We can view the current java version installed in our system with:

java -version

My java version:

java version "1.7.0_141"
OpenJDK Runtime Environment (rhel- u141-b02)
OpenJDK 64-Bit Server VM (build 24.141-b02, mixed mode)

Viewing the list of Java version installed and selecting the default one

sudo update-alternatives --config java

This command will give you the list of installed Java versions.

My output was:

There are 2 programs which provide 'java'.

  Selection    Command
*+ 1           java-1.7.0-openjdk.x86_64 (/usr/lib/jvm/java-1.7.0-openjdk-
   2           /usr/java/jdk1.8.0_131/jre/bin/java

1 is the selected currently in use, to make use of 2 typing 2 and pressing enter will choose jdk1.8.0_131 for me.

Now, if I do

java -version

The output is:

java version "1.8.0_131"
Java(TM) SE Runtime Environment (build 1.8.0_131-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.131-b11, mixed mode)