Ubuntu安装JDK7
CLASSPATH=.:/usr/lib/jvm/jdk1.7.0/lib ?
JAVA_HOME=/usr/lib/jvm/jdk1.7.0 ?

PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/lib/jvm/jdk1.7.0/bin"
CLASSPATH=.:/usr/lib/jvm/jdk1.7.0/lib
JAVA_HOME=/usr/lib/jvm/jdk1.7.0

And then reload .bashrc file:

source ~/.bashrc


5. Run the following commands:?

$sudo update-alternatives --install /usr/bin/java java /usr/lib/jvm/jdk1.7.0/bin/java 300
$sudo update-alternatives --install /usr/bin/javac javac /usr/lib/jvm/jdk1.7.0/bin/javac 300
$sudo update-alternatives --install /usr/bin/javap javap /usr/lib/jvm/jdk1.7.0/bin/javap 300
$sudo update-alternatives --install /usr/bin/javadoc javadoc /usr/lib/jvm/jdk1.7.0/bin/javadoc 300

Installation has now done. Run $java -version for a test.