Run

sudo update-java-alternatives --list

to list off all the Java installations on a machine by name and directory,and then run

sudo update-java-alternatives --set [JDK/JRE name e.g. java-8-oracle]

to choose which JRE/JDK to use.

If you want to use different JDKs/JREs for each Java task,you can run update-alternatives to configure one java executable at a time; you can run

sudo update-alternatives --config java[Tab]

to see the Java commands that can be configured (java,javac,javah,javaws,etc). And then

sudo update-alternatives --config [javac|java|javadoc|etc.]

will associate that Java task/command to a particular JDK/JRE.

You may also need to set JAVA_HOME for some applications: from this answer you can use

export JAVA_HOME=$(readlink -f /usr/bin/java | sed "s:bin/java::")

for JREs,or

export JAVA_HOME=$(readlink -f /usr/bin/java | sed "s:jre/bin/java::")

for JDKs.