How to install Java on two major Linux distributions.

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+# Installing Java on Ubuntu
+
+Accurate as of: Java 7, Ubuntu 12.04. The instructions below assume an amd64
+(64-bit) installation. If you're still using a 32-bit OS, work out the
+differences yourself.
+
+## Via Package Management (Apt)
+
+OpenJDK 7 is available via apt by default.
+
+To install the JDK:
+
+    sudo aptitude update
+    sudo aptitude install openjdk-7-jdk
+
+To install the JRE only (without the JDK):
+
+    sudo aptitude update
+    sudo aptitude install openjdk-7-jre
+
+To install the JRE without GUI support (appropriate for headless servers):
+
+    sudo aptitude update
+    sudo aptitude install openjdk-7-jre-headless
+
+(You can also use `apt-get` instead of `aptitude`.)
+
+These packages interact with [the `alternatives`
+system](http://manpages.ubuntu.com/manpages/hardy/man8/update-alternatives.8.html),
+and have [a dedicated `alternatives` manager
+script](http://manpages.ubuntu.com/manpages/hardy/man8/update-java-alternatives.8.html).
+The `alternatives` system affects `/usr/bin/java`, `/usr/bin/javac`, and
+browser plugins for applets and Java Web Start applications for browsers
+installed via package management. It also affects the symlinks under
+`/etc/alternatives` related to Java.
+
+To list Java versions available, with at least one Java version installed via
+Apt:
+
+    update-java-alternatives --list
+
+To switch to `java-1.7.0-openjdk-amd64` for all Java invocations:
+
+    update-java-alternatives --set java-1.7.0-openjdk-amd64
+
+The value should be taken from the first column of the `--list` output.
+
+### Tool support
+
+Most modern Java tools will pick up the installed JDK via `$PATH` and do not
+need the `JAVA_HOME` environment variable set explicitly. For applications old
+enough not to be able to detect the JDK, you can set `JAVA_HOME` to
+`/usr/lib/jvm/java-1.7.0-openjdk-amd64`.
+
+## By Hand
+
+The [Java SE Development Kit
+7](http://www.oracle.com/technetwork/java/javase/downloads/jdk7-downloads-1880260.html)
+tarballs can be installed by hand. Download the "Linux x64" `.tar.gz` version,
+then unpack it in `/opt`:
+
+    cd /opt
+    tar xzf ~/jdk-7u45-linux-x64.tar.gz
+
+This will create a directory named `/opt/jdk1.7.0_45` (actual version number
+may vary) containing a ready-to-use Java dev kit.
+
+You will need to add the JDK's `bin` directory to `PATH` if you want commands
+like `javac` and `java` to work without fully-qualifying the directory:
+
+    cat > /etc/profile.d/oracle_jdk <<'ORACLE_JDK'
+    PATH="${PATH}:/opt/jdk1.7.0_45/bin"
+    export PATH
+    ORACLE_JDK
+
+(This will not affect non-interactive use; setting PATH for non-interactive
+programs like build servers is beyond the scope of this document. Learn to use
+your OS.)
+
+Installation this way does _not_ interact with the alternatives system (but
+you can set that up by hand if you need to).
+
+For tools that cannot autodetect the JDK via `PATH`, you may need to set
+`JAVA_HOME` to `/opt/jdk1.7.0_45`.