before install, you should update your JRE and JDK to 8:
Final Update
JDK
<span class="pln">sudo apt</span><span class="pun">-</span><span class="pln">get install openjdk</span><span class="pun">-</span><span class="lit">8</span><span class="pun">-</span><span class="pln">jdk</span>JRE
<span class="pln">sudo apt</span><span class="pun">-</span><span class="pln">get install openjdk</span><span class="pun">-</span><span class="lit">8</span><span class="pun">-</span><span class="pln">jre</span>Old Update
I found two repository but I do not recommend
-
OpenJDK builds (all archs) <pre class="lang-java prettyprint prettyprinted">
<span class="pln">ppa</span><span class="pun">:</span><span class="pln">openjdk</span><span class="pun">-</span><span class="pln">r</span><span class="pun">/</span><span class="pln">ppa</span></pre> -
OpenJDK 8 backport for trusty <pre class="lang-java prettyprint prettyprinted">
<span class="pln">ppa</span><span class="pun">:</span><span class="pln">jochenkemnade</span><span class="pun">/</span><span class="pln">openjdk</span><span class="pun">-</span><span class="lit">8</span></pre>
Original Message
If you really want to use OpenJDK, you have to compile from source. There is not still any PPA for OpenJDK.
It has been requested at https://bugs.launchpad.net/ubuntu/+bug/1297065
I recommend you to use Webup8 Oracle Java8 Installer
<span class="pln">sudo add</span><span class="pun">-</span><span class="pln">apt</span><span class="pun">-</span><span class="pln">repository ppa</span><span class="pun">:</span><span class="pln">webupd8team</span><span class="pun">/</span><span class="pln">java </span><span class="pun">-</span><span class="pln">y
sudo apt</span><span class="pun">-</span><span class="pln">get update
sudo apt</span><span class="pun">-</span><span class="pln">get install oracle</span><span class="pun">-</span><span class="pln">java8</span><span class="pun">-</span><span class="pln">installer</span>To automatically set up the Java 8 environment variables
<span class="pln">sudo apt</span><span class="pun">-</span><span class="pln">get install oracle</span><span class="pun">-</span><span class="pln">java8</span><span class="pun">-</span><span class="pln">set</span><span class="pun">-</span><span class="kwd">default</span>Check it
<span class="pln">java </span><span class="pun">-</span><span class="pln">version</span>So you have to wait to use OpenJDK8
Then download eclipse from https://www.eclipse.org/downloads/
unpack and install it