
    esac
    if [ ! -d $jdir ]; then
	echo >&2 "$prog: directory does not exist: $jdir"
	exit 1
    fi
    if [ -f $top/.$jname.jinfo ]; then
	jinfo=$top/.$jname.jinfo
    elif [ -f $top/$jname.jinfo ]; then
	jinfo=$top/$jname.jinfo
    else
	echo >&2 "$prog: file does not exist: $top/.$jname.jinfo"
	exit 1
    fi
fi
vecho()
{
    [ -z "$verbose" ]  || echo >&2 "$@"
}
jinfo_files=
do_auto()
{
    vecho "resetting java alternatives"
    awk "/$which/ {print \$2}" $top/*.jinfo | sort -u \
	| \
    while read name; do
	update-alternatives $uaopts --auto $name
    done
}
do_list()
{
    vecho "listing java alternatives:"
    for i in ${jinfo:-$top/*.jinfo}; do
	alias=$(basename ${i%.jinfo})
	alias=${alias#.}
	prio=$(awk -F= '/priority=/ {print $2}' $i)
	printf "%-30s %-10s %s\n" $alias "$prio " $top/$alias
	[ -n "$verbose" ] && egrep "$which" $i
    done
}
do_set()
{
    do_auto
    awk "/$which/ {print}" $jinfo | sort -u \
	| \
    while read type name location; do
	if [ $type = jdk -a -z "$hlonly$jreonly$pluginonly" -a ! -f $location ]; then
	    echo >&2 "$prog: jdk alternative does not exist: $location"
	    continue
	fi
	if [ $type = plugin -a -z "$hlonly$jreonly" -a ! -f $location ]; then
	    echo >&2 "$prog: plugin alternative does not exist: $location"
	    continue
	fi
	update-alternatives $uaopts --set $name $location
    done
}
if [ "$action" != list ] && [ "$(id -u)" -ne 0 ]; then
    echo >&2 "$prog: no root privileges"
    exit 1
fi
do_$action
 |