I'm tring to install jdk 8 for weeks, but it fails.Following are some outputs,

sudo update-alternatives --config java

There is only one alternative in link group java (providing /usr/bin/java): /opt/java-oracle/jdk1.8.0/bin/java
Nothing to configure.

which java

   /usr/bin/java

java

 bash: /usr/local/java/jdk1.7.0_51/bin/java: No such file or directory

ls -l /usr/bin/java

 /usr/bin/java -> /etc/alternatives/java

ls -l /etc/alternatives/java

/etc/alternatives/java -> /opt/java-oracle/jdk1.8.0/bin/java

sudo update-alternatives --config javac

/opt/java-oracle/jdk1.8.0/bin/javac 1 manual mode

javac

  Error: could not find libjava.so
  Error: Could not find Java SE Runtime Environment.

If you can please give me a help to slove this problem. Thank You