Ubuntu 出现apt-get: Package has no installation candidate问题

When I run

sudo apt-get install cmake

I get the following response:

Package cmake is not available, but is referred to by another package.This may mean the package is missing, has been obsoleted, or is only available from another sourceE: Package 'cmake' has no installation candidate

解决方法如下：
# apt-get update
# apt-get upgrade
# apt-get install <packagename>
这样就可以正常使用apt-get了～